Transcriber's Note

The punctuation and spelling from the original text have been faithfullypreserved. Only obvious typographical errors have been corrected. Theadvertisement from the beginning of the book has been joined with theother advertisements near the end of the book. 

Greek words are spelled out and represented as [word]. Greek letters arerepresented as [a] "for alpha". 

ASTRONOMY OF TO-DAY

[Illustration: THE TOTAL ECLIPSE OF THE SUN OF AUGUST 30TH, 1905. 

The Corona; from a water-colour sketch, made at Burgos, in Spain, duringthe total phase, by the French Artist, Mdlle. ANDRÉE MOCH. ]

 ASTRONOMY OF TO-DAY

 _A POPULAR INTRODUCTION IN NON-TECHNICAL LANGUAGE_

 By

 CECIL G. DOLMAGE, M. A. , LL. D. , D. C. L. 

 Fellow of the Royal Astronomical Society; Member of the British Astronomical Association; Member of the Astronomical Society of the Pacific; Membre de la Société Astronomique de France; Membre de la Société Belge d'Astronomie

 With a Frontispiece in Colour and 45 Illustrations & Diagrams

 _THIRD EDITION_

 LONDON SEELEY AND CO. LIMITED 38 GREAT RUSSELL STREET 1910

PREFACE

The object of this book is to give an account of the science ofAstronomy, as it is known at the present day, in a manner acceptable tothe _general reader_. 

It is too often supposed that it is impossible to acquire any usefulknowledge of Astronomy without much laborious study, and withoutadventuring into quite a new world of thought. The reasoning applied tothe study of the celestial orbs is, however, of no different order fromthat which is employed in the affairs of everyday life. The science ofmathematics is perhaps responsible for the idea that some kind ofdifference does exist; but mathematical processes are, in effect, nomore than ordinary logic in concentrated form, the _shorthand ofreasoning_, so to speak. I have attempted in the following pages to takethe main facts and theories of Astronomy out of those mathematical formswhich repel the general reader, and to present them in the _ordinarylanguage of our workaday world_. 

The few diagrams introduced are altogether supplementary, and are notconnected with the text by any wearying cross-references. Each diagramis complete in itself, being intended to serve as a pictorial aid, incase the wording of the text should not have perfectly conveyed thedesired meaning. The full page illustrations are also described asadequately as possible at the foot of each. 

As to the coloured frontispiece, this must be placed in a category byitself. It is the work of the _artist_ as distinct from the scientist. 

The book itself contains incidentally a good deal of matter concernedwith the Astronomy of the past, the introduction of which has been foundnecessary in order to make clearer the Astronomy of our time. 

It would be quite impossible for me to enumerate here the many sourcesfrom which information has been drawn. But I acknowledge my especialindebtedness to Professor F. R. Moulton's _Introduction to Astronomy_(Macmillan, 1906), to the works on Eclipses of the late Rev. S. J. Johnson and of Mr. W. T. Lynn, and to the excellent _Journals of theBritish Astronomical Association_. Further, for those grand questionsconcerned with the Stellar Universe at large, I owe a very deep debt tothe writings of the famous American astronomer, Professor Simon Newcomb, and of our own countryman, Mr. John Ellard Gore; to the latter of whom Iam under an additional obligation for much valuable informationprivately rendered. 

In my search for suitable illustrations, I have been greatly aided bythe kindly advice of Mr. W. H. Wesley, the Assistant Secretary of theRoyal Astronomical Society. To those who have been so good as to permitme to reproduce pictures and photographs, I desire to record my bestthanks as follows:--To the French Artist, Mdlle. Andrée Moch; to theAstronomer Royal; to Sir David Gill, K. C. B. , LL. D. , F. R. S. ; to theCouncil of the Royal Astronomical Society; to Professor E. B. Frost, Director of the Yerkes Observatory; to M. P. Puiseux, of the ParisObservatory; to Dr. Max Wolf, of Heidelberg; to Professor PercivalLowell; to the Rev. Theodore E. R. Phillips, M. A. , F. R. A. S. ; to Mr. W. H. Wesley; to the Warner and Swasey Co. , of Cleveland, Ohio, U. S. A. ; to thepublishers of _Knowledge_, and to Messrs. Sampson, Low & Co. Forpermission to reproduce the beautiful photograph of the Spiral Nebula inCanes Venatici (Plate XXII. ), I am indebted to the distinguishedastronomer, the late Dr. W. E. Wilson, D. Sc. , F. R. S. , whose untimelydeath, I regret to state, occurred in the early part of this year. 

Finally, my best thanks are due to Mr. John Ellard Gore, F. R. A. S. , M. R. I. A. , to Mr. W. H. Wesley, and to Mr. John Butler Burke, M. A. , ofCambridge, for their kindness in reading the proof-sheets. 

CECIL G. DOLMAGE. 

LONDON, S. W. , _August 4, 1908. _

PREFATORY NOTE TO THE SECOND EDITION

The author of this book lived only long enough to hear of the favourwith which it had been received, and to make a few corrections in viewof the second edition which it has so soon reached. 

_December 1908. _

CONTENTS

 CHAPTER I PAGE THE ANCIENT VIEW 17

 CHAPTER II THE MODERN VIEW 20

 CHAPTER III THE SOLAR SYSTEM 29

 CHAPTER IV CELESTIAL MECHANISM 38

 CHAPTER V CELESTIAL DISTANCES 46

 CHAPTER VI CELESTIAL MEASUREMENT 55

 CHAPTER VII ECLIPSES AND KINDRED PHENOMENA 61

 CHAPTER VIII FAMOUS ECLIPSES OF THE SUN 83

 CHAPTER IX FAMOUS ECLIPSES OF THE MOON 101

 CHAPTER X THE GROWTH OF OBSERVATION 105

 CHAPTER XI SPECTRUM ANALYSIS 121

 CHAPTER XII THE SUN 127

 CHAPTER XIII THE SUN--_continued_ 134

 CHAPTER XIV THE INFERIOR PLANETS 146

 CHAPTER XV THE EARTH 158

 CHAPTER XVI THE MOON 183

 CHAPTER XVII THE SUPERIOR PLANETS 209

 CHAPTER XVIII THE SUPERIOR PLANETS--_continued_ 229

 CHAPTER XIX COMETS 247

 CHAPTER XX REMARKABLE COMETS 259

 CHAPTER XXI METEORS OR SHOOTING STARS 266

 CHAPTER XXII THE STARS 278

 CHAPTER XXIII THE STARS--_continued_ 287

 CHAPTER XXIV SYSTEMS OF STARS 300

 CHAPTER XXV THE STELLAR UNIVERSE 319

 CHAPTER XXVI THE STELLAR UNIVERSE--_continued_ 329

 CHAPTER XXVII THE BEGINNING OF THINGS 333

 CHAPTER XXVIII THE END OF THINGS 342

 INDEX 351

LIST OF ILLUSTRATIONS

LIST OF PLATES

 PLATE THE TOTAL ECLIPSE OF THE SUN OF AUGUST 30, 1905 _Frontispiece_

 I. THE TOTAL ECLIPSE OF THE SUN OF MAY 17, 1882 _To face page_ 96

 II. GREAT TELESCOPE OF HEVELIUS " " 110

 III. A TUBELESS, OR "AERIAL" TELESCOPE " " 112

 IV. THE GREAT YERKES TELESCOPE " " 118

 V. THE SUN, SHOWING SEVERAL GROUPS OF SPOTS " " 134

 VI. PHOTOGRAPH OF A SUNSPOT " " 136

 VII. FORMS OF THE SOLAR CORONA AT THE EPOCHS OF SUNSPOT MAXIMUM AND SUNSPOT MINIMUM RESPECTIVELY. (A) THE TOTAL ECLIPSE OF THE SUN OF DECEMBER 22, 1870. (B) THE TOTAL ECLIPSE OF THE SUN OF MAY 28, 1900 " " 142

 VIII. THE MOON _To face page_ 196

 IX. MAP OF THE MOON, SHOWING THE PRINCIPAL "CRATERS, " MOUNTAIN RANGES AND "SEAS" " " 198

 X. ONE OF THE MOST INTERESTING REGIONS ON THE MOON " " 200

 XI. THE MOON (SHOWING SYSTEMS OF "RAYS") " " 204

 XII. A MAP OF THE PLANET MARS " " 216

 XIII. MINOR PLANET TRAILS " " 226

 XIV. THE PLANET JUPITER " " 230

 XV. THE PLANET SATURN " " 236

 XVI. EARLY REPRESENTATIONS OF SATURN " " 242

 XVII. DONATI'S COMET " " 256

 XVIII. DANIEL'S COMET OF 1907 " " 258

 XIX. THE SKY AROUND THE NORTH POLE " " 292

 XX. ORION AND HIS NEIGHBOURS " " 296

 XXI. THE GREAT GLOBULAR CLUSTER IN THE SOUTHERN CONSTELLATION OF CENTAURUS " " 306

 XXII. SPIRAL NEBULA IN THE CONSTELLATION OF CANES VENATICI " " 314

 XXIII. THE GREAT NEBULA IN THE CONSTELLATION OF ANDROMEDA _To face page_ 316

 XXIV. THE GREAT NEBULA IN THE CONSTELLATION OF ORION " " 318

LIST OF DIAGRAMS

 FIG. PAGE 1. THE PTOLEMAIC IDEA OF THE UNIVERSE 19

 2. THE COPERNICAN THEORY OF THE SOLAR SYSTEM 21

 3. TOTAL AND PARTIAL ECLIPSES OF THE MOON 64

 4. TOTAL AND PARTIAL ECLIPSES OF THE SUN 67

 5. "BAILY'S BEADS" 70

 6. MAP OF THE WORLD ON MERCATOR'S PROJECTION, SHOWING A PORTION OF THE PROGRESS OF THE TOTAL SOLAR ECLIPSE OF AUGUST 30, 1905, ACROSS THE SURFACE OF THE EARTH 81

 7. THE "RING WITH WINGS" 87

 8. THE VARIOUS TYPES OF TELESCOPE 113

 9. THE SOLAR SPECTRUM 123

 10. A SECTION THROUGH THE SUN, SHOWING HOW THE PROMINENCES RISE FROM THE CHROMOSPHERE 131

 11. ORBIT AND PHASES OF AN INFERIOR PLANET 148

 12. THE "BLACK DROP" 153

 13. SUMMER AND WINTER 176

 14. ORBIT AND PHASES OF THE MOON 184

 15. THE ROTATION OF THE MOON ON HER AXIS 187

 16. LAPLACE'S "PERENNIAL FULL MOON" 191

 17. ILLUSTRATING THE AUTHOR'S EXPLANATION OF THE APPARENT ENLARGEMENT OF CELESTIAL OBJECTS 195

 18. SHOWING HOW THE TAIL OF A COMET IS DIRECTED AWAY FROM THE SUN 248

 19. THE COMET OF 1066, AS REPRESENTED IN THE BAYEUX TAPESTRY 263

 20. PASSAGE OF THE EARTH THROUGH THE THICKEST PORTION OF A METEOR SWARM 269

ASTRONOMY OF TO-DAY

CHAPTER I

THE ANCIENT VIEW

It is never safe, as we know, to judge by appearances, and this isperhaps more true of astronomy than of anything else. 

For instance, the idea which one would most naturally form of the earthand heaven is that the solid earth on which we live and move extends toa great distance in every direction, and that the heaven is an immensedome upon the inner surface of which the stars are fixed. Such mustneeds have been the idea of the universe held by men in the earliesttimes. In their view the earth was of paramount importance. The sun andmoon were mere lamps for the day and for the night; and these, if notgods themselves, were at any rate under the charge of special deities, whose task it was to guide their motions across the vaulted sky. 

Little by little, however, this simple estimate of nature began to beoverturned. Difficult problems agitated the human mind. On what, forinstance, did the solid earth rest, and what prevented the vaultedheaven from falling in upon men and crushing them out of existence?Fantastic myths sprang from the vain attempts to solve these riddles. The Hindoos, for example, imagined the earth as supported by fourelephants which stood upon the back of a gigantic tortoise, which, inits turn, floated on the surface of an elemental ocean. The earlyWestern civilisations conceived the fable of the Titan Atlas, who, as apunishment for revolt against the Olympian gods, was condemned to holdup the expanse of sky for ever and ever. 

Later on glimmerings of the true light began to break in upon men. TheGreek philosophers, who busied themselves much with such matters, gradually became convinced that the earth was spherical in shape, thatis to say, round like a ball. In this opinion we now know that they wereright; but in their other important belief, viz. That the earth wasplaced at the centre of all things, they were indeed very far from thetruth. 

By the second century of the Christian era, the ideas of the earlyphilosophers had become hardened into a definite theory, which, thoughit appears very incorrect to us to-day, nevertheless demands exceptionalnotice from the fact that it was everywhere accepted as the trueexplanation until so late as some four centuries ago. This theory of theuniverse is known by the name of the Ptolemaic System, because it wasfirst set forth in definite terms by one of the most famous of theastronomers of antiquity, Claudius Ptolemæus Pelusinensis (100-170A. D. ), better known as Ptolemy of Alexandria. 

In his system the Earth occupied the centre; while around it circled inorder outwards the Moon, the planets Mercury and Venus, the Sun, andthen the planets Mars, Jupiter, and Saturn. Beyond these again revolvedthe background of the heaven, upon which it was believed that the starswere fixed--

 "Stellis ardentibus aptum, "

as Virgil puts it (see Fig. 1). 

[Illustration: FIG. 1. --The Ptolemaic idea of the Universe. ]

The Ptolemaic system persisted unshaken for about fourteen hundred yearsafter the death of its author. Clearly men were flattered by the notionthat their earth was the most important body in nature, that it stoodstill at the centre of the universe, and was the pivot upon which allthings revolved. 

CHAPTER II

THE MODERN VIEW

It is still well under four hundred years since the modern, orCopernican, theory of the universe supplanted the Ptolemaic, which hadheld sway during so many centuries. In this new theory, propoundedtowards the middle of the sixteenth century by Nicholas Copernicus(1473-1543), a Prussian astronomer, the earth was dethroned from itscentral position and considered merely as one of a number of planetarybodies which revolve around the sun. As it is not a part of our purposeto follow in detail the history of the science, it seems advisable tobegin by stating in a broad fashion the conception of the universe asaccepted and believed in to-day. 

The Sun, the most important of the celestial bodies so far as we areconcerned, occupies the central position; not, however, in the wholeuniverse, but only in that limited portion which is known as the SolarSystem. Around it, in the following order outwards, circle the planetsMercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune(see Fig. 2, p. 21). At an immense distance beyond the solar system, andscattered irregularly through the depth of space, lie the stars. The twofirst-mentioned members of the solar system, Mercury and Venus, areknown as the Inferior Planets; and in their courses about the sun, theyalways keep well inside the path along which our earth moves. Theremaining members (exclusive of the earth) are called Superior Planets, and their paths lie all outside that of the earth. 

[Illustration: FIG. 2. --The Copernican theory of the Solar System. ]

The five planets, Mercury, Venus, Mars, Jupiter, and Saturn, have beenknown from all antiquity. Nothing then can bring home to us morestrongly the immense advance which has taken place in astronomy duringmodern times than the fact that it is only 127 years since observationof the skies first added a planet to that time-honoured number. It wasindeed on the 13th of March 1781, while engaged in observing theconstellation of the Twins, that the justly famous Sir William Herschelcaught sight of an object which he did not recognise as having met withbefore. He at first took it for a comet, but observations of itsmovements during a few days showed it to be a planet. This body, whichthe power of the telescope alone had thus shown to belong to the solarfamily, has since become known to science under the name of Uranus. Byits discovery the hitherto accepted limits of the solar system were atonce pushed out to twice their former extent, and the hope naturallyarose that other planets would quickly reveal themselves in theimmensities beyond. 

For a number of years prior to Herschel's great discovery, it had beennoticed that the distances at which the then known planets circulatedappeared to be arranged in a somewhat orderly progression outwards fromthe sun. This seeming plan, known to astronomers by the name of Bode'sLaw, was closely confirmed by the distance of the new planet Uranus. There still lay, however, a broad gap between the planets Mars andJupiter. Had another planet indeed circuited there, the solar systemwould have presented an appearance of almost perfect order. But the voidbetween Mars and Jupiter was unfilled; the space in which one wouldreasonably expect to find another planet circling was unaccountablyempty. 

On the first day of the nineteenth century the mystery was howeverexplained, a body being discovered[1] which revolved in the space thathad hitherto been considered planetless. But it was a tiny globe hardlyworthy of the name of planet. In the following year a second body wasdiscovered revolving in the same space; but it was even smaller in sizethan the first. During the ensuing five years two more of these littleplanets were discovered. Then came a pause, no more such bodies beingadded to the system until half-way through the century, when suddenlythe discovery of these so-called "minor planets" began anew. Since thenadditions to this portion of our system have rained thick and fast. Thesmall bodies have received the name of Asteroids or Planetoids; and upto the present time some six hundred of them are known to exist, allrevolving in the previously unfilled space between Mars and Jupiter. 

In the year 1846 the outer boundary of the solar system was againextended by the discovery that a great planet circulated beyond Uranus. The new body, which received the name of Neptune, was brought to lightas the result of calculations made at the same time, though quiteindependently, by the Cambridge mathematician Adams, and the Frenchastronomer Le Verrier. The discovery of Neptune differed, however, fromthat of Uranus in the following respect. Uranus was found merely in thecourse of ordinary telescopic survey of the heavens. The position ofNeptune, on the other hand, was predicted as the result of rigorousmathematical investigations undertaken with the object of fixing theposition of an unseen and still more distant body, the attraction ofwhich, in passing by, was disturbing the position of Uranus in itsrevolution around the sun. Adams actually completed his investigationfirst; but a delay at Cambridge in examining that portion of the sky, where he announced that the body ought just then to be, allowed Franceto snatch the honour of discovery, and the new planet was found by theobserver Galle at Berlin, very near the place in the heavens which LeVerrier had mathematically predicted for it. 

Nearly fifty years later, that is to say, in our own time, anotherimportant planetary discovery was made. One of the recent additions tothe numerous and constantly increasing family of the asteroids, a tinybody brought to light in 1898, turned out after all not to becirculating in the customary space between Mars and Jupiter, butactually in that between our earth and Mars. This body is very small, not more than about twenty miles across. It has received the name ofEros (the Greek equivalent for Cupid), in allusion to its insignificantsize as compared with the other leading members of the system. 

This completes the list of the planets which, so far, have revealedthemselves to us. Whether others exist time alone will show. Two orthree have been suspected to revolve beyond the path of Neptune; and ithas even been asserted, on more than one occasion, that a planetcirculates nearer to the sun than Mercury. This supposed body, to whichthe name of "Vulcan" was provisionally given, is said to have been"discovered" in 1859 by a French doctor named Lescarbault, of Orgèresnear Orleans; but up to the present there has been no sufficientevidence of its existence. The reason why such uncertainty should existupon this point is easy enough to understand, when we consider theoverpowering glare which fills our atmosphere all around the sun's placein the sky. Mercury, the nearest known planet to the sun, is for thisreason always very difficult to see; and even when, in its course, itgets sufficiently far from the sun to be left for a short time above thehorizon after sunset, it is by no means an easy object to observe onaccount of the mists which usually hang about low down near the earth. One opportunity, however, offers itself from time to time to solve theriddle of an "intra-Mercurial" planet, that is to say, of a planet whichcirculates within the path followed by Mercury. The opportunity inquestion is furnished by a total eclipse of the sun; for when, during aneclipse of that kind, the body of the moon for a few minutes entirelyhides the sun's face, and the dazzling glare is thus completely cut off, astronomers are enabled to give an unimpeded, though all too hurried, search to the region close around. A goodly number of total eclipses ofthe sun have, however, come and gone since the days of Lescarbault, andno planet, so far, has revealed itself in the intra-Mercurial space. Itseems, therefore, quite safe to affirm that no globe of sufficient sizeto be seen by means of our modern telescopes circulates nearer to thesun than the planet Mercury. 

Next in importance to the planets, as permanent members of the solarsystem, come the relatively small and secondary bodies known by the nameof Satellites. The name _satellite_ is derived from a Latin wordsignifying _an attendant_; for the bodies so-called move along always inclose proximity to their respective "primaries, " as the planets whichthey accompany are technically termed. 

The satellites cannot be considered as allotted with any particularregularity among the various members of the system; several of theplanets, for instance, having a goodly number of these bodiesaccompanying them, while others have but one or two, and some again havenone at all. Taking the planets in their order of distance outward fromthe Sun, we find that neither Mercury nor Venus are provided withsatellites; the Earth has only one, viz. Our neighbour the Moon; whileMars has but two tiny ones, so small indeed that one might imagine themto be merely asteroids, which had wandered out of their proper regionand attached themselves to that planet. For the rest, so far as we atpresent know, Jupiter possesses seven, [2] Saturn ten, Uranus four, andNeptune one. It is indeed possible, nay more, it is extremely probable, that the two last-named planets have a greater number of these secondarybodies revolving around them; but, unfortunately, the Uranian andNeptunian systems are at such immense distances from us, that even themagnificent telescopes of to-day can extract very little informationconcerning them. 

From the distribution of the satellites, the reader will notice that theplanets relatively near to the sun are provided with few or none, whilethe more distant planets are richly endowed. The conclusion, therefore, seems to be that nearness to the sun is in some way unfavourable eitherto the production, or to the continued existence, of satellites. 

A planet and its satellites form a repetition of the solar system on atiny scale. Just as the planets revolve around the sun, so do thesesecondary bodies revolve around their primaries. When Galileo, in 1610, turned his newly invented telescope upon Jupiter, he quickly recognisedin the four circling moons which met his gaze, a miniature edition ofthe solar system. 

Besides the planets and their satellites, there are two other classes ofbodies which claim membership of the solar system. These are Comets andMeteors. Comets differ from the bodies which we have just beendescribing in that they appear filmy and transparent, whereas the othersare solid and opaque. Again, the paths of the planets around the sun andof the satellites around their primaries are not actually circles; theyare ovals, but their ovalness is not of a marked degree. The paths ofcomets on the other hand are usually _very_ oval; so that in theircourses many of them pass out as far as the known limits of the solarsystem, and even far beyond. It should be mentioned that nowadays thetendency is to consider comets as permanent members of the system, though this was formerly not by any means an article of faith withastronomers. 

Meteors are very small bodies, as a rule perhaps no larger than pebbles, which move about unseen in space, and of which we do not become awareuntil they arrive very close to the earth. They are then made visible tous for a moment or two in consequence of being heated to a white heat bythe friction of rushing through the atmosphere, and are thus usuallyturned into ashes and vapour long before they reach the surface of ourglobe. Though occasionally a meteoric body survives the fiery ordeal, and reaches the earth more or less in a solid state to bury itself deepin the soil, the majority of these celestial visitants constitute nosource of danger whatever for us. Any one who will take the trouble togaze at the sky for a short time on a clear night, is fairly certain tobe rewarded with the view of a meteor. The impression received is as ifone of the stars had suddenly left its accustomed place, and dashedacross the heavens, leaving in its course a trail of light. It is forthis reason that meteors are popularly known under the name of "shootingstars. "

[1] By the Italian astronomer, Piazzi, at Palermo. 

[2] Probably eight. (See note, page 232. )

CHAPTER III

THE SOLAR SYSTEM

We have seen, in the course of the last chapter, that the solar systemis composed as follows:--there is a central body, the sun, around whichrevolve along stated paths a number of important bodies known asplanets. Certain of these planets, in their courses, carry along incompany still smaller bodies called satellites, which revolve aroundthem. With regard, however, to the remaining members of the system, viz. The comets and the meteors, it is not advisable at this stage to addmore to what has been said in the preceding chapter. For the time being, therefore, we will devote our attention merely to the sun, the planets, and the satellites. 

Of what shape then are these bodies? Of one shape, and that one alonewhich appears to characterise all solid objects in the celestial spaces:they are spherical, which means _round like a ball_. 

Each of these spherical bodies rotates; that is to say, turns round andround, as a top does when it is spinning. This rotation is said to takeplace "upon an axis, " a statement which may be explained asfollows:--Imagine a ball with a knitting-needle run right through itscentre. Then imagine this needle held pointing in one fixed directionwhile the ball is turned round and round. Well, it is the same thingwith the earth. As it journeys about the sun, it keeps turning round andround continually as if pivoted upon a mighty knitting needletransfixing it from North Pole to South Pole. In reality, however, thereis no such material axis to regulate the constant direction of therotation, just as there are no actual supports to uphold the earthitself in space. The causes which keep the celestial spheres poised, andwhich control their motions, are far more wonderful than any mechanicaldevice. 

At this juncture it will be well to emphasise the sharp distinctionbetween the terms _rotation_ and _revolution_. The term "rotation" isinvariably used by astronomers to signify the motion which a celestialbody has upon an axis; the term "revolution, " on the other hand, is usedfor the movement of one celestial body around another. Speaking of theearth, for instance, we say, that it _rotates_ on its axis, and that it_revolves_ around the sun. 

So far, nothing has been said about the sizes of the members of oursystem. Is there any stock size, any pattern according to which they maybe judged? None whatever! They vary enormously. Very much the largest ofall is the Sun, which is several hundred times larger than all theplanets and satellites of the system rolled together. Next comes Jupiterand afterwards the other planets in the following order ofsize:--Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury. Very much smaller than any of these are the asteroids, of which Ceres, the largest, is less than 500 miles in diameter. It is, by the way, astrange fact that the zone of asteroids should mark the separation ofthe small planets from the giant ones. The following table, givingroughly the various diameters of the sun and the principal planets inmiles, will clearly illustrate the great discrepancy in size whichprevails in the system. 

Sun 866, 540 milesMercury 2, 765 "Venus 7, 826 "Earth 7, 918 "Mars 4, 332 "

ZONE OF ASTEROIDS

Jupiter 87, 380 "Saturn 73, 125 "Uranus[3] 34, 900 "Neptune[3] 32, 900 "

It does not seem possible to arrive at any generalisation from the abovedata, except it be to state that there is a continuous increase in sizefrom Mercury to the earth, and a similar decrease in size from Jupiteroutwards. Were Mars greater than the earth, the planets could then withtruth be said to increase in size up to Jupiter, and then to decrease. But the zone of asteroids, and the relative smallness of Mars, negativeany attempt to regard the dimensions of the planets as an orderlysequence. 

Next with respect to relative distance from the sun, Venus circulatesnearly twice as far from it as Mercury, the earth nearly three times asfar, and Mars nearly four times. After this, just as we found a suddenincrease in size, so do we meet with a sudden increase in distance. Jupiter, for instance, is about thirteen times as far as Mercury, Saturnabout twenty-five times, Uranus about forty-nine times, and Neptuneabout seventy-seven. (See Fig. 2, p. 21. )

It will thus be seen how enormously the solar system was enlarged inextent by the discovery of the outermost planets. The finding of Uranusplainly doubled its breadth; the finding of Neptune made it more thanhalf as broad again. Nothing indeed can better show the import of thesegreat discoveries than to take a pair of compasses and roughly set outthe above relative paths in a series of concentric circles upon a largesheet of paper, and then to consider that the path of Saturn was thesupposed boundary of our solar system prior to the year 1781. 

We have seen that the usual shape of celestial bodies themselves isspherical. Of what form then are their paths, or _orbits_, as these arecalled? One might be inclined at a venture to answer "circular, " butthis is not the case. The orbits of the planets cannot be regarded astrue circles. They are ovals, or, to speak in technical language, "ellipses. " Their ovalness or "ellipticity" is, however, in each casenot by any means of the same degree. Some orbits--for instance, that ofthe earth--differ only slightly from circles; while others--those ofMars or Mercury, for example--are markedly elliptic. The orbit of thetiny planet Eros is, however, far and away the most elliptic of all, aswe shall see when we come to deal with that little planet in detail. 

It has been stated that the sun and planets are always rotating. Thevarious rates at which they do so will, however, be best appreciated bya comparison with the rate at which the earth itself rotates. 

But first of all, let us see what ground we have, if any, for assertingthat the earth rotates at all?

If we carefully watch the heavens we notice that the background of thesky, with all the brilliant objects which sparkle in it, appears to turnonce round us in about twenty-four hours; and that the pivot upon whichthis movement takes place is situated somewhere near what is known to usas the _Pole Star_. This was one of the earliest facts noted with regardto the sky; and to the men of old it therefore seems as if the heavensand all therein were always revolving around the earth. It was naturalenough for them to take this view, for they had not the slightest ideaof the immense distance of the celestial bodies, and in the absence ofany knowledge of the kind they were inclined to imagine themcomparatively near. It was indeed only after the lapse of manycenturies, when men had at last realised the enormous gulf whichseparated them from even the nearest object in the sky, that a morereasonable opinion began to prevail. It was then seen that thisrevolution of the heavens about the earth could be more easily and moresatisfactorily explained by supposing a mere rotation of the solid earthabout a fixed axis, pointed in the direction of the polar star. Theprobability of such a rotation on the part of the earth itself wasfurther strengthened by the observations made with the telescope. Whenthe surfaces of the sun and planets were carefully studied these bodieswere seen to be rotating. This being the case, there could not surelybe much hesitation in granting that the earth rotated also; particularlywhen it so simply explained the daily movement of the sky, and saved menfrom the almost inconceivable notion that the whole stupendous vaultedheaven was turning about them once in every twenty-four hours. 

If the sun be regularly observed through a telescope, it will graduallybe gathered from the slow displacement of sunspots across its face, their disappearance at one edge and their reappearance again at theother edge, that it is rotating on an axis in a period of abouttwenty-six days. The movement, too, of various well-known markings onthe surfaces of the planets Mars, Jupiter, and Saturn proves to us thatthese bodies are rotating in periods, which are about twenty-four hoursfor the first, and about ten hours for each of the other two. Withregard, however, to Uranus and Neptune there is much more uncertainty, as these planets are at such great distances that even our besttelescopes give but a confused view of the markings which they display;still a period of rotation of from ten to twelve hours appears to beaccepted for them. On the other hand the constant blaze of sunlight inthe neighbourhood of Mercury and Venus equally hampers astronomers inthis quest. The older telescopic observers considered that the rotationperiods of these two planets were about the same as that of the earth;but of recent years the opinion has been gaining ground that they turnround on their axes in exactly the same time as they revolve about thesun. This question is, however, a very doubtful one, and will be againreferred to later on; but, putting it on one side, it will be seen fromwhat we have said above, that the rotation periods of the other planetsof our system are usually about twenty-four hours, or under. The factthat the rotation period of the sun should run into _days_ need not seemextraordinary when one considers its enormous size. 

The periods taken by the various planets to revolve around the sun isthe next point which has to be considered. Here, too, it is well tostart with the earth's period of revolution as the standard, and to seehow the periods taken by the other planets compare with it. 

The earth takes about 365-1/4 days to revolve around the sun. Thisperiod of time is known to us as a "year. " The following table shows indays and years the periods taken by each of the other planets to make acomplete revolution round the sun:--

Mercury about 88 days. Venus " 226 "Mars " 1 year and 321 days. Jupiter " 11 years and 313 days. Saturn " 29 years and 167 days. Uranus " 84 years and 7 days. Neptune " 164 years and 284 days. 

From these periods we gather an important fact, namely, that the nearera planet is to the sun the faster it revolves. 

Compared with one of our years what a long time does an Uranian, orNeptunian, "year" seem? For instance, if a "year" had commenced inNeptune about the middle of the reign of George II. , that "year" wouldbe only just coming to a close; for the planet is but now arriving backto the position, with regard to the sun, which it then occupied. Uranus, too, has only completed a little more than 1-1/2 of its "years" sinceHerschel discovered it. 

Having accepted the fact that the planets are revolving around the sun, the next point to be inquired into is:--What are the positions of theirorbits, or paths, relatively to each other?

Suppose, for instance, the various planetary orbits to be represented bya set of hoops of different sizes, placed one within the other, and thesun by a small ball in the middle of the whole; in what positions willthese hoops have to be arranged so as to imitate exactly the truecondition of things?

First of all let us suppose the entire arrangement, ball and hoops, tobe on one level, so to speak. This may be easily compassed by imaginingthe hoops as floating, one surrounding the other, with the ball in themiddle of all, upon the surface of still water. Such a set of objectswould be described in astronomical parlance as being _in the sameplane_. Suppose, on the other hand, that some of these floating hoopsare tilted with regard to the others, so that one half of a hoop risesout of the water and the other half consequently sinks beneath thesurface. This indeed is the actual case with regard to the planetaryorbits. They do not by any means lie all exactly in the same plane. Eachone of them is tilted, or _inclined_, a little with respect to the planeof the earth's orbit, which astronomers, for convenience, regard as the_level_ of the solar system. This tilting, or "inclination, " is, in thelarger planets, greatest for the orbit of Mercury, least for that ofUranus. Mercury's orbit is inclined to that of the earth at an angle ofabout 7°, that of Venus at a little over 3°, that of Saturn 2-1/2°;while in those of Mars, Neptune, and Jupiter the inclination is lessthan 2°. But greater than any of these is the inclination of the orbitof the tiny planet Eros, viz. Nearly 11°. 

The systems of satellites revolving around their respective planetsbeing, as we have already pointed out, mere miniature editions of thesolar system, the considerations so far detailed, which regulate thebehaviour of the planets in their relations to the sun, will ofnecessity apply to the satellites very closely. In one respect, however, a system of satellites differs materially from a system of planets. Thecentral body around which planets are in motion is self-luminous, whereas the planetary body around which a satellite revolves is not. True, planets shine, and shine very brightly too; as, for instance, Venus and Jupiter. But they do not give forth any light of their own, asthe sun does; they merely reflect the sunlight which they receive fromhim. Putting this one fact aside, the analogy between the planetarysystem and a satellite system is remarkable. The satellites arespherical in form, and differ markedly in size; they rotate, so far aswe know, upon their axes in varying times; they revolve around theirgoverning planets in orbits, not circular, but elliptic; and theseorbits, furthermore, do not of necessity lie in the same plane. Last ofall the satellites revolve around their primaries at rates which aredirectly comparable with those at which the planets revolve around thesun, the rule in fact holding good that the nearer a satellite is to itsprimary the faster it revolves. 

[3] As there seems to be much difference of opinion concerning thediameters of Uranus and Neptune, it should here be mentioned that theabove figures are taken from Professor F. R. Moulton's _Introduction toAstronomy_ (1906). They are there stated to be given on the authority of"Barnard's many measures at the Lick Observatory. "

CHAPTER IV

CELESTIAL MECHANISM

As soon as we begin to inquire closely into the actual condition of thevarious members of the solar system we are struck with a certaindistinction. We find that there are two quite different points of viewfrom which these bodies can be regarded. For instance, we may make ourestimates of them either as regards _volume_--that is to say, the mereroom which they take up; or as regards _mass_--that is to say, theamount of matter which they contain. 

Let us imagine two globes of equal volume; in other words, which take upan equal amount of space. One of these globes, however, may be composedof material much more tightly put together than in the other; or ofgreater _density_, as the term goes. That globe is said to be thegreater of the two in mass. Were such a pair of globes to be weighed inscales, one globe in each pan, we should see at once, by its weighingdown the other, which of the two was composed of the more tightly packedmaterials; and we should, in astronomical parlance, say of this one thatit had the greater mass. 

Volume being merely another word for size, the order of the members ofthe solar system, with regard to their volumes, will be as follows, beginning with the greatest:--the Sun, Jupiter, Saturn, Uranus, Neptune, the Earth, Venus, Mars, and Mercury. 

With regard to mass the same order strangely enough holds good. Theactual densities of the bodies in question are, however, very different. The densest or closest packed body of all is the Earth, which is aboutfive and a half times as dense as if it were composed entirely of water. Venus follows next, then Mars, and then Mercury. The remaining bodies, on the other hand, are relatively loose in structure. Saturn is theleast dense of all, less so than water. The density of the Sun is alittle greater than that of water. 

This method of estimating is, however, subject to a qualification. Itmust be remembered that in speaking of the Sun, for instance, as beingonly a little denser than water, we are merely treating the questionfrom the point of view of an average. Certain parts of it in fact willbe ever so much denser than water: those are the parts in the centre. Other portions, for instance, the outside portions, will be very muchless dense. It will easily be understood that in all such bodies thedensest or most compressed portions are to be found towards the centre;while the portions towards the exterior being less pressed upon, will beless dense. 

We now reach a very important point, the question of Gravitation. _Gravitation_, or _gravity_, as it is often called, is the attractiveforce which, for instance, causes objects to fall to the earth. Now itseems rather strange that one should say that it is owing to a certainforce that things fall towards the earth. All things seem to us to fallso of their own accord, as if it were quite natural, or rather mostunnatural if they did not. Why then require a "force" to make them fall?

The story goes that the great Sir Isaac Newton was set a-thinking onthis subject by seeing an apple fall from a tree to the earth. He thencarried the train of thought further; and, by studying the movements ofthe moon, he reached the conclusion that a body even so far off as oursatellite would be drawn towards the earth in the same manner. Thisbeing the case, one will naturally ask why the moon herself does notfall in upon the earth. The answer is indeed found to be that the moonis travelling round and round the earth at a certain rapid pace, and itis this very same rapid pace which keeps her from falling in upon us. Any one can test this simple fact for himself. If we tie a stone to theend of a string, and keep whirling it round and round fast enough, therewill be a strong pull from the stone in an outward direction, and thestring will remain tight all the time that the stone is being whirled. If, however, we gradually slacken the speed at which we are making thestone whirl, a moment will come at length when the string will becomelimp, and the stone will fall back towards our hand. 

It seems, therefore, that there are two causes which maintain the stoneat a regular distance all the time it is being steadily whirled. One ofthese is the continual pull inward towards our hand by means of thestring. The other is the continual pull away from us caused by the rateat which the stone is travelling. When the rate of whirling is soregulated that these pulls exactly balance each other, the stone travelscomfortably round and round, and shows no tendency either to fall backupon our hand or to break the string and fly away into the air. It isindeed precisely similar with regard to the moon. The continual pull ofthe earth's gravitation takes the place of the string. If the moon wereto go round and round slower than it does, it would tend to fall intowards the earth; if, on the other hand, it were to go faster, it wouldtend to rush away into space. 

The same kind of pull which the earth exerts upon the objects at itssurface, or upon its satellite, the moon, exists through space so far aswe know. Every particle of matter in the universe is found in fact toattract every other particle. The moon, for instance, attracts the earthalso, but the controlling force is on the side of the much greater massof the earth. This force of gravity or attraction of gravitation, as itis also called, is perfectly regular in its action. Its power dependsfirst of all exactly upon the mass of the body which exerts it. Thegravitational pull of the sun, for instance, reaches out to an enormousdistance, controlling perhaps, in their courses, unseen planets circlingfar beyond the orbit of Neptune. Again, the strength with which theforce of gravity acts depends upon distance in a regularly diminishingproportion. Thus, the nearer an object is to the earth, for instance, the stronger is the gravitational pull which it gets from it; thefarther off it is, the weaker is this pull. If then the moon were to bebrought nearer to the earth, the gravitational pull of the latter wouldbecome so much stronger that the moon's rate of motion would have alsoto increase in due proportion to prevent her from being drawn into theearth. Last of all, the point in a body from which the attraction ofgravitation acts, is not necessarily the centre of the body, but ratherwhat is known as its _centre of gravity_, that is to say, the balancingpoint of all the matter which the body contains. 

It should here be noted that the moon does not actually revolve aroundthe centre of gravity of the earth. What really happens is that bothorbs revolve around their _common_ centre of gravity, which is a pointwithin the body of the earth, and situated about three thousand milesfrom its centre. In the same manner the planets and the sun revolvearound the centre of gravity of the solar system, which is a pointwithin the body of the sun. 

The neatly poised movements of the planets around the sun, and of thesatellites around their respective planets, will therefore be readilyunderstood to result from a nice balance between gravitation and speedof motion. 

The mass of the earth is ascertained to be about eighty times that ofthe moon. Our knowledge of the mass of a planet is learned fromcomparing the revolutions of its satellite or satellites around it, withthose of the moon around the earth. We are thus enabled to deduce whatthe mass of such a planet would be compared to the earth's mass; that isto say, a study, for instance, of Jupiter's satellite system shows thatJupiter must have a mass nearly three hundred and eighteen times that ofour earth. In the same manner we can argue out the mass of the sun fromthe movements of the planets and other bodies of the system around it. With regard, however, to Venus and Mercury, the problem is by no meanssuch an easy one, as these bodies have no satellites. For information inthis latter case we have to rely upon such uncertain evidence as, forinstance, the slight disturbances caused in the motion of the earth bythe attraction of these planets when they pass closest to us, or theirobserved effect upon the motions of such comets as may happen to passnear to them. 

Mass and weight, though often spoken of as one and the same thing, areby no means so. Mass, as we have seen, merely means the amount of matterwhich a body contains. The weight of a body, on the other hand, dependsentirely upon the gravitational pull which it receives. The force ofgravity at the surface of the earth is, for instance, about six times asgreat as that at the surface of the moon. All bodies, therefore, weighabout six times as much on the earth as they would upon the moon; or, rather, a body transferred to the moon's surface would weigh only aboutone-sixth of what it did on the terrestrial surface. It will thereforebe seen that if a body of given _mass_ were to be placed upon planetafter planet in turn, its _weight_ would regularly alter according tothe force of gravity at each planet's surface. 

Gravitation is indeed one of the greatest mysteries of nature. What itis, the means by which it acts, or why such a force should exist at all, are questions to which so far we have not had even the merest hint of ananswer. Its action across space appears to be instantaneous. 

The intensity of gravitation is said in mathematical parlance "to varyinversely with the square of the distance. " This means that at _twice_the distance the pull will become only _one-quarter_ as strong, and notone-half as otherwise might be expected. At _four_ times the distance, therefore, it will be _one-sixteenth_ as strong. At the earth's surfacea body is pulled by the earth's gravitation, or "falls, " as weordinarily term it, through 16 feet in one _second_ of time; whereas atthe distance of the moon the attraction of the earth is so very muchweakened that a body would take as long as one _minute_ to fall throughthe same space. 

Newton's investigations showed that if a body were to be placed _atrest_ in space entirely away from the attraction of any other body itwould remain always in a motionless condition, because there wouldplainly be no reason why it should move in any one direction rather thanin another. And, similarly, if a body were to be projected in a certaindirection and at a certain speed, it would move always in the samedirection and at the same speed so long as it did not come within thegravitational attraction of any other body. 

The possibility of an interaction between the celestial orbs hadoccurred to astronomers before the time of Newton; for instance, in theninth century to the Arabian Musa-ben-Shakir, to Camillus Agrippa in1553, and to Kepler, who suspected its existence from observation of thetides. Horrox also, writing in 1635, spoke of the moon as moved by an_emanation_ from the earth. But no one prior to Newton attempted toexamine the question from a mathematical standpoint. 

Notwithstanding the acknowledged truth and far-reaching scope of the lawof gravitation--for we find its effects exemplified in every portion ofthe universe--there are yet some minor movements which it does notaccount for. For instance, there are small irregularities in themovement of Mercury which cannot be explained by the influence ofpossible intra-Mercurial planets, and similarly there are slightunaccountable deviations in the motions of our neighbour the Moon. 

CHAPTER V

CELESTIAL DISTANCES

Up to this we have merely taken a general view of the solar system--abird's-eye view, so to speak, from space. 

In the course of our inquiry we noted in a rough way the _relative_distances at which the various planets move around the sun. But we havenot yet stated what these distances _actually_ are, and it weretherefore well now to turn our attention to this important matter. 

Each of us has a fair idea of what a mile is. It is a quarter of anhour's sharp walk, for instance; or yonder village or building, we know, lies such and such a number of miles away. 

The measurements which have already been given of the diameters of thevarious bodies of the solar system appear very great to us, who findthat a walk of a few miles at a time taxes our strength; but they are amere nothing when we consider the distances from the sun at which thevarious planets revolve in their orbits. 

The following table gives these distances in round numbers. As herestated they are what are called "mean" distances; for, as the orbits areoval, the planets vary in their distances from the sun, and we aretherefore obliged to strike a kind of average for each case:--

Mercury about 36, 000, 000 miles. Venus " 67, 200, 000 "Earth " 92, 900, 000 "Mars " 141, 500, 000 "Jupiter " 483, 300, 000 "Saturn " 886, 000, 000 "Uranus " 1, 781, 900, 000 "Neptune " 2, 791, 600, 000 "

From the above it will be seen at a glance that we have entered upon astill greater scale of distance than in dealing with the diameters ofthe various bodies of the system. In that case the distances werelimited to thousands of miles; in this, however, we have to deal withmillions. A million being ten hundred thousand, it will be noticed thateven the diameter of the huge sun is well under a million miles. 

How indeed are we to get a grasp of such distances, when those to whichwe are ordinarily accustomed--the few miles' walk, the little stretch ofsea or land which we gaze upon around us--are so utterly minute incomparison? The fact is, that though men may think that they can picturein their minds such immense distances, they actually can not. In matterslike these we unconsciously employ a kind of convention, and we estimatea thing as being two or three or more times the size of another. Morethan this we are unable to do. For instance, our ordinary experience ofa mile enables us to judge, in a way, of a stretch of several miles, such as one can take in with a glance; but in our estimation of athousand miles, or even of one hundred, we are driven back upon a mentaltrick, so to speak. 

In our attempts to realise such immense distances as those in the solarsystem we are obliged to have recourse to analogies; to comparisons withother and simpler facts, though this is at the best a mere self-cheatingdevice. The analogy which seems most suited to our purpose here, and onewhich has often been employed by writers, is borrowed from the rate atwhich an express train travels. 

Let us imagine, for instance, that we possess an express train which iscapable of running anywhere, never stops, never requires fuel, andalways goes along at sixty miles an hour. Suppose we commence byemploying it to gauge the size of our own planet, the earth. Let us sendit on a trip around the equator, the span of which is about 24, 000miles. At its sixty-miles-an-hour rate of going, this journey will takenearly 17 days. Next let us send it from the earth to the moon. Thisdistance, 240, 000 miles, being ten times as great as the last, will ofcourse take ten times as long to cover, namely, 170 days; that is tosay, nearly half a year. Again, let us send it still further afield, tothe sun, for example. Here, however, it enters upon a journey which isnot to be measured in thousands of miles, as the others were, but inmillions. The distance from the earth to the sun, as we have seen in theforegoing table, is about 93 millions of miles. Our express train wouldtake about 178 _years_ to traverse this. 

Having arrived at the sun, let us suppose that our train makes a tourright round it. This will take more than five years. 

Supposing, finally, that our train were started from the sun, and madeto run straight out to the known boundaries of the solar system, that isto say, as far as the orbit of Neptune, it would take over 5000 years totraverse the distance. 

That sixty miles an hour is a very great speed any one, I think, willadmit who has stood upon the platform of a country station while one ofthe great mail trains has dashed past. But are not the immensities ofspace appalling to contemplate, when one realises that a body movingincessantly at such a rate would take so long as 10, 000 years totraverse merely the breadth of our solar system? Ten thousand years!Just try to conceive it. Why, it is only a little more than half thattime since the Pyramids were built, and they mark for us the Dawn ofHistory. And since then half-a-dozen mighty empires have come and gone!

Having thus concluded our general survey of the appearance anddimensions of the solar system, let us next inquire into its positionand size in relation to what we call the Universe. 

A mere glance at the night sky, when it is free from clouds, shows usthat in every direction there are stars; and this holds good, no matterwhat portion of the globe we visit. The same is really true of the skyby day, though in that case we cannot actually see the stars, for theirlight is quite overpowered by the dazzling light of the sun. 

We thus reach the conclusion that our earth, that our solar system infact, lies plunged within the midst of a great tangle of stars. Whatposition, by the way, do we occupy in this mighty maze? Are we at thecentre, or anywhere near the centre, or where?

It has been indeed amply proved by astronomical research that the starsare bodies giving off a light of their own, just as our sun does; thatthey are in fact suns, and that our sun is merely one, perhaps indeed avery unimportant member, of this great universe of stars. Each of thesestars, or suns, besides, may be the centre of a system similar to whatwe call our solar system, comprising planets and satellites, comets andmeteors;--or perchance indeed some further variety of attendant bodiesof which we have no example in our tiny corner of space. But as towhether one is right in a conjecture of this kind, there is up to thepresent no proof whatever. No telescope has yet shown a planet inattendance upon one of these distant suns; for such bodies, even if theydo exist, are entirely out of the range of our mightiest instruments. Onwhat then can we ground such an assumption? Merely upon analogy; uponthe common-sense deduction that as the stars have characteristicssimilar to our particular star, the sun, it would seem unlikely thatours should be the only such body in the whole of space which isattended by a planetary system. 

"The Stars, " using that expression in its most general sense, do not lieat one fixed distance from us, set here and there upon a background ofsky. There is in fact no background at all. The brilliant orbs are allaround us in space, at different distances from us and from each other;and we can gaze between them out into the blackness of the void which, perhaps, continues to extend unceasingly long after the very outposts ofthe stellar universe has been left behind. Shall we then start ourimaginary express train once more, and send it out towards the nearestof the stars? This would, however, be a useless experiment. Ourexpress-train method of gauging space would fail miserably in theattempt to bring home to us the mighty gulf by which we are now faced. Let us therefore halt for a moment and look back upon the orders ofdistance with which we have been dealing. First of all we dealt withthousands of miles. Next we saw how they shrank into insignificance whenwe embarked upon millions. We found, indeed, that our sixty-mile-an-hourtrain, rushing along without ceasing, would consume nearly the whole ofhistorical time in a journey from the sun to Neptune. 

In the spaces beyond the solar system we are faced, however, by a neworder of distance. From sun to planets is measured in millions of miles, but from sun to sun is measured in billions. But does the mere statingof this fact convey anything? I fear not. For the word "billion" runs asglibly off the tongue as "million, " and both are so wholly unrealisableby us that the actual difference between them might easily passunnoticed. 

Let us, however, make a careful comparison. What is a million? It is athousand thousands. But what is a billion? It is a million millions. Consider for a moment! A million of millions. That means a million, eachunit of which is again a million. In fact every separate "1" in thismillion is itself a million. Here is a way of trying to realise thisgigantic number. A million seconds make only eleven and a half days andnights. But a billion seconds will make actually more than thirtythousand years!

Having accepted this, let us try and probe with our express train even alittle of the new gulf which now lies before us. At our old rate ofgoing it took almost two years to cover a million miles. To cover abillion miles--that is to say, a million times this distance--would thustake of course nearly two million years. Alpha Centauri, the neareststar to our earth, is some twenty-five billions of miles away. Ourexpress train would thus take about fifty millions of years to reach it!

This shows how useless our illustration, appropriate though it seemedfor interplanetary space, becomes when applied to the interstellarspaces. It merely gives us millions in return for billions; and so themind, driven in upon itself, whirls round and round like a squirrel inits revolving cage. There is, however, a useful illustration still leftus, and it is the one which astronomers usually employ in dealing withthe distances of the stars. The illustration in question is taken fromthe velocity of light. 

Light travels at the tremendous speed of about 186, 000 miles a second. It therefore takes only about a second and a quarter to come to us fromthe moon. It traverses the 93, 000, 000 of miles which separate us fromthe sun in about eight minutes. It travels from the sun out to Neptunein about four hours, which means that it would cross the solar systemfrom end to end in eight. To pass, however, across the distance whichseparates us from Alpha Centauri it would take so long as about fourand a quarter years!

Astronomers, therefore, agree in estimating the distances of the starsfrom the point of view of the time which light would take to pass fromthem to our earth. They speak of that distance which light takes a yearto traverse as a "light year. " According to this notation, AlphaCentauri is spoken of as being about four and a quarter light yearsdistant from us. 

Now as the rays of light coming from Alpha Centauri to us are chasingone another incessantly across the gulf of space, and as each ray leftthat star some four years before it reaches us, our view of the staritself must therefore be always some four years old. Were then this starto be suddenly removed from the universe at any moment, we shouldcontinue to see it still in its place in the sky for some four yearsmore, after which it would suddenly disappear. The rays which hadalready started upon their journey towards our earth must indeedcontinue travelling, and reaching us in their turn until the last onehad arrived; after which no more would come. 

We have drawn attention to Alpha Centauri as the nearest of the stars. The majority of the others indeed are ever so much farther. We can onlyhazard a guess at the time it takes for the rays from many of them toreach our globe. Suppose, for instance, we see a sudden change in thelight of any of these remote stars, we are inclined to ask ourselveswhen that change did actually occur. Was it in the days of QueenElizabeth, or at the time of the Norman Conquest; or was it when Romewas at the height of her glory, or perhaps ages before that when thePyramids of Egypt were being built? Even the last of these suppositionscannot be treated lightly. We have indeed no real knowledge of thedistance from us of those stars which our giant telescopes have broughtinto view out of the depths of the celestial spaces. 

CHAPTER VI

CELESTIAL MEASUREMENT

Had the telescope never been invented our knowledge of astronomy wouldbe trifling indeed. 

Prior to the year 1610, when Galileo first turned the new instrumentupon the sky, all that men knew of the starry realms was gathered fromobservation with their own eyes unaided by any artificial means. In suchresearches they had been very much at a disadvantage. The sun and moon, in their opinion, were no doubt the largest bodies in the heavens, forthe mere reason that they looked so! The mighty solar disturbances, which are now such common-places to us, were then quite undreamed of. The moon displayed a patchy surface, and that was all; her craters andring-mountains were surprises as yet in store for men. Nothing of coursewas known about the surfaces of the planets. These objects had indeed noparticular characteristics to distinguish them from the great host ofthe stars, except that they continually changed their positions in thesky while the rest did not. The stars themselves were considered asfixed inalterably upon the vault of heaven. The sun, moon, and planetsapparently moved about in the intermediate space, supported in theircourses by strange and fanciful devices. The idea of satellites was asyet unknown. Comets were regarded as celestial portents, and meteors assmall conflagrations taking place in the upper air. 

In the entire absence of any knowledge with regard to the actual sizesand distances of the various celestial bodies, men naturally consideredthem as small; and, concluding that they were comparatively near, assigned to them in consequence a permanent connection with terrestrialaffairs. Thus arose the quaint and erroneous beliefs of astrology, according to which the events which took place upon our earth wereconsidered to depend upon the various positions in which the planets, for instance, found themselves from time to time. 

It must, however, be acknowledged that the study of astrology, fallacious though its conclusions were, indirectly performed a greatservice to astronomy by reason of the accurate observations and diligentstudy of the stars which it entailed. 

We will now inquire into the means by which the distances and sizes ofthe celestial orbs have been ascertained, and see how it was that theancients were so entirely in the dark in this matter. 

There are two distinct methods of finding out the distance at which anyobject happens to be situated from us. 

One method is by actual measurement. 

The other is by moving oneself a little to the right or left, andobserving whether the distant object appears in any degree altered inposition by our own change of place. 

One of the best illustrations of this relative change of position whichobjects undergo as a result of our own change of place, is to observethe landscape from the window of a moving railway carriage. As we areborne rapidly along we notice that the telegraph posts which are setclose to the line appear to fly past us in the contrary direction; thetrees, houses, and other things beyond go by too, but not so fast;objects a good way off displace slowly; while some spire, or talllandmark, in the far distance appears to remain unmoved during acomparatively long time. 

Actual change of position on our own part is found indeed to beinvariably accompanied by an apparent displacement of the objects aboutus, such apparent displacement as a result of our own change of positionbeing known as "parallax. " The dependence between the two is somathematically exact, that if we know the amount of our own change ofplace, and if we observe the amount of the consequent displacement ofany object, we are enabled to calculate its precise distance from us. Thus it comes to pass that distances can be measured without thenecessity of moving over them; and the breadth of a river, for instance, or the distance from us of a ship at sea, can be found merely by suchmeans. 

It is by the application of this principle to the wider field of the skythat we are able to ascertain the distance of celestial bodies. We havenoted that it requires a goodly change of place on our own part to shiftthe position in which some object in the far distance is seen by us. Totwo persons separated by, say, a few hundred yards, a ship upon thehorizon will appear pretty much in the same direction. They wouldrequire, in fact, to be much farther apart in order to displace itsufficiently for the purpose of estimating their distance from it. Itis the same with regard to the moon. Two observers, standing upon ourearth, will require to be some thousands of miles apart in order to seethe position of our satellite sufficiently altered with regard to thestarry background, to give the necessary data upon which to ground theircalculations. 

The change of position thus offered by one side of the earth's surfaceat a time is, however, not sufficient to displace any but the nearestcelestial bodies. When we have occasion to go farther afield we have toseek a greater change of place. This we can get as a consequence of theearth's movement around the sun. Observations, taken several days apart, will show the effect of the earth's change of place during the intervalupon the positions of the other bodies of our system. But when we desireto sound the depths of space beyond, and to reach out to measure thedistance of the nearest star, we find ourselves at once thrown upon thegreatest change of place which we can possibly hope for; and this we getduring the long journey of many millions of miles which our earthperforms around the sun during the course of each year. But even thislast change of place, great as it seems in comparison with terrestrialmeasurements, is insufficient to show anything more than the tiniestdisplacements in a paltry forty-three out of the entire host of thestars. 

We can thus realise at what a disadvantage the ancients were. Themeasuring instruments at their command were utterly inadequate to detectsuch small displacements. It was reserved for the telescope to revealthem; and even then it required the great telescopes of recent times toshow the slight changes in the position of the nearer stars, which werecaused by the earth's being at one time at one end of its orbit, andsome six months later at the other end--stations separated from eachother by a gulf of about one hundred and eighty-six millions of miles. 

The actual distances of certain celestial bodies being thusascertainable, it becomes a matter of no great difficulty to determinethe actual sizes of the measurable ones. It is a matter of everydayexperience that the size which any object appears to have, dependsexactly upon the distance it is from us. The farther off it is thesmaller it looks; the nearer it is the bigger. If, then, an object whichlies at a known distance from us looks such and such a size, we can ofcourse ascertain its real dimensions. Take the moon, for instance. As wehave already shown, we are able to ascertain its distance. We observealso that it looks a certain size. It is therefore only a matter ofcalculation to find what its actual dimensions should be, in order thatit may look that size at that distance away. Similarly we can ascertainthe real dimensions of the sun. The planets, appearing to us as pointsof light, seem at first to offer a difficulty; but, by means of thetelescope, we can bring them, as it were, so much nearer to us, thattheir broad expanses may be seen. We fail, however, signally with regardto the stars; for they are so very distant, and therefore such tinypoints of light, that our mightiest telescopes cannot magnify themsufficiently to show any breadth of surface. 

Instead of saying that an object looks a certain breadth across, suchas a yard or a foot, a statement which would really mean nothing, astronomers speak of it as measuring a certain angle. Such angles areestimated in what are called "degrees of arc"; each degree being dividedinto sixty minutes, and each minute again into sixty seconds. Popularlyconsidered the moon and sun _look_ about the same size, or, as anastronomer would put it, they measure about the same angle. This is anangle, roughly, of thirty-two minutes of arc; that is to say, slightlymore than half a degree. The broad expanse of surface which a celestialbody shows to us, whether to the naked eye, as in the case of the sunand moon, or in the telescope, as in the case of other members of oursystem, is technically known as its "disc. "

CHAPTER VII

ECLIPSES AND KINDRED PHENOMENA

Since some members of the solar system are nearer to us than others, andall are again much nearer than any of the stars, it must often happenthat one celestial body will pass between us and another, and thusintercept its light for a while. The moon, being the nearest object inthe universe, will, of course, during its motion across the sky, temporarily blot out every one of the others which happen to lie in itspath. When it passes in this manner across the face of the sun, it issaid to _eclipse_ it. When it thus hides a planet or star, it is said to_occult_ it. The reason why a separate term is used for what is merely acase of obscuring light in exactly the same way, will be plain when oneconsiders that the disc of the sun is almost of the same apparent sizeas that of the moon, and so the complete hiding of the sun can last buta few minutes at the most; whereas a planet or a star looks so verysmall in comparison, that it is always _entirely swallowed up for sometime_ when it passes behind the body of our satellite. 

The sun, of course, occults planets and stars in exactly the same manneras the moon does, but we cannot see these occultations on account of theblaze of sunlight. 

By reason of the small size which the planets look when viewed with thenaked eye, we are not able to note them in the act of passing over starsand so blotting them out; but such occurrences may be seen in thetelescope, for the planetary bodies then display broad discs. 

There is yet another occurrence of the same class which is known as a_transit_. This takes place when an apparently small body passes acrossthe face of an apparently large one, the phenomenon being in fact theexact reverse of an occultation. As there is no appreciable body nearerto us than the moon, we can never see anything in transit across herdisc. But since the planets Venus and Mercury are both nearer to us thanthe sun, they will occasionally be seen to pass across his face, andthus we get the well-known phenomena called Transits of Venus andTransits of Mercury. 

As the satellites of Jupiter are continually revolving around him, theywill often pass behind or across his disc. Such occultations andtransits of satellites can be well observed in the telescope. 

There is, however, a way in which the light of a celestial body may beobscured without the necessity of its being hidden from us by onenearer. It will no doubt be granted that any opaque object casts ashadow when a strong light falls directly upon it. Thus the earth, underthe powerful light which is directed upon it from the sun, casts anextensive shadow, though we are not aware of the existence of thisshadow until it falls upon something. The shadow which the earth castsis indeed not noticeable to us until some celestial body passes into it. As the sun is very large, and the earth in comparison very small, theshadow thrown by the earth is comparatively short, and reaches out inspace for only about a million miles. There is no visible object exceptthe moon, which circulates within that distance from our globe, andtherefore she is the only body which can pass into this shadow. Wheneversuch a thing happens, her surface at once becomes dark, for the reasonthat she never emits any light of her own, but merely reflects that ofthe sun. As the moon is continually revolving around the earth, onewould be inclined to imagine that once in every month, namely at what iscalled _full moon_, when she is on the other side of the earth withrespect to the sun, she ought to pass through the shadow in question. But this does not occur every time, because the moon's orbit is notquite _upon the same plane_ with the earth's. It thus happens that timeafter time the moon passes clear of the earth's shadow, sometimes aboveit, and sometimes below it. It is indeed only at intervals of about sixmonths that the moon can be thus obscured. This darkening of her lightis known as an _eclipse of the moon_. It seems a great pity that customshould oblige us to employ the one term "eclipse" for this and also forthe quite different occurrence, an eclipse of the sun; in which thesun's face is hidden as a consequence of the moon's body coming directly_between_ it and our eyes. 

The popular mind seems always to have found it more difficult to graspthe causes of an eclipse of the moon than an eclipse of the sun. As Mr. J. E. Gore[4] puts it: "The darkening of the sun's light by theinterposition of the moon's body seems more obvious than the passing ofthe moon through the earth's shadow. "

Eclipses of the moon furnish striking spectacles, but really add littleto our knowledge. They exhibit, however, one of the most remarkableevidences of the globular shape of our earth; for the outline of itsshadow when seen creeping over the moon's surface is always circular. 

[Illustration: FIG. 3. --Total and Partial Eclipses of the Moon. The Moonis here shown in two positions; i. E. _entirely_ plunged in the earth'sshadow and therefore totally eclipsed, and only _partly_ plunged in itor partially eclipsed. ]

_Eclipses of the Moon_, or Lunar Eclipses, as they are also called, areof two kinds--_Total_, and _Partial_. In a total lunar eclipse the moonpasses entirely into the earth's shadow, and the whole of her surface isconsequently darkened. This darkening lasts for about two hours. In apartial lunar eclipse, a portion only of the moon passes through theshadow, and so only _part_ of her surface is darkened (see Fig. 3). Avery striking phenomenon during a total eclipse of the moon, is that thedarkening of the lunar surface is usually by no means so intense as onewould expect, when one considers that the sunlight at that time shouldbe _wholly_ cut off from it. The occasions indeed upon which the moonhas completely disappeared from view during the progress of a totallunar eclipse are very rare. On the majority of these occasions she hasappeared of a coppery-red colour, while sometimes she has assumed anashen hue. The explanations of these variations of colour is to be foundin the then state of the atmosphere which surrounds our earth. Whenthose portions of our earth's atmosphere through which the sun's rayshave to filter on their way towards the moon are free from wateryvapour, the lunar surface will be tinged with a reddish light, such aswe ordinarily experience at sunset when our air is dry. The ashen colouris the result of our atmosphere being laden with watery vapour, and issimilar to what we see at sunset when rain is about. Lastly, when theair around the earth is thickly charged with cloud, no light at all canpass; and on such occasions the moon disappears altogether for the timebeing from the night sky. 

_Eclipses of the Sun_, otherwise known as Solar Eclipses, are dividedinto _Total_, _Partial_, and _Annular_. A total eclipse of the sun takesplace when the moon comes between the sun and the earth, in such amanner that it cuts off the sunlight _entirely_ for the time being froma _portion_ of the earth's surface. A person situated in the region inquestion will, therefore, at that moment find the sun temporarilyblotted out from his view by the body of the moon. Since the moon is avery much smaller body than the sun, and also very much the nearer to usof the two, it will readily be understood that the portion of the earthfrom which the sun is seen thus totally eclipsed will be of smallextent. In places not very distant from this region, the moon willappear so much shifted in the sky that the sun will be seen onlypartially eclipsed. The moon being in constant movement round the earth, the portion of the earth's surface from which an eclipse is seen astotal will be always a comparatively narrow band lying roughly from westto east. This band, known as the _track of totality_, can, at theutmost, never be more than about 165 miles in width, and as a rule isvery much less. For about 2000 miles on either side of it the sun isseen partially eclipsed. Outside these limits no eclipse of any kind isvisible, as from such regions the moon is not seen to come in the way ofthe sun (see Fig. 4 (i. ), p. 67). 

It may occur to the reader that eclipses can also take place in thecourse of which the positions, where the eclipse would ordinarily beseen as total, will lie outside the surface of the earth. Such aneclipse is thus not dignified with the name of total eclipse, but iscalled a partial eclipse, because from the earth's surface the sun isonly seen _partly eclipsed at the utmost_ (see Fig. 4 (ii. ), p. 67). 

[Illustration: (i. ) Total Eclipse of the Sun. ]

[Illustration: (ii. ) Partial Eclipse of the Sun. 

FIG. 4. --Total and Partial Eclipses of the Sun. From the position A theSun cannot be seen, as it is entirely blotted out by the Moon. From B itis seen partially blotted out, because the Moon is to a certain degreein the way. From C no eclipse is seen, because the Moon does not come inthe way. 

It is to be noted that in a Partial Eclipse of the Sun, the position Alies _outside_ the surface of the Earth. ]

An _Annular eclipse_ is an eclipse which just fails to become total foryet another reason. We have pointed out that the orbits of the variousmembers of the solar system are not circular, but oval. Such ovalfigures, it will be remembered, are technically known as ellipses. In anelliptic orbit the controlling body is situated not in the middle of thefigure, but rather towards one of the ends; the actual point which itoccupies being known as the _focus_. The sun being at the focus of theearth's orbit, it follows that the earth is, at times, a little nearerto him than at others. The sun will therefore appear to us to vary alittle in size, looking sometimes slightly larger than at other times. It is so, too, with the moon, at the focus of whose orbit the earth issituated. She therefore also appears to us at times to vary slightly insize. The result is that when the sun is eclipsed by the moon, and themoon at the time appears the larger of the two, she is able to blot outthe sun completely, and so we can get a total eclipse. But when, on theother hand, the sun appears the larger, the eclipse will not be quitetotal, for a portion of the sun's disc will be seen protruding allaround the moon like a ring of light. This is what is known as anannular eclipse, from the Latin word _annulus_, which means a ring. Theterm is consecrated by long usage, but it seems an unfortunate one onaccount of its similarity to the word "annual. " The Germans speak ofthis kind of eclipse as "ring-formed, " which is certainly much more tothe point. 

There can never be a year without an eclipse of the sun. Indeed theremust be always two such eclipses _at least_ during that period, thoughthere need be no eclipse of the moon at all. On the other hand, thegreatest number of eclipses which can ever take place during a year areseven; that is to say, either five solar eclipses and two lunar, or foursolar and three lunar. This general statement refers merely to eclipsesin their broadest significance, and informs us in no way whether theywill be total or partial. 

Of all the phenomena which arise from the hiding of any celestial bodyby one nearer coming in the way, a total eclipse of the sun is far themost important. It is, indeed, interesting to consider how much poorermodern astronomy would be but for the extraordinary coincidence whichmakes a total solar eclipse just possible. The sun is about 400 timesfarther off from us than the moon, and enormously greater than her inbulk. Yet the two are relatively so distanced from us as to look aboutthe same size. The result of this is that the moon, as has been seen, can often blot out the sun entirely from our view for a short time. Whenthis takes place the great blaze of sunlight which ordinarily dazzlesour eyes is completely cut off, and we are thus enabled, unimpeded, tonote what is going on in the immediate vicinity of the sun itself. 

In a total solar eclipse, the time which elapses from the moment whenthe moon's disc first begins to impinge upon that of the sun at hiswestern edge until the eclipse becomes total, lasts about an hour. During all this time the black lunar disc may be watched making its waysteadily across the solar face. Notwithstanding the gradual obscurationof the sun, one does not notice much diminution of light until aboutthree-quarters of his disc are covered. Then a wan, unearthly appearancebegins to pervade all things, the temperature falls noticeably, andnature seems to halt in expectation of the coming of something unusual. The decreasing portion of sun becomes more and more narrow, until atlength it is reduced to a crescent-shaped strip of exceeding fineness. Strange, ill-defined, flickering shadows (known as "Shadow Bands") mayat this moment be seen chasing each other across any white expanse suchas a wall, a building, or a sheet stretched upon the ground. The westernside of the sky has now assumed an appearance dark and lowering, as if arainstorm of great violence were approaching. This is caused by themighty mass of the lunar shadow sweeping rapidly along. It flies onwardat the terrific velocity of about half a mile a second. 

If the gradually diminishing crescent of sun be now watched through atelescope, the observer will notice that it does not eventually vanishall at once, as he might have expected. Rather, it breaks up first ofall along its length into a series of brilliant dots, known as "Baily'sBeads. " The reason of this phenomenon is perhaps not entirely agreedupon, but the majority of astronomers incline to the opinion that theso-called "beads" are merely the last remnants of sunlight peepingbetween those lunar mountain peaks which happen at the moment to fringethe advancing edge of the moon. The beads are no sooner formed than theyrapidly disappear one after the other, after which no portion of thesolar surface is left to view, and the eclipse is now total (see Fig. 5). 

[Illustration: _In a total Eclipse_ _In an annular Eclipse_

FIG. 5. --"Baily's Beads. "]

But with the disappearance of the sun there springs into view a new andstrange appearance, ordinarily unseen because of the blaze of sunlight. It is a kind of aureole, or halo, pearly white in colour, which is seento surround the black disc of the moon. This white radiance is noneother than the celebrated phenomenon widely known as the _Solar Corona_. It was once upon a time thought to belong to the moon, and to be perhapsa lunar atmosphere illuminated by the sunlight shining through it frombehind. But the suddenness with which the moon always blots out starswhen occulting them, has amply proved that she possesses no atmosphereworth speaking about. It is now, however, satisfactorily determined thatthe corona belongs to the sun, for during the short time that it remainsin view the black body of the moon can be seen creeping across it. 

All the time that the _total phase_ (as it is called) lasts, the coronaglows with its pale unearthly light, shedding upon the earth's surfacean illumination somewhat akin to full moonlight. Usually the planetVenus and a few stars shine out the while in the darkened heaven. Meantime around the observer animal and plant life behave as atnightfall. Birds go to roost, bats fly out, worms come to the surface ofthe ground, flowers close up. In the Norwegian eclipse of 1896 fish wereseen rising to the surface of the water. When the total phase at lengthis over, and the moon in her progress across the sky has allowed thebrilliant disc of the sun to spring into view once more at the otherside, the corona disappears. 

There is another famous accompaniment of the sun which partly revealsitself during total solar eclipses. This is a layer of red flame whichclosely envelops the body of the sun and lies between it and the corona. This layer is known by the name of the _Chromosphere_. Just as atordinary times we cannot see the corona on account of the blaze ofsunlight, so are we likewise unable to see the chromosphere because ofthe dazzling white light which shines through from the body of the sununderneath and completely overpowers it. When, however, during a solareclipse, the lunar disc has entirely hidden the brilliant face of thesun, we are still able for a few moments to see an edgewise portion ofthe chromosphere in the form of a narrow red strip, fringing theadvancing border of the moon. Later on, just before the moon begins touncover the face of the sun from the other side, we may again get a viewof a strip of chromosphere. 

The outer surface of the chromosphere is not by any means even. It isrough and billowy, like the surface of a storm-tossed sea. Portions ofit, indeed, rise at times to such heights that they may be seen standingout like blood-red points around the black disc of the moon, and remainthus during a good part of the total phase. These projections are knownas the _Solar Prominences_. In the same way as the corona, thechromosphere and prominences were for a time supposed to belong to themoon. This, however, was soon found not to be the case, for the lunardisc was noticed to creep slowly across them also. 

The total phase, or "totality, " as it is also called, lasts fordifferent lengths of time in different eclipses. It is usually of abouttwo or three minutes' duration, and at the utmost it can never lastlonger than about eight minutes. 

When totality is over and the corona has faded away, the moon's disccreeps little by little from the face of the sun, light and heat returnsonce more to the earth, and nature recovers gradually from the gloom inwhich she has been plunged. About an hour after totality, the lastremnant of moon draws away from the solar disc, and the eclipse isentirely at an end. 

The corona, the chromosphere, and the prominences are the most importantof these accompaniments of the sun which a total eclipse reveals to us. Our further consideration of them must, however, be reserved for asubsequent chapter, in which the sun will be treated of at length. 

Every one who has had the good fortune to see a total eclipse of the sunwill, the writer feels sure, agree with the verdict of Sir NormanLockyer that it is at once one of the "grandest and most awe-inspiringsights" which man can witness. Needless to say, such an occurrence usedto cause great consternation in less civilised ages; and that it has notin modern times quite parted with its terrors for some persons, is shownby the fact that in Iowa, in the United States, a woman died from frightduring the eclipse of 1869. 

To the serious observer of a total solar eclipse every instant isextremely precious. Many distinct observations have to be crowded into atime all too limited, and this in an eclipse-party necessitates constantrehearsals in order that not a moment may be wasted when the longed-fortotality arrives. Such preparation is very necessary; for the rarity anduncommon nature of a total eclipse of the sun, coupled with itsexceeding short duration, tends to flurry the mind, and to render itslow to seize upon salient points of detail. And, even after everyprecaution has been taken, weather possibilities remain to be reckonedwith, so that success is rather a lottery. 

Above all things, therefore, a total solar eclipse is an occurrence forthe proper utilisation of which personal experience is of absolutenecessity. It was manifestly out of the question that such experiencecould be gained by any individual in early times, as the imperfectionof astronomical theory and geographical knowledge rendered thepredicting of the exact position of the track of totality well-nighimpossible. Thus chance alone would have enabled one in those days towitness a total phase, and the probabilities, of course, were muchagainst a second such experience in the span of a life-time. And even inmore modern times, when the celestial motions had come to be betterunderstood, the difficulties of foreign travel still were in the way;for it is, indeed, a notable fact that during many years following theinvention of the telescope the tracks were placed for the most part infar-off regions of the earth, and Europe was visited by singularly fewtotal solar eclipses. Thus it came to pass that the building up of abody of organised knowledge upon this subject was greatly delayed. 

Nothing perhaps better shows the soundness of modern astronomical theorythan the almost exact agreement of the time predicted for an eclipsewith its actual occurrence. Similarly, by calculating backwards, astronomers have discovered the times and seasons at which many ancienteclipses took place, and valuable opportunities have thus arisen forchecking certain disputed dates in history. 

It should not be omitted here that the ancients were actually able, _ina rough way_, to predict eclipses. The Chaldean astronomers had indeednoticed very early a curious circumstance, _i. E. _ that eclipses tend torepeat themselves after a lapse of slightly more than eighteen years. 

In this connection it must, however, be pointed out, in the firstinstance, that the eclipses which occur in any particular year are inno way associated with those which occurred in the previous year. Inother words, the mere fact that an eclipse takes place upon a certainday this year will not bring about a repetition of it at the same timenext year. However, the nicely balanced behaviour of the solar system, an equilibrium resulting from æons of orbital ebb and flow, naturallytends to make the members which compose that family repeat their ancientcombinations again and again; so that after definite lapses of time thesame order of things will _almost exactly_ recur. Thus, as a consequenceof their beautifully poised motions, the sun, the moon, and the earthtend, after a period of 18 years and 10-1/3 days, [5] to occupy verynearly the same positions with regard to each other. The result of thisis that, during each recurring period, the eclipses comprised within itwill be repeated in their order. 

To give examples:--

The total solar eclipse of August 30, 1905, was a repetition of that ofAugust 19, 1887. 

The partial solar eclipse of February 23, 1906, corresponded to thatwhich took place on February 11, 1888. 

The annular eclipse of July 10, 1907, was a recurrence of that of June28, 1889. 

In this way we can go on until the eighteen year cycle has run out, andwe come upon a total solar eclipse predicted for September 10, 1923, which will repeat the above-mentioned ones of 1905 and 1887; and so ontoo with the others. 

From mere observation alone, extending no doubt over many ages, thosetime-honoured watchers of the sky, the early Chaldeans, had arrived atthis remarkable generalisation; and they used it for the roughprediction of eclipses. To the period of recurrence they give the nameof "Saros. "

And here we find ourselves led into one of the most interesting andfascinating by-paths in astronomy, to which writers, as a rule, pay alltoo little heed. 

In order not to complicate matters unduly, the recurrence of solareclipses alone will first be dealt with. This limitation will, however, not affect the arguments in the slightest, and it will be all the moreeasy in consequence to show their application to the case of eclipses ofthe moon. 

The reader will perhaps have noticed that, with regard to the repetitionof an eclipse, it has been stated that the conditions which bring it onat each recurrence are reproduced _almost exactly_. Here, then, lies the_crux_ of the situation. For it is quite evident that were theconditions _exactly_ reproduced, the recurrences of each eclipse wouldgo on for an indefinite period. For instance, if the lapse of a sarosperiod found the sun, moon, and earth again in the precise relativesituations which they had previously occupied, the recurrences of asolar eclipse would tend to duplicate its forerunner with regard to theposition of the shadow upon the terrestrial surface. But the conditions_not_ being exactly reproduced, the shadow-track does not pass acrossthe earth in quite the same regions. It is shifted a little, so tospeak; and each time the eclipse comes round it is found to be shifted alittle farther. Every solar eclipse has therefore a definite "life" ofits own upon the earth, lasting about 1150 years, or 64 saros returns, and working its way little by little across our globe from north tosouth, or from south to north, as the case may be. Let us take animaginary example. A _partial_ eclipse occurs, say, somewhere near theNorth Pole, the edge of the "partial" shadow just grazing the earth, andthe "track of totality" being as yet cast into space. Here we have thebeginning of a series. At each saros recurrence the partial shadowencroaches upon a greater extent of earth-surface. At length, in itsturn, the track of totality begins to impinge upon the earth. This trackstreaks across our globe at each return of the eclipse, repeating itselfevery time in a slightly more southerly latitude. South and south itmoves, passing in turn the Tropic of Cancer, the Equator, the Tropic ofCapricorn, until it reaches the South Pole; after which it touches theearth no longer, but is cast into space. The rear portion of the partialshadow, in its turn, grows less and less in extent; and it too in timefinally passes off. Our imaginary eclipse series is now no more--its"life" has ended. 

We have taken, as an example, an eclipse series moving from north tosouth. We might have taken one moving from south to north, for theyprogress in either direction. 

From the description just given the reader might suppose that, if thetracks of totality of an eclipse series were plotted upon a chart of theworld, they would lie one beneath another like a set of steps. This is, however, _not_ the case, and the reason is easily found. It depends uponthe fact that the saros does not comprise an exact number of days, butincludes, as we have seen, one-third of a day in addition. 

It will be granted, of course, that if the number of days was exact, the_same_ parts of the earth would always be brought round by the axialrotation _to front the sun_ at the moment of the recurrence of theeclipse. But as there is still one-third of a day to complete the sarosperiod, the earth has yet to make one-third of a rotation upon its axisbefore the eclipse takes place. Thus at every recurrence the track oftotality finds itself placed one-third of the earth's circumference tothe _westward_. Three of the recurrences will, of course, complete thecircuit of the globe; and so the fourth recurrence will duplicate theone which preceded it, three saros returns, or 54 years and 1 monthbefore. This duplication, as we have already seen, will, however, besituated in a latitude to the south or north of its predecessor, according as the eclipse series is progressing in a southerly ornortherly direction. 

Lastly, every eclipse series, after working its way across the earth, will return again to go through the same process after some 12, 000years; so that, at the end of that great lapse of time, the entire"life" of every eclipse should repeat itself, provided that theconditions of the solar system have not altered appreciably during theinterval. 

We are now in a position to consider this gradual southerly ornortherly progress of eclipse recurrences in its application to the caseof eclipses of the moon. It should be evident that, just as in solareclipses the lunar shadow is lowered or raised (as the case may be) eachtime it strikes the terrestrial surface, so in lunar eclipses will thebody of the moon shift its place at each recurrence relatively to theposition of the earth's shadow. Every lunar eclipse, therefore, willcommence on our satellite's disc as a partial eclipse at the northern orsouthern extremity, as the case may be. Let us take, as an example, animaginary series of eclipses of the moon progressing from north tosouth. At each recurrence the partial phase will grow greater, itsboundary encroaching more and more to the southward, until eventuallythe whole disc is enveloped by the shadow, and the eclipse becomestotal. It will then repeat itself as total during a number ofrecurrences, until the entire breadth of the shadow has been passedthrough, and the northern edge of the moon at length springs out intosunlight. This illuminated portion will grow more and more extensive ateach succeeding return, the edge of the shadow appearing to recede fromit until it finally passes off at the south. Similarly, when a lunareclipse commences as partial at the south of the moon, the edge of theshadow at each subsequent recurrence finds itself more and more to thenorthward. In due course the total phase will supervene, and willpersist during a number of recurrences until the southerly trend of themoon results in the uncovering of the lunar surface at the south. Thus, as the boundary of the shadow is left more and more to the northward, the illuminated portion on the southern side of the moon becomes at eachrecurrence greater and the darkened portion on the northern side less, until the shadow eventually passes off at the north. 

The "life" of an eclipse of the moon happens, for certain reasons, to bemuch shorter than that of an eclipse of the sun. It lasts during onlyabout 860 years, or 48 saros returns. 

Fig. 6, p. 81, is a map of the world on Mercator's Projection, showing aportion of the march of the total solar eclipse of August 30, 1905, across the surface of the earth. The projection in question has beenemployed because it is the one with which people are most familiar. Thiseclipse began by striking the neighbourhood of the North Pole in theguise of a partial eclipse during the latter part of the reign of QueenElizabeth, and became total on the earth for the first time on the 24thof June 1797. Its next appearance was on the 6th of July 1815. It hasnot been possible to show the tracks of totality of these two earlyvisitations on account of the distortion of the polar regions consequenton the _fiction_ of Mercator's Projection. It is therefore made tocommence with the track of its third appearance, viz. On July 17, 1833. In consequence of those variations in the apparent sizes of the sun andmoon, which result, as we have seen, from the variations in theirdistances from the earth, this eclipse will change from a total into anannular eclipse towards the end of the twenty-first century. By thattime the track will have passed to the southern side of the equator. Thetrack will eventually leave the earth near the South Pole about thebeginning of the twenty-sixth century, and the rear portion of thepartial shadow will in its turn be clear of the terrestrial surface byabout 2700 A. D. , when the series comes to an end. 

[Illustration: FIG. 6. --Map of the World on Mercator's Projection, showing a portion of the progress of the Total Solar Eclipse of August30, 1905, across the surface of the earth. ]

[4] Astronomical Essays (p. 40), London, 1907. 

[5] In some cases the periods between the dates of the correspondingeclipses _appear_ to include a greater number of days than ten; but thisis easily explained when allowance is made for intervening _leap_ years(in each of which an _extra_ day has of course been added), and also forvariations in local time. 

CHAPTER VIII

FAMOUS ECLIPSES OF THE SUN

What is thought to be the earliest reference to an eclipse comes down tous from the ancient Chinese records, and is over four thousand yearsold. The eclipse in question was a solar one, and occurred, so far ascan be ascertained, during the twenty-second century B. C. The story runsthat the two state astronomers, Ho and Hi by name, being exceedinglyintoxicated, were unable to perform their required duties, whichconsisted in superintending the customary rites of beating drums, shooting arrows, and the like, in order to frighten away the mightydragon which it was believed was about to swallow up the Lord of Day. This eclipse seems to have been only partial; nevertheless a greatturmoil ensued, and the two astronomers were put to death, no doubt withthe usual _celestial_ cruelty. 

The next eclipse mentioned in the Chinese annals is also a solareclipse, and appears to have taken place more than a thousand yearslater, namely in 776 B. C. Records of similar eclipses follow from thesame source; but as they are mere notes of the events, and do not enterinto any detail, they are of little interest. Curiously enough theChinese have taken practically no notice of eclipses of the moon, buthave left us a comparatively careful record of comets, which has beenof value to modern astronomy. 

The earliest mention of a _total_ eclipse of the sun (for it should benoted that the ancient Chinese eclipse above-mentioned was merelypartial) was deciphered in 1905, on a very ancient Babylonian tablet, byMr. L. W. King of the British Museum. This eclipse took place in the year1063 B. C. 

Assyrian tablets record three solar eclipses which occurred betweenthree and four hundred years later than this. The first of these was in763 B. C. ; the total phase being visible near Nineveh. 

The next record of an eclipse of the sun comes to us from a Greciansource. This eclipse took place in 585 B. C. , and has been the subject ofmuch investigation. Herodotus, to whom we are indebted for the account, tells us that it occurred during a battle in a war which had been wagingfor some years between the Lydians and Medes. The sudden coming on ofdarkness led to a termination of the contest, and peace was afterwardsmade between the combatants. The historian goes on to state that theeclipse had been foretold by Thales, who is looked upon as the Founderof Grecian astronomy. This eclipse is in consequence known as the"Eclipse of Thales. " It would seem as if that philosopher wereacquainted with the Chaldean saros. 

The next solar eclipse worthy of note was an annular one, and occurredin 431 B. C. , the first year of the Peloponnesian War. Plutarch relatesthat the pilot of the ship, which was about to convey Pericles to thePeloponnesus, was very much frightened by it; but Pericles calmed him byholding up a cloak before his eyes, and saying that the only differencebetween this and the eclipse was that something larger than the cloakprevented his seeing the sun for the time being. 

An eclipse of great historical interest is that known as the "Eclipse ofAgathocles, " which occurred on the morning of the 15th of August, 310B. C. Agathocles, Tyrant of Syracuse, had been blockaded in the harbourof that town by the Carthaginian fleet, but effected the escape of hissquadron under cover of night, and sailed for Africa in order to invadethe enemy's territory. During the following day he and his vesselsexperienced a total eclipse, in which "day wholly put on the appearanceof night, and the stars were seen in all parts of the sky. "

A few solar eclipses are supposed to be referred to in early Romanhistory, but their identity is very doubtful in comparison with thosewhich the Greeks have recorded. Additional doubt is cast upon them bythe fact that they are usually associated with famous events. The birthand death of Romulus, and the Passage of the Rubicon by Julius Cæsar, are stated indeed to have been accompanied by these marks of theapproval or disapproval of the gods!

Reference to our subject in the Bible is scanty. Amos viii. 9 is thoughtto refer to the Nineveh eclipse of 763 B. C. , to which allusion hasalready been made; while the famous episode of Hezekiah and the shadowon the dial of Ahaz has been connected with an eclipse which was partialat Jerusalem in 689 B. C. 

The first solar eclipse, recorded during the Christian Era, is known asthe "Eclipse of Phlegon, " from the fact that we are indebted for theaccount to a pagan writer of that name. This eclipse took place in A. D. 29, and the total phase was visible a little to the north of Palestine. It has sometimes been confounded with the "darkness of the Crucifixion, "which event took place near the date in question; but it is sufficienthere to say that the Crucifixion is well known to have occurred duringthe Passover of the Jews, which is always celebrated at the _full_ moon, whereas an eclipse of the sun can only take place at _new_ moon. 

Dion Cassius, commenting on the Emperor Claudius about the year A. D. 45, writes as follows:--

"As there was going to be an eclipse on his birthday, through fear of adisturbance, as there had been other prodigies, he put forth a publicnotice, not only that the obscuration would take place, and about thetime and magnitude of it, but also about the causes that produce such anevent. "

This is a remarkable piece of information; for the Romans, anessentially military nation, appear hitherto to have troubled themselvesvery little about astronomical matters, and were content, as we haveseen, to look upon phenomena, like eclipses, as mere celestialprodigies. 

What is thought to be the first definite mention of the solar coronaoccurs in a passage of Plutarch. The eclipse to which he refers isprobably one which took place in A. D. 71. He says that the obscurationcaused by the moon "has no time to last and no extensiveness, but somelight shows itself round the sun's circumference, which does not allowthe darkness to become deep and complete. " No further reference to thisphenomenon occurs until near the end of the sixteenth century. Itshould, however, be here mentioned that Mr. E. W. Maunder has pointedout the probability[6] that we have a very ancient symbolicrepresentation of the corona in the "winged circle, " "winged disc, " or"ring with wings, " as it is variously called, which appears so oftenupon Assyrian and Egyptian monuments, as the symbol of the Deity (Fig. 7). 

[Illustration: FIG. 7. --The "Ring with Wings. " The upper is the Assyrianform of the symbol, the lower the Egyptian. (From _Knowledge_. ) Comparethe form of the corona on Plate VII. (B), p. 142. ]

The first solar eclipse recorded to have been seen in England is that ofA. D. 538, mention of which is found in the _Anglo-Saxon Chronicle_. Thetrack of totality did not, however, come near our islands, for onlytwo-thirds of the sun's disc were eclipsed at London. 

In 840 a great eclipse took place in Europe, which was total for morethan five minutes across what is now Bavaria. Terror at this eclipse issaid to have hastened the death of Louis le Debonnaire, Emperor of theWest, who lay ill at Worms. 

In 878--_temp. _ King Alfred--an eclipse of the sun took place which wastotal at London. From this until 1715 no other eclipse was total atLondon itself; though this does not apply to other portions of England. 

An eclipse, generally known as the "Eclipse of Stiklastad, " is said tohave taken place in 1030, during the sea-fight in which Olaf of Norwayis supposed to have been slain. Longfellow, in his _Saga of King Olaf_, has it that

"The Sun hung redAs a drop of blood, "

but, as in the case of most poets, the dramatic value of an eclipseseems to have escaped his notice. 

In the year 1140 there occurred a total eclipse of the sun, the last tobe visible in England for more than five centuries. Indeed there havebeen only two such since--namely, those of 1715 and 1724, to which weshall allude in due course. The eclipse of 1140 took place on the 20thMarch, and is thus referred to in the _Anglo-Saxon Chronicle_:--

"In the Lent, the sun and the day darkened, about the noon-tide of theday, when men were eating, and they lighted candles to eat by. That wasthe 13th day before the calends of April. Men were very much struck withwonder. "

Several of the older historians speak of a "fearful eclipse" as havingtaken place on the morning of the Battle of Crecy, 1346. Lingard, forinstance, in his _History of England_, has as follows:--

"Never, perhaps, were preparations for battle made under circumstancesso truly awful. On that very day the sun suffered a partial eclipse:birds, in clouds, the precursors of a storm, flew screaming over the twoarmies, and the rain fell in torrents, accompanied by incessant thunderand lightning. About five in the afternoon the weather cleared up; thesun in full splendour darted his rays in the eyes of the enemy. "

Calculations, however, show that no eclipse of the sun took place inEurope during that year. This error is found to have arisen from themistranslation of an obsolete French word _esclistre_ (lightning), whichis employed by Froissart in his description of the battle. 

In 1598 an eclipse was total over Scotland and part of North Germany. Itwas observed at Torgau by Jessenius, an Hungarian physician, who noticeda bright light around the moon during the time of totality. This is saidto be the first reference to the corona since that of Plutarch, to whichwe have already drawn attention. 

Mention of Scotland recalls the fact that an unusual number of eclipseshappen to have been visible in that country, and the occult bent naturalto the Scottish character has traditionalised a few of them in suchterms as the "Black Hour" (an eclipse of 1433), "Black Saturday" (theeclipse of 1598 which has been alluded to above), and "Mirk Monday"(1652). The track of the last-named also passed over Carrickfergus inIreland, where it was observed by a certain Dr. Wybord, in whose accountthe term "corona" is first employed. This eclipse is the last which hasbeen total in Scotland, and it is calculated that there will not beanother eclipse seen as total there until the twenty-second century. 

An eclipse of the sun which took place on May 30, 1612, is recorded ashaving been seen "through a tube. " This probably refers to the thenrecent invention--the telescope. 

The eclipses which we have been describing are chiefly interesting froman historical point of view. The old mystery and confusion to thebeholders seem to have lingered even into comparatively enlightenedtimes, for we see how late it is before the corona attracts definiteattention for the sake of itself alone. 

It is not a far cry from notice of the corona to that of otheraccompaniments of a solar eclipse. Thus the eclipse of 1706, the totalphase of which was visible in Switzerland, is of great interest; for itwas on this occasion that the famous red prominences seem first to havebeen noted. A certain Captain Stannyan observed this eclipse from Bernein Switzerland, and described it in a letter to Flamsteed, the thenAstronomer Royal. He says the sun's "getting out of his eclipse waspreceded by a blood-red streak of light from its left limb, whichcontinued not longer than six or seven seconds of time; then part of theSun's disc appeared all of a sudden, as bright as Venus was ever seen inthe night, nay brighter; and in that very instant gave a Light andShadow to things as strong as Moonlight uses to do. " How little was thenexpected of the sun is, however, shown by Flamsteed's words, whencommunicating this information to the Royal Society:--

"The Captain is the first man I ever heard of that took notice of a RedStreak of Light preceding the Emersion of the Sun's body from a totalEclipse. And I take notice of it to you because it infers that _the Moonhas an atmosphere_; and its short continuance of only six or sevenseconds of time, tells us that _its height is not more than the five orsix hundredth part of her diameter_. "

What a change has since come over the ideas of men! The sun has proved averitable mine of discovery, while the moon has yielded up nothing new. 

The eclipse of 1715, the first total at London since that of 878, wasobserved by the famous astronomer, Edmund Halley, from the rooms of theRoyal Society, then in Crane Court, Fleet Street. On this occasion boththe corona and a red projection were noted. Halley further makesallusion to that curious phenomenon, which later on became celebratedunder the name of "Baily's beads. " It was also on the occasion of thiseclipse that the _earliest recorded drawings of the corona_ were made. Cambridge happened to be within the track of totality; and a certainProfessor Cotes of that University, who is responsible for one of thedrawings in question, forwarded them to Sir Isaac Newton together with aletter describing his observations. 

In 1724 there occurred an eclipse, the total phase of which was visiblefrom the south-west of England, but not from London. The weather wasunfavourable, and the eclipse consequently appears to have been seen byonly one person, a certain Dr. Stukeley, who observed it from HaradenHill near Salisbury Plain. This is the last eclipse of which the totalphase was seen in any part of England. The next will not be until June29, 1927, and will be visible along a line across North Wales andLancashire. The discs of the sun and moon will just then be almost ofthe same apparent size, and so totality will be of extremely shortduration; in fact only a few seconds. London itself will not see atotality until the year 2151--a circumstance which need hardly distressany of us personally!

It is only from the early part of the nineteenth century that seriousscientific attention to eclipses of the sun can be dated. An _annular_eclipse, visible in 1836 in the south of Scotland, drew the carefulnotice of Francis Baily of Jedburgh in Roxburghshire to that curiousphenomenon which we have already described, and which has ever sincebeen known by the name of "Baily's beads. " Spurred by his observation, the leading astronomers of the day determined to pay particularattention to a total eclipse, which in the year 1842 was to be visiblein the south of France and the north of Italy. The public interestaroused on this occasion was also very great, for the region acrosswhich the track of totality was to pass was very populous, and inhabitedby races of a high degree of culture. 

This eclipse occurred on the morning of the 8th July, and from it may bedated that great enthusiasm with which total eclipses of the sun haveever since been received. Airy, our then Astronomer Royal, observed itfrom Turin; Arago, the celebrated director of the Paris Observatory, from Perpignan in the south of France; Francis Baily from Pavia; and SirJohn Herschel from Milan. The corona and three large red prominenceswere not only well observed by the astronomers, but drew tremendousapplause from the watching multitudes. 

The success of the observations made during this eclipse promptedastronomers to pay similar attention to that of July 28, 1851, the totalphase of which was to be visible in the south of Norway and Sweden, andacross the east of Prussia. This eclipse was also a success, and it wasnow ascertained that the red prominences belonged to the sun and not tothe moon; for the lunar disc, as it moved onward, was seen to cover andto uncover them in turn. It was also noted that these prominences weremerely uprushes from a layer of glowing gaseous matter, which was seenclosely to envelop the sun. 

The total eclipse of July 18, 1860, was observed in Spain, andphotography was for the first time _systematically_ employed in itsobservation. [7] In the photographs taken the stationary appearance ofboth the corona and prominences with respect to the moving moon, definitely confirmed the view already put forward that they were actualappendages of the sun. 

The eclipse of August 18, 1868, the total phase of which lasted nearlysix minutes, was visible in India, and drew thither a large concourse ofastronomers. In this eclipse the spectroscope came to the front, andshowed that both the prominences, and the chromospheric layer from whichthey rise, are composed of glowing vapours--chief among which is thevapour of hydrogen. The direct result of the observations made on thisoccasion was the spectroscopic method of examining prominences at anytime in full daylight, and without a total eclipse. This method, whichhas given such an immense impetus to the study of the sun, was theoutcome of independent and simultaneous investigation on the part of theFrench astronomer, the late M. Janssen, and the English astronomer, Professor (now Sir Norman) Lockyer, a circumstance strangely reminiscentof the discovery of Neptune. The principles on which the method wasfounded seem, however, to have occurred to Dr. (now Sir William) Hugginssome time previously. 

The eclipse of December 22, 1870, was total for a little more than twominutes, and its track passed across the Mediterranean. M. Janssen, ofwhom mention has just been made, escaped in a balloon from then besiegedParis, taking his instruments with him, and made his way to Oran, inAlgeria, in order to observe it; but his expectations were disappointedby cloudy weather. The expedition sent out from England had themisfortune to be shipwrecked off the coast of Sicily. But the occasionwas redeemed by a memorable observation made by the American astronomer, the late Professor Young, which revealed the existence of what is nowknown as the "Reversing Layer. " This is a shallow layer of gases whichlies immediately beneath the chromosphere. An illustration of thecorona, as it was seen during the above eclipse, will be found on PlateVII. (A), p. 142. 

In the eclipse of December 12, 1871, total across Southern India, thephotographs of the corona obtained by Mr. Davis, assistant to LordLindsay (now the Earl of Crawford), displayed a wealth of detailhitherto unapproached. 

The eclipse of July 29, 1878, total across the western states of NorthAmerica, was a remarkable success, and a magnificent view of the coronawas obtained by the well-known American astronomer and physicist, thelate Professor Langley, from the summit of Pike's Peak, Colorado, over14, 000 feet above the level of the sea. The coronal streamers were seento extend to a much greater distance at this altitude than at pointsless elevated, and the corona itself remained visible during more thanfour minutes after the end of totality. It was, however, not entirely aquestion of altitude; the coronal streamers were actually very muchlonger on this occasion than in most of the eclipses which hadpreviously been observed. 

The eclipse of May 17, 1882, observed in Upper Egypt, is notable fromthe fact that, in one of the photographs taken by Dr. Schuster at Sohag, a bright comet appeared near the outer limit of the corona (see PlateI. , p. 96). The comet in question had not been seen before the eclipse, and was never seen afterwards. This is the third occasion on whichattention has been drawn to a comet _merely_ by a total eclipse. Thefirst is mentioned by Seneca; and the second by Philostorgius, in anaccount of an eclipse observed at Constantinople in A. D. 418. A fourthcase of the kind occurred in 1893, when faint evidences of one of thesefilmy objects were found on photographs of the corona taken by theAmerican astronomer, Professor Schaeberle, during the total eclipse ofApril 16 of that year. 

The eclipse of May 6, 1883, had a totality of over five minutes, butthe central track unfortunately passed across the Pacific Ocean, and thesole point of land available for observing it from was one of theMarquesas Group, Caroline Island, a coral atoll seven and a half mileslong by one and a half broad. Nevertheless astronomers did not hesitateto take up their posts upon that little spot, and were rewarded withgood weather. 

The next eclipse of importance was that of April 16, 1893. It stretchedfrom Chili across South America and the Atlantic Ocean to the West Coastof Africa, and, as the weather was fine, many good results wereobtained. Photographs were taken at both ends of the track, and theseshowed that the appearance of the corona remained unchanged during theinterval of time occupied by the passage of the shadow across the earth. It was on the occasion of this eclipse that Professor Schaeberle foundupon his photographs those traces of the presence of a comet, to whichallusion has already been made. 

Extensive preparations were made to observe the eclipse of August 9, 1896. Totality lasted from two to three minutes, and the track stretchedfrom Norway to Japan. Bad weather disappointed the observers, with theexception of those taken to Nova Zembla by Sir George Baden Powell inhis yacht _Otaria_. 

The eclipse of January 22, 1898, across India _viâ_ Bombay and Benares, was favoured with good weather, and is notable for a photograph obtainedby Mrs. E. W. Maunder, which showed a ray of the corona extending to amost unusual distance. 

[Illustration: PLATE I. THE TOTAL ECLIPSE OF THE SUN OF MAY 17TH, 1882

A comet is here shown in the immediate neighbourhood of the corona. 

Drawn by Mr. W. H. Wesley from the photographs. 

(Page 95)]

Of very great influence in the growth of our knowledge with regard tothe sun, is the remarkable piece of good fortune by which the countriesaround the Mediterranean, so easy of access, have been favoured with acomparatively large number of total eclipses during the past sixtyyears. Tracks of totality have, for instance, traversed the Spanishpeninsula on no less than five occasions during that period. Two ofthese are among the most notable eclipses of recent years, namely, thoseof May 28, 1900, and of August 30, 1905. In the former the track oftotality stretched from the western seaboard of Mexico, through theSouthern States of America, and across the Atlantic Ocean, after whichit passed over Portugal and Spain into North Africa. The total phaselasted for about a minute and a half, and the eclipse was well observedfrom a great many points along the line. A representation of the corona, as it appeared on this occasion, will be found on Plate VII. (B), p. 142. 

The track of the other eclipse to which we have alluded, _i. E. _ that ofAugust 30, 1905, crossed Spain about 200 miles to the northward of thatof 1900. It stretched from Winnipeg in Canada, through Labrador, andover the Atlantic; then traversing Spain, it passed across the BalearicIslands, North Africa, and Egypt, and ended in Arabia (see Fig. 6, p. 81). Much was to be expected from a comparison between the photographstaken in Labrador and Egypt on the question as to whether the coronawould show any alteration in shape during the time that the shadow wastraversing the intervening space--some 6000 miles. The duration of thetotal phase in this eclipse was nearly four minutes. Bad weather, however, interfered a good deal with the observations. It was notpossible, for instance, to do anything at all in Labrador. In Spain theweather conditions were by no means favourable; though at Burgos, wherean immense number of people had assembled, the total phase was, fortunately, well seen. On the whole, the best results were obtained atGuelma in Algeria. The corona on the occasion of this eclipse was a veryfine one, and some magnificent groups of prominences were plainlyvisible to the naked eye (see the Frontispiece). 

The next total eclipse after that of 1905 was one which occurred onJanuary 14, 1907. It passed across Central Asia and Siberia, and had atotality lasting two and a half minutes at most; but it was not observedas the weather was extremely bad, a circumstance not surprising withregard to those regions at that time of year. 

The eclipse of January 3, 1908, passed across the Pacific Ocean. Onlytwo small coral islands--Hull Island in the Phoenix Group, and FlintIsland about 400 miles north of Tahiti--lay in the track. Twoexpeditions set out to observe it, _i. E. _ a combined American party fromthe Lick Observatory and the Smithsonian Institution of Washington, anda private one from England under Mr. F. K. McClean. As Hull Islandafforded few facilities, both parties installed their instruments onFlint Island, although it was very little better. The duration of thetotal phase was fairly long--about four minutes, and the sun veryfavourably placed, being nearly overhead. Heavy rain and clouds, however, marred observation during the first minute of totality, but theremaining three minutes were successfully utilised, good photographs ofthe corona being obtained. 

The next few years to come are unfortunately by no means favourablefrom the point of view of the eclipse observer. An eclipse will takeplace on June 17, 1909, the track stretching from Greenland across theNorth Polar regions into Siberia. The geographical situation is, however, a very awkward one, and totality will be extremely short--onlysix seconds in Greenland and twenty-three seconds in Siberia. 

The eclipse of May 9, 1910, will be visible in Tasmania. Totality willlast so long as four minutes, but the sun will be at the time much toolow in the sky for good observation. 

The eclipse of the following year, April 28, 1911, will also beconfined, roughly speaking, to the same quarter of the earth, the trackpassing across the old convict settlement of Norfolk Island, and thenout into the Pacific. 

The eclipse of April 17, 1912, will stretch from Portugal, throughFrance and Belgium into North Germany. It will, however, be ofpractically no service to astronomy. Totality, for instance, will lastfor only three seconds in Portugal; and, though Paris lies in thecentral track, the eclipse, which begins as barely total, will havechanged into an _annular_ one by the time it passes over that city. 

The first really favourable eclipse in the near future will be that ofAugust 21, 1914. Its track will stretch from Greenland across Norway, Sweden, and Russia. This eclipse is a return, after one saros, of theeclipse of August 9, 1896. 

The last solar eclipse which we will touch upon is that predicted forJune 29, 1927. It has been already alluded to as the first of those inthe future to be _total_ in England. The central line will stretch fromWales in a north-easterly direction. Stonyhurst Observatory, inLancashire, will lie in the track; but totality there will be veryshort, only about twenty seconds in duration. 

[6] _Knowledge_, vol. Xx. P. 9, January 1897. 

[7] The _first photographic representation of the corona_ had, however, been made during the eclipse of 1851. This was a daguerreotype taken byDr. Busch at Königsberg in Prussia. 

CHAPTER IX

FAMOUS ECLIPSES OF THE MOON

The earliest lunar eclipse, of which we have any trustworthyinformation, was a total one which took place on the 19th March, 721B. C. , and was observed from Babylon. For our knowledge of this eclipsewe are indebted to Ptolemy, the astronomer, who copied it, along withtwo others, from the records of the reign of the Chaldean king, Merodach-Baladan. 

The next eclipse of the moon worth noting was a total one, which tookplace some three hundred years later, namely, in 425 B. C. This eclipsewas observed at Athens, and is mentioned by Aristophanes in his play, _The Clouds_. 

Plutarch relates that a total eclipse of the moon, which occurred in 413B. C. , so greatly frightened Nicias, the general of the Athenians, thenwarring in Sicily, as to cause a delay in his retreat from Syracusewhich led to the destruction of his whole army. 

Seven years later--namely, in 406 B. C. , the twenty-sixth year of thePeloponnesian War--there took place another total lunar eclipse of whichmention is made by Xenophon. 

Omitting a number of other eclipses alluded to by ancient writers, wecome to one recorded by Josephus as having occurred a little before thedeath of Herod the Great. It is probable that the eclipse in questionwas the total lunar one, which calculation shows to have taken place onthe 15th September 5 B. C. , and to have been visible in Western Asia. This is very important, for we are thus enabled to fix that year as thedate of the birth of Christ, for Herod is known to have died in theearly part of the year following the Nativity. 

In those accounts of total lunar eclipses, which have come down to usfrom the Dark and Middle Ages, the colour of the moon is nearly alwayslikened to "blood. " On the other hand, in an account of the eclipse ofJanuary 23, A. D. 753, our satellite is described as "covered with ahorrid black shield. " We thus have examples of the two distinctappearances alluded to in Chapter VII. , _i. E. _ when the moon appears ofa coppery-red colour, and when it is entirely darkened. 

It appears, indeed, that, in the majority of lunar eclipses on record, the moon has appeared of a ruddy, or rather of a coppery hue, and thedetails on its surface have been thus rendered visible. One of the bestexamples of a _bright_ eclipse of this kind is that of the 19th March1848, when the illumination of our satellite was so great that manypersons could not believe that an eclipse was actually taking place. Acertain Mr. Foster, who observed this eclipse from Bruges, states thatthe markings on the lunar disc were almost as visible as on an "ordinarydull moonlight night. " He goes on to say that the British Consul atGhent, not knowing that there had been any eclipse, wrote to him for anexplanation of the red colour of the moon on that evening. 

Out of the _dark_ eclipses recorded, perhaps the best example is thatof May 18, 1761, observed by Wargentin at Stockholm. On this occasionthe lunar disc is said to have disappeared so completely, that it couldnot be discovered even with the telescope. Another such instance is theeclipse of June 10, 1816, observed from London. The summer of that yearwas particularly wet--a point worthy of notice in connection with thetheory that these different appearances are due to the varying state ofour earth's atmosphere. 

Sometimes, indeed, it has happened that an eclipse of the moon haspartaken of both appearances, part of the disc being visible and partinvisible. An instance of this occurred in the eclipse of July 12, 1870, when the late Rev. S. J. Johnson, one of the leading authorities oneclipses, who observed it, states that he found one-half the moon'ssurface quite invisible, both with the naked eye and with the telescope. 

In addition to the examples given above, there are three total lunareclipses which deserve especial mention. 

1. A. D. 755, November 23. During the progress of this eclipse the moonocculted the star Aldebaran in the constellation of Taurus. 

2. A. D. 1493, April 2. This is the celebrated eclipse which is said tohave so well served the purposes of Christopher Columbus. Certainnatives having refused to supply him with provisions when in sorestraits, he announced to them that the moon would be darkened as a signof the anger of heaven. When the event duly came to pass, the savageswere so terrified that they brought him provisions as much as he needed. 

3. A. D. 1610, July 6. The eclipse in question is notable as having beenseen through the telescope, then a recent invention. It was withoutdoubt the first so observed, but unfortunately the name of the observerhas not come down to us. 

CHAPTER X

THE GROWTH OF OBSERVATION

The earliest astronomical observations must have been made in the Dawnof Historic Time by the men who tended their flocks upon the greatplains. As they watched the clear night sky they no doubt soon noticedthat, with the exception of the moon and those brilliant wanderingobjects known to us as the planets, the individual stars in the heavenremained apparently fixed with reference to each other. These seeminglychangeless points of light came in time to be regarded as sign-posts toguide the wanderer across the trackless desert, or the voyager upon thewide sea. 

Just as when looking into the red coals of a fire, or when watching theclouds, our imagination conjures up strange and grotesque forms, so didthe men of old see in the grouping of the stars the outlines of weirdand curious shapes. Fed with mythological lore, they imagined these tobe rough representations of ancient heroes and fabled beasts, whom theysupposed to have been elevated to the heavens as a reward for greatdeeds done upon the earth. We know these groupings of stars to-day underthe name of the Constellations. Looking up at them we find it extremelydifficult to fit in the majority with the figures which the ancientsbelieved them to represent. Nevertheless, astronomy has accepted thearrangement, for want of a better method of fixing the leading stars inthe memory. 

Our early ancestors lived the greater part of their lives in the openair, and so came to pay more attention in general to the heavenly orbsthan we do. Their clock and their calendar was, so to speak, in thecelestial vault. They regulated their hours, their days, and theirnights by the changing positions of the sun, the moon, and the stars;and recognised the periods of seed-time and harvest, of calm and stormyweather, by the rising or setting of certain well-known constellations. Students of the classics will recall many allusions to this, especiallyin the Odes of Horace. 

As time went on and civilisation progressed, men soon devised measuringinstruments, by means of which they could note the positions of thecelestial bodies in the sky with respect to each other; and, fromobservations thus made, they constructed charts of the stars. Theearliest complete survey of this kind, of which we have a record, is thegreat Catalogue of stars which was made, in the second century B. C. , bythe celebrated Greek astronomer, Hipparchus, and in which he is said tohave noted down about 1080 stars. 

It is unnecessary to follow in detail the tedious progress ofastronomical discovery prior to the advent of the telescope. Certain itis that, as time went on, the measuring instruments to which we havealluded had become greatly improved; but, had they even been perfect, they would have been utterly inadequate to reveal those minutedisplacements, from which we have learned the actual distance of thenearest of the celestial orbs. From the early times, therefore, untilthe mediæval period of our own era, astronomy grew up upon a faultybasis, for the earth ever seemed so much the largest body in theuniverse, that it continued from century to century to be regarded asthe very centre of things. 

To the Arabians is due the credit of having kept alive the study of thestars during the dark ages of European history. They erected some fineobservatories, notably in Spain and in the neighbourhood of Bagdad. Following them, some of the Oriental peoples embraced the science inearnest; Ulugh Beigh, grandson of the famous Tamerlane, founding, forinstance, a great observatory at Samarcand in Central Asia. The Mongolemperors of India also established large astronomical instruments in thechief cities of their empire. When the revival of learning took place inthe West, the Europeans came to the front once more in science, andrapidly forged ahead of those who had so assiduously kept alight thelamp of knowledge through the long centuries. 

The dethronement of the older theories by the Copernican system, inwhich the earth was relegated to its true place, was fortunately soonfollowed by an invention of immense import, the invention of theTelescope. It is to this instrument, indeed, that we are indebted forour knowledge of the actual scale of the celestial distances. Itpenetrated the depths of space; it brought the distant orbs so near, that men could note the detail on the planets, or measure the smallchanges in their positions in the sky which resulted from the movementof our own globe. 

It was in the year 1609 that the telescope was first constructed. Ayear or so previous to this a spectacle-maker of Middleburgh in Holland, one Hans Lippershey, had, it appears, hit upon the fact that distantobjects, when viewed through certain glass lenses suitably arranged, looked nearer. [8] News of this discovery reached the ears of GalileoGalilei, of Florence, the foremost philosopher of the day, and he atonce applied his great scientific attainments to the construction of aninstrument based upon this principle. The result was what was called an"optick tube, " which magnified distant objects some few times. It wasnot much larger than what we nowadays contemptuously refer to as a"spy-glass, " yet its employment upon the leading celestial objectsinstantly sent astronomical science onward with a bound. In rapidsuccession Galileo announced world-moving discoveries; large spots uponthe face of the sun; crater-like mountains upon the moon; foursubordinate bodies, or satellites, circling around the planet Jupiter;and a strange appearance in connection with Saturn, which latertelescopic observers found to be a broad flat ring encircling thatplanet. And more important still, the magnified image of Venus showeditself in the telescope at certain periods in crescent and other forms;a result which Copernicus is said to have announced should of necessityfollow if his system were the true one. 

The discoveries made with the telescope produced, as time went on, agreat alteration in the notions of men with regard to the universe atlarge. It must have been, indeed, a revelation to find that those pointsof light which they called the planets, were, after all, globes of asize comparable with the earth, and peopled perchance with sentientbeings. Even to us, who have been accustomed since our early youth tosuch an idea, it still requires a certain stretch of imagination toenlarge, say, the Bright Star of Eve, into a body similar in size to ourearth. The reader will perhaps recollect Tennyson's allusion to this in_Locksley Hall, Sixty Years After_:--

"Hesper--Venus--were we native to that splendour or in Mars, We should see the Globe we groan in, fairest of their evening stars. 

"Could we dream of wars and carnage, craft and madness, lust and spite, Roaring London, raving Paris, in that point of peaceful light?"

The form of instrument as devised by Galileo is called the RefractingTelescope, or "Refractor. " As we know it to-day it is the same inprinciple as his "optick tube, " but it is not quite the same inconstruction. The early _object-glass_, or large glass at the end, was asingle convex lens (see Fig. 8, p. 113, "Galilean"); the modern one is, on the other hand, composed of two lenses fitted together. The attemptsto construct large telescopes of the Galilean type met in course of timewith a great difficulty. The magnified image of the object observed wasnot quite pure; its edges, indeed, were fringed with rainbow-likecolours. This defect was found to be aggravated with increase in thesize of object-glasses. A method was, however, discovered ofdiminishing this colouration, or _chromatic aberration_ as it is calledfrom the Greek word [chrôma] (_chroma_), which means colour, viz. Bymaking telescopes of great length and only a few inches in width. Butthe remedy was, in a way, worse than the disease; for telescopes thusbecame of such huge proportions as to be too unwieldy for use. Attemptswere made to evade this unwieldiness by constructing them with skeletontubes (see Plate II. , p. 110), or, indeed, even without tubes at all;the object-glass in the tubeless or "aerial" telescope being fixed atthe top of a high post, and the _eye-piece_, that small lens orcombination of lenses, which the eye looks directly into, being kept inline with it by means of a string and manoeuvred about near the ground(Plate III. , p. 112). The idea of a telescope without a tube may appeara contradiction in terms; but it is not really so, for the tube addsnothing to the magnifying power of the instrument, and is, in fact, nomore than a mere device for keeping the object-glass and eye-piece in astraight line, and for preventing the observer from being hindered bystray lights in his neighbourhood. It goes without saying, of course, that the image of a celestial object will be more clear and defined whenexamined in the darkness of a tube. 

The ancients, though they knew nothing of telescopes, had, however, found out the merit of a tube in this respect; for they employed simpletubes, blackened on the inside, in order to obtain a clearer view ofdistant objects. It is said that Julius Cæsar, before crossing theChannel, surveyed the opposite coast of Britain through a tube of thiskind. 

[Illustration: PLATE II. GREAT TELESCOPE OF HEVELIUS

This instrument, 150 feet in length, with a _skeleton_ tube, wasconstructed by the celebrated seventeenth century astronomer, Heveliusof Danzig. From an illustration in the _Machina Celestis_. 

(Page 110)]

A few of the most famous of the immensely long telescopes above alludedto are worthy of mention. One of these, 123 feet in length, waspresented to the Royal Society of London by the Dutch astronomerHuyghens. Hevelius of Danzig constructed a skeleton one of 150 feet inlength (see Plate II. , p. 110). Bradley used a tubeless one 212 feetlong to measure the diameter of Venus in 1722; while one of 600 feet issaid to have been constructed, but to have proved quite unworkable!

Such difficulties, however, produced their natural result. They set menat work to devise another kind of telescope. In the new form, called theReflecting Telescope, or "Reflector, " the light coming from the objectunder observation was _reflected_ into the eye-piece from the surface ofa highly polished concave metallic mirror, or _speculum_, as it wascalled. It is to Sir Isaac Newton that the world is indebted for thereflecting telescope in its best form. That philosopher had set himselfto investigate the causes of the rainbow-like, or prismatic colourswhich for a long time had been such a source of annoyance to telescopicobservers; and he pointed out that, as the colours were produced in thepassage of the rays of light _through_ the glass, they would be entirelyabsent if the light were reflected from the _surface_ of a mirrorinstead. 

The reflecting telescope, however, had in turn certain drawbacks of itsown. A mirror, for instance, can plainly never be polished to such ahigh degree as to reflect as much light as a piece of transparent glasswill let through. Further, the position of the eye-piece is by no meansso convenient. It cannot, of course, be pointed directly towards themirror, for the observer would then have to place his head right in theway of the light coming from the celestial object, and would thus, ofcourse, cut it off. In order to obviate this difficulty, the followingdevice was employed by Newton in his telescope, of which he constructedhis first example in 1668. A small, flat mirror was fixed by thin wiresin the centre of the tube of the telescope, and near to its open end. Itwas set slant-wise, so that it reflected the rays of light directly intothe eye-piece, which was screwed into a hole at the side of the tube(see Fig. 8, p. 113, "Newtonian"). 

Although the Newtonian form of telescope had the immense advantage ofdoing away with the prismatic colours, yet it wasted a great deal oflight; for the objection in this respect with regard to loss of light byreflection from the large mirror applied, of course, to the small mirroralso. In addition, the position of the "flat, " as the small mirror iscalled, had the further effect of excluding from the great mirror acertain proportion of light. But the reflector had the advantage, on theother hand, of costing less to make than the refractor, as it was notnecessary to procure flawless glass for the purpose. A disc of a certainmetallic composition, an alloy of copper and tin, known in consequenceas _speculum metal_, had merely to be cast; and this had to be groundand polished _upon one side only_, whereas a lens has to be thus treated_upon both its sides_. It was, therefore, possible to make a much largerinstrument at a great deal less labour and expense. 

[Illustration: PLATE III. A TUBELESS, OR "AERIAL" TELESCOPE

From an illustration in the _Opera Varia_ of Christian Huyghens. 

(Page 110)]

[Illustration: FIG. 8. --The various types of Telescope. All the abovetelescopes are _pointed_ in the same direction; that is to say, the raysof light from the object are coming from the left-hand side. ]

We have given the Newtonian form as an example of the principle of thereflecting telescope. A somewhat similar instrument had, however, beenprojected, though not actually constructed, by James Gregory a few yearsearlier than Newton's, _i. E. _ in 1663. In this form of reflector, knownas the "Gregorian" telescope, a hole was made in the big concave mirror;and a small mirror, also concave, which faced it at a certain distance, received the reflected rays, and reflected them back again through thehole in question into the eye-piece, which was fixed just behind (seeFig. 8, p. 113, "Gregorian"). The Gregorian had thus the sentimentaladvantage of being _pointed directly at the object_. The hole in the bigmirror did not cause any loss of light, for the central portion in whichit was made was anyway unable to receive light through the small mirrorbeing directly in front of it. An adaptation of the Gregorian was the"Cassegrainian" telescope, devised by Cassegrain in 1672, which differedfrom it chiefly in the small mirror being convex instead of concave (seeFig. 8, p. 113, "Cassegrainian"). These _direct-view_ forms of thereflecting telescope were much in vogue about the middle of theeighteenth century, when many beautiful examples of Gregorians were madeby the famous optician, James Short, of Edinburgh. 

An adaptation of the Newtonian type of telescope is known as the"Herschelian, " from being the kind favoured by Sir William Herschel. Itis, however, only suitable in immense instruments, such as Herschel wasin the habit of employing. In this form the object-glass is set at aslight slant, so that the light coming from the object is reflectedstraight into the eye-piece, which is fixed facing it in the side of thetube (see Fig. 8, p. 113, "Herschelian"). This telescope has anadvantage over the other forms of reflector through the saving of lightconsequent on doing away with the _second_ reflection. There is, however, the objection that the slant of the object-glass is productiveof some distortion in the appearance of the object observed; but thisslant is of necessity slight when the length of the telescope is verygreat. 

The principle of this type of telescope had been described to theFrench Academy of Sciences as early as 1728 by Le Maire, but no oneavailed himself of the idea until 1776, when Herschel tried it. Atfirst, however, he rejected it; but in 1786 he seems to have found thatit suited the huge instruments which he was then making. Herschel'slargest telescope, constructed in 1789, was about four feet in diameterand forty feet in length. It is generally spoken of as the "Forty-footTelescope, " though all other instruments have been known by their_diameters_, rather than by their lengths. 

To return to the refracting telescope. A solution of the colourdifficulty was arrived at in 1729 (two years after Newton's death) by anEssex gentleman named Chester Moor Hall. He discovered that by making adouble object-glass, composed of an outer convex lens and an innerconcave lens, made respectively of different kinds of glass, _i. E. __crown_ glass and _flint_ glass, the troublesome colour effects couldbe, _to a very great extent_, removed. Hall's investigations appear tohave been rather of an academic nature; and, although he is believed tohave constructed a small telescope upon these lines, yet he seems tohave kept the matter so much to himself that it was not until the year1758 that the first example of the new instrument was given to theworld. This was done by John Dollond, founder of the well-known opticalfirm of Dollond, of Ludgate Hill, London, who had, quite independently, re-discovered the principle. 

This "Achromatic" telescope, or telescope "free from colour effects, " isthe kind ordinarily in use at present, whether for astronomical or forterrestrial purposes (see Fig. 8, p. 113, "Achromatic"). The expense ofmaking large instruments of this type is very great, for, in theobject-glass alone, no less than _four_ surfaces have to be ground andpolished to the required curves; and, usually, the two lenses of whichit is composed have to fit quite close together. 

With the object of evading the expense referred to, and of securing_complete_ freedom from colour effects, telescopes have even been made, the object-glasses of which were composed of various transparent liquidsplaced between thin lenses; but leakages, and currents set up withinthem by changes of temperature, have defeated the ingenuity of those whodevised these substitutes. 

The solution of the colour difficulty by means of Dollond's achromaticrefractor has not, however, ousted the reflecting telescope in its best, or Newtonian form, for which great concave mirrors made of glass, covered with a thin coating of silver and highly polished, have beenused since about 1870 instead of metal mirrors. They are very muchlighter in weight and cheaper to make than the old specula; and thoughthe silvering, needless to say, deteriorates with time, it can berenewed at a comparatively trifling cost. Also these mirrors reflectmuch more light, and give a clearer view, than did the old metallicones. 

When an object is viewed through the type of astronomical telescopeordinarily in use, it is seen _upside down_. This is, however, a matterof very small moment in dealing with celestial objects; for, as they areusually round, it is really not of much consequence which part we regardas top and which as bottom. Such an inversion would, of course, be mostinconvenient when viewing terrestrial objects. In order to observe thelatter we therefore employ what is called a terrestrial telescope, whichis merely a refractor with some extra lenses added in the eye portionfor the purpose of turning the inverted image the right way up again. These extra lenses, needless to say, absorb a certain amount of light;wherefore it is better in astronomical observation to save light bydoing away with them, and putting up with the slight inconvenience ofseeing the object inverted. 

This inversion of images by the astronomical telescope must be speciallyborne in mind with regard to the photographs of the moon in Chapter XVI. 

In the year 1825 the largest achromatic refractor in existence was oneof nine and a half inches in diameter constructed by Fraunhofer for theObservatory of Dorpat in Russia. The largest refractors in the worldto-day are in the United States, _i. E. _ the forty-inch of the YerkesObservatory (see Plate IV. , p. 118), and the thirty-six inch of theLick. The object-glasses of these and of the thirty-inch telescope ofthe Observatory of Pulkowa, in Russia, were made by the great opticalhouse of Alvan Clark & Sons, of Cambridge, Massachusetts, U. S. A. Thetubes and other portions of the Yerkes and Lick telescopes were, however, constructed by the Warner and Swasey Co. , of Cleveland, Ohio. 

The largest reflector, and so the largest telescope in the world, isstill the six-foot erected by the late Lord Rosse at Parsonstown inIreland, and completed in the year 1845. It is about fifty-six feet inlength. Next come two of five feet, with mirrors of silver on glass;one of them made by the late Dr. Common, of Ealing, and the other by theAmerican astronomer, Professor G. W. Ritchey. The latter of these isinstalled in the Solar Observatory belonging to Carnegie Institution ofWashington, which is situated on Mount Wilson in California. The formeris now at the Harvard College Observatory, and is considered byProfessor Moulton to be probably the most efficient reflector in use atpresent. Another large reflector is the three-foot made by Dr. Common. It came into the possession of Mr. Crossley of Halifax, who presented itto the Lick Observatory, where it is now known as the "CrossleyReflector. "

Although to the house of Clark belongs, as we have seen, the credit ofconstructing the object-glasses of the largest refracting telescopes ofour time, it has nevertheless keen competitors in Sir Howard Grubb, ofDublin, and such well-known firms as Cooke of York and Steinheil ofMunich. In the four-foot reflector, made in 1870 for the Observatory ofMelbourne by the firm of Grubb, the Cassegrainian principle wasemployed. 

With regard to the various merits of refractors and reflectors muchmight be said. Each kind of instrument has, indeed, its specialadvantages; though perhaps, on the whole, the most perfect type oftelescope is the achromatic refractor. 

[Illustration: PLATE IV. THE GREAT YERKES TELESCOPE

Great telescope at the Yerkes Observatory of the University of Chicago, Williams Bay, Wisconsin, U. S. A. It was erected in 1896-7, and is thelargest refracting telescope in the world. Diameter of object-glass, 40inches; length of telescope, about 60 feet. The object-glass was made bythe firm of Alvan Clark and Sons, of Cambridge, Massachusetts; the otherportions of the instrument by the Warner and Swasey Co. , of Cleveland, Ohio. 

(Page 117)]

In connection with telescopes certain devices have from time to timebeen introduced, but these merely aim at the _convenience_ of theobserver and do not supplant the broad principles upon which are basedthe various types of instrument above described. Such, for instance, arethe "Siderostat, " and another form of it called the "Coelostat, " inwhich a plane mirror is made to revolve in a certain manner, so as toreflect those portions of the sky which are to be observed, into thetube of a telescope kept fixed. Such too are the "Equatorial Coudé" ofthe late M. Loewy, Director of the Paris Observatory, and the"Sheepshanks Telescope" of the Observatory of Cambridge, in which atelescope is separated into two portions, the eye-piece portion beingfixed upon a downward slant, and the object-glass portion jointed to itat an angle and pointed up at the sky. In these two instruments (which, by the way, differ materially) an arrangement of slanting mirrors in thetubes directs the journey of the rays of light from the object-glass tothe eye-piece. The observer can thus sit at the eye-end of his telescopein the warmth and comfort of his room, and observe the stars in the sameunconstrained manner as if he were merely looking down into amicroscope. 

Needless to say, devices such as these are subject to the drawback thatthe mirrors employed sap a certain proportion of the rays of light. Itwill be remembered that we made allusion to loss of light in this way, when pointing out the advantage in light grasp of the Herschelian formof telescope, where only _one_ reflection takes place, over theNewtonian in which there are _two_. 

It is an interesting question as to whether telescopes can be made muchlarger. The American astronomer, Professor G. E. Hale, concludes that thelimit of refractors is about five feet in diameter, but he thinks thatreflectors as large as nine feet in diameter might now be made. Asregards refractors there are several strong reasons against augmentingtheir proportions. First of all comes the great cost. Secondly, sincethe lenses are held in position merely round their rims, they will bendby their weight in the centres if they are made much larger. On theother hand, attempts to obviate this, by making the lenses thicker, would cause a decrease in the amount of light let through. 

But perhaps the greatest stumbling-block to the construction of largertelescopes is the fact that the unsteadiness of the air will beincreasingly magnified. And further, the larger the tubes become, themore difficult will it be to keep the air within them at one constanttemperature throughout their lengths. 

It would, indeed, seem as if telescopes are not destined greatly toincrease in size, but that the means of observation will break out insome new direction, as it has already done in the case of photographyand the spectroscope. The direct use of the eye is gradually givingplace to indirect methods. We are, in fact, now _feeling_ rather thanseeing our way about the universe. Up to the present, for instance, wehave not the slightest proof that life exists elsewhere than upon ourearth. But who shall say that the twentieth century has not that instore for us, by which the presence of life in other orbs may beperceived through some form of vibration transmitted across illimitablespace? There is no use speaking of the impossible or the inconceivable. After the extraordinary revelations of the spectroscope--nay, after theastounding discovery of Röntgen--the word impossible should be castaside, and inconceivability cease to be regarded as any criterion. 

[8] The principle upon which the telescope is based appears to have beenknown _theoretically_ for a long time previous to this. The monk RogerBacon, who lived in the thirteenth century, describes it very clearly;and several writers of the sixteenth century have also dealt with theidea. Even Lippershey's claims to a practical solution of the questionwere hotly contested at the time by two of his own countrymen, _i. E. _ acertain Jacob Metius, and another spectacle-maker of Middleburgh, namedJansen. 

CHAPTER XI

SPECTRUM ANALYSIS

If white light (that of the sun, for instance) be passed through a glassprism, namely, a piece of glass of triangular shape, it will issue fromit in rainbow-tinted colours. It is a common experience with any of usto notice this when the sunlight shines through cut-glass, as in thependant of a chandelier, or in the stopper of a wine-decanter. 

The same effect may be produced when light passes through water. TheRainbow, which we all know so well, is merely the result of the sunlightpassing through drops of falling rain. 

White light is composed of rays of various colours. Red, orange, yellow, green, blue, indigo, and violet, taken all together, go, in fact, tomake up that effect which we call white. 

It is in the course of the _refraction_, or bending of a beam of light, when it passes in certain conditions through a transparent and densermedium, such as glass or water, that the constituent rays are sorted outand spread in a row according to their various colours. This productionof colour takes place usually near the edges of a lens; and, as will berecollected, proved very obnoxious to the users of the old form ofrefracting telescope. 

It is, indeed, a strange irony of fate that this very same productionof colour, which so hindered astronomy in the past, should have aided itin recent years to a remarkable degree. If sunlight, for instance, beadmitted through a narrow slit before it falls upon a glass prism, itwill issue from the latter in the form of a band of variegated colour, each colour blending insensibly with the next. The colours arrangethemselves always in the order which we have mentioned. This seemingband is, in reality, an array of countless coloured images of theoriginal slit ranged side by side; the colour of each image being theslightest possible shade different from that next to it. This strip ofcolour when produced by sunlight is called the "Solar Spectrum" (seeFig. 9, p. 123). A similar strip, or _spectrum_, will be produced by anyother light; but the appearance of the strip, with regard topreponderance of particular colours, will depend upon the character ofthat light. Electric light and gas light yield spectra not unlike thatof sunlight; but that of gas is less rich in blue and violet than thatof the sun. 

The Spectroscope, an instrument devised for the examination of spectra, is, in its simplest form, composed of a small tube with a narrow slitand prism at one end, and an eye-piece at the other. If we drop ordinarytable salt into the flame of a gas light, the flame becomes stronglyyellow. If, then, we observe this yellow flame with the spectroscope, wefind that its spectrum consists almost entirely of two bright yellowtransverse lines. Chemically considered ordinary table salt is sodiumchloride; that is to say, a compound of the metal sodium and the gaschlorine. Now if other compounds of sodium be experimented with in thesame manner, it will soon be found that these two yellow lines arecharacteristic of sodium when turned into vapour by great heat. In thesame manner it can be ascertained that every element, when heated to acondition of vapour, gives as its spectrum a set of lines peculiar toitself. Thus the spectroscope enables us to find out the composition ofsubstances when they are reduced to vapour in the laboratory. 

[Illustration: FIG. 9. --The Solar Spectrum. ]

In order to increase the power of a spectroscope, it is necessary toadd to the number of prisms. Each extra prism has the effect oflengthening the coloured strip still more, so that lines, which at firstappeared to be single merely through being crowded together, areeventually drawn apart and become separately distinguishable. 

On this principle it has gradually been determined that the sun iscomposed of elements similar to those which go to make up our earth. Further, the composition of the stars can be ascertained in the samemanner; and we find them formed on a like pattern, though with certainelements in greater or less proportion as the case may be. It is inconsequence of our thus definitely ascertaining that the stars areself-luminous, and of a sun-like character, that we are enabled to speakof them as _suns_, or to call the sun a _star_. 

In endeavouring to discover the elements of which the planets andsatellites of our system are composed, we, however, find ourselvesbaffled, for the simple reason that these bodies emit no real light oftheir own. The light which reaches us from them, being merely reflectedsunlight, gives only the ordinary solar spectrum when examined with thespectroscope. But in certain cases we find that the solar spectrum thusviewed shows traces of being weakened, or rather of sufferingabsorption; and it is concluded that this may be due to the sunlighthaving had to pass through an atmosphere on its way to and from thesurface of the planet from which it is reflected to us. 

Since the sun is found to be composed of elements similar to those whichgo to make up our earth, we need not be disheartened at this failure ofthe spectroscope to inform us of the composition of the planets andsatellites. We are justified, indeed, in assuming that more or less thesame constituents run through our solar system; and that the elements ofwhich these bodies are composed are similar to those which are foundupon our earth and in the sun. 

The spectroscope supplies us with even more information. It tells us, indeed, whether the sun-like body which we are observing is moving awayfrom us or towards us. A certain slight shifting of the lines towardsthe red or violet end of the spectrum respectively, is found to followsuch movement. This method of observation is known by the name of_Doppler's Method_, [9] and by it we are enabled to confirm the evidencewhich the sunspots give us of the rotation of the sun; for we find thusthat one edge of that body is continually approaching us, and the otheredge is continually receding from us. Also, we can ascertain in the samemanner that certain of the stars are moving towards us, and certain ofthem away from us. 

[9] The idea, initiated by Christian Doppler at Prague in 1842, wasoriginally applied to sound. The approach or recession of a source fromwhich sound is coming is invariably accompanied by alterations of pitch, as the reader has no doubt noticed when a whistling railway-engine hasapproached him or receded from him. It is to Sir William Huggins, however, that we are indebted for the application of the principle tospectroscopy. This he gave experimental proof of in the year 1868. 

CHAPTER XII

THE SUN

The sun is the chief member of our system. It controls the motions ofthe planets by its immense gravitative power. Besides this it is themost important body in the entire universe, so far as we are concerned;for it pours out continually that flood of light and heat, without whichlife, as we know it, would quickly become extinct upon our globe. 

Light and heat, though not precisely the same thing, may be regarded, however, as next-door neighbours. The light rays are those whichdirectly affect the eye and are comprised in the visible spectrum. We_feel_ the heat rays, the chief of which are beyond the red portion ofthe spectrum. They may be investigated with the _bolometer_, aninstrument invented by the late Professor Langley. Chemical rays--forinstance, those radiations which affect the photographic plate--are forthe most part also outside the visible spectrum. They are, however, atthe other end of it, namely, beyond the violet. 

Such a scale of radiations may be compared to the keyboard of animaginary piano, the sound from only one of whose octaves is audible tous. 

The brightest light we know on the earth is dull compared with the lightof the sun. It would, indeed, look quite dark if held up against it. 

It is extremely difficult to arrive at a precise notion of thetemperature of the body of the sun. However, it is far in excess of anytemperature which we can obtain here, even in the most powerful electricfurnace. 

A rough idea of the solar heat may be gathered from the calculation thatif the sun's surface were coated all over with a layer of ice 4000 feetthick, it would melt through this completely in one hour. 

The sun cannot be a hot body merely cooling; for the rate at which it isat present giving off heat could not in such circumstances be kept up, according to Professor Moulton, for more than 3000 years. Further, it isnot a mere burning mass, like a coal fire, for instance; as in that caseabout a thousand years would show a certain drop in temperature. Noperceptible diminution of solar heat having taken place within historicexperience, so far as can be ascertained, we are driven to seek somemore abstruse explanation. 

The theory which seems to have received most acceptance is that putforward by Helmholtz in 1854. His idea was that gravitation producescontinual contraction, or falling in of the outer parts of the sun; andthat this falling in, in its turn, generates enough heat to compensatefor what is being given off. The calculations of Helmholtz showed that acontraction of about 100 feet a year from the surface towards the centrewould suffice for the purpose. In recent years, however, this estimatehas been extended to about 180 feet. Nevertheless, even with thisincreased figure, the shrinkage required is so slight in comparison withthe immense girth of the sun, that it would take a continualcontraction at this rate for about 6000 years, to show even in ourfinest telescopes that any change in the size of that body was takingplace at all. Upon this assumption of continuous contraction, a timeshould, however, eventually be reached when the sun will have shrunk tosuch a degree of solidity, that it will not be able to shrink anyfurther. Then, the loss of heat not being made up for any longer, thebody of the sun should begin to grow cold. But we need not be distressedon this account; for it will take some 10, 000, 000 years, according tothe above theory, before the solar orb becomes too cold to support lifeupon our earth. 

Since the discovery of radium it has, on the other hand, been suggested, and not unreasonably, that radio-active matter may possibly play animportant part in keeping up the heat of the sun. But the body ofscientific opinion appears to consider the theory of contraction as aresult of gravitation, which has been outlined above, to be of itselfquite a sound explanation. Indeed, the late Lord Kelvin is said to haveheld to the last that it was amply sufficient to account for theunderground heat of the earth, the heat of the sun, and that of all thestars in the universe. 

One great difficulty in forming theories with regard to the sun, is thefact that the temperature and gravitation there are enormously in excessof anything we meet with upon our earth. The force of gravity at thesun's surface is, indeed, about twenty-seven times that at the surfaceof our globe. 

The earth's atmosphere appears to absorb about one-half of theradiations which come to us from the sun. This absorptive effect is verynoticeable when the solar orb is low down in our sky, for its light andheat are then clearly much reduced. Of the light rays, the blue ones arethe most easily absorbed in this way; which explains why the sun looksred when near the horizon. It has then, of course, to shine through amuch greater thickness of atmosphere than when high up in the heavens. 

What astonishes one most about the solar radiation, is the immenseamount of it that is apparently wasted into space in comparison withwhat falls directly upon the bodies of the solar system. Only about theone-hundred-millionth is caught by all the planets together. Whatbecomes of the rest we cannot tell. 

That brilliant white body of the sun, which we see, is enveloped byseveral layers of gases and vaporous matter, in the same manner as ourglobe is enveloped by its atmosphere (see Fig. 10, p. 131). These aretransparent, just as our atmosphere is transparent; and so we see thewhite bright body of the sun right through them. 

This white bright portion is called the _Photosphere_. From it comesmost of that light and heat which we see and feel. We do not know whatlies under the photosphere, but, no doubt, the more solid portions ofthe sun are there situated. Just above the photosphere, and lying closeupon it, is a veil of smoke-like haze. 

Next upon this is what is known as the _Reversing Layer_, which isbetween 500 and 1000 miles in thickness. It is cooler than theunderlying photosphere, and is composed of glowing gases. Many of theelements which go to make up our earth are present in the reversinglayer in the form of vapour. 

The _Chromosphere_, of which especial mention has already been made indealing with eclipses of the sun, is another layer lying immediatelyupon the last one. It is between 5000 and 10, 000 miles in thickness. Like the reversing layer, it is composed of glowing gases, chief amongwhich is the vapour of hydrogen. The colour of the chromosphere is, inreality, a brilliant scarlet; but, as we have already said, theintensely white light of the photosphere shines through it from behind, and entirely overpowers its redness. The upper portion of thechromosphere is in violent agitation, like the waves of a stormy sea, and from it rise those red prominences which, it will be recollected, are such a notable feature in total solar eclipses. 

[Illustration: FIG. 10. --A section through the Sun, showing how theprominences rise from the chromosphere. ]

The _Corona_ lies next in order outside the chromosphere, and is, sofar as we know, the outermost of the accompaniments of the sun. Thishalo of pearly-white light is irregular in outline, and fades away intothe surrounding sky. It extends outwards from the sun to severalmillions of miles. As has been stated, we can never see the coronaunless, when during a total solar eclipse, the moon has, for the timebeing, hidden the brilliant photosphere completely from our view. 

The solar spectrum is really composed of three separate spectracommingled, _i. E. _ those of the photosphere, of the reversing layer, andof the chromosphere respectively. 

If, therefore, the photosphere could be entirely removed, or covered up, we should see only the spectra of those layers which lie upon it. Such astate of things actually occurs in a total eclipse of the sun. When themoon's body has crept across the solar disc, and hidden the last pieceof photosphere, the solar spectrum suddenly becomes what is technicallycalled "reversed, "--the dark lines crossing it changing into brightlines. This occurs because a strip of those layers which lie immediatelyupon the photosphere remains still uncovered. The lower of these layershas therefore been called the "reversing layer, " for want of a bettername. After a second or two this reversed spectrum mostly vanishes, andan altered spectrum is left to view. Taking into consideration the rateat which the moon is moving across the face of the sun, and the veryshort time during which the spectrum of the reversing layer lasts, thethickness of that layer is estimated to be not more than a few hundredmiles. In the same way the last of the three spectra--namely, that ofthe chromosphere--remains visible for such a time as allows us toestimate its depth at about ten times that of the reversing layer, orseveral thousand miles. 

When the chromosphere, in its turn during a total eclipse, has beencovered by the moon, the corona alone is left. This has a distinctspectrum of its own also; wherein is seen a strange line in the greenportion, which does not tally with that of any element we are acquaintedwith upon the earth. This unknown element has received for the timebeing the name of "Coronium. "

CHAPTER XIII

THE SUN--_continued_

The various parts of the Sun will now be treated of in detail. 

I. PHOTOSPHERE. 

The photosphere, or "light-sphere, " from the Greek [phôs] (_phos_), which means _light_, is, as we have already said, the innermost portionof the sun which can be seen. Examined through a good telescope it showsa finely mottled structure, as of brilliant granules, somewhat like ricegrains, with small dark spaces lying in between them. It has beensupposed that we have here the process of some system of circulation bywhich the sun keeps sending forth its radiations. In the bright granuleswe perhaps see masses of intensely heated matter, rising from theinterior of the sun. The dark interspaces may represent matter which hasbecome cooled and darkened through having parted with its heat andlight, and is falling back again into the solar furnace. 

The _sun spots_, so familiar to every one nowadays, are dark patcheswhich are often seen to break out in the photosphere (see Plate V. , p. 134). They last during various periods of time; sometimes only for a fewdays, sometimes so long as a month or more. A spot is usually composedof a dark central portion called the _umbra_, and a less dark fringearound this called the _penumbra_ (see Plate VI. , p. 136). The umbraordinarily has the appearance of a deep hole in the photosphere; but, that it is a hole at all, has by no means been definitely proved. 

[Illustration: PLATE V. THE SUN, SHOWING SEVERAL GROUPS OF SPOTS

From a photograph taken at the Royal Observatory, Greenwich. Thecross-lines seen on the disc are in no way connected with the Sun, butbelong to the telescope through which the photograph was taken. 

(Page 134)]

Sun spots are, as a rule, some thousands of miles across. The umbra ofa good-sized spot could indeed engulf at once many bodies the size ofour earth. 

Sun spots do not usually appear singly, but in groups. The total area ofa group of this kind may be of immense extent; even so great as to coverthe one-hundredth part of the whole surface of the sun. Very largespots, when such are present, may be seen without any telescope; eitherthrough a piece of smoked glass, or merely with the naked eye when theair is misty, or the sun low on the horizon. 

The umbra of a spot is not actually dark. It only appears so in contrastwith the brilliant photosphere around. 

Spots form, grow to a large size in comparatively short periods of time, and then quickly disappear. They seem to shrink away as a consequence ofthe photosphere closing in upon them. 

That the sun is rotating upon an axis, is shown by the continual changeof position of all spots in one constant direction across his disc. Thetime in which a spot is carried completely round depends, however, uponthe position which it occupies upon the sun's surface. A spot situatednear the equator of the sun goes round once in about twenty-five days. The further a spot is situated from this equator, the longer it takes. About twenty-seven days is the time taken by a spot situated midwaybetween the equator and the solar poles. Spots occur to the north ofthe sun's equator, as well as to the south; though, since regularobservations have been made--that is to say, during the past fifty yearsor so--they appear to have broken out a little more frequently in thesouthern parts. 

From these considerations it will be seen that the sun does not rotateas the earth does, but that different portions appear to move atdifferent speeds. Whether in the neighbourhood of the solar poles thetime of rotation exceeds twenty-seven days we are unable to ascertain, for spots are not seen in those regions. No explanation has yet beengiven of this peculiar rotation; and the most we can say on the subjectis that the sun is not by any means a solid body. 

_Faculæ_ (Latin, little torches) are brilliant patches which appear hereand there upon the sun's surface, and are in some way associated withspots. Their displacement, too, across the solar face confirms theevidence which the spots give us of the sun's rotation. 

Our proofs of this rotation are still further strengthened by theDoppler spectroscopic method of observation alluded to in Chapter XI. Aswas then stated, one edge of the sun is thus found to be continuallyapproaching us, and the other side continually receding from us. Thevarying rates of rotation, which the spots and faculæ give us, are dulyconfirmed by this method. 

[Illustration: PLATE VI. PHOTOGRAPH OF A SUNSPOT

This fine picture was taken by the late M. Janssen. The granularstructure of the Sun's surface is here well represented. (From_Knowledge_. )

(Page 135)]

The first attempt to bring some regularity into the question ofsunspots was the discovery by Schwabe, in 1852, that they were subjectto a regular variation. As a matter of fact they wax and wane in theirnumber, and the total area which they cover, in the course of a period, or cycle, of on an average about 11-1/4 years; being at one part of thisperiod large and abundant, and at another few and small. This period of11-1/4 years is known as the sun spot cycle. No explanation has yet beengiven of the curious round of change, but the period in question seemsto govern most of the phenomena connected with the sun. 

II. REVERSING LAYER. 

This is a layer of relatively cool gases lying immediately upon thephotosphere. We never see it directly; and the only proof we have of itspresence is that remarkable reversal of the spectrum already described, when during an instant or two in a total eclipse, the advancing edge ofthe moon, having just hidden the brilliant photosphere, is moving acrossthe fine strip which the layer then presents edgewise towards us. Thefleeting moments during which this reversed spectrum lasts, informs usthat the layer is comparatively shallow; little more indeed than about500 miles in depth. 

The spectrum of the reversing layer, or "flash spectrum, " as it issometimes called on account of the instantaneous character with whichthe change takes place, was, as we have seen, first noticed by Young in1870; and has been successfully photographed since then during severaleclipses. The layer itself appears to be in a fairly quiescent state; amarked contrast to the seething photosphere beneath, and the agitatedchromosphere above. 

III. THE CHROMOSPHERE. 

The Chromosphere--so called from the Greek [chrôma] (_chroma_), whichsignifies _colour_--is a layer of gases lying immediately upon thepreceding one. Its thickness is, however, plainly much the greater ofthe two; for whereas the reversing layer is only revealed to us_indirectly_ by the spectroscope, a portion of the chromosphere mayclearly be _seen_ in a total eclipse in the form of a strip of scarletlight. The time which the moon's edge takes to traverse it tells us thatit must be about ten times as deep as the reversing layer, namely, from5000 to 10, 000 miles in depth. Its spectrum shows that it is composedchiefly of hydrogen, calcium and helium, in the state of vapour. Its redcolour is mainly due to glowing hydrogen. The element helium, which italso contains, has received its appellation from [hêlios] (_helios_), the Greek name for the sun; because, at the time when it first attractedattention, there appeared to be no element corresponding to it upon ourearth, and it was consequently imagined to be confined to the sun alone. Sir William Ramsay, however, discovered it to be also a terrestrialelement in 1895, and since then it has come into much prominence as oneof the products given off by radium. 

Taking into consideration the excessive force of gravity on the sun, onewould expect to find the chromosphere and reversing layer growinggradually thicker in the direction of the photosphere. This, however, isnot the case. Both these layers are strangely enough of the samedensities all through; which makes it suspected that, in these regions, the force of gravity may be counteracted by some other force or forces, exerting a powerful pressure outwards from the sun. 

IV. THE PROMINENCES. 

We have already seen, in dealing with total eclipses, that the exteriorsurface of the chromosphere is agitated like a stormy sea, and from itbillows of flame are tossed up to gigantic heights. These flaming jetsare known under the name of prominences, because they were first noticedin the form of brilliant points projecting from behind the rim of themoon when the sun was totally eclipsed. Prominences are of two kinds, _eruptive_ and _quiescent_. The eruptive prominences spurt up directlyfrom the chromosphere with immense speeds, and change their shape withgreat rapidity. Quiescent prominences, on the other hand, have a formsomewhat like trees, and alter their shape but slowly. In the eruptiveprominences glowing masses of gas are shot up to altitudes sometimes ashigh as 300, 000 miles, [10] with velocities even so great as from 500 to600 miles a second. It has been noticed that the eruptive prominencesare mostly found in those portions of the sun where spots usuallyappear, namely, in the regions near the solar equator. The quiescentprominences, on the other hand, are confined, as a rule, to theneighbourhood of the sun's poles. 

Prominences were at first never visible except during total eclipses ofthe sun. But in the year 1868, as we have already seen, a method ofemploying the spectroscope was devised, by means of which they could beobserved and studied at any time, without the necessity of waiting foran eclipse. 

A still further development of the spectroscope, the_Spectroheliograph_, an instrument invented almost simultaneously byProfessor Hale and the French astronomer, M. Deslandres, permits ofphotographs being taken of the sun, with the light emanating from _onlyone_ of its glowing gases at a time. For instance, we can thus obtain arecord of what the glowing hydrogen alone is doing on the solar body atany particular moment. With this instrument it is also possible toobtain a series of photographs, showing what is taking place upon thesun at various levels. This is very useful in connection with the studyof the spots; for we are, in consequence, enabled to gather moreevidence on the subject of their actual form than is given us by theirhighly foreshortened appearances when observed directly in thetelescope. 

V. CORONA. (Latin, _a Crown_. )

This marvellous halo of pearly-white light, which displays itself to ourview only during the total phase of an eclipse of the sun, is by nomeans a layer like those other envelopments of the sun of which we havejust been treating. It appears, on the other hand, to be composed offilmy matter, radiating outwards in every direction, and fading awaygradually into space. Its structure is noted to bear a strongresemblance to the tails of comets, or the streamers of the auroraborealis. 

Our knowledge concerning the corona has, however, advanced very slowly. We have not, so far, been as fortunate with regard to it as with regardto the prominences; and, for all we can gather concerning it, we arestill entirely dependent upon the changes and chances of total solareclipses. All attempts, in fact, to apply the spectroscopic method, soas to observe the corona at leisure in full sunlight in the way in whichthe prominences can be observed, have up to the present met withfailure. 

The general form under which the corona appears to our eyes variesmarkedly at different eclipses. Sometimes its streamers are many, andradiate all round; at other times they are confined only to the middleportions of the sun, and are very elongated, with short feathery-lookingwisps adorning the solar poles. It is noticed that this change of shapevaries in close accordance with that 11-1/4 year period during which thesun spots wax and wane; the many-streamered regular type correspondingto the time of great sunspot activity, while the irregular type with thelong streamers is present only when the spots are few (see Plate VII. , p. 142). Streamers have often been noted to issue from those regions ofthe sun where active prominences are at the moment in existence; but itcannot be laid down that this is always the case. 

No hypothesis has yet been formulated which will account for thestructure of the corona, or for its variation in shape. The greatdifficulty with regard to theorising upon this subject, is the factthat we see so much of the corona under conditions of markedforeshortening. Assuming, what indeed seems natural, that the rays ofwhich it is composed issue in every direction from the solar body, in amanner which may be roughly imitated by sticking pins all over a ball;it is plainly impossible to form any definite idea concerning streamers, which actually may owe most of the shape they present to us, to themixing up of multitudes of rays at all kinds of angles to the line ofsight. In a word, we have to try and form an opinion concerning anarrangement which, broadly speaking, is _spherical_, but which, onaccount of its distance, must needs appear to us as absolutely _flat_. 

The most known about the composition of the corona is that it is made upof particles of matter, mingled with a glowing gas. It is an element inthe composition of this gas which, as has been stated, is not found totally with any known terrestrial element, and has, therefore, receivedthe name of coronium for want of a better designation. 

One definite conclusion appears to be reached with regard to the corona, _i. E. _ that the matter of which it is composed, must be exceedinglyrarefied; as it is not found, for instance, to retard appreciably thespeed of comets, on occasions when these bodies pass very close to thesun. A calculation has indeed been made which would tend to show thatthe particles composing the coronal matter, are separated from eachother by a distance of perhaps between two and three yards! The densityof the corona is found not to increase inwards towards the sun. This iswhat has already been noted with regard to the layers lying beneath it. Powerful forces, acting in opposition to gravity, must hold sway herealso. 

[Illustration: (A. ) THE TOTAL ECLIPSE OF THE SUN OF DECEMBER 22ND, 1870

Drawn by Mr. W. H. Wesley from a photograph taken at Syracuse by Mr. Brothers. This is the type of corona seen at the time of _greatest_sunspot activity. The coronas of 1882 (Plate I. , p. 96) and of 1905(Frontispiece) are of the same type. 

(B. ) THE TOTAL ECLIPSE OF THE SUN OF MAY 28TH, 1900

Drawn by Mr. W. H. Wesley from photographs taken by Mr. E. W. Maunder. This is the type of corona seen when the sunspots are _least_ active. Compare the "Ring with Wings, " Fig. 7, p. 87. 

PLATE VII. FORMS OF THE SOLAR CORONA AT THE EPOCHS OF SUNSPOT MAXIMUMAND SUNSPOT MINIMUM, RESPECTIVELY

(Page 141)]

The 11-1/4 year period, during which the sun spots vary in number andsize, appears to govern the activities of the sun much in the same waythat our year does the changing seasonal conditions of our earth. Notonly, as we have seen, does the corona vary its shape in accordance withthe said period, but the activity of the prominences, and of the faculæ, follow suit. Further, this constant round of ebb and flow is notconfined to the sun itself, but, strangely enough, affects the earthalso. The displays of the aurora borealis, which we experience here, coincide closely with it, as does also the varying state of the earth'smagnetism. The connection may be still better appreciated when a greatspot, or group of spots, has made its appearance upon the sun. It has, for example, often been noted that when the solar rotation carries aspot, or group of spots, across the middle of the visible surface of thesun, our magnetic and electrical arrangements are disturbed for the timebeing. The magnetic needles in our observatories are, for instance, seento oscillate violently, telegraphic communication is for a while upset, and magnificent displays of the aurora borealis illumine our nightskies. Mr. E. W. Maunder, of Greenwich Observatory, who has made a verycareful investigation of this subject, suspects that, when elongatedcoronal streamers are whirled round in our direction by the solarrotation, powerful magnetic impulses may be projected upon us at themoments when such streamers are pointing towards the earth. 

Some interesting investigations with regard to sunspots have recentlybeen published by Mrs. E. W. Maunder. In an able paper, communicated tothe Royal Astronomical Society on May 10, 1907, she reviews theGreenwich Observatory statistics dealing with the number and extent ofthe spots which have appeared during the period from 1889 to 1901--awhole sunspot cycle. From a detailed study of the dates in question, shefinds that the number of those spots which are formed on the side of thesun turned away from us, and die out upon the side turned towards us, ismuch greater than the number of those which are formed on the sideturned towards us and die out upon the side turned away. It used, forinstance, to be considered that the influence of a planet might_produce_ sunspots; but these investigations make it look rather as ifsome influence on the part of the earth tends, on the contrary, to_extinguish_ them. Mrs. Maunder, so far, prefers to call the influencethus traced an _apparent_ influence only, for, as she very fairly pointsout, it seems difficult to attribute a real influence in this matter tothe earth, which is so small a thing in comparison not only with thesun, but even with many individual spots. 

The above investigation was to a certain degree anticipated by Mr. HenryCorder in 1895; but Mrs. Maunder's researches cover a much longerperiod, and the conclusions deduced are of a wider and more definednature. 

With regard to its chemical composition, the spectroscope shows us thatthirty-nine of the elements which are found upon our earth are also tobe found in the sun. Of these the best known are hydrogen, oxygen, helium, carbon, calcium, aluminium, iron, copper, zinc, silver, tin, andlead. Some elements of the metallic order have, however, not been foundthere, as, for instance, gold and mercury; while a few of the otherclass of element, such as nitrogen, chlorine, and sulphur, are alsoabsent. It must not, indeed, be concluded that the elements apparentlymissing do not exist at all in the solar body. Gold and mercury have, inconsequence of their great atomic weight, perhaps sunk away into thecentre. Again, the fact that we cannot find traces of certain otherelements, is no real proof of their entire absence. Some of them may, for instance, be resolved into even simpler forms, under the unusualconditions which exist in the sun; and so we are unable to trace themwith the spectroscope, the experience of which rests on laboratoryexperiments conducted, at best, in conditions which obtain upon theearth. 

[10] On November 15, 1907, Dr. A. Rambaut, Radcliffe Observer at OxfordUniversity, noted a prominence which rose to a height of 324, 600 miles. 

CHAPTER XIV

THE INFERIOR PLANETS

Starting from the centre of the solar system, the first body we meetwith is the planet Mercury. It circulates at an average distance fromthe sun of about thirty-six millions of miles. The next body to it isthe planet Venus, at about sixty-seven millions of miles, namely, aboutdouble the distance of Mercury from the sun. Since our earth comes nextagain, astronomers call those planets which circulate within its orbit, _i. E. _ Mercury and Venus, the Inferior Planets, while those whichcirculate outside it they call the Superior Planets. [11]

In studying the inferior planets, the circumstances in which we make ourobservations are so very similar with regard to each, that it is best totake them together. Let us begin by considering the various positions ofan inferior planet, as seen from the earth, during the course of itsjourneys round the sun. When furthest from us it is at the other side ofthe sun, and cannot then be seen owing to the blaze of light. As itcontinues its journey it passes to the left of the sun, and is thensufficiently away from the glare to be plainly seen. It next draws inagain towards the sun, and is once more lost to view in the blaze atthe time of its passing nearest to us. Then it gradually comes out toview on the right hand, separates from the sun up to a certain distanceas before, and again recedes beyond the sun, and is for the time beingonce more lost to view. 

To these various positions technical names are given. When the inferiorplanet is on the far side of the sun from us, it is said to be in_Superior Conjunction_. When it has drawn as far as it can to the lefthand, and is then as east as possible of the sun, it is said to be atits _Greatest Eastern Elongation_. Again, when it is passing nearest tous, it is said to be in _Inferior Conjunction_; and, finally, when ithas drawn as far as it can to the right hand, it is spoken of as beingat its _Greatest Western Elongation_ (see Fig. 11, p. 148). 

The continual variation in the distance of an interior planet from us, during its revolution around the sun, will of course be productive ofgreat alterations in its apparent size. At superior conjunction itought, being then farthest away, to show the smallest disc; while atinferior conjunction, being the nearest, it should look much larger. When at greatest elongation, whether eastern or western, it shouldnaturally present an appearance midway in size between the two. 

[Illustration: Various positions, and illumination by the Sun, of anInferior Planet in the course of its orbit. 

Corresponding views of the same situations of an Inferior Planet as seenfrom the Earth, showing consequent phases and alterations in apparentsize. 

FIG. 11. --Orbit and Phases of an Inferior Planet. ]

From the above considerations one would be inclined to assume that thebest time for studying the surface of an interior planet with thetelescope is when it is at inferior conjunction, or, nearest to us. Butthat this is not the case will at once appear if we consider that thesunlight is then falling upon the side away from us, leaving the sidewhich is towards us unillumined. In superior conjunction, on the otherhand, the light falls full upon the side of the planet facing us; butthe disc is then so small-looking, and our view besides is so dazzled bythe proximity of the sun, that observations are of little avail. In theelongations, however, the sunlight comes from the side, and so we seeone half of the planet lit up; the right half at eastern elongation, andthe left half at western elongation. Piecing together the results givenus at these more favourable views, we are enabled, bit by bit, to gathersome small knowledge concerning the surface of an inferior planet. 

From these considerations it will be seen at once that the inferiorplanets show various phases comparable to the waxing and waning of ourmoon in its monthly round. Superior conjunction is, in fact, similar tofull moon, and inferior conjunction to new moon; while the eastern andwestern elongations may be compared respectively to the moon's first andlast quarters. It will be recollected how, when these phases were firstseen by the early telescopic observers, the Copernican theory was feltto be immensely strengthened; for it had been pointed out that if thissystem were the correct one, the planets Venus and Mercury, were itpossible to see them more distinctly, would of necessity present phaseslike these when viewed from the earth. It should here be noted that thetelescope was not invented until nearly seventy years after the death ofCopernicus. 

The apparent swing of an inferior planet from side to side of the sun, at one time on the east side, then passing into and lost in the sun'srays to appear once more on the west side, is the explanation of what ismeant when we speak of an _evening_ or a _morning star_. An inferiorplanet is called an evening star when it is at its eastern elongation, that is to say, on the left-hand of the sun; for, being then on theeastern side, it will set after the sun sets, as both sink in their turnbelow the western horizon at the close of day. Similarly, when such aplanet is at its western elongation, that is to say, to the right-handof the sun, it will go in advance of him, and so will rise above theeastern horizon before the sun rises, receiving therefore thedesignation of morning star. In very early times, however, before anydefinite ideas had been come to with regard to the celestial motions, itwas generally believed that the morning and evening stars were quitedistinct bodies. Thus Venus, when a morning star, was known to theancients under the name of Phosphorus, or Lucifer; whereas they calledit Hesperus when it was an evening star. 

Since an inferior planet circulates between us and the sun, one would beinclined to expect that such a body, each time it passed on the sidenearest to the earth, should be seen as a black spot against the brightsolar disc. Now this would most certainly be the case were the orbit ofan inferior planet in the same plane with the orbit of the earth. But wehave already seen how the orbits in the solar system, whether those ofplanets or of satellites, are by no means in the one plane; and that itis for this very reason that the moon is able to pass time after time inthe direction of the sun, at the epoch known as new moon, and yet not toeclipse him save after the lapse of several such passages. Transits, then, as the passages of an inferior planet across the sun's disc arecalled, take place, for the same reason, only after certain regularlapses of time; and, as regards the circumstances of their occurrence, are on a par with eclipses of the sun. The latter, however, happen muchmore frequently, because the moon passes in the neighbourhood of thesun, roughly speaking, once a month, whereas Venus comes to eachinferior conjunction at intervals so long apart as a year and a half, and Mercury only about every four months. From this it will be furthergathered that transits of Mercury take place much oftener than transitsof Venus. 

Until recent years _Transits of Venus_ were phenomena of greatimportance to astronomers, for they furnished the best means thenavailable of calculating the distance of the sun from the earth. Thiswas arrived at through comparing the amount of apparent displacement inthe planet's path across the solar disc, when the transit was observedfrom widely separated stations on the earth's surface. The last transitof Venus took place in 1882, and there will not be another until theyear 2004. 

_Transits of Mercury_, on the other hand, are not of much scientificimportance. They are of no interest as a popular spectacle; for thedimensions of the planet are so small, that it can be seen only with theaid of a telescope when it is in the act of crossing the sun's disc. Thelast transit of Mercury took place on November 14, 1907, and there willbe another on November 6, 1914. 

The first person known to have observed a transit of an inferior planetwas the celebrated French philosopher, Gassendi. This was the transit ofMercury which took place on the 7th of December 1631. 

The first time a transit of Venus was ever seen, so far as is known, wason the 24th of November 1639. The observer was a certain JeremiahHorrox, curate of Hoole, near Preston, in Lancashire. The transit inquestion commenced shortly before sunset, and his observations inconsequence were limited to only about half-an-hour. Horrox happened tohave a great friend, one William Crabtree, of Manchester, whom he hadadvised by letter to be on the look out for the phenomenon. The weatherin Crabtree's neighbourhood was cloudy, with the result that he only gota view of the transit for about ten minutes before the sun set. 

That this transit was observed at all is due entirely to the remarkableability of Horrox. According to the calculations of the great Kepler, notransit could take place that year (1639), as the planet would just passclear of the lower edge of the sun. Horrox, however, not being satisfiedwith this, worked the question out for himself, and came to theconclusion that the planet would _actually_ traverse the lower portionof the sun's disc. The event, as we have seen, proved him to be quite inthe right. Horrox is said to have been a veritable prodigy ofastronomical skill; and had he lived longer would, no doubt, have becomevery famous. Unfortunately he died about two years after his celebratedtransit, in his _twenty-second_ year only, according to the accounts. His friend Crabtree, who was then also a young man, is said to have beenkilled at the battle of Naseby in 1645. 

There is an interesting phenomenon in connection with transits which isknown as the "Black Drop. " When an inferior planet has just made its wayon to the face of the sun, it is usually seen to remain for a short timeas if attached to the sun's edge by what looks like a dark ligament (seeFig. 12, p. 153). This gives to the planet for the time being anelongated appearance, something like that of a pear; but when theligament, which all the while keeps getting thinner and thinner, has atlast broken, the black body of the planet is seen to stand out roundagainst the solar disc. 

[Illustration: FIG. 12. --The "Black Drop. "]

This appearance may be roughly compared to the manner in which a drop ofliquid (or, preferably, of some glutinous substance) tends for a whileto adhere to an object from which it is falling. 

When the planet is in turn making its way off the face of the sun, theligament is again seen to form and to attach it to the sun's edge beforeits due time. 

The phenomenon of the black drop, or ligament, is entirely an illusion, and, broadly speaking, of an optical origin. Something very similar willbe noticed if one brings one's thumb and forefinger _slowly_ togetheragainst a very bright background. 

This peculiar phenomenon has proved one of the greatest drawbacks to theproper observation of transits, for it is quite impossible to note theexact instant of the planet's entrance upon and departure from the solardisc in conditions such as these. 

The black drop seems to bear a family resemblance, so to speak, to thephenomenon of Baily's beads. In the latter instance the lunar peaks, asthey approach the sun's edge, appear to lengthen out in a similar mannerand bridge the intervening space before their time, thus givingprominence to an effect which otherwise should scarcely be noticeable. 

The last transit of Mercury, which, as has been already stated, tookplace on November 14, 1907, was not successfully observed by astronomersin England, on account of the cloudiness of the weather. In France, however, Professor Moye, of Montpellier, saw it under good conditions, and mentions that the black drop remained very conspicuous for fully aminute. The transit was also observed in the United States, the reportsfrom which speak of the black drop as very "troublesome. "

Before leaving the subject of transits it should be mentioned that itwas in the capacity of commander of an expedition to Otaheite, in thePacific, to observe the transit of Venus of June 3, 1769, that CaptainCook embarked upon the first of his celebrated voyages. 

In studying the surfaces of Venus and Mercury with the telescope, observers are, needless to say, very much hindered by the proximity ofthe sun. Venus, when at the greatest elongations, certainly draws somedistance out of the glare; but her surface is, even then, so dazzlinglybright, that the markings upon it are difficult to see. Mercury, on theother hand, is much duller in contrast, but the disc it shows in thetelescope is exceedingly small; and, in addition, when that planet isleft above the horizon for a short time after sunset, as necessarilyhappens after certain intervals, the mists near the earth's surfacerender observation of it very difficult. 

Until about twenty-five years ago, it was generally believed that boththese planets rotated on their axes in about twenty-four hours, anotion, no doubt, originally founded upon an unconscious desire to bringthem into some conformity with our earth. But Schiaparelli, observing inItaly, and Percival Lowell, in the clear skies of Arizona and Mexico, have lately come to the conclusion that both planets rotate upon theiraxes in the same time as they revolve in their orbits, [12] the resultbeing that they turn one face ever towards the sun in the same mannerthat the moon turns one face ever towards the earth--a curious state ofthings, which will be dealt with more fully when we come to treat of oursatellite. 

The marked difference in the brightness between the two planets hasalready been alluded to. The surface of Venus is, indeed, about fivetimes as bright as that of Mercury. The actual brightness of Mercury isabout equivalent to that of our moon, and astronomers are, therefore, inclined to think that it may resemble her in having a very ruggedsurface and practically no atmosphere. This probable lack of atmosphereis further corroborated by two circumstances. One of these is that whenMercury is just about to transit the face of the sun, no ring ofdiffused light is seen to encircle its disc as would be the case if itpossessed an atmosphere. Such a lack of atmosphere is, indeed, only tobe expected from what is known as the _Kinetic Theory of Gases_. According to this theory, which is based upon the behaviour of variouskinds of gas, it is found that these elements tend to escape into spacefrom the surface of bodies whose force of gravitation is weak. Hydrogengas, for example, tends to fly away from our earth, as any one may seefor himself when a balloon rises into the air. The gravitation of theearth seems, however, powerful enough to hold down other gases, as, forinstance, those of which the air is chiefly composed, namely, oxygen andnitrogen. In due accordance with the Kinetic theory, we find the moonand Mercury, which are much about the same size, destitute ofatmospheres. Mars, too, whose diameter is only about double that of themoon, has very little atmosphere. We find, on the other hand, thatVenus, which is about the same size as our earth, clearly possesses anatmosphere, as just before the planet is in transit across the sun, theoutline of its dark body is seen to be surrounded by a bright ring oflight. 

The results of telescopic observation show that more markings arevisible on Mercury than on Venus. The intense brilliancy of Venus is, indeed, about the same as that of our white clouds when the sun isshining directly upon them. It has, therefore, been supposed that theplanet is thickly enveloped in cloud, and that we do not ever see anypart of its surface, except perchance the summit of some lofty mountainprojecting through the fleecy mass. 

With regard to the great brilliancy of Venus, it may be mentioned thatshe has frequently been seen in England, with the naked eye in fullsunshine, when at the time of her greatest brightness. The writer hasseen her thus at noonday. Needless to say, the sky at the moment wasintensely blue and clear. 

The orbit of Mercury is very oval, and much more so than that of anyother planet. The consequence is that, when Mercury is nearest to thesun, the heat which it receives is twice as great as when it is farthestaway. The orbit of Venus, on the other hand, is in marked contrast withthat of Mercury, and is, besides, more nearly of a circular shape thanthat of any of the other planets. Venus, therefore, always keeps aboutthe same distance from the sun, and so the heat which she receivesduring the course of her year can only be subject to very slightvariations. 

[11] In employing the terms Inferior and Superior the writer bows toastronomical custom, though he cannot help feeling that, in thecircumstances, Interior and Exterior would be much more appropriate. 

[12] This question is, however, uncertain, for some very recentspectroscopic observations of Venus seem to show a rotation period ofabout twenty-four hours. 

CHAPTER XV

THE EARTH

We have already seen (in Chapter I. ) how, in very early times, mennaturally enough considered the earth to be a flat plane extending to avery great distance in every direction; but that, as years went on, certain of the Greek philosophers suspected it to be a sphere. One ortwo of the latter are, indeed, said to have further believed in itsrotation about an axis, and even in its revolution around the sun; but, as the ideas in question were founded upon fancy, rather than upon anydirect evidence, they did not generally attract attention. The smalleffect, therefore, which these theories had upon astronomy, may well begathered from the fact that in the Ptolemaic system the earth wasconsidered as fixed and at the centre of things; and this belief, as wehave seen, continued unaltered down to the days of Copernicus. It was, indeed, quite impossible to be certain of the real shape of the earth orthe reality of its motions until knowledge became more extended andscientific instruments much greater in precision. 

We will now consider in detail a few of the more obvious arguments whichcan be put forward to show that our earth is a sphere. 

If, for instance, the earth were a plane surface, a ship sailing awayfrom us over the sea would appear to grow smaller and smaller as itreceded into the distance, becoming eventually a tiny speck, and fadinggradually from our view. This, however, is not at all what actuallytakes place. As we watch a vessel receding, its hull appears bit by bitto slip gently down over the horizon, leaving the masts alone visible. Then, in their turn, the masts are seen to slip down in the same manner, until eventually every trace of the vessel is gone. On the other hand, when a ship comes into view, the masts are the first portions to appear. They gradually rise up from below the horizon, and the hull follows inits turn, until the whole vessel is visible. Again, when one is upon aship at sea, a set of masts will often be seen sticking up alone abovethe horizon, and these may shorten and gradually disappear from viewwithout the body of the ship to which they belong becoming visible atall. Since one knows from experience that there is no _edge_ at thehorizon over which a vessel can drop down, the appearance which we havebeen describing can only be explained by supposing that the surface ofthe earth is always curving gradually in every direction. 

The distance at which what is known as the _horizon_ lies away from usdepends entirely upon the height above the earth's surface where wehappen at the moment to be. A ship which has appeared to sink below thehorizon for a person standing on the beach, will be found to come backagain into view if he at once ascends a high hill. Experiment shows thatthe horizon line lies at about three miles away for a person standing atthe water's edge. The curving of the earth's surface is found, indeed, to be at the rate of eight inches in every mile. Now it can beascertained, by calculation, that a body curving at this rate in everydirection must be a globe about 8000 miles in diameter. 

Again, the fact that, if not stopped by such insuperable obstacles asthe polar ice and snow, those who travel continually in any onedirection upon the earth's surface always find themselves back again atthe regions from which they originally set out, is additional ground forconcluding that the earth is a globe. 

We can find still further evidence. For instance, in an eclipse of themoon the earth's shadow, when seen creeping across the moon's face, isnoted to be _always_ circular in shape. One cannot imagine how such athing could take place unless the earth were a sphere. 

Also, it is found from observation that the sun, the planets, and thesatellites are, all of them, round. This roundness cannot be theroundness of a flat plate, for instance, for then the objects inquestion would sometimes present their thin sides to our view. Ithappens, also, that upon the discs which these bodies show, we seecertain markings shifting along continually in one direction, todisappear at one side and to reappear again at the other. Such bodiesmust, indeed, be spheres in rotation. 

The crescent and other phases, shown by the moon and the inferiorplanets, should further impress the truth of the matter upon us, as suchappearances can only be caused by the sunlight falling from variousdirections upon the surfaces of spherical bodies. 

Another proof, perhaps indeed the weightiest of all, is the continuousmanner in which the stars overhead give place to others as one travelsabout the surface of the earth. When in northern regions the Pole Starand its neighbours--the stars composing the Plough, for instance--areover our heads. As one journeys south these gradually sink towards thenorthern horizon, while other stars take their place, and yet others areuncovered to view from the south. The regularity with which thesechanges occur shows that every point on the earth's surface faces adifferent direction of the sky, and such an arrangement would only bepossible if the earth were a sphere. The celebrated Greek philosopher, Aristotle, is known to have believed in the globular shape of the earth, and it was by this very argument that he had convinced himself that itwas so. 

The idea of the sphericity of the earth does not appear, however, tohave been generally accepted until the voyages of the great navigatorsshowed that it could be sailed round. 

The next point we have to consider is the rotation of the earth aboutits axis. From the earliest times men noticed that the sky andeverything in it appeared to revolve around the earth in one fixeddirection, namely, towards what is called the West, and that it made onecomplete revolution in the period of time which we know as twenty-fourhours. The stars were seen to come up, one after another, from below theeastern horizon, to mount the sky, and then to sink in turn below thewestern horizon. The sun was seen to perform exactly the same journey, and the moon, too, whenever she was visible. One or two of the ancientGreek philosophers perceived that this might be explained, either by amovement of the entire heavens around the earth, or by a turning motionon the part of the earth itself. Of these diverse explanations, thatwhich supposed an actual movement of the heavens appealed to them themost, for they could hardly conceive that the earth should continuallyrotate and men not be aware of its movement. The question may becompared to what we experience when borne along in a railway train. Wesee the telegraph posts and the trees and buildings near the line flypast us one after another in the contrary direction. Either these mustbe moving, or we must be moving; and as we happen to _know_ that it is, indeed, we who are moving, there can be no question therefore about thematter. But it would not be at all so easy to be sure of this movementwere one unable to see the objects close at hand displacing themselves. For instance, if one is shut up in a railway carriage at night with theblinds down, there is really nothing to show that one is moving, exceptthe jolting of the train. And even then it is hard to be sure in whichdirection one is actually travelling. 

The way we are situated upon the earth is therefore as follows. Thereare no other bodies sufficiently near to be seen flying past us in turn;our earth spins without a jolt; we and all things around us, includingthe atmosphere itself, are borne along together with precisely the sameimpetus, just as all the objects scattered about a railway carriageshare in the forward movement of the train. Such being the case, whatwonder that we are unconscious of the earth's rotation, of which weshould know nothing at all, were it not for that slow displacement ofthe distant objects in the heavens, as we are borne past them in turn. 

If the night sky be watched, it will be soon found that its apparentturning movement seems to take place around a certain point, whichappears as if fixed. This point is known as the north pole of theheavens; and a rather bright star, which is situated very close to thishub of movement, is in consequence called the Pole Star. For thedwellers in southern latitudes there is also a point in their sky whichappears to remain similarly fixed, and this is known as the south poleof the heavens. Since, however, the heavens do not turn round at all, but the earth does, it will easily be seen that these apparentlystationary regions in the sky are really the points towards which theaxis of the earth is directed. The positions on the earth's surfaceitself, known as the North and South Poles, are merely the places wherethe earth's axis, if there were actually such a thing, would be expectedto jut out. The north pole of the earth will thus be situated exactlybeneath the north pole of the heavens, and the south pole of the earthexactly beneath the south pole of the heavens. 

We have seen that the earth rotates upon its imaginary axis once inabout every twenty-four hours. This means that everything upon thesurface of the earth is carried round once during that time. Themeasurement around the earth's equator is about 24, 000 miles; and, therefore, an object situated at the equator must be carried roundthrough a distance of about 24, 000 miles in each twenty-four hours. Everything at the equator is thus moving along at the rapid rate ofabout 1000 miles an hour, or between sixteen and seventeen times asfast as an express train. If, however, one were to take measurementsaround the earth parallel to the equator, one would find thesemeasurements becoming less and less, according as the poles wereapproached. It is plain, therefore, that the speed with which any pointmoves, in consequence of the earth's rotation, will be greatest at theequator, and less and less in the direction of the poles; while at thepoles themselves there will be practically no movement, and objectsthere situated will merely turn round. 

The considerations above set forth, with regard to the different speedsat which different portions of a rotating globe will necessarily bemoving, is the foundation of an interesting experiment, which gives usfurther evidence of the rotation of our earth. The measurement aroundthe earth at any distance below the surface, say, for instance, at thedepth of a mile, will clearly be less than a similar measurement at thesurface itself. The speed of a point at the bottom of a mine, whichresults from the actual rotation of the earth, must therefore be lessthan the speed of a point at the surface overhead. This can bedefinitely proved by dropping a heavy object down a mine shaft. Theobject, which starts with the greater speed of the surface, will, whenit reaches the bottom of the mine, be found, as might be indeedexpected, to be a little ahead (_i. E. _ to the east) of the point whichoriginally lay exactly underneath it. The distance by which the objectgains upon this point is, however, very small. In our latitudes itamounts to about an inch in a fall of 500 feet. 

The great speed at which, as we have seen, the equatorial regions ofthe earth are moving, should result in giving to the matter theresituated a certain tendency to fly outwards. Sir Isaac Newton was thefirst to appreciate this point, and he concluded from it that the earthmust be _bulged_ a little all round the equator. This is, indeed, foundto be the case, the diameter at the equator being nearly twenty-sevenmiles greater than it is from pole to pole. The reader will, no doubt, be here reminded of the familiar comparison in geographies between theshape of the earth and that of an orange. 

In this connection it is interesting to consider that, were the earth torotate seventeen times as fast as it does (_i. E. _ in one hourtwenty-five minutes, instead of twenty-four hours), bodies at theequator would have such a strong tendency to fly outwards that the forceof terrestrial gravity acting upon them would just be counterpoised, andthey would virtually have _no weight_. And, further, were the earth torotate a little faster still, objects lying loose upon its surface wouldbe shot off into space. 

The earth is, therefore, what is technically known as an _oblatespheroid_; that is, a body of spherical shape flattened at the poles. Itfollows of course from this, that objects at the polar regions areslightly nearer to the earth's centre than objects at the equatorialregions. We have already seen that gravitation acts from the centralparts of a body, and that its force is greater the nearer are thosecentral parts. The result of this upon our earth will plainly be thatobjects in the polar regions will be pulled with a slightly strongerpull, and will therefore _weigh_ a trifle more than objects in theequatorial regions. This is, indeed, found by actual experiment to bethe case. As an example of the difference in question, Professor Young, in his _Manual of Astronomy_, points out that a man who weighs 190pounds at the equator would weigh 191 at the pole. In such an experimentthe weighing would, however, have to be made with a _spring balance_, and _not with scales_; for, in the latter case, the "weights" used wouldalter in their weight in exactly the same degree as the objects to beweighed. 

It used to be thought that the earth was composed of a relatively thincrust, with a molten interior. Scientific men now believe, on the otherhand, that such a condition cannot after all prevail, and that the earthmust be more or less solid all through, except perhaps in certainisolated places where collections of molten matter may exist. 

The _atmosphere_, or air which we breathe, is in the form of a layer oflimited depth which closely envelops the earth. Actually, it is amixture of several gases, the most important being nitrogen and oxygen, which between them practically make up the air, for the proportion ofthe other gases, the chief of which is carbonic acid gas, is exceedinglysmall. 

It is hard to picture our earth, as we know it, without this atmosphere. Deprived of it, men at once would die; but even if they could be made togo on living without it by any miraculous means, they would be like untodeaf beings, for they would never hear any sound. What we call _sounds_are merely vibrations set up in the air, which travel along and strikeupon the drum of the ear. 

The atmosphere is densest near the surface of the earth, and becomesless and less dense away from it, as a result of diminishing pressure ofair from above. The greater portion of it is accumulated within four orfive miles of the earth's surface. 

It is impossible to determine exactly at what distance from the earth'ssurface the air ceases altogether, for it grows continually more andmore rarefied. There are, however, two distinct methods of ascertainingthe distance beyond which it can be said practically not to exist. Oneof these methods we get from twilight. Twilight is, in fact, merelylight reflected to us from those upper regions of the air, which stillcontinue to be illuminated by the sun after it has disappeared from ourview below the horizon. The time during which twilight lasts, shows usthat the atmosphere must be at least fifty miles high. 

But the most satisfactory method of ascertaining the height to which theatmosphere extends is from the observation of meteors. It is found thatthese bodies become ignited, by the friction of passing into theatmosphere, at a height of about 100 miles above the surface of theearth. We thus gather that the atmosphere has a certain degree ofdensity even at this height. It may, indeed, extend as far as about 150miles. 

The layer of atmosphere surrounding our earth acts somewhat in themanner of the glass covering of a greenhouse, bottling in the sun'srays, and thus storing up their warmth for our benefit. Were this notso, the heat which we get from the sun would, after falling upon theearth, be quickly radiated again into space. 

It is owing to the unsteadiness of the air that stars are seen totwinkle. A night when this takes place, though it may please the averageperson, is worse than useless to the astronomer, for the unsteadiness isgreatly magnified in the telescope. This twinkling is, no doubt, in agreat measure responsible for the conventional "points" with which Arthas elected to embellish stars, and which, of course, have no existencein fact. 

The phenomena of _Refraction_, [13] namely, that bending which rays oflight undergo, when passing _slant-wise_ from a rare into a densetransparent medium, are very marked with regard to the atmosphere. Thedenser the medium into which such rays pass, the greater is this bendingfound to be. Since the layer of air around us becomes denser and densertowards the surface of the earth, it will readily be granted that therays of light reaching our eyes from a celestial object, will suffer thegreater bending the lower the object happens to be in the sky. Celestialobjects, unless situated directly overhead, are thus not seen in theirtrue places, and when nearest to the horizon are most out of place. Thebending alluded to is upwards. Thus the sun and the moon, for instance, when we see them resting upon the horizon, are actually _entirely_beneath it. 

When the sun, too, is sinking towards the horizon, the lower edge of itsdisc will, for the above reason, look somewhat more raised than theupper. The result is a certain appearance of flattening; which mayplainly be seen by any one who watches the orb at setting. 

In observations to determine the exact positions of celestial objectscorrection has to be made for the effects of refraction, according tothe apparent elevation of these objects in the sky. Such effects areleast when the objects in question are directly overhead, for then therays of light, coming from them to the eye, enter the atmosphereperpendicularly, and not at any slant. 

A very curious effect, due to refraction, has occasionally been observedduring a total eclipse of the moon. To produce an eclipse of this kind, _the earth must, of course, lie directly between the sun and the moon_. Therefore, when we see the shadow creeping over the moon's surface, thesun should actually be well below the horizon. But when a lunar eclipsehappens to come on just about sunset, the sun, although really sunkbelow the horizon, appears still above it through refraction, and theeclipsed moon, situated, of course, exactly opposite to it in the sky, is also lifted up above the horizon by the same cause. Pliny, writing inthe first century of the Christian era, describes an eclipse of thiskind, and refers to it as a "prodigy. " The phenomenon is known as a"horizontal eclipse. " It was, no doubt, partly owing to it that theancients took so long to decide that an eclipse of the moon was reallycaused by the shadow cast by the earth. Plutarch, indeed, remarks thatit was easy enough to understand that a solar eclipse was caused by theinterposition of the moon, but that one could not imagine by theinterposition _of what body_ the moon itself could be eclipsed. 

In that apparent movement of the heavens about the earth, which men nowknow to be caused by the mere rotation of the earth itself, a slightchange is observed to be continually taking place. The stars, indeed, are always found to be gradually drawing westward, _i. E. _ towards thesun, and losing themselves one after the other in the blaze of hislight, only to reappear, however, on the other side of him after acertain lapse of time. This is equivalent to saying that the sun itselfseems always creeping slowly _eastward_ in the heaven. The rate at whichthis appears to take place is such that the sun finds itself back againto its original position, with regard to the starry background, at theend of a year's time. In other words, the sun seems to make a completetour of the heavens in the course of a year. Here, however, we haveanother illusion, just as the daily movement of the sky around the earthwas an illusion. The truth indeed is, that this apparent movement of thesun eastward among the stars during a year, arises merely from a_continuous displacement of his position_ caused by an actual motion ofthe earth itself around him in that very time. In a word, it is theearth which really moves around the sun, and not the sun around theearth. 

The stress laid upon this fundamental point by Copernicus, marks theseparation of the modern from the ancient view. Not that Copernicus, indeed, had obtained any real proof that the earth is merely a planetrevolving around the sun; but it seemed to his profound intellect that amovement of this kind on the part of our globe was the more likelyexplanation of the celestial riddle. The idea was not new; for, as wehave already seen, certain of the ancient Greeks (Aristarchus of Samos, for example) had held such a view; but their notions on the subject werevery fanciful, and unsupported by any good argument. 

What Copernicus, however, really seems to have done was to _insist_ uponthe idea that the sun occupied the _centre_, as being more consonantwith common sense. No doubt, he was led to take up this position by thefact that the sun appeared entirely of a different character from theother members of the system. The one body in the scheme, which performedthe important function of dispenser of light and heat, would indeed bemore likely to occupy a position apart from the rest; and what positionmore appropriate for its purposes than the centre!

But here Copernicus only partially solved the difficult question. Heunfortunately still clung to an ancient belief, which as yet remainedunquestioned; _i. E. _ the great virtue, one might almost say, the_divineness_, of circular motion. The ancients had been hag-ridden, soto speak, by the circle; and it appeared to them that such a perfectlyformed curve was alone fitted for the celestial motions. Ptolemyemployed it throughout his system. According to him the "planets" (whichincluded, under the ancient view, both the sun and the moon), movedaround the earth in circles; but, as their changing positions in the skycould not be altogether accounted for in this way, it was furthersupposed that they performed additional circular movements, aroundpeculiarly placed centres, during the course of their orbitalrevolutions. Thus the Ptolemaic system grew to be extremelycomplicated; for astronomers did not hesitate to add new circularmovements whenever the celestial positions calculated for the planetswere found not to tally with the positions observed. In this manner, indeed, they succeeded in doctoring the theory, so that it fairlysatisfied the observations made with the rough instruments ofpre-telescopic times. 

Although Copernicus performed the immense service to astronomy of boldlydirecting general attention to the central position of the sun, heunfortunately took over for the new scheme the circular machinery of thePtolemaic system. It therefore remained for the famous Kepler, who livedabout a century after him, to find the complete solution. Just asCopernicus, for instance, had broken free from tradition with regard tothe place of the sun; so did Kepler, in turn, break free from the spellof circular motion, and thus set the coping-stone to the newastronomical edifice. This astronomer showed, in fact, that if the pathsof the planets around the sun, and of the moon around the earth, werenot circles, but _ellipses_, the movements of these bodies about the skycould be correctly accounted for. The extreme simplicity of such anarrangement was far more acceptable than the bewildering intricacy ofmovement required by the Ptolemaic theory. The Copernican system, asamended by Kepler, therefore carried the day; and was furtherstrengthened, as we have already seen, by the telescopic observations ofGalileo and the researches of Newton into the effects of gravitation. 

And here a word on the circle, now fallen from its high estate. Theancients were in error in supposing that it stood entirely apart--thecurve of curves. As a matter of fact it is merely _a special kind ofellipse_. To put it paradoxically, it is an ellipse which has noellipticity, an oval without any ovalness!

Notwithstanding all this, astronomy had to wait yet a long time for adefinite proof of the revolution of the earth around the sun. Theleading argument advanced by Aristotle, against the reality of anymovement of the earth, still held good up to about seventy years ago. That philosopher had pointed out that the earth could not move about inspace to any great extent, or the stars would be found to alter theirapparent places in the sky, a thing which had never been observed tohappen. Centuries ran on, and instruments became more and more perfect, yet no displacements of stars were noted. In accepting the Copernicantheory men were therefore obliged to suppose these objects asimmeasurably distant. At length, however, between the years 1835 and1840, it was discovered by the Prussian astronomer, Bessel, that a starknown as 61 Cygni--that is to say, the star marked in celestial atlasesas No. 61 in the constellation of the Swan--appeared, during the courseof a year, to perform a tiny circle in the heavens, such as would resultfrom a movement on our own part around the sun. Since then aboutforty-three stars have been found to show minute displacements of asimilar kind, which cannot be accounted for upon any other suppositionthan that of a continuous revolution of the earth around the sun. Thetriumph of the Copernican system is now at last supreme. 

If the axis of the earth stood "straight up, " so to speak, while theearth revolved in its orbit, the sun would plainly keep always on alevel with the equator. This is equivalent to stating that, in suchcircumstances, a person at the equator would see it rise each morningexactly in the east, pass through the _zenith_, that is, the pointdirectly overhead of him, at midday, and set in the evening due in thewest. As this would go on unchangingly at the equator every daythroughout the year, it should be clear that, at any particular placeupon the earth, the sun would in these conditions always be seen to movein an unvarying manner across the sky at a certain altitude dependingupon the latitude of the place. Thus the more north one went upon theearth's surface, the more southerly in the sky would the sun's path lie;while at the north pole itself, the sun would always run round and roundthe horizon. Similarly, the more south one went from the equator themore northerly would the path of the sun lie, while at the south pole itwould be seen to skirt the horizon in the same manner as at the northpole. The result of such an arrangement would be, that each place uponthe earth would always have one unvarying climate; in which case therewould not exist any of those beneficial changes of season to which weowe so much. 

The changes of season, which we fortunately experience, are due, however, to the fact that the sun does not appear to move across the skyeach day at one unvarying altitude, but is continually altering theposition of its path; so that at one period of the year it passes acrossthe sky low down, and remains above the horizon for a short time only, while at another it moves high up across the heavens, and is above thehorizon for a much longer time. Actually, the sun seems little by littleto creep up the sky during one half of the year, namely, from mid-winterto mid-summer, and then, just as gradually, to slip down it again duringthe other half, namely, from mid-summer to mid-winter. It will thereforebe clear that every region of the earth is much more thoroughly warmedduring one portion of the year than during another, _i. E. _ when thesun's path is high in the heavens than when it is low down. 

Once more we find appearances exactly the contrary from the truth. Theearth is in this case the real cause of the deception, just as it was inthe other cases. The sun does not actually creep slowly up the sky, andthen slowly dip down it again, but, owing to the earth's axis being setaslant, different regions of the earth's surface are presented to thesun at different times. Thus, in one portion of its orbit, the northerlyregions of the earth are presented to the sun, and in the other portionthe southerly. It follows of course from this, that when it is summer inthe northern hemisphere it is winter in the southern, and _vice versâ_(see Fig. 13, p. 176). 

[Illustration: FIG. 13. --Summer and Winter. ]

The fact that, in consequence of this slant of the earth's axis, the sunis for part of the year on the north side of the equator and part of theyear on the south side, leads to a very peculiar result. The path of themoon around the earth is nearly on the same plane with the earth's patharound the sun. The moon, therefore, always keeps to the same regions ofthe sky as the sun. The slant of the earth's axis thus regularlydisplaces the position of both the sun and the moon to the north andsouth sides of the equator respectively in the manner we have beendescribing. Were the earth, however, a perfect sphere, such change ofposition would not produce any effect. We have shown, however, that theearth is not a perfect sphere, but that it is bulged out all round theequator. The result is that this bulged-out portion swings slowly underthe pulls of solar and lunar gravitation, in response to thedisplacements of the sun and moon to the north and to the south of it. This slow swing of the equatorial regions results, of course, in acertain slow change of the direction of the earth's axis, so that thenorth pole does not go on pointing continually to the same region of thesky. The change in the direction of the axis is, however, so extremelyslight, that it shows up only after the lapse of ages. The north pole ofthe heavens, that is, the region of the sky towards which the north poleof the earth's axis points, displaces therefore extremely slowly, tracing out a wide circle, and arriving back again to the same positionin the sky only after a period of about 25, 000 years. At present thenorth pole of the heavens is quite close to a bright star in the tail ofthe constellation of the Little Bear, which is consequently known as thePole Star; but in early Greek times it was at least ten times as faraway from this star as it is now. After some 12, 000 years the pole willpoint to the constellation of Lyra, and Vega, the most brilliant star inthat constellation, will then be considered as the pole star. This slowtwisting of the earth's axis is technically known as _Precession_, orthe _Precession of the Equinoxes_ (see Plate XIX. , p. 292). 

The slow displacement of the celestial pole appears to have attractedthe attention of men in very early times, but it was not until thesecond century B. C. That precession was established as a fact by thecelebrated Greek astronomer, Hipparchus. For the ancients this strangecyclical movement had a mystic significance; and they looked towards theend of the period as the end, so to speak, of a "dispensation, " afterwhich the life of the universe would begin anew:--

"Magnus ab integro sæclorum nascitur ordo. Jam redit et Virgo, redeunt Saturnia regna; . . . . . . Alter erit tum Tiphys, et altera quæ vehat ArgoDelectos heroas; erunt etiam altera bella, Atque iterum ad Trojam magnus mittetur Achilles. "

We have seen that the orbit of the earth is an ellipse, and that the sunis situated at what is called the _focus_, a point not in the middle ofthe ellipse, but rather towards one of its ends. Therefore, during thecourse of the year the distance of the earth from the sun varies. Thesun, in consequence of this, is about 3, 000, 000 miles _nearer_ to us inour northern _winter_ than it is in our northern summer, a statementwhich sounds somewhat paradoxical. This variation in distance, large asit appears in figures, can, however, not be productive of muchalteration in the amount of solar heat which we receive, for during thefirst week in January, when the distance is least, the sun only looksabout _one-eighteenth_ broader than at the commencement of July, whenthe distance is greatest. The great disparity in temperature betweenwinter and summer depends, as we have seen, upon causes of quite anotherkind, and varies between such wide limits that the effects of thisslight alteration in the distance of the sun from the earth may beneglected for practical purposes. 

The Tides are caused by the gravitational pull of the sun and moon uponthe water of the earth's surface. Of the two, the moon, being so muchthe nearer, exerts the stronger pull, and therefore may be regarded asthe chief cause of the tides. This pull always draws that portion of thewater, which happens to be right underneath the moon at the time, into aheap; and there is also a _second_ heaping of water at the same moment_at the contrary side of the earth_, the reasons for which can be shownmathematically, but cannot be conveniently dealt with here. 

As the earth rotates on its axis each portion of its surface passesbeneath the moon, and is swelled up by this pull; the watery portionsbeing, however, the only ones to yield visibly. A similar swelling up, as we have seen, takes place at the point exactly away from the moon. Thus each portion of our globe is borne by the rotation through two"tide-areas" every day, and this is the reason why there are two tidesduring every twenty-four hours. 

The crest of the watery swelling is known as high tide. The journey ofthe moon around the earth takes about a month, and this brings her pasteach place in turn by about fifty minutes later each day, which is thereason why high tide is usually about twenty-five minutes later eachtime. 

The moon is, however, not the sole cause of the tides, but the sun, aswe have said, has a part in the matter also. When it is new moon thegravitational attractions of both sun and moon are clearly actingtogether from precisely the same direction, and, therefore, the tidewill be pulled up higher than at other times. At full moon, too, thesame thing happens; for, although the bodies are now acting fromopposite directions, they do not neutralise each other's pulls as onemight imagine, since the sun, in the same manner as the moon, produces atide both under it and also at the opposite side of the earth. Thus boththese tides are actually increased in height. The exceptionally hightides which we experience at new and full moons are known as _SpringTides_, in contradistinction to the minimum high tides, which are knownas _Neap Tides_. 

The ancients appear to have had some idea of the cause of the tides. Itis said that as early as 1000 B. C. The Chinese noticed that the moonexerted an influence upon the waters of the sea. The Greeks and Romans, too, had noticed the same thing; and Cæsar tells us that when he wasembarking his troops for Britain the tide was high _because_ the moonwas full. Pliny went even further than this, in recognising a similarconnection between the waters and the sun. 

From casual observation one is inclined to suppose that the high tidealways rises many feet. But that this is not the case is evidenced bythe fact that the tides in the midst of the great oceans are only fromthree to four feet high. However, in the seas and straits around ourIsles, for instance, the tides rise very many feet indeed, but this ismerely owing to the extra heaping up which the large volumes of waterundergo in forcing their passage through narrow channels. 

As the earth, in rotating, is continually passing through thesetide-areas, one might expect that the friction thus set up would tend toslow down the rotation itself. Such a slowing down, or "tidal drag, " asit is called, is indeed continually going on; but the effects producedare so exceedingly minute that it will take many millions of years tomake the rotation appreciably slower, and so to lengthen the day. 

Recently it has been proved that the axis of the earth is subject to avery small displacement, or rather, "wobbling, " in the course of aperiod of somewhat over a year. As a consequence of this, the poleshifts its place through a circle of, roughly, a few yards in widthduring the time in question. This movement is, perhaps, the combinedresult of two causes. One of these is the change of place during theyear of large masses of material upon our earth; such as occurs, forinstance, when ice and snow melt, or when atmospheric and oceancurrents transport from place to place great bodies of air and water. The other cause is supposed to be the fact that the earth is notabsolutely rigid, and so yields to certain strains upon it. In thecourse of investigation of this latter point the interesting conclusionhas been reached by the famous American astronomer, Professor SimonNewcomb, that our globe as a whole is _a little more rigid than steel_. 

We will bring this chapter to a close by alluding briefly to two strangeappearances which are sometimes seen in our night skies. These are knownrespectively as the Zodiacal Light and the Gegenschein. 

The _Zodiacal Light_ is a faint cone-shaped illumination which is seento extend upwards from the western horizon after evening twilight hasended, and from the eastern horizon before morning twilight has begun. It appears to rise into the sky from about the position where the sunwould be at that time. The proper season of the year for observing itduring the evening is in the spring, while in autumn it is best seen inthe early morning. In our latitudes its light is not strong enough torender it visible when the moon is full, but in the tropics it isreported to be very bright, and easily seen in full moonlight. Onetheory regards it as the reflection of light from swarms of meteorsrevolving round the sun; another supposes it to be a very rarefiedextension of the corona. 

The _Gegenschein_ (German for "counter-glow") is a faint oval patch oflight, seen in the sky exactly opposite to the place of the sun. It isusually treated of in connection with the zodiacal light, and one theoryregards it similarly as of meteoric origin. Another theory, however--that of Mr. Evershed--considers it a sort of _tail_ to theearth (like a comet's tail) composed of hydrogen and helium--the two_lightest_ gases we know--driven off from our planet in the directioncontrary to the sun. 

[13] Every one knows the simple experiment in which a coin lying at thebottom of an empty basin, and hidden from the eye by its side, becomesvisible when a certain quantity of water has been poured in. This is anexample of refraction. The rays of light coming from the coin ought_not_ to reach the eye, on account of the basin's side being in the way;yet by the action of the water they are _refracted_, or bent over itsedge, in such a manner that they do. 

CHAPTER XVI

THE MOON

What we call the moon's "phases" are merely the various ways in which wesee the sun shining upon her surface during the course of her monthlyrevolutions around the earth (see Fig. 14, p. 184). When she passes inthe neighbourhood of the sun all his light falls upon that side which isturned away from us, and so the side which is turned towards us isunillumined, and therefore invisible. When in this position the moon isspoken of as _new_. 

As she continues her motion around the earth, she draws gradually to theeast of the sun's place in the sky. The sunlight then comes somewhatfrom the side; and so we see a small portion of the right side of thelunar disc illuminated. This is the phase known as the _crescent_ moon. 

As she moves on in her orbit more and more of her illuminated surface isbrought into view; and so the crescent of light becomes broader andbroader, until we get what is called half-moon, or _first quarter_, whenwe see exactly one-half of her surface lit up by the sun's rays. As shedraws still further round yet more of her illuminated surface is broughtinto view, until three-quarters of the disc appear lighted up. She isthen said to be _gibbous_. 

Eventually she moves round so that she faces the sun completely, andthe whole of her disc appears illuminated. She is then spoken of as_full_. When in this position it is clear that she is on the contraryside of the earth to the sun, and therefore rises about the same timethat he is setting. She is now, in fact, at her furthest from the sun. 

[Illustration: Direction from which the sun's rays are coming. 

Various positions and illumination of the mooon by the sun during herrevolution around the earth. 

The corresponding positions as viewed from the earth, showing theconsequent phases. 

FIG. 14. --Orbit and Phases of the Moon. ]

After this, the motion of the moon in her orbit carries her on backagain in the direction of the sun. She thus goes through her phases asbefore, only these of course are _in the reverse order_. The full phaseis seen to give place to the gibbous, and this in turn to the half-moonand to the crescent; after which her motion carries her into theneighbourhood of the sun, and she is once more new, and lost to oursight in the solar glare. Following this she draws away to the east ofthe sun again, and the old order of phases repeat themselves as before. 

The early Babylonians imagined that the moon had a bright and a darkside, and that her phases were caused by the bright side coming more andmore into view during her movement around the sky. The Greeks, notablyAristotle, set to work to examine the question from a mathematicalstandpoint, and came to the conclusion that the crescent and otherappearances were such as would necessarily result if the moon were adark body of spherical shape illumined merely by the light of the sun. 

Although the true explanation of the moon's phases has thus been knownfor centuries, it is unfortunately not unusual to seepictures--advertisement posters, for instance--in which stars appear_within_ the horns of a crescent moon! Can it be that there are to-dayeducated persons who believe that the moon is a thing which _grows_ to acertain size and then wastes away again; who, in fact, do not know thatthe entire body of the moon is there all the while?

When the moon shows a very thin crescent, we are able dimly to see herstill dark portion standing out against the sky. This appearance ispopularly known as the "old moon in the new moon's arms. " The dark partof her surface must, indeed, be to some degree illumined, or we shouldnot be able to see it at all. Whence then comes the light whichillumines it, since it clearly cannot come from the sun? The riddle iseasily solved, if we consider what kind of view of our earth an observersituated on this darkened part of the moon would at that moment get. Hewould, as a matter of fact, just then see nearly the whole disc of theearth brightly lit up by sunlight. The lunar landscape all around would, therefore, be bathed in what to _him_ would be "earthlight, " which ofcourse takes the place there of what _we_ call moonlight. If, then, werecollect how much greater in size the earth is than the moon, it shouldnot surprise us that this earthlight will be many times brighter thanmoonlight. It is considered, indeed, to be some twenty times brighter. It is thus not at all astonishing that we can see the dark portion ofthe moon illumined merely by sunlight reflected upon it from our earth. 

The ancients were greatly exercised in their minds to account for this"earthlight, " or "earthshine, " as it is also called. Posidonius (135-51B. C. ) tried to explain it by supposing that the moon was partiallytransparent, and that some sunlight consequently filtered through fromthe other side. It was not, however, until the fifteenth century thatthe correct solution was arrived at. 

[Illustration: One side of the moon only is ever presented to theearth. This side is here indicated by the letters S. F. E. (side facingearth). 

By placing the above positions in a row, we can see at once that themoon makes one complete rotation on her axis in exactly the same time asshe revolves around the earth. 

FIG. 15. --The Rotation of the Moon on her Axis. ]

Perhaps the most remarkable thing which one notices about the moon isthat she always turns the same side towards us, and so we never see herother side. One might be led from this to jump to the conclusion thatshe does not rotate upon an axis, as do the other bodies which we see;but, paradoxical as it may appear, the fact that she turns one facealways towards the earth, is actually a proof that she _does_ rotateupon an axis. The rotation, however, takes place with such slowness, that she turns round but once during the time in which she revolvesaround the earth (see Fig. 15). In order to understand the matterclearly, let the reader place an object in the centre of a room and walkaround it once, _keeping his face turned towards it the whole time_, While he is doing this, it is evident that he will face every one of thefour walls of the room in succession. Now in order to face each of thefour walls of a room in succession one would be obliged _to turn oneselfentirely round_. Therefore, during the act of walking round an objectwith his face turned directly towards it, a person at the same timeturns his body once entirely round. 

In the long, long past the moon must have turned round much faster thanthis. Her rate of rotation has no doubt been slowed down by the actionof some force. It will be recollected how, in the course of the previouschapter, we found that the tides were tending, though exceedinglygradually, to slow down the rotation of the earth upon its axis. But, onaccount of the earth's much greater mass, the force of gravitationexercised by it upon the surface of the moon is, of course, much morepowerful than that which the moon exercises upon the surface of theearth. The tendency to tidal action on the moon itself must, therefore, be much in excess of anything which we here experience. It is, inconsequence, probable that such a tidal drag, extending over a very longperiod of time, has resulted in slowing down the moon's rotation to itspresent rate. 

The fact that we never see but one side of the moon has given rise fromtime to time to fantastic speculations with regard to the other side. Some, indeed, have wished to imagine that our satellite is shaped likean egg, the more pointed end being directed away from us. We are here, of course, faced with a riddle, which is all the more tantalising fromits appearing for ever insoluble to men, chained as they are to theearth. However, it seems going too far to suppose that any abnormalconditions necessarily exist at the other side of the moon. As a matterof fact, indeed, small portions of that side are brought into our viewfrom time to time in consequence of slight irregularities in the moon'smovement; and these portions differ in no way from those which weordinarily see. On the whole, we obtain a view of about 60 per cent. Ofthe entire lunar surface; that is to say, a good deal more thanone-half. 

The actual diameter of the moon is about 2163 miles, which is somewhatmore than one-quarter the diameter of the earth. For a satellite, therefore, she seems very large compared with her primary, the earth;when we consider that Jupiter's greatest satellite, although nearlytwice as broad as our moon, has a diameter only one twenty-fifth that ofJupiter. Furthermore, the moon moves around the earth comparativelyslowly, making only about thirteen revolutions during the entire year. Seen from space, therefore, she would not give the impression of acircling body, as other satellites do. Her revolutions are, indeed, relatively so very slow that she would appear rather like a smallerplanet accompanying the earth in its orbit. In view of all this, someastronomers are inclined to regard the earth and moon rather as a"double planet" than as a system of planet and satellite. 

When the moon is full she attracts more attention perhaps than in any ofher other phases. The moon, in order to be full, must needs be in thatregion of the heavens exactly opposite to the sun. The sun _appears_ togo once entirely round the sky in the course of a year, and the moonperforms the same journey in the space of about a month. The moon, whenfull, having got half-way round this journey, occupies, therefore, thatregion of the sky which the sun itself will occupy half a year later. Thus in winter the full moon will be found roughly to occupy the sun'ssummer position in the sky, and in summer the sun's winter position. Ittherefore follows that the full moon in winter time is high up in theheavens, while in summer time it is low down. We thus get the greatestamount of full moonlight when it is the most needed. 

The great French astronomer, Laplace, being struck by the fact that the"lesser light" did not rule the night to anything like the same extentthat the "greater light" ruled the day, set to work to examine theconditions under which it might have been made to do so. The result ofhis speculations showed that if the moon were removed to such a distancethat she took a year instead of a month to revolve around the earth; andif she were started off in her orbit at full moon, she would alwayscontinue to remain full--a great advantage for us. Whewell, however, pointed out that in order to get the moon to move with the requisitedegree of slowness, she would have to revolve so far from the earth thatshe would only look one-sixteenth as large as she does at present, whichrather militates against the advantage Laplace had in mind! Finally, however, it was shown by M. Liouville, in 1845, that the position of a_perennial full moon_, such as Laplace dreamed of, would beunstable--that is to say, the body in question could not for long remainundisturbed in the situation suggested (see Fig. 16, p. 191). 

[Illustration: Various positions of Laplace's "Moon" with regard to theearth and sun during the course of a year. 

The same positions of Laplace's "Moon, " arranged around the earth, showthat it would make only one revolution in a year. 

FIG. 16. --Laplace's "Perennial Full Moon. "]

There is a well-known phenomenon called _harvest moon_, concerning thenature of which there seems to be much popular confusion. An idea infact appears to prevail among a good many people that the moon is aharvest moon when, at rising, it looks bigger and redder than usual. Such an appearance has, however, nothing at all to say to the matter;for the moon always _looks_ larger when low down in the sky, and, furthermore, it usually looks red in the later months of the year, whenthere is more mist and fog about than there is in summer. Whatastronomers actually term the harvest moon is, indeed, somethingentirely different from this. About the month of September the slant atwhich the full moon comes up from below the horizon happens to be suchthat, during several evenings together, she _rises almost at the samehour_, instead of some fifty minutes later, as is usually the case. Asthe harvest is being gathered in about that time, it has come to bepopularly considered that this is a provision of nature, according towhich the sunlight is, during several evenings, replaced without delayby more or less full-moonlight, in order that harvesters may continuetheir work straight on into the night, and not be obliged to break offafter sunset to wait until the moon rises. The same phenomenon is almostexactly repeated a month later, but by reason of the pursuits thencarried on it is known as the "hunter's moon. "

In this connection should be mentioned that curious phenomenon abovealluded to, and which seems to attract universal notice, namely, thatthe moon _looks much larger when near the horizon_--at its rising, forinstance, than when higher up in the sky. This seeming enlargement is, however, by no means confined to the moon. That the sun also looks muchlarger when low down in the sky than when high up, seems to strike eventhe most casual watcher of a sunset. The same kind of effect will, indeed, be noted if close attention be paid to the stars themselves. Aconstellation, for instance, appears more spread out when low down inthe sky than when high up. This enlargement of celestial objects when inthe neighbourhood of the horizon is, however, only _apparent_ and notreal. It must be entirely an _illusion_; for the most carefulmeasurements of the discs of the sun and of the moon fail to show thatthe bodies are any larger when near the horizon than when high up in thesky. In fact, if there be any difference in measurements with regard tothe moon, it will be found to be the other way round; for her disc, whencarefully measured, is actually the slightest degree _greater_ when_high_ in the sky, than when low down. The reason for this is that, onaccount of the rotundity of the earth's surface, she is a trifle nearerthe observer when overhead of him. 

This apparent enlargement of celestial objects, when low down in thesky, is granted on all sides to be an illusion; but although thequestion has been discussed with animation time out of mind, none of theexplanations proposed can be said to have received unreservedacceptance. The one which usually figures in text-books is that weunconsciously compare the sun and moon, when low down in the sky, withthe terrestrial objects in the same field of view, and are thereforeinclined to exaggerate the size of these orbs. Some persons, on theother hand, imagine the illusion to have its source in the structure ofthe human eye; while others, again, put it down to the atmosphere, maintaining that the celestial objects in question _loom_ large in thethickened air near the horizon, in the same way that they do when viewedthrough fog or mist. 

The writer[14] ventures, however, to think that the illusion has itsorigin in our notion of the shape of the celestial vault. One would beinclined, indeed, to suppose that this vault ought to appear to us asthe half of a hollow sphere; but he maintains that it does not soappear, as a consequence of the manner in which the eyes of men are setquite close together in their heads. If one looks, for instance, high upin the sky, the horizon cannot come within the field of view, and sothere is nothing to make one think that the expanse then gazed upon isother than quite _flat_--let us say like the ceiling of a room. But, asthe eyes are lowered, a portion of the _encircling_ horizon comesgradually into the field of view, and the region of the sky then gazedupon tends in consequence to assume a _hollowed-out_ form. From this itwould seem that our idea of the shape of the celestial vault is, that itis _flattened down over our heads and hollowed out all around in theneighbourhood of the horizon_ (see Fig. 17, p. 195). Now, as aconsequence of their very great distance, all the objects in the heavensnecessarily appear to us to move as if they were placed on thebackground of the vault; the result being that the mind is obliged toconceive them as expanded or contracted, in its unconscious attempts tomake them always fill their due proportion of space in the various partsof this abnormally shaped sky. 

From such considerations the writer concludes that the apparentenlargement in question is merely the natural consequence of the idea wehave of the shape of the celestial vault--an idea gradually built up inchildhood, to become later on what is called "second nature. " And insupport of this contention, he would point to the fact that theenlargement is not by any means confined to the sun and moon, but isevery whit as marked in the case of the constellations. To one who hasnot noticed this before, it is really quite a revelation to compare theappearance of one of the large constellations (Orion, for instance) whenhigh up in the sky and when low down. The widening apart of the variousstars composing the group, when in the latter position, is verynoticeable indeed. 

[Illustration: FIG. 17. --Illustrating the author's explanation of theapparent enlargement of celestial objects. ]

Further, if a person were to stand in the centre of a large dome, hewould be exactly situated as if he were beneath the vaulted heaven, andone would consequently expect him to suffer the same illusion as to theshape of the dome. Objects fixed upon its background would thereforeappear to him under the same conditions as objects in the sky, and theillusions as to their apparent enlargement should hold good here also. 

Some years ago a Belgian astronomer, M. Stroobant, in an investigationof the matter at issue, chanced to make a series of experiments underthe very conditions just detailed. To various portions of the innersurface of a large dome he attached pairs of electric lights; and onplacing himself at the centre of the building, he noticed that, in everycase, those pairs which were high up appeared closer together than thosewhich were low down! He does not, however, seem to have sought for thecause in the vaulted expanse. On the contrary, he attributed the effectto something connected with our upright stature, to some physiologicalreason which regularly makes us estimate objects as larger when in frontthan when overhead. 

In connection with this matter, it may be noted that it always appearsextremely difficult to estimate with the eye the exact height above thehorizon at which any object (say a star) happens to be. Even skilledobservers find themselves in error in attempting to do so. This seems tobear out the writer's contention that the form under which the celestialvault really appears to us is a peculiar one, and tends to give rise tofalse judgments. 

Before leaving this question, it should also be mentioned that nothingperhaps is more deceptive than the size which objects in the sky appearto present. The full moon looks so like a huge plate, that it astonishesone to find that a threepenny bit held at arm's length will a long waymore than cover its disc. 

[Illustration: PLATE VIII. THE MOON

From a photograph taken at the Paris Observatory by M. P. Puiseux. 

(Page 197)]

The moon is just too far off to allow us to see the actual detail onher surface with the naked eye. When thus viewed she merely displays apatchy appearance, [15] and the imaginary forms which her darker markingssuggest to the fancy are popularly expressed by the term "Man in theMoon. " An examination of her surface with very moderate optical aid is, however, quite a revelation, and the view we then get is not easilycomparable to what we see with the unaided eye. 

Even with an ordinary opera-glass, an observer will be able to note agood deal of detail upon the lunar disc. If it be his first observationof the kind, he cannot fail to be struck by the fact to which we havejust made allusion, namely, the great change which the moon appears toundergo when viewed with magnifying power. "Cain and his Dog, " the "Manin the Moon gathering sticks, " or whatever indeed his fancy was wont toconjure up from the lights and shades upon the shining surface, have nowcompletely disappeared; and he sees instead a silvery globe marked hereand there with extensive dark areas, and pitted all over withcrater-like formations (see Plate VIII. , p. 196). The dark areas retaineven to the present day their ancient name of "seas, " for Galileo andthe early telescopic observers believed them to be such, and they arestill catalogued under the mystic appellations given to them in the longago; as, for instance, "Sea of Showers, " "Bay of Rainbows, " "Lake ofDreams. "[16] The improved telescopes of later times showed, however, that they were not really seas (there is no water on the moon), butmerely areas of darker material. 

The crater-like formations above alluded to are the "lunar mountains. " Aperson examining the moon for the first time with telescopic aid, willperhaps not at once grasp the fact that his view of lunar mountains mustneeds be what is called a "bird's-eye" one, namely, a view from above, like that from a balloon and that he cannot, of course, expect to seethem from the side, as he does the mountains upon the earth. But once hehas realised this novel point of view, he will no doubt marvel at theformations which lie scattered as it were at his feet. The type of lunarmountain is indeed in striking contrast to the terrestrial type. On ourearth the range-formation is supreme; on the moon the crater-formationis the rule, and is so-called from analogy to our volcanoes. A typicallunar crater may be described as a circular wall, enclosing a centralplain, or "floor, " which is often much depressed below the level of thesurface outside. These so-called "craters, " or "ring-mountains, " as theyare also termed, are often of gigantic proportions. For instance, thecentral plain of one of them, known as Ptolemæus, [17] is about 115 milesacross, while that of Plato is about 60. The walls of craters often riseto great heights; which, in proportion to the small size of the moon, are very much in excess of our highest terrestrial elevations. Nevertheless, a person posted at the centre of one of the larger cratersmight be surprised to find that he could not see the encompassingcrater-walls, which would in every direction be below his horizon. Thiswould arise not alone from the great breadth of the crater itself, butalso from the fact that the curving of the moon's surface is very sharpcompared with that of our earth. 

[Illustration: PLATE IX. MAP OF THE MOON, SHOWING THE PRINCIPAL"CRATERS, " MOUNTAIN RANGES, AND "SEAS"

In this, as in the other plates of the Moon, the _South_ will be foundat the top of the picture; such being the view given by the ordinaryastronomical telescope, in which all objects are seen _inverted_. 

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We have mentioned Ptolemæus as among the very large craters, orring-mountains, on the moon. Its encompassing walls rise to nearly13, 000 feet, and it has the further distinction of being almost in thecentre of the lunar disc. There are, however, several others much wider, but they are by no means in such a conspicuous position. For instance, Schickard, close to the south-eastern border, is nearly 130 miles indiameter, and its wall rises in one point to over 10, 000 feet. Grimaldi, almost exactly at the east point, is nearly as large as Schickard. Another crater, Clavius, situated near the south point, is about 140miles across; while its neighbour Bailly--named after a famous Frenchastronomer of the eighteenth century--is 180, and the largest of thosewhich we can see (see Plate IX. , p. 198). 

Many of the lunar craters encroach upon one another; in fact there isnot really room for them all upon the visible hemisphere of the moon. About 30, 000 have been mapped; but this is only a small portion, foraccording to the American astronomer, Professor W. H. Pickering, thereare more than 200, 000 in all. 

Notwithstanding the fact that the crater is the type of mountainassociated in the mind with the moon, it must not be imagined that uponour satellite there are no mountains at all of the terrestrial type. There are indeed many isolated peaks, but strangely enough they arenearly always to be found in the centres of craters. Some of these peaksare of great altitude, that in the centre of the crater Copernicus beingover 11, 000 feet high. A few mountain ranges also exist; the best knownof which are styled, the Lunar Alps and Lunar Apennines (see Plate X. , p. 200). 

Since the _mass_ of the moon is only about one-eightieth that of theearth, it will be understood that the force of gravity which sheexercises is much less. It is calculated that, at her surface, this isonly about one-sixth of what we experience. A man transported to themoon would thus be able to jump _six times as high_ as he can here. Abuilding could therefore be six times as tall as upon our earth, withoutcausing any more strain upon its foundations. It should not, then, beany subject for wonder, that the highest peaks in the Lunar Apenninesattain to such heights as 22, 000 feet. Such a height, upon acomparatively small body like the moon, for her _volume_ is onlyone-fiftieth that of the earth, is relatively very much in excess of the29, 000 feet of Himalayan structure, Mount Everest, the boast of ourplanet, 8000 miles across!

High as are the Lunar Apennines, the highest peaks on the moon are yetnot found among them. There is, for instance, on the extreme southernedge of the lunar disc, a range known as the Leibnitz Mountains; severalpeaks of which rise to a height of nearly 30, 000 feet, one peak inparticular being said to attain to 36, 000 feet (see Plate IX. , p. 198). 

[Illustration: PLATE X. ONE OF THE MOST INTERESTING REGIONS ON THE MOON

We have here (see "Map, " Plate IX. , p. 198) the mountain ranges of theApennines, the Caucasus and the Alps; also the craters Plato, Aristotle, Eudoxus, Cassini, Aristillus, Autolycus, Archimedes and Linné. Thecrater Linné is the very bright spot in the dark area at the upper lefthand side of the picture. From a photograph taken at the ParisObservatory by M. M. Loewy and Puiseux. 

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But the reader will surely ask the question: "How is it possible todetermine the actual height of a lunar mountain, if one cannot go uponthe moon to measure it?" The answer is, that we can calculate its heightfrom noting the length of the shadow which it casts. Any one will allowthat the length of a shadow cast by the sun depends upon two things:firstly, upon the height of the object which causes the shadow, andsecondly, upon the elevation of the sun at the moment in the sky. Themost casual observer of nature upon our earth can scarcely have failedto notice that shadows are shortest at noonday, when the sun is at itshighest in the sky; and that they lengthen out as the sun declinestowards its setting. Here, then, we have the clue. To ascertain, therefore, the height of a lunar mountain, we have first to consider atwhat elevation the sun is at that moment above the horizon of the placewhere the mountain in question is situated. Then, having measured theactual length in miles of the shadow extended before us, all that isleft is to ask ourselves the question: "What height must an object bewhose shadow cast by the sun, when at that elevation in the sky, willextend to this length?"

There is no trace whatever of water upon the moon. The opinion, indeed, which seems generally held, is that water has never existed upon itssurface. Erosions, sedimentary deposits, and all those marks which pointto a former occupation by water are notably absent. 

Similarly there appears to be no atmosphere on the moon; or, at anyrate, such an excessively rare one, as to be quite inappreciable. Ofthis there are several proofs. For instance, in a solar eclipse themoon's disc always stands out quite clear-cut against that of the sun. Again during occultations, stars disappear behind the moon with asuddenness, which could not be the case were there any appreciableatmosphere. Lastly, we see no traces of twilight upon the lunar surface, nor any softening at the edges of shadows; both which effects would beapparent if there were an atmosphere. 

The moon's surface is rough and rocky, and displays no marks of the"weathering" that would necessarily follow, had it possessed anything ofan atmosphere in the past. This makes us rather inclined to doubt thatit ever had one at all. Supposing, however, that it did possess anatmosphere in the past, it is interesting to inquire what may havebecome of it. In the first place it might have gradually disappeared, inconsequence of the gases which composed it uniting chemically with thematerials of which the lunar body is constructed; or, again, itsconstituent gases may have escaped into space, in accordance with theprinciples of that kinetic theory of which we have already spoken. Thelatter solution seems, indeed, the most reasonable of the two, for theforce of gravity at the lunar surface appears too weak to hold down anyknown gases. This argument seems also to dispose of the question ofabsence of water; for Dr. George Johnstone Stoney, in a carefulinvestigation of the subject, has shown that the liquid in question, when in the form of vapour, will escape from a planet if its mass isless than _one-fourth_ that of our earth. And the mass of the moon isvery much less than this; indeed only the _one-eightieth_, as we havealready stated. 

In consequence of this lack of atmosphere, the condition of things uponthe moon will be in marked contrast to what we experience upon theearth. The atmosphere here performs a double service in shielding usfrom the direct rays of the sun, and in bottling the heat as aglass-house does. On the moon, however, the sun beats down in theday-time with a merciless force; but its rays are reflected away fromthe surface as quickly as they are received, and so the cold of thelunar night is excessive. It has been calculated that the daytemperature on the moon may, indeed, be as high as our boiling-point, while the night temperature may be more than twice as low as thegreatest cold known in our arctic regions. 

That a certain amount of solar heat is reflected to us from the moon isshown by the sharp drop in temperature which certain heat-measuringinstruments record when the moon becomes obscured in a lunar eclipse. The solar heat which is thus reflected to us by the moon is, however, onthe whole extremely small; more light and heat, indeed, reach us_direct_ from the sun in half a minute than we get by _reflection_ fromthe moon during the entire course of the year. 

With regard to the origin of the lunar craters there has been muchdiscussion. Some have considered them to be evidence of violent volcanicaction in the dim past; others, again, as the result of the impact ofmeteorites upon the lunar surface, when the moon was still in a plasticcondition; while a third theory holds that they were formed by thebursting of huge bubbles during the escape into space of gases from theinterior. The question is, indeed, a very difficult one. Thoughvolcanic action, such as would result in craters of the size ofPtolemæus, is hard for us to picture, and though the lone peaks whichadorn the centres of many craters have nothing reminiscent of them inour terrestrial volcanoes, nevertheless the volcanic theory seems toreceive more favour than the others. 

In addition to the craters there are two more features which demandnotice, namely, what are known as _rays_ and _rills_. The rays are long, light-coloured streaks which radiate from several of the large craters, and extend to a distance of some hundreds of miles. That they are meremarkings on the surface is proved by the fact that they cast no shadowsof any kind. One theory is, that they were originally great cracks whichhave been filled with lighter coloured material, welling up frombeneath. The rills, on the other hand, are actually fissures, about amile or so in width and about a quarter of a mile in depth. 

The rays are seen to the best advantage in connection with the cratersTycho and Copernicus (see Plate XI. , p. 204). In consequence of itsfairly forward position on the lunar disc, and of the remarkable systemof rays which issue from it like spokes from the axle of a wheel, Tychocommands especial attention. The late Rev. T. W. Webb, a famous observer, christened it, very happily, the "metropolitan crater of the moon. "

[Illustration: PLATE XI. THE MOON

The systems of rays from the craters Tycho, Copernicus and Kepler arewell shown here. From a photograph taken at the Paris Observatory byM. P. Puiseux. 

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A great deal of attention is, and has been, paid by certain astronomersto the moon, in the hope of finding out if any changes are actually inprogress at present upon her surface. Sir William Herschel, indeed, oncethought that he saw a lunar volcano in eruption, but this proved to bemerely the effect of the sunlight striking the top of the craterAristarchus, while the region around it was still in shadow--sunriseupon Aristarchus, in fact! No change of any real importance has, however, been noted, although it is suspected that some minoralterations have from time to time taken place. For instance, slightvariations of tint have been noticed in certain areas of the lunarsurface. Professor W. H. Pickering puts forward the conjecture that thesemay be caused by the growth and decay of some low form of vegetation, brought into existence by vapours of water, or carbonic acid gas, makingtheir way out from the interior through cracks near at hand. 

Again, during the last hundred years one small crater known as Linné(Linnæus), situated in the Mare Serenitatis (Sea of Serenity), hasappeared to undergo slight changes, and is even said to have beeninvisible for a while (see Plate X. , p. 200). It is, however, believedthat the changes in question may be due to the varying angles at whichthe sunlight falls upon the crater; for it is an understood fact thatthe irregularities of the moon's motion give us views of her surfacewhich always differ slightly. 

The suggestion has more than once been put forward that the surface ofthe moon is covered with a thick layer of ice. This is generallyconsidered improbable, and consequently the idea has received verylittle support. It first originated with the late Mr. S. E. Peal, anEnglish observer of the moon, and has recently been resuscitated by theGerman observer, Herr Fauth. 

The most unfavourable time for telescopic study of the moon is when sheis full. The sunlight is then falling directly upon her visiblehemisphere, and so the mountains cast no shadows. We thus do not getthat impression of hill and hollow which is so very noticeable in theother phases. 

The first map of the moon was constructed by Galileo. Tobias Mayerpublished another in 1775; while during the nineteenth century greatlyimproved ones were made by Beer and Mädler, Schmidt, Neison and others. In 1903, Professor W. H. Pickering brought out a complete photographiclunar atlas; and a similar publication has recently appeared, the workof MM. Loewy and Puiseux of the Observatory of Paris. 

The so-called "seas" of the moon are, as we have seen, merely darkareas, and there appears to be no proof that they were ever occupied byany liquid. They are for the most part found in the _northern_ portionof the moon; a striking contrast to our seas and oceans, which take upso much of the _southern_ hemisphere of the earth. 

There are many erroneous ideas popularly held with regard to certaininfluences which the moon is supposed to exercise upon the earth. Forinstance, a change in the weather is widely believed to depend upon achange in the moon. But the word "change" as here used is meaningless, for the moon is continually changing her phase during the whole of hermonthly round. Besides, the moon is visible over a great portion of theearth _at the same moment_, and certainly all the places from which itcan then be seen do not get the same weather! Further, carefulobservations, and records extending over the past one hundred years andmore, fail to show any reliable connection between the phases of themoon and the condition of the weather. 

It has been stated, on very good authority, that no telescope ever showsthe surface of the moon as clearly as we could see it with the naked eyewere it only 240 miles distant from us. 

Supposing, then, that we were able to approach our satellite, and viewit without optical aid at such comparatively close quarters, it isinteresting to consider what would be the smallest detail which our eyecould take in. The question of the limit of what can be appreciated withthe naked eye is somewhat uncertain, but it appears safe to say that ata distance of 240 miles the _minutest speck_ visible would have to be_at least_ some 60 yards across. 

Atmosphere and liquid both wanting, the lunar surface must be the seatof an eternal calm; where no sound breaks the stillness and wherechange, as we know it, does not exist. The sun beats down upon the aridrocks, and inky shadows lie athwart the valleys. There is no mellowingof the harsh contrasts. 

We cannot indeed absolutely affirm that Life has no place at all uponthis airless and waterless globe, since we know not under what strangeconditions it may manifest its presence; and our most powerfultelescopes, besides, do not bring the lunar surface sufficiently near tous to disprove the existence there of even such large creatures asdisport themselves upon our planet. Still, we find it hard to ridourselves of the feeling that we are in the presence of a dead world. Onshe swings around the earth month after month, with one face everturned towards us, leaving a certain mystery to hang around that hiddenside, the greater part of which men can never hope to see. The rotationof the moon upon her axis--the lunar day--has become, as we have seen, equal to her revolution around the earth. An epoch may likewiseeventually be reached in the history of our own planet, when the lengthof the terrestrial day has been so slowed down by tidal friction that itwill be equal to the year. Then will the earth revolve around thecentral orb, with one side plunged in eternal night and the other ineternal sunshine. But such a vista need not immediately distress us. Itis millions of years forward in time. 

[14] _Journal of the British Astronomical Association_, vol. X. (1899-1900), Nos. 1 and 3. 

[15] Certain of the ancient Greeks thought the markings on the moon tobe merely the reflection of the seas and lands of our earth, as in abadly polished mirror. 

[16] Mare Imbrium, Sinus Iridum, Lacus Somniorum. 

[17] The lunar craters have, as a rule, received their names fromcelebrated persons, usually men of science. This system of nomenclaturewas originated by Riccioli, in 1651. 

CHAPTER XVII

THE SUPERIOR PLANETS

Having, in a previous chapter, noted the various aspects which aninferior planet presents to our view, in consequence of its orbit beingnearer to the sun than the orbit of the earth, it will be well here toconsider in the same way the case of a superior planet, and to markcarefully the difference. 

To begin with, it should be quite evident that we cannot ever have atransit of a superior planet. The orbit of such a body being entirely_outside_ that of the earth, the body itself can, of course, never passbetween us and the sun. 

A superior planet will be at its greatest distance from us when on thefar side of the sun. It is said then to be in _conjunction_. As it comesround in its orbit it eventually passes, so to speak, at the _back_ ofus. It is then at its nearest, or in _opposition_, as this istechnically termed, and therefore in the most favourable position fortelescopic observation of its surface. Being, besides, seen by us atthat time in the direction of the heavens exactly opposite to where thesun is, it will thus at midnight be high up in the south side of thesky, a further advantage to the observer. 

Last of all, a superior planet cannot show crescent shapes like aninterior; for whether it be on the far side of the sun, or behind us, or again to our right or left, the sunlight must needs appear to fallmore or less full upon its face. 

THE PLANETOID EROS

The nearest to us of the superior planets is the tiny body, Eros, which, as has been already stated, was discovered so late as the year 1898. Inpoint of view, however, of its small size, it can hardly be consideredas a true planet, and the name "planetoid" seems much more appropriateto it. 

Eros was not discovered, like Uranus, in the course of telescopicexamination of the heavens, nor yet, like Neptune, as the direct resultof difficult calculations, but was revealed by the impress of its lightupon a photographic plate, which had been exposed for some length oftime to the starry sky. Since many of the more recent additions to theasteroids have been discovered in the same manner, we shall havesomewhat more to say about this special employment of photography whenwe come to deal with those bodies later on. 

The path of Eros around the sun is so very elliptical, or, to use theexact technical term, so very "eccentric, " that the planetoid does notkeep all the time entirely in the space between our orbit and that ofMars, which latter happens to be the next body in the order of planetarysuccession outwards. In portions of its journey Eros, indeed, actuallygoes outside the Martian orbit. The paths of the planetoid and of Marsare, however, _not upon the same plane_, so the bodies always pass clearof each other, and there is thus as little chance of their dashingtogether as there would be of trains which run across a bridge at anupper level, colliding with those which pass beneath it at a lowerlevel. 

When Eros is in opposition, it comes within about 13-1/2 million milesof our earth, and, after the moon, is therefore by a long way ournearest neighbour in space. It is, however, extremely small, not more, perhaps, than twenty miles in diameter, and is subject to markedvariations in brightness, which do not appear up to the present to meetwith a satisfactory explanation. But, insignificant as is this littlebody, it is of great importance to astronomy; for it happens to furnishthe best method known of calculating the sun's distance from ourearth--a method which Galle, in 1872, and Sir David Gill, in 1877, suggested that asteroids might be employed for, and which has inconsequence supplanted the old one founded upon transits of Venus. Thesun's distance is now an ascertained fact to within 100, 000 miles, orless than half the distance of the moon. 

THE PLANET MARS

We next come to the planet Mars. This body rotates in a period ofslightly more than twenty-four hours. The inclination, or slant, of itsaxis is about the same as that of the earth, so that, putting aside itsgreater distance from the sun, the variations of season which itexperiences ought to be very much like ours. 

The first marking detected upon Mars was the notable one called theSyrtis Major, also known, on account of its shape, as the Hour-GlassSea. This observation was made by the famous Huyghens in 1659; and, fromthe movement of the marking in question across the disc, he inferredthat the planet rotated on its axis in a period of about twenty-fourhours. 

There appears to be very little atmosphere upon Mars, the result beingthat we almost always obtain a clear view of the detail on its surface. Indeed, it is only to be expected from the kinetic theory that Marscould not retain much of an atmosphere, as the force of gravity at itssurface is less than one-half of what we experience upon the earth. Itshould here be mentioned that recent researches with the spectroscopeseem to show that, whatever atmosphere there may be upon Mars, itsdensity at the surface of the planet cannot be more than the one-fourthpart of the density of the air at the surface of the earth. ProfessorLowell, indeed, thinks it may be more rarefied than that upon ourhighest mountain-tops. 

Seen with the naked eye, Mars appears of a red colour. Viewed in thetelescope, its surface is found to be in general of a ruddy hue, variedhere and there with darker patches of a bluish-green colour. Thesemarkings are permanent, and were supposed by the early telescopicobservers to imply a distribution of the planet's surface into land andwater, the ruddy portions being considered as continental areas (perhapssandy deserts), and the bluish-green as seas. The similarity to ourearth thus suggested was further heightened by the fact that broad whitecaps, situated at the poles, were seen to vary with the planet'sseasons, diminishing greatly in extent during the Martian summer (thesouthern cap in 1894 even disappearing altogether), and developing againin the Martian winter. [18] Readers of Oliver Wendell Holmes will nodoubt recollect that poet's striking lines:--

"The snows that glittered on the disc of MarsHave melted, and the planet's fiery orbRolls in the crimson summer of its year. "

A state of things so strongly analogous to what we experience here, naturally fired the imaginations of men, and caused them to look on Marsas a world like ours, only upon a much smaller scale. Being smaller, itwas concluded to have cooled quicker, and to be now long past its prime;and its "inhabitants" were, therefore, pictured as at a later stage ofdevelopment than the inhabitants of our earth. 

Notwithstanding the strong temptation to assume that the whiteness ofthe Martian polar caps is due to fallen snow, such a solution is, however, by no means so simple as it looks. The deposition of water inthe form of snow, or even of hoar frost, would at least imply that theatmosphere of Mars should now and then display traces of aqueous vapour, which it does not appear to do. [19] It has, indeed, been suggested thatthe whiteness may not after all be due to this cause, but to carbonicacid gas (carbon dioxide), which is known to freeze at a _very low_temperature. The suggestion is plainly based upon the assumption that, as Mars is so much further from the sun than we are, it would receivemuch less heat, and that the little thus received would be quicklyradiated away into space through lack of atmosphere to bottle it in. 

We now come to those well-known markings, popularly known as the"canals" of Mars, which have been the subject of so much discussionsince their discovery thirty years ago. 

It was, in fact, in the year 1877, when Mars was in opposition, and thusat its nearest to us, that the famous Italian astronomer, Schiaparelli, announced to the world that he had found that the ruddy areas, thoughtto be continents, were intersected by a network of straight dark lines. These lines, he reported, appeared in many cases to be of great length, so long, indeed, as several thousands of miles, and from about twenty tosixty miles in width. He christened the lines _channels_, the Italianword for which, "canali, " was unfortunately translated into English as"canals. " The analogy, thus accidentally suggested, gave rise to theidea that they might be actual waterways. [20]

In the winter of 1881-1882, when Mars was again in opposition, Schiaparelli further announced that he had found some of these linesdoubled; that is to say, certain of them were accompanied by similarlines running exactly parallel at no great distance away. There was atfirst a good deal of scepticism on the subject of Schiaparelli'sdiscoveries, but gradually other observers found themselves seeing boththe lines and their doublings. We have in this a good example of acurious circumstance in astronomical observation, namely, the fact thatwhen fine detail has once been noted by a competent observer, it is notlong before other observers see the same detail with ease. 

An immense amount of close attention has been paid to the planet Marsduring recent years by the American observer, Professor Percival Lowell, at his famous observatory, 7300 feet above the sea, near the town ofFlagstaff, Arizona, U. S. A. His observations have not, like those of mostastronomers, been confined merely to "oppositions, " but he hassystematically kept the planet in view, so far as possible, since theyear 1894. 

The instrumental equipment of his observatory is of the very best, andthe "seeing" at Flagstaff is described as excellent. In support of thelatter statement, Mr. Lampland, of the Lowell Observatory, maintainsthat the faintest stars shown on charts made at the Lick Observatorywith the 36-inch telescope there, are _perfectly visible_ with the24-inch telescope at Flagstaff. 

Professor Lowell is, indeed, generally at issue with the other observersof Mars. He finds the canals extremely narrow and sharply defined, andhe attributes the blurred and hazy appearance, which they have presentedto other astronomers, to the unsteady and imperfect atmosphericconditions in which their observations have been made. He assigns to thethinnest a width of two or three miles, and from fifteen to twenty tothe larger. Relatively to their width, however, he finds their lengthenormous. Many of them are 2000 miles long, while one is even as muchas 3540. Such lengths as these are very great in comparison with thesmallness of the planet. He considers that the canals stand in somepeculiar relation to the polar cap, for they crowd together in itsneighbourhood. In place, too, of ill-defined condensations, he seessharp black spots where the canals meet and intersect, and to these hegives the name of "Oases. " He further lays particular stress upon a darkband of a blue tint, which is always seen closely to surround the edgesof the polar caps all the time that they are disappearing; and this hetakes to be a proof that the white material is something which actually_melts_. Of all substances which we know, water alone, he affirms, wouldact in such a manner. 

The question of melting at all may seem strange in a planet which issituated so far from the sun, and possesses such a rarefied atmosphere. But Professor Lowell considers that this very thinness of the atmosphereallows the direct solar rays to fall with great intensity upon theplanet's surface, and that this heating effect is accentuated by thegreat length of the Martian summer. In consequence he concludes that, although the general climate of Mars is decidedly cold, it is above thefreezing point of water. 

The observations at Flagstaff appear to do away with the old idea thatthe darkish areas are seas, for numerous lines belonging to theso-called "canal system" are seen to traverse them. Again, there is nostar-like image of the sun reflected from them, as there would be, ofcourse, from the surface of a great sheet of water. Lastly, they areobserved to vary in tone and colour with the changing Martian seasons, the blue-green changing into ochre, and later on back again intoblue-green. Professor Lowell regards these areas as great tracts ofvegetation, which are brought into activity as the liquid reaches themfrom the melting snows. 

[Illustration: PLATE XII. A MAP OF THE PLANET MARS

We see here the Syrtis Major (or "Hour-Glass Sea"), the polar caps, several "oases, " and a large number of "canals, " some of which aredouble. The South is at the top of the picture, in accordance with the_inverted_ view given by an astronomical telescope. From a drawing byProfessor Percival Lowell. 

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With respect to the canals, the Lowell observations further inform usthat these are invisible during the Martian winter, but begin to appearin the spring when the polar cap is disappearing. Professor Lowell, therefore, inclines to the view that in the middle of the so-calledcanals there exist actual waterways which serve the purposes ofirrigation, and that what we see is not the waterways themselves, forthey are too narrow, but the fringe of vegetation which springs up alongthe banks as the liquid is borne through them from the melting of thepolar snows. He supports this by his observation that the canals beginto appear in the neighbourhood of the polar caps, and gradually grow, asit were, in the direction of the planet's equator. 

It is the idea of life on Mars which has given this planet such afascination in the eyes of men. A great deal of nonsense has, however, been written in newspapers upon the subject, and many persons have thusbeen led to think that we have obtained some actual evidence of theexistence of living beings upon Mars. It must be clearly understood, however, that Professor Lowell's advocacy of the existence of life uponthat planet is by no means of this wild order. At the best he merelyindulges in such theories as his remarkable observations naturally callforth. His views are as follows:--He considers that the planet hasreached a time when "water" has become so scarce that the "inhabitants"are obliged to employ their utmost skill to make their scanty supplysuffice for purposes of irrigation. The changes of tone and colour uponthe Martian surface, as the irrigation produces its effects, are similarto what a telescopic observer--say, upon Venus--would notice on ourearth when the harvest ripens over huge tracts of country; that is, ofcourse, if the earth's atmosphere allowed a clear view of theterrestrial surface--a very doubtful point indeed. Professor Lowellthinks that the perfect straightness of the lines, and the geometricalmanner in which they are arranged, are clear evidences of artificiality. On a globe, too, there is plainly no reason why the liquid which resultsfrom the melting of the polar caps should trend at all in the directionof the equator. Upon our earth, for instance, the transference of water, as in rivers, merely follows the slope of the ground, and nothing else. The Lowell observations show, however, that the Martian liquid isapparently carried from one pole towards the equator, and then past itto the other pole, where it once more freezes, only to melt again in dueseason, and to reverse the process towards and across the equator asbefore. Professor Lowell therefore holds, and it seems a strong point infavour of his theory, that the liquid must, in some artificial manner, as by pumping, for instance, be _helped_ in its passage across thesurface of the planet. 

A number of attempts have been made to explain the _doubling_ of thecanals merely as effects of refraction or reflection; and it has evenbeen suggested that it may arise from the telescope not being accuratelyfocussed. 

The actual doubling of the canals once having been doubted, it was aneasy step to the casting of doubt on the reality of the canalsthemselves. The idea, indeed, was put forward that the human eye, indealing with detail so very close to the limit of visibility, mayunconsciously treat as an actual line several point-like markings whichmerely happen to lie in a line. In order to test this theory, experiments were carried out in 1902 by Mr. E. W. Maunder of GreenwichObservatory, and Mr. J. E. Evans of the Royal Hospital School atGreenwich, in which certain schoolboys were set to make drawings of awhite disc with some faint markings upon it. The boys were placed atvarious distances from the disc in question; and it was found that thedrawings made by those who were just too far off to see distinctly, boreout the above theory in a remarkable manner. Recently, however, theplausibility of the _illusion_ view has been shaken by photographs ofMars taken during the opposition of 1905 by Mr. Lampland at the LowellObservatory, in which a number of the more prominent canals come out asstraight dark lines. Further still, in some photographs made there quitelately, several canals are said to appear visibly double. 

Following up the idea alluded to in Chapter XVI. , that the moon may becovered with a layer of ice, Mr. W. T. Lynn has recently suggested thatthis may be the case on Mars; and that, at certain seasons, the watermay break through along definite lines, and even along lines parallel tothese. This, he maintains, would account for the canals becominggradually visible across the disc, without the necessity of ProfessorLowell's "pumping" theory. 

And now for the views of Professor Lowell himself with regard to thedoubling of the canals. From his observations, he considers that nopairs of railway lines could apparently be laid down with greaterparallelism. He draws attention to the fact that the doubling does nottake place by any means in every canal; indeed, out of 400 canals seenat Flagstaff, only fifty-one--or, roughly, one-eighth--have at any timebeen seen double. He lays great stress upon this, which he considerspoints strongly against the duplication being an optical phenomenon. Hefinds that the distance separating pairs of canals is much less in somedoubles than in others, and varies on the whole from 75 to 200 miles. According to him, the double canals appear to be confined to within 40degrees of the equator: or, to quote his own words, they are "anequatorial feature of the planet, confined to the tropic and temperatebelts. " Finally, he points out that they seem to _avoid_ the blue-greenareas. But, strangely enough, Professor Lowell does not so far attemptto fit in the doubling with his body of theory. He makes the obviousremark that they may be "channels and return channels, " and with that heleaves us. 

The conclusions of Professor Lowell have recently been subjected tostrenuous criticism by Professor W. H. Pickering and Dr. Alfred RusselWallace. It was Professor Pickering who discovered the "oases, " and whooriginated the idea that we did not see the so-called "canals"themselves, but only the growth of vegetation along their borders. Heholds that the oases are craterlets, and that the canals are crackswhich radiate from them, as do the rifts and streaks from craters uponthe moon. He goes on to suggest that vapours of water, or of carbonicacid gas, escaping from the interior, find their way out through thesecracks, and promote the growth of a low form of vegetation on eitherside of them. In support of this view he draws attention to theexistence of long "steam-cracks, " bordered by vegetation, in the desertsof the highly volcanic island of Hawaii. We have already seen, in anearlier chapter, how he has applied this idea to the explanation ofcertain changes which are suspected to be taking place upon the moon. 

In dealing with the Lowell canal system, Professor Pickering points outthat under such a slight atmospheric pressure as exists on Mars, theevaporation of the polar caps--supposing them to be formed ofsnow--would take place with such extraordinary rapidity that theresulting water could never be made to travel along open channels, butthat a system of gigantic tubes or water-mains would have to beemployed!

As will be gathered from his theories regarding vegetation, ProfessorPickering does not deny the existence of a form of life upon Mars. Buthe will not hear of civilisation, or of anything even approaching it. Hethinks, however, that as Mars is intermediate physically between themoon and earth, the form of life which it supports may be higher thanthat on the moon and lower than that on the earth. 

In a small book published in the latter part of 1907, and entitled _IsMars Habitable?_ Dr. Alfred Russel Wallace sets himself, among otherthings, to combat the idea of a comparatively high temperature, such asProfessor Lowell has allotted to Mars. He shows the immense servicewhich the water-vapour in our atmosphere exercises, through keeping thesolar heat from escaping from the earth's surface. He then drawsattention to the fact that there is no spectroscopic evidence ofwater-vapour on Mars[21]; and points out that its absence is only to beexpected, as Dr. George Johnstone Stoney has shown that it will escapefrom a body whose mass is less than one-quarter the mass of the earth. The mass of Mars is, in fact, much less than this, _i. E. _ onlyone-ninth. Dr. Wallace considers, therefore, that the temperature ofMars ought to be extremely low, unless the constitution of itsatmosphere is very different from ours. With regard to the latterstatement, it should be mentioned that the Swedish physicist, Arrhenius, has recently shown that the carbonic acid gas in our atmosphere has animportant influence upon climate. The amount of it in our air is, as wehave seen, extremely small; but Arrhenius shows that, if it weredoubled, the temperature would be more uniform and much higher. We thussee how futile it is, with our present knowledge, to dogmatise on theexistence or non-existence of life in other celestial orbs. 

As to the canals Dr. Wallace puts forward a theory of his own. Hecontends that after Mars had cooled to a state of solidity, a greatswarm of meteorites and small asteroids fell in upon it, with the resultthat a thin molten layer was formed all over the planet. As this layercooled, the imprisoned gases escaped, producing vents or craterlets; andas it attempted to contract further upon the solid interior, it split infissures radiating from points of weakness, such, for instance, as thecraterlets. And he goes on to suggest that the two tiny Martiansatellites, with which we shall deal next, are the last survivors of hishypothetical swarm. Finally, with regard to the habitability of Mars, Dr. Wallace not only denies it, but asserts that the planet is"absolutely uninhabitable. "

For a long time it was supposed that Mars did not possess anysatellites. In 1877, however, during that famous opposition in whichSchiaparelli first saw the canals, two tiny satellites were discoveredat the Washington Observatory by an American astronomer, Professor AsaphHall. These satellites are so minute, and so near to the planet, thatthey can only be seen with very large telescopes; and even then thebright disc of the planet must be shielded off. They have beenchristened Phobos and Deimos (Fear and Dread); these being the names ofthe two subordinate deities who, according to Homer, attended upon Mars, the god of war. 

It is impossible to measure the exact sizes of these satellites, as theyare too small to show any discs, but an estimate has been formed fromtheir brightness. The diameter of Phobos was at first thought to be sixmiles, and that of Deimos, seven. As later estimates, however, considerably exceed this, it will, perhaps, be not far from the truth tostate that they are each roughly about the size of the planetoid Eros. Phobos revolves around Mars in about 7-1/2 hours, at a distance of aboutonly 4000 miles from the planet's surface, and Deimos in about 30 hours, at a distance of about 12, 000 miles. As Mars rotates on its axis inabout 24 hours, it will be seen that Phobos makes more than threerevolutions while the planet is rotating once--a very interestingcondition of things. 

A strange foreshadowing of the discovery of the satellites of Mars willbe familiar to readers of _Gulliver's Travels_. According to DeanSwift's hero, the astronomers on the Flying Island of Laputa had foundtwo tiny satellites to Mars, one of which revolved around the planet inten hours. The correctness of this guess is extraordinarily close, though at best it is, of course, nothing more than a pure coincidence. 

It need not be at all surprising that much uncertainty should exist withregard to the actual condition of the surface of Mars. The circumstancesin which we are able to see that planet at the best are, indeed, hardlysufficient to warrant us in propounding any hard and fast theories. Oneof the most experienced of living observers, the American astronomer, Professor E. E. Barnard, considers that the view we get of Mars with thebest telescope may be fairly compared with our naked eye view of themoon. Since we have seen that a view with quite a small telescopeentirely alters our original idea of the lunar surface, a slightmagnification revealing features of whose existence we had notpreviously the slightest conception, it does not seem too much to saythat a further improvement in optical power might entirely subvert thepresent notions with regard to the Martian canals. Therefore, until weget a still nearer view of these strange markings, it seems somewhatfutile to theorise. The lines which we see are perhaps, indeed, aforeshortened and all too dim view of some type of formation entirelynovel to us, and possibly peculiar to Mars. Differences of gravity andother conditions, such as obtain upon different planets, may perhapsproduce very diverse results. The earth, the moon, and Mars differgreatly from one another in size, gravitation, and other suchcharacteristics. Mountain-ranges so far appear typical of our globe, andring-mountains typical of the moon. May not the so-called "canals" bemerely some special formation peculiar to Mars, though quite a naturalresult of its particular conditions and of its past history?

THE ASTEROIDS (OR MINOR PLANETS)

We now come to that belt of small planets which are known by the name ofasteroids. In the general survey of the solar system given in ChapterII. , we saw how it was long ago noticed that the distances of theplanetary orbits from the sun would have presented a marked appearanceof orderly sequence, were it not for a gap between the orbits of Marsand Jupiter where no large planet was known to circulate. The suspicionthus aroused that some planet might, after all, be moving in thisseemingly empty space, gave rise to the gradual discovery of a greatnumber of small bodies; the largest of which, Ceres, is less than 500miles in diameter. Up to the present day some 600 of these bodies havebeen discovered; the four leading ones, in order of size, being namedCeres, Pallas, Juno, and Vesta. All the asteroids are invisible to thenaked eye, with the exception of Vesta, which, though by no means thelargest, happens to be the brightest. It is, however, only just visibleto the eye under favourable conditions. No trace of an atmosphere hasbeen noted upon any of the asteroids, but such a state of things is onlyto be expected from the kinetic theory. 

For a good many years the discoveries of asteroids were made by means ofthe telescope. When, in the course of searching the heavens, an objectwas noticed which did not appear upon any of the recognised star charts, it was kept under observation for several nights to see whether itchanged its place in the sky. Since asteroids move around the sun inorbits, just as planets do, they, of course, quickly reveal themselvesby their change of position against the starry background. 

The year 1891 started a new era in the discovery of asteroids. Itoccurred to the Heidelberg observer, Dr. Max Wolf, one of the mostfamous of the hunters of these tiny planets, that photography might beemployed in the quest with success. This photographic method, to whichallusion has already been made in dealing with Eros, is an extremelysimple one. If a photograph of a portion of the heavens be taken throughan "equatorial"--that is, a telescope, moving by machinery, so as tokeep the stars, at which it is pointed, always exactly in the field ofview during their apparent movement across the sky--the images of thesestars will naturally come out in the photograph as sharply definedpoints. If, however, there happens to be an asteroid, or other planetarybody, in the same field of view, its image will come out as a shortwhite streak; because the body has a comparatively rapid motion of itsown, and will, during the period of exposure, have moved sufficientlyagainst the background of the stars to leave a short trail, instead of adot, upon the photographic plate. By this method Wolf himself hassucceeded in discovering more than a hundred asteroids (see Plate XIII. , p. 226). It was, indeed, a little streak of this kind, appearing upon aphotograph taken by the astronomer Witt, at Berlin, in 1898, which firstinformed the world of the existence of Eros. 

[Illustration: PLATE XIII. MINOR PLANET TRAILS

Two trails of minor planets (asteroids) imprinted _at the same time_upon one photographic plate. In the white streak on the left-hand sideof the picture we witness the _discovery_ of a new minor planet. Thestreak on the right was made by a body already known--the minor planet"Fiducia. " This photograph was taken by Dr. Max Wolf, at Heidelberg, onthe 4th of November, 1901, with the aid of a 16-inch telescope. The timeof exposure was two hours. 

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It has been calculated that the total mass of the asteroids must bemuch less than one-quarter that of the earth. They circulate as a rulewithin a space of some 30, 000, 000 miles in breadth, lying about midwaybetween the paths of Mars and Jupiter. Two or three, however, of themost recently discovered of these small bodies have been found to passquite close to Jupiter. The orbits of the asteroids are by no means inthe one plane, that of Pallas being the most inclined to the plane ofthe earth's orbit. It is actually three times as much inclined as thatof Eros. 

Two notable theories have been put forward to account for the origin ofthe asteroids. The first is that of the celebrated German astronomer, Olbers, who was the discoverer of Pallas and Vesta. He suggested thatthey were the fragments of an exploded planet. This theory was for atime generally accepted, but has now been abandoned in consequence ofcertain definite objections. The most important of these objections isthat, in accordance with the theory of gravitation, the orbits of suchfragments would all have to pass through the place where the explosionoriginally occurred. But the wide area over which the asteroids arespread points rather against the notion that they all set out originallyfrom one particular spot. Another objection is that it does not appearpossible that, within a planet already formed, forces could originatesufficiently powerful to tear the body asunder. 

The second theory is that for some reason a planet here failed in themaking. Possibly the powerful gravitational action of the huge body ofJupiter hard by, disturbed this region so much that the matterdistributed through it was never able to collect itself into a singlemass. 

[18] Sir William Herschel was the first to note these polar changes. 

[19] Quite recently, however, Professor Lowell has announced that hisobserver, Mr. E. C. Slipher, finds with the spectroscope faint traces ofwater vapour in the Martian atmosphere. 

[20] In a somewhat similar manner the term "crater, " as applied to thering-mountain formation on the moon, has evidently given a bias infavour of the volcanic theory as an explanation of that peculiarstructure. 

[21] Mr. Slipher's results (see note 2, page 213) were not then known. 

CHAPTER XVIII

THE SUPERIOR PLANETS--_continued_

The planets, so far, have been divided into inferior and superior. Sucha division, however, refers merely to the situation of their orbits withregard to that of our earth. There is, indeed, another manner in whichthey are often classed, namely, according to size. On this principlethey are divided into two groups; one group called the _TerrestrialPlanets_, or those which have characteristics like our earth, and theother called the _Major Planets_, because they are all of very greatsize. The terrestrial planets are Mercury, Venus, the earth, and Mars. The major planets are the remainder, namely, Jupiter, Saturn, Uranus, and Neptune. As the earth's orbit is the boundary which separates theinferior from the superior planets, so does the asteroidal belt dividethe terrestrial from the major planets. We found the division intoinferior and superior useful for emphasising the marked difference inaspect which those two classes present as seen from our earth; theinferior planets showing phases like the moon when viewed in thetelescope, whereas the superior planets do not. But the division intoterrestrial and major planets is the more far-reaching classification ofthe two, for it includes the whole number of planets, whereas the otherarrangement necessarily excludes the earth. The members of each ofthese classes have many definite characteristics in common. Theterrestrial planets are all of them relatively small in size, comparatively near together, and have few or no satellites. They are, moreover, rather dense in structure. The major planets, on the otherhand, are huge bodies, circulating at great distances from each other, and are, as a rule, provided with a number of satellites. With respectto structure, they may be fairly described as being loosely puttogether. Further, the markings on the surfaces of the terrestrialplanets are permanent, whereas those on the major planets arecontinually shifting. 

THE PLANET JUPITER

Jupiter is the greatest of the major planets. It has been justly calledthe "Giant" planet, for both in volume and in mass it exceeds all theother planets put together. When seen through the telescope it exhibitsa surface plentifully covered with markings, the most remarkable being aseries of broad parallel belts. The chief belt lies in the central partsof the planet, and is at present about 10, 000 miles wide. It is boundedon either side by a reddish brown belt of about the same width. Brightspots also appear upon the surface of the planet, last for a while, andthen disappear. The most notable of the latter is one known as the"Great Red Spot. " This is situated a little beneath the southern redbelt, and appeared for the first time about thirty years ago. It hasundergone a good many changes in colour and brightness, and is stillfaintly visible. This spot is the most permanent marking which has yetbeen seen upon Jupiter. In general, the markings change so often thatthe surface which we see is evidently not solid, but of a fleetingnature akin to cloud (see Plate XIV. , p. 230). 

[Illustration: PLATE XIV. THE PLANET JUPITER

The Giant Planet as seen at 11. 30 p. M. , on the 11th of January, 1908, with a 12-1/2-inch reflecting telescope. The extensive oval marking inthe upper portion of the disc is the "Great Red Spot. " The South is atthe top of the picture, the view being the _inverted_ one given by anastronomical telescope. From a drawing by the Rev. Theodore E. R. Phillips, M. A. , F. R. A. S. , Director of the Jupiter Section of the BritishAstronomical Association. 

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Observations of Jupiter's markings show that on an average the planetrotates on its axis in a period of about 9 hours 54 minutes. The mentionhere of _an average_ with reference to the rotation will, no doubt, recall to the reader's mind the similar case of the sun, the differentportions of which rotate with different velocities. The parts of Jupiterwhich move quickest take 9 hours 50 minutes to go round, while thosewhich move slowest take 9 hours 57 minutes. The middle portions rotatethe fastest, a phenomenon which the reader will recollect was also thecase with regard to the sun. 

Jupiter is a very loosely packed body. Its density is on an average onlyabout 1-1/2 times that of water, or about one-fourth the density of theearth; but its bulk is so great that the gravitation at that surfacewhich we see is about 2-1/2 times what it is on the surface of theearth. In accordance, therefore, with the kinetic theory, we may expectthe planet to retain an extensive layer of gases around it; and this isconfirmed by the spectroscope, which gives evidence of the presence of adense atmosphere. 

All things considered, it may be safely inferred that the interior ofJupiter is very hot, and that what we call its surface is not the actualbody of the planet, but a voluminous layer of clouds and vapours drivenupwards from the heated mass underneath. The planet was indeed formerlythought to be self-luminous; but this can hardly be the case, for thoseportions of the surface which happen to lie at any moment in theshadows cast by the satellites appear to be quite black. Again, when asatellite passes into the great shadow cast by the planet it becomesentirely invisible, which would not be the case did the planet emit anyperceptible light of its own. 

In its revolutions around the sun, Jupiter is attended, so far as weknow, by seven[22] satellites. Four of these were among the firstcelestial objects which Galileo discovered with his "optick tube, " andhe named them the "Medicean Stars" in honour of his patron, Cosmo deMedici. Being comparatively large bodies they might indeed just be seenwith the naked eye, were it not for the overpowering glare of theplanet. 

It was only in quite recent times, namely, in 1892, that a fifthsatellite was added to the system of Jupiter. This body, discovered byProfessor E. E. Barnard, is very small. It circulates nearer to theplanet than the innermost of Galileo's moons; and, on account of theglare, is a most difficult object to obtain a glimpse of, even in thebest of telescopes. In December 1904 and January 1905 respectively, twomore moons were added to the system, these being found by _photography_, by the American astronomer, Professor C. D. Perrine. Both the bodies inquestion revolve at a greater distance from the planet than theoutermost of the older known satellites. 

Galileo's moons, though the largest bodies of Jupiter's satellitesystem, are, as we have already pointed out, very small indeed whencompared with the planet itself. The diameters of two of them, Europaand Io, are, however, about the same as that of our moon, while those ofthe other two, Callisto and Ganymede, are more than half as large again. The recently discovered satellites are, on the other hand, insignificant; that found by Barnard, for example, being only about 100miles in diameter. 

Of the four original satellites Io is the nearest to Jupiter, and, seenfrom the planet, it would show a disc somewhat larger than that of ourmoon. The others would appear somewhat smaller. However, on account ofthe great distance of the sun, the entire light reflected to Jupiter byall the satellites should be very much less than what we get from ourmoon. 

Barnard's satellite circles around Jupiter at a distance less than ourmoon is from us, and in a period of about 12 hours. Galileo's foursatellites revolve in periods of about 2, 3-1/2, 7, and 16-1/2 daysrespectively, at distances lying roughly between a quarter of a millionand one million miles. Perrine's two satellites are at a distance ofabout seven million miles, and take about nine months to complete theirrevolutions. 

The larger satellites, when viewed in the telescope, exhibit certaindefined markings; but the bodies are so far away from us, that onlythose details which are of great extent can be seen. The satellite Io, according to Professor Barnard, shows a darkish disc, with a broad whitebelt across its middle regions. Mr. Douglass, one of the observers atthe Lowell Observatory, has noted upon Ganymede a number of markingssomewhat resembling those seen on Mars, and he concludes, from theirmovement, that this satellite rotates on its axis in about seven days. Professor Barnard, on the other hand, does not corroborate this, thoughhe claims to have discovered bright polar caps on both Ganymede andCallisto. 

In an earlier chapter we dealt at length with eclipses, occultations, and transits, and endeavoured to make clear the distinction betweenthem. The system of Jupiter's satellites furnishes excellent examples ofall these phenomena. The planet casts a very extensive shadow, and thesatellites are constantly undergoing obscuration by passing through it. Such occurrences are plainly comparable to our lunar eclipses. Again, the satellites may, at one time, be occulted by the huge disc of theplanet, and at another time seen in transit over its face. A fourthphenomenon is what is known as an _eclipse of the planet by asatellite_, which is the exact equivalent of what we style on the earthan eclipse of the sun. In this last case the shadow, cast by thesatellite, appears as a round black spot in movement across the planet'ssurface. 

In the passages of these attendant bodies behind the planet, into itsshadow, or across its face, respectively, it occasionally happens thatGalileo's four satellites all disappear from view, and the planet isthen seen for a while in the unusual condition of being apparentlywithout its customary attendants. An instance of this phenomenon tookplace on the 3rd of October 1907. On that occasion, the satellites knownas I. And III. (_i. E. _ Io and Ganymede) were eclipsed, that is to say, obscured by passing into the planet's shadow; Satellite IV. (Callisto)was occulted by the planet's disc; while Satellite II. (Europa), beingat the same moment in transit across the planet's face, was invisibleagainst that brilliant background. A number of instances of this kind ofoccurrence are on record. Galileo, for example, noted one on the 15th ofMarch 1611, while Herschel observed another on the 23rd of May 1802. 

It was indirectly to Jupiter's satellites that the world was firstindebted for its knowledge of the velocity of light. When the periods ofrevolution of the satellites were originally determined, Jupiterhappened, at the time, to be at his nearest to us. From the periods thusfound tables were made for the prediction of the moments at which theeclipses and other phenomena of the satellites should take place. AsJupiter, in the course of his orbit, drew further away from the earth, it was noticed that the disappearances of the satellites into the shadowof the planet occurred regularly later than the time predicted. In theyear 1675, Roemer, a Danish astronomer, inferred from this, not that thepredictions were faulty, but that light did not travel instantaneously. It appeared, in fact, to take longer to reach us, the greater thedistance it had to traverse. Thus, when the planet was far from theearth, the last ray given out by the satellite, before its passage intothe shadow, took a longer time to cross the intervening space, than whenthe planet was near. Modern experiments in physics have quite confirmedthis, and have proved for us that light does not travel across space inthe twinkling of an eye, as might hastily be supposed, but actuallymoves, as has been already stated, at the rate of about 186, 000 milesper second. 

THE PLANET SATURN

Seen in the telescope the planet Saturn is a wonderful and verybeautiful object. It is distinguished from all the other planets, infact from all known celestial bodies, through being girt around itsequator by what looks like a broad, flat ring of exceeding thinness. This, however, upon closer examination, is found to be actually composedof three concentric rings. The outermost of these is nearly of the samebrightness as the body of the planet itself. The ring which comesimmediately within it is also bright, and is separated from the outerone all the way round by a relatively narrow space, known as "Cassini'sdivision, " because it was discovered by the celebrated Frenchastronomer, J. D. Cassini, in the year 1675. Inside the second ring, andmerging insensibly into it, is a third one, known as the "crape ring, "because it is darker in hue than the others and partly transparent, thebody of Saturn being visible through it. The inner boundary of thisthird and last ring does not adjoin the planet, but is everywhereseparated from it by a definite space. This ring was discovered_independently_[23] in 1850 by Bond in America and Dawes in England. 

[Illustration: PLATE XV. THE PLANET SATURN

From a drawing made by Professor Barnard with the Great Lick Telescope. The black band fringing the outer ring, where it crosses the disc, isportion of the _shadow which the rings cast upon the planet_. The blackwedge-shaped mark, where the rings disappear behind the disc at theleft-hand side, is portion of the _shadow which the planet casts uponthe rings_. 

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As distinguished from the crape ring, the bright rings must have aconsiderable closeness of texture; for the shadow of the planet may beseen projected upon them, and their shadows in turn projected upon thesurface of the planet (see Plate XV. , p. 236). 

According to Professor Barnard, the entire breadth of the ring system, that is to say, from one side to the other of the outer ring, is 172, 310miles, or somewhat more than double the planet's diameter. 

In the varying views which we get of Saturn, the system of the rings ispresented to us at very different angles. Sometimes we are enabled togaze upon its broad expanse; at other times, however, its thin edge isturned exactly towards us, an occurrence which takes place afterintervals of about fifteen years. When this happened in 1892 the ringsare said to have disappeared entirely from view in the great Licktelescope. We thus get an idea of their small degree of thickness, whichwould appear to be only about 50 miles. The last time the system ofrings was exactly edgewise to the earth was on the 3rd of October 1907. 

The question of the composition of these rings has given rise to a gooddeal of speculation. It was formerly supposed that they were eithersolid or liquid, but in 1857 it was proved by Clerk Maxwell that astructure of this kind would not be able to stand. He showed, however, that they could be fully explained by supposing them to consist of animmense number of separate solid particles, or, as one might otherwiseput it, extremely small satellites, circling in dense swarms around themiddle portions of the planet. It is therefore believed that we havehere the materials ready for the formation of a satellite or satellites;but that the powerful gravitative action, arising through the planet'sbeing so near at hand, is too great ever to allow these materials toaggregate themselves into a solid mass. There is, as a matter of fact, aminimum distance from the body of any planet within which it can beshown that a satellite will be unable to form on account ofgravitational stress. This is known as "Roche's limit, " from the name ofa French astronomer who specially investigated the question. 

There thus appears to be a certain degree of analogy between Saturn'srings and the asteroids. Empty spaces, too, exist in the asteroidalzone, the relative position of one of which bears a striking resemblanceto that of "Cassini's division. " It is suggested, indeed, that thisdivision had its origin in gravitational disturbances produced by theattraction of the larger satellites, just as the empty spaces in theasteroidal zone are supposed to be the result of perturbations caused bythe Giant Planet hard by. 

It has long been understood that the system of the rings must berotating around Saturn, for if they were not in motion his intensegravitational attraction would quickly tear them in pieces. This was atlength proved to be the fact by the late Professor Keeler, Director ofthe Lick Observatory, who from spectroscopic observations found thatthose portions of the rings situated near to the planet rotated fasterthan those farther from it. This directly supports the view that therings are composed of satellites; for, as we have already seen, thenearer a satellite is to its primary the faster it will revolve. On theother hand, were the rings solid, their outer portions would move thefastest; as we have seen takes place in the body of the earth, forexample. The mass of the ring system, however, must be exceedinglysmall, for it does not appear to produce any disturbances in themovements of Saturn's satellites. From the kinetic theory, therefore, one would not expect to find any atmosphere on the rings, and theabsence of it is duly shown by spectroscopic observations. 

The diameter of Saturn, roughly speaking, is about one-fifth less thanthat of Jupiter. The planet is very flattened at the poles, thisflattening being quite noticeable in a good telescope. For instance, thediameter across the equator is about 76, 470 miles, while from pole topole it is much less, namely, 69, 770. 

The surface of Saturn bears a strong resemblance to that of Jupiter. Itsmarkings, though not so well defined, are of the same belt-likedescription; and from observation of them it appears that the planetrotates _on an average_ in a little over ten hours. The rotation is infact of the same peculiar kind as that of the sun and Jupiter; but thedifference of speed at which the various portions of Saturn go round areeven more marked than in the case of the Giant Planet. The density ofSaturn is less than that of Jupiter; so that it must be largely in acondition of vapour, and in all probability at a still earlier stage ofplanetary evolution. 

Up to the present we know of as many as ten satellites circling aroundSaturn, which is more than any other planet of the solar system can layclaim to. Two of these, however, are very recent discoveries; one, Phoebe, having been found by photography in August 1898, and the other, Themis, in 1904, also by the same means. For both of these we areindebted to Professor W. H. Pickering. Themis is said to be _the faintestobject in the solar system_. It cannot be _seen_, even with the largesttelescope in existence; a fact which should hardly fail to impress uponone the great advantage the photographic plate possesses in theseresearches over the human eye. 

The most important of the whole Saturnian family of satellites are thetwo known as Titan and Japetus. These were discovered respectively byHuyghens in 1655 and by Cassini in 1671. Japetus is about the same sizeas our moon; while the diameter of Titan, the largest of the satellites, is about half as much again. Titan takes about sixteen days to revolvearound Saturn, while Japetus takes more than two months and a half. Theformer is about three-quarters of a million miles distant from theplanet, and the latter about two and a quarter millions. To Sir WilliamHerschel we are indebted for the discovery of two more satellites, oneof which he found on the evening that he used his celebrated 40-foottelescope for the first time. The ninth satellite, Phoebe, one of thetwo discovered by Professor Pickering, is perhaps the most remarkablebody in the solar system, for all the other known members of that systemperform their revolutions in one fixed direction, whereas this satelliterevolves in the _contrary_ direction. 

In consequence of the great distance of Saturn, the sun, as seen fromthe planet, would appear so small that it would scarcely show any disc. The planet, indeed, only receives from the sun about one-ninetieth ofthe heat and light which the earth receives. Owing to this diminishedintensity of illumination, the combined light reflected to Saturn by thewhole of its satellites must be very small. 

With the sole exception of Jupiter, not one of the planets circulatingnearer to the sun could be seen from Saturn, as they would be entirelylost in the solar glare. For an observer upon Saturn, Jupiter would, therefore, fill much the same position as Venus does for us, regularlydisplaying phases and being alternately a morning and an evening star. 

It is rather interesting to consider the appearances which would beproduced in our skies were the earth embellished with a system of ringssimilar to those of Saturn. In consequence of the curving of theterrestrial surface, they would not be seen at all from within theArctic or Antarctic circles, as they would be always below the horizon. From the equator they would be continually seen edgewise, and so wouldappear merely as line of light stretching right across the heaven andpassing through the zenith. But the dwellers in the remaining regionswould find them very objectionable, for they would cut off the light ofthe sun during lengthy periods of time. 

Saturn was a sore puzzle to the early telescopic observers. They did notfor a long time grasp the fact that it was surrounded by a ring--so slowis the human mind to seek for explanations out of the ordinary course ofthings. The protrusions of the ring on either side of the planet, atfirst looked to Galileo like two minor globes placed on opposite sidesof it, and slightly overlapping the disc. He therefore informed Keplerthat "Saturn consists of three stars in contact with one another. " Yethe was genuinely puzzled by the fact that the two attendant bodies (ashe thought them) always retained the same position with regard to theplanet's disc, and did not appear to revolve around it, nor to be in anywise shifted as a consequence of the movements of our earth. 

About a year and a half elapsed before he again examined Saturn; and, ifhe was previously puzzled, he was now thoroughly amazed. It happenedjust then to be one of those periods when the ring is edgewise towardsthe earth, and of course he only saw a round disc like that of Jupiter. What, indeed, had become of the attendant orbs? Was some demon mockinghim? Had Saturn devoured his own children? He was, however, fated to bestill more puzzled, for soon the minor orbs reappeared, and, becominglarger and larger as time went on, they ended by losing their globularappearance and became like two pairs of arms clasping the planet fromeach side! (see Plate XVI. , p. 242). 

Galileo went to his grave with the riddle still unsolved, and itremained for the famous Dutch astronomer, Huyghens, to clear up thematter. It was, however, some little time before he hit upon the realexplanation. Having noticed that there were dark spaces between thestrange appendages and the body of the planet, he imagined Saturn to bea globe fitted with handles at each side; "ansæ" these came to becalled, from the Latin _ansa_, which means a handle. At length, in theyear 1656, he solved the problem, and this he did by means of that123-foot tubeless telescope, of which mention has already been made. Thering happened then to be at its edgewise period, and a careful study ofthe behaviour of the ansæ when disappearing and reappearing soonrevealed to Huyghens the true explanation. 

[Illustration: PLATE XVI. EARLY REPRESENTATIONS OF SATURN

From an illustration in the _Systema Saturnium_ of Christian Huyghens. 

(Page 242)]

THE PLANETS URANUS AND NEPTUNE

We have already explained (in Chapter II. ) the circumstances in whichboth Uranus and Neptune were discovered. It should, however, be addedthat after the discovery of Uranus, that planet was found to have beenalready noted upon several occasions by different observers, but alwayswithout the least suspicion that it was other than a mere faint star. Again, with reference to the discovery of Neptune, it may here bementioned that the apparent amount by which that planet had pulledUranus out of its place upon the starry background was exceedinglysmall--so small, indeed, that no eye could have detected it without theaid of a telescope!

Of the two predictions of the place of Neptune in the sky, that of LeVerrier was the nearer. Indeed, the position calculated by Adams wasmore than twice as far out. But Adams was by a long way the first in thefield with his results, and only for unfortunate delays the prize wouldcertainly have fallen to him. For instance, there was no star-map atCambridge, and Professor Challis, the director of the observatory there, was in consequence obliged to make a laborious examination of the starsin the suspected region. On the other hand, all that Galle had to do wasto compare that part of the sky where Le Verrier told him to look withthe Berlin star-chart which he had by him. This he did on September 23, 1846, with the result that he quickly noted an eighth magnitude starwhich did not figure in that chart. By the next night this star hadaltered its position in the sky, thus disclosing the fact that it wasreally a planet. 

Six days later Professor Challis succeeded in finding the planet, but ofcourse he was now too late. On reviewing his labours he ascertained thathe had actually noted down its place early in August, and had he onlybeen able to sift his observations as he made them, the discovery wouldhave been made then. 

Later on it was found that Neptune had only just missed being discoveredabout fifty years earlier. In certain observations made during 1795, thefamous French astronomer, Lalande, found that a star, which he hadmapped in a certain position on the 8th of May of that year, was in adifferent position two days later. The idea of a planet does not appearto have entered his mind, and he merely treated the first observation asan error!

The reader will, no doubt, recollect how the discovery of the asteroidswas due in effect to an apparent break in the seemingly regular sequenceof the planetary orbits outwards from the sun. This curious sequence ofrelative distances is usually known as "Bode's Law, " because it wasfirst brought into general notice by an astronomer of that name. It had, however, previously been investigated mathematically by Titius in 1772. Long before this, indeed, the unnecessarily wide space between theorbits of Mars and Jupiter had attracted the attention of the greatKepler to such a degree, that he predicted that a planet would some daybe found to fill the void. Notwithstanding the service which theso-called Law of Bode has indirectly rendered to astronomy, it hasstrangely enough been found after all not to rest upon any scientificfoundation. It will not account for the distance from the sun of theorbit of Neptune, and the very sequence seems on the whole to be in thenature of a mere coincidence. 

Neptune is invisible to the naked eye; Uranus is just at the limit ofvisibility. Both planets are, however, so far from us that we can getbut the poorest knowledge of their condition and surroundings. Uranus, up to the present, is known to be attended by four satellites, andNeptune by one. The planets themselves are about equal in size; theirdiameters, roughly speaking, being about one-half that of Saturn. Somemarkings have, indeed, been seen upon the disc of Uranus, but they arevery indistinct and fleeting. From observation of them, it is assumedthat the planet rotates on its axis in a period of some ten to twelvehours. No definite markings have as yet been seen upon Neptune, whichbody is described by several observers as resembling a faint planetarynebula. 

With regard to their physical condition, the most that can be said aboutthese two planets is that they are probably in much the same vaporousstate as Jupiter and Saturn. On account of their great distance from thesun they can receive but little solar heat and light. Seen fromNeptune, in fact, the sun would appear only about the size of Venus ather best, though of a brightness sufficiently intense to illumine theNeptunian landscape with about seven hundred times our full moonlight. 

[22] Mr. P. Melotte, of Greenwich Observatory, while examining aphotograph taken there on February 28, 1908, discovered upon it a veryfaint object which it is firmly believed will prove to be an _eighth_satellite of Jupiter. This object was afterwards found on plates exposedas far back as January 27. It has since been photographed several timesat Greenwich, and also at Heidelberg (by Dr. Max Wolf) and at the LickObservatory. Its movement is probably _retrograde_, like that of Phoebe(p. 240). 

[23] In the history of astronomy two salient points stand out. 

The first of these is the number of "independent" discoveries which havetaken place; such, for instance, as in the cases of Le Verrier and Adamswith regard to Neptune, and of Lockyer and Janssen in the matter of thespectroscopic method of observing solar prominences. 

The other is the great amount of "anticipation. " Copernicus, as we haveseen, was anticipated by the Greeks; Kepler was not actually the firstwho thought of elliptic orbits; others before Newton had imagined anattractive force. 

Both these points furnish much food for thought!

CHAPTER XIX

COMETS

The reader has, no doubt, been struck by the marked uniformity whichexists among those members of the solar system with which we have dealtup to the present. The sun, the planets, and their satellites are allwhat we call solid bodies. The planets move around the sun, and thesatellites around the planets, in orbits which, though strictlyspeaking, ellipses, are yet not in any instance of a very oval form. Tworesults naturally follow from these considerations. Firstly, the bodiesin question hide the light coming to us from those further off, whenthey pass in front of them. Secondly, the planets never get so far fromthe sun that we lose sight of them altogether. 

With the objects known as Comets it is, however, quite the contrary. These objects do not conform to our notions of solidity. They are sotransparent that they can pass across the smallest star without dimmingits light in the slightest degree. Again, they are only visible to usduring a portion of their orbits. A comet may be briefly described as anilluminated filmy-looking object, made up usually of three portions--ahead, a nucleus, or brighter central portion within this head, and atail. The heads of comets vary greatly in size; some, indeed, appearquite small, like stars, while others look even as large as the moon. Occasionally the nucleus is wanting, and sometimes the tail also. 

[Illustration: FIG. 18. --Showing how the Tail of a Comet is directedaway from the Sun. ]

These mysterious visitors to our skies come up into view out of theimmensities beyond, move towards the sun at a rapidly increasing speed, and, having gone around it, dash away again into the depths of space. Asa comet approaches the sun, its body appears to grow smaller andsmaller, while, at the same time, it gradually throws out behind it anappendage like a tail. As the comet moves round the central orb thistail is always directed _away_ from the sun; and when it departs againinto space the tail goes in advance. As the comet's distance from thesun increases, the tail gradually shrinks away and the head once moregrows in size (see Fig. 18). In consequence of these changes, and of thefact that we lose sight of comets comparatively quickly, one is muchinclined to wonder what further changes may take place after the bodieshave passed beyond our ken. 

The orbits of comets are, as we have seen, very elliptic. In someinstances this ellipticity is so great as to take the bodies out intospace to nearly six times the distance of Neptune from the sun. For along time, indeed, it was considered that comets were of two kinds, namely, those which actually _belonged_ to the solar system, and thosewhich were merely _visitors_ to it for the first and only time--rushingin from the depths of space, rapidly circuiting the sun, and finallydashing away into space again, never to return. On the contrary, nowadays, astronomers are generally inclined to regard comets aspermanent members of the solar system. 

The difficulty, however, of deciding absolutely whether the orbits ofcomets are really always _closed_ curves, that is to say, curves whichmust sooner or later bring the bodies back again towards the sun, is, indeed, very great. Comets, in the first place, are always so diffuse, that it is impossible to determine their exact position, or, rather, theexact position of that important point within them, known as the centreof gravity. Secondly, that stretch of its orbit along which we canfollow a comet, is such a very small portion of the whole path, that theslightest errors of observation which we make will result inconsiderably altering our estimate of the actual shape of the orbit. 

Comets have been described as so transparent that they can pass acrossthe sky without dimming the lustre of the smallest stars, which thethinnest fog or mist would do. This is, indeed, true of every portionof a comet except the nucleus, which is, as its name implies, thedensest part. And yet, in contrast to this ghostlike character, is thestrange fact that when comets are of a certain brightness they mayactually be seen in full daylight. 

As might be gathered from their extreme tenuity, comets are soexceedingly small in mass that they do not appear to exert anygravitational attraction upon the other bodies of our system. It is, indeed, a known fact that in the year 1886 a comet passed right amidstthe satellites of Jupiter without disturbing them in the slightestdegree. The attraction of the planet, on the other hand, so altered thecomet's orbit, as to cause it to revolve around the sun in a period ofseven years, instead of twenty-seven, as had previously been the case. Also, in 1779, the comet known as Lexell's passed quite close toJupiter, and its orbit was so changed by that planet's attraction thatit has never been seen since. The density of comets must, as a rule, bevery much less than the one-thousandth part of that of the air at thesurface of our globe; for, if the density of the comet were even sosmall as this, its mass would _not_ be inappreciable. 

If comets are really undoubted members of the solar system, thecircumstances in which they were evolved must have been different fromthose which produced the planets and satellites. The axial rotations ofboth the latter, and also their revolutions, take place in one certaindirection;[24] their orbits, too, are ellipses which do not differ muchfrom circles, and which, furthermore, are situated fairly in the oneplane. Comets, on the other hand, do not necessarily travel round thesun in the same fixed direction as the planets. Their orbits, besides, are exceedingly elliptic; and, far from keeping to one plane, or evennear it, they approach the sun from all directions. 

Broadly speaking, comets may be divided into two distinct classes, or"families. " In the first class, the same orbit appears to be shared incommon by a series of comets which travel along it, one following theother. The comets which appeared in the years 1668, 1843, 1880, 1882, and 1887 are instances of a number of different bodies pursuing the samepath around the sun. The members of a comet family of this kind areobserved to have similar characteristics. The idea is that such cometsare merely portions of one much larger cometary body, which becamebroken up by the gravitational action of other bodies in the system, orthrough violent encounter with the sun's surroundings. 

The second class is composed of comets which are supposed to have beenseized by the gravitative action of certain planets, and thus forced torevolve in short ellipses around the sun, well within the limits of thesolar system. These comets are, in consequence, spoken of as "captures. "They move around the sun in the same direction as the planets do. Jupiter has a fairly large comet family of this kind attached to him. Asa result of his overpowering gravitation, it is imagined that during theages he must have attracted a large number of these bodies on his ownaccount, and, perhaps, have robbed other planets of their captures. Hisfamily at present numbers about thirty. Of the other planets, so far aswe know, Saturn possesses a comet family of two, Uranus three, andNeptune six. There are, indeed, a few comets which appear as if underthe influence of some force situated outside the known bounds of thesolar system, a circumstance which goes to strengthen the idea thatother planets may revolve beyond the orbit of Neptune. The terrestrialplanets, on the other hand, cannot have comet families; because theenormous gravitative action of the sun in their vicinity entirelyoverpowers the attractive force which they exert upon those comets whichpass close to them. Besides this, a comet, when in the inner regions ofthe solar system, moves with such rapidity, that the gravitational pullof the planets there situated is not powerful enough to deflect it toany extent. It must not be presumed, however, that a comet once capturedshould always remain a prisoner. Further disturbing causes mightunsettle its newly acquired orbit, and send it out again into thecelestial spaces. 

With regard to the matter of which comets are composed, the spectroscopeshows the presence in them of hydrocarbon compounds (a notablecharacteristic of these bodies), and at times, also, of sodium and iron. Some of the light which we get from comets is, however, merely reflectedsunlight. 

The fact that the tails of comets are always directed away from the sun, has given rise to the idea that this is caused by some repelling actionemanating from the sun itself, which is continually driving off thesmallest particles. Two leading theories have been formulated to accountfor the tails themselves upon the above assumption. One of these, firstsuggested by Olbers in 1812, and now associated with the name of theRussian astronomer, the late Professor Brédikhine, who carefully workedit out, presumes an electrical action emanating from the sun; the other, that of Arrhenius, supposes a pressure exerted by the solar light in itsradiation outwards into space. It is possible, indeed, that repellingforces of both these kinds may be at work together. Minute particles areprobably being continually produced by friction and collisions among themore solid parts in the heads of comets. Supposing that such particlesare driven off altogether, one may therefore assume that the so-calledcaptured comets are disintegrating at a comparatively rapid rate. Keplerlong ago maintained that "comets die, " and this actually appears to bethe case. The ordinary periodic ones, such, for instance, as Encke'sComet, are very faint, and becoming fainter at each return. Certain ofthese comets have, indeed, failed altogether to reappear. It is notablethat the members of Jupiter's comet family are not very conspicuousobjects. They have small tails, and even in some cases have none at all. The family, too, does not contain many members, and yet one cannot butsuppose that Jupiter, on account of his great mass, has had manyopportunities for making captures adown the ages. 

Of the two theories to which allusion has above been made, that ofBrédikhine has been worked out so carefully, and with such a show ofplausibility, that it here calls for a detailed description. It appearsbesides to explain the phenomena of comets' tails so much moresatisfactorily than that of Arrhenius, that astronomers are inclined toaccept it the more readily of the two. According to Brédikhine's theorythe electrical repulsive force, which he assumes for the purposes of hisargument, will drive the minutest particles of the comet in a directionaway from the sun much more readily than the gravitative action of thatbody will pull them towards it. This may be compared to the ease withwhich fine dust may be blown upwards, although the earth's gravitationis acting upon it all the time. 

The researches of Brédikhine, which began seriously with hisinvestigation of Coggia's Comet of 1874, led him to classify the tailsof comets in _three types_. Presuming that the repulsive force emanatingfrom the sun did not vary, he came to the conclusion that the differentforms assumed by cometary tails must be ascribed to the special actionof this force upon the various elements which happen to be present inthe comet. The tails which he classes as of the first type, are thosewhich are long and straight and point directly away from the sun. Examples of such tails are found in the comets of 1811, 1843, and 1861. Tails of this kind, he thinks, are in all probability formed of_hydrogen_. His second type comprises those which are pointed away fromthe sun, but at the same time are considerably curved, as was seen inthe comets of Donati and Coggia. These tails are formed of _hydrocarbongas_. The third type of tail is short, brush-like, and strongly bent, and is formed of the _vapour of iron_, mixed with that of sodium andother elements. It should, however, be noted that comets haveoccasionally been seen which possess several tails of these varioustypes. 

We will now touch upon a few of the best known comets of modern times. 

The comet of 1680 was the first whose orbit was calculated according tothe laws of gravitation. This was accomplished by Newton, and he foundthat the comet in question completed its journey round the sun in aperiod of about 600 years. 

In 1682 there appeared a great comet, which has become famous under thename of Halley's Comet, in consequence of the profound investigationsmade into its motion by the great astronomer, Edmund Halley. He fixedits period of revolution around the sun at about seventy-five years, andpredicted that it would reappear in the early part of 1759. He did not, however, live to see this fulfilled, but the comet duly returned--_thefirst body of the kind to verify such a prediction_--and was detected onChristmas Day, 1758, by George Palitzch, an amateur observer living nearDresden. Halley also investigated the past history of the comet, andtraced it back to the year 1456. The orbit of Halley's comet passes outslightly beyond the orbit of Neptune. At its last visit in 1835, thiscomet passed comparatively close to us, namely, within five millionmiles of the earth. According to the calculations of Messrs P. H. Cowelland A. C. D. Crommelin of Greenwich Observatory, its next return will bein the spring of 1910; the nearest approach to the earth taking placeabout May 12. 

On the 26th of March, 1811, a great comet appeared, which remainedvisible for nearly a year and a half. It was a magnificent object; thetail being about 100 millions of miles in length, and the head about127, 000 miles in diameter. A detailed study which he gave to this cometprompted Olbers to put forward that theory of electrical repulsionwhich, as we have seen, has since been so carefully worked out byBrédikhine. Olbers had noticed that the particles expelled from the headappeared to travel to the end of the tail in about eleven minutes, thusshowing a velocity per second very similar to that of light. 

The discovery in 1819 of the comet known as Encke's, because its orbitwas determined by an astronomer of that name, drew attention for thefirst time to Jupiter's comet family, and, indeed, to short-periodcomets in general. This comet revolves around the sun in the shortestknown period of any of these bodies, namely, 3-1/3 years. Enckepredicted that it would return in 1822. This duly occurred, the cometpassing at its nearest to the sun within three hours of the timeindicated; being thus the second instance of the fulfilment of aprediction of the kind. A certain degree of irregularity which Encke'sComet displays in the dates of its returns to the sun, has been supposedto indicate that it passes in the course of its orbit through someretarding medium, but no definite conclusions have so far been arrivedat in this matter. 

A comet, which appeared in 1826, goes by the name of Biela's Comet, because of its discovery by an Austrian military officer, Wilhelm vonBiela. This comet was found to have a period of between six and sevenyears. Certain calculations made by Olbers showed that, at its return in1832, it would pass _through the earth's orbit_. The announcement ofthis gave rise to a panic; for people did not wait to inquire whetherthe earth would be anywhere near that part of its orbit when the cometpassed. The panic, however, subsided when the French astronomer, Arago, showed that at the moment in question the earth would be some 50millions of miles away from the point indicated!

[Illustration: PLATE XVII. DONATI'S COMET

From a drawing made on October 9th, 1858, by G. P. Bond, of HarvardCollege Observatory, U. S. A. A good illustration of Brédikhine's theory:note the straight tails of his _first_ type, and the curved tail of his_second_. 

(Page 257)]

In 1846, shortly after one of its returns, Biela's Comet divided intotwo portions. At its next appearance (1852) these portions had separatedto a distance of about 1-1/2 millions of miles from each other. Thiscomet, or rather its constituents, have never since been seen. 

Perhaps the most remarkable comet of recent times was that of 1858, known as Donati's, it having been discovered at Florence by the Italianastronomer, G. B. Donati. This comet, a magnificent object, was visiblefor more than three months with the naked eye. Its tail was then 54millions of miles in length. It was found to revolve around the sun in aperiod of over 2000 years, and to go out in its journey to about 5-1/2times the distance of Neptune. Its motion is retrograde, that is to say, in the contrary direction to the usual movement in the solar system. Anumber of beautiful drawings of Donati's Comet were made by the Americanastronomer, G. P. Bond. One of the best of these is reproduced on PlateXVII. , p. 256. 

In 1861 there appeared a great comet. On the 30th of June of that yearthe earth and moon actually passed through its tail; but no effects werenoticed, other than a peculiar luminosity in the sky. 

In the year 1881 there appeared another large comet, known as Tebbutt'sComet, from the name of its discoverer. This was the _first comet ofwhich a satisfactory photograph was obtained_. The photograph inquestion was taken by the late M. Janssen. 

The comet of 1882 was of vast size and brilliance. It approached soclose to the sun that it passed through some 100, 000 miles of the solarcorona. Though its orbit was not found to have been altered by thisexperience, its nucleus displayed signs of breaking up. Some very finephotographs of this comet were obtained at the Cape of Good Hope by Mr. (now Sir David) Gill. 

The comet of 1889 was followed with the telescope nearly up to the orbitof Saturn, which seems to be the greatest distance at which a comet hasever been seen. 

The _first discovery of a comet by photographic means_[25] was made byProfessor Barnard in 1892; and, since then, photography has beenemployed with marked success in the detection of small periodic comets. 

The best comet seen in the Northern hemisphere since that of 1882, appears to have been Daniel's Comet of 1907 (see Plate XVIII. , p. 258). This comet was discovered on June 9, 1907, by Mr. Z. Daniel, atPrinceton Observatory, New Jersey, U. S. A. It became visible to the nakedeye about mid-July of that year, and reached its greatest brilliancyabout the end of August. It did not, however, attract much popularattention, as its position in the sky allowed it to be seen only justbefore dawn. 

[24] With the exception, of course, of such an anomaly as the retrogrademotion of the ninth satellite of Saturn. 

[25] If we except the case of the comet which was photographed near thesolar corona in the eclipse of 1882. 

[Illustration: PLATE XVIII. DANIEL'S COMET OF 1907

From a photograph taken, on August 11th, 1907, by Dr. Max Wolf, at theAstrophysical Observatory, Heidelberg. The instrument used was a 28-inchreflecting telescope, and the time of exposure was fifteen minutes. Asthe telescope was guided to follow the moving comet, the stars haveimprinted themselves upon the photographic plate as short trails. Thisis clearly the opposite to what is depicted on Plate XIII. 

(Page 258)]

CHAPTER XX

REMARKABLE COMETS

If eclipses were a cause of terror in past ages, comets appear to havebeen doubly so. Their much longer continuance in the sight of men had nodoubt something to say to this, and also the fact that they arrivedwithout warning; it not being then possible to give even a roughprediction of their return, as in the case of eclipses. As both thesephenomena were occasional, and out of the ordinary course of things, they drew exceptional attention as unusual events always do; for it mustbe allowed that quite as wonderful things exist, but they pass unnoticedmerely because men have grown accustomed to them. 

For some reason the ancients elected to class comets along with meteors, the aurora borealis, and other phenomena of the atmosphere, rather thanwith the planets and the bodies of the spaces beyond. The suddenappearance of these objects led them to be regarded as signs sent by thegods to announce remarkable events, chief among these being the deathsof monarchs. Shakespeare has reminded us of this in those celebratedlines in _Julius Cæsar_:--

"When beggars die there are no comets seen, The heavens themselves blaze forth the death of princes. "

Numbed by fear, the men of old blindly accepted these presages of fate;and did not too closely question whether the threatened danger was totheir own nation or to some other, to their ruler or to his enemy. Nowand then, as in the case of the Roman Emperor Vespasian, there was acynical attempt to apply some reasoning to the portent. That emperor, inalluding to the comet of A. D. 79, is reported to have said: "This hairystar does not concern me; it menaces rather the King of the Parthians, for he is hairy and I am bald. " Vespasian, all the same, died shortlyafterwards!

Pliny, in his natural history, gives several instances of the terriblesignificance which the ancients attached to comets. "A comet, " he says, "is ordinarily a very fearful star; it announces no small effusion ofblood. We have seen an example of this during the civil commotion ofOctavius. "

A very brilliant comet appeared in 371 B. C. , and about the same time anearthquake caused Helicè and Bura, two towns in Achaia, to be swallowedup by the sea. The following remark made by Seneca concerning it showsthat the ancients did not consider comets merely as precursors, but evenas actual _causes_ of fatal events: "This comet, so anxiously observedby every one, _because of the great catastrophe which it produced assoon as it appeared_, the submersion of Bura and Helicè. "

Comets are by no means rare visitors to our skies, and very few yearshave elapsed in historical times without such objects making theirappearance. In the Dark and Middle Ages, when Europe was split up intomany small kingdoms and principalities, it was, of course, hardlypossible for a comet to appear without the death of some ruler occurringnear the time. Critical situations, too, were continually arising inthose disturbed days. The end of Louis le Debonnaire was hastened, asthe reader will, no doubt, recollect, by the great eclipse of 840; butit was firmly believed that a comet which had appeared a year or twopreviously presaged his death. The comet of 1556 is reported to have_influenced_ the abdication of the Emperor Charles V. ; but curiouslyenough, this event had already taken place before the comet made itsappearance! Such beliefs, no doubt, had a very real effect upon rulersof a superstitious nature, or in a weak state of health. For instance, Gian Galeazzo Visconti, Duke of Milan, was sick when the comet of 1402appeared. After seeing it, he is said to have exclaimed: "I renderthanks to God for having decreed that my death should be announced tomen by this celestial sign. " His malady then became worse, and he diedshortly afterwards. 

It is indeed not improbable that such superstitious fears in monarchswere fanned by those who would profit by their deaths, and yet did notwish to stain their own hands with blood. 

Evil though its effects may have been, this morbid interest which pastages took in comets has proved of the greatest service to our science. Had it not been believed that the appearance of these objects wasattended with far-reaching effects, it is very doubtful whether the oldchroniclers would have given themselves the trouble of alluding to themat all; and thus the modern investigators of cometary orbits would havelacked a great deal of important material. 

We will now mention a few of the most notable comets which historianshave recorded. 

A comet which appeared in 344 B. C. Was thought to betoken the successof the expedition undertaken in that year by Timoleon of Corinth againstSicily. "The gods by an extraordinary prodigy announced his success andfuture greatness: a burning torch appeared in the heavens throughout thenight and preceded the fleet of Timoleon until it arrived off the coastof Sicily. "

The comet of 43 B. C. Was generally believed to be the soul of Cæsar onits way to heaven. 

Josephus tells us that in A. D. 69 several prodigies, and amongst them acomet in the shape of a sword, announced the destruction of Jerusalem. This comet is said to have remained over the city for the space of ayear!

A comet which appeared in A. D. 336 was considered to have announced thedeath of the Emperor Constantine. 

But perhaps the most celebrated comet of early times was the one whichappeared in A. D. 1000. That year was, in more than one way, big withportent, for there had long been a firm belief that the Christian eracould not possibly run into four figures. Men, indeed, steadfastlybelieved that when the thousand years had ended, the millennium wouldimmediately begin. Therefore they did not reap neither did they sow, they toiled not, neither did they spin, and the appearance of the cometstrengthened their convictions. The fateful year, however, passed bywithout anything remarkable taking place; but the neglect of husbandrybrought great famine and pestilence over Europe in the years whichfollowed. 

In April 1066, that year fraught with such immense consequences forEngland, a comet appeared. No one doubted but that it was a presage ofthe success of the Conquest, and perhaps, indeed, it had its due weightin determining the minds and actions of the men who took part in theexpedition. _Nova stella, novus rex_ ("a new star, a new sovereign") wasa favourite proverb of the time. The chroniclers, with one accord, havedelighted to relate that the Normans, "guided by a comet, " invadedEngland. A representation of this object appears in the Bayeux Tapestry(see Fig. 19, p. 263). [26]

[Illustration: FIG. 19. --The comet of 1066, as represented in the BayeuxTapestry. 

(From the _World of Comets_. )]

We have mentioned Halley's Comet of 1682, and how it revisits theneighbourhood of the earth at intervals of seventy-six years. The cometof 1066 has for many years been supposed to be Halley's Comet on one ofits visits. The identity of these two, however, was only quite recentlyplaced beyond all doubt by the investigations of Messrs Cowell andCrommelin. This comet appeared also in 1456, when John Huniades wasdefending Belgrade against the Turks led by Mahomet II. , the conquerorof Constantinople, and is said to have paralysed both armies with fear. 

The Middle Ages have left us descriptions of comets, which show only toowell how the imagination will run riot under the stimulus of terror. Forinstance, the historian, Nicetas, thus describes the comet of the year1182: "After the Romans were driven from Constantinople a prognostic wasseen of the excesses and crimes to which Andronicus was to abandonhimself. A comet appeared in the heavens similar to a writhing serpent;sometimes it extended itself, sometimes it drew itself in; sometimes, tothe great terror of the spectators, it opened a huge mouth; it seemedthat, as if thirsting for human blood, it was upon the point ofsatiating itself. " And, again, the celebrated Ambrose Paré, the fatherof surgery, has left us the following account of the comet of 1528, which appeared in his own time: "This comet, " said he, "was so horrible, so frightful, and it produced such great terror in the vulgar, that somedied of fear, and others fell sick. It appeared to be of excessivelength, and was of the colour of blood. At the summit of it was seen thefigure of a bent arm, holding in its hand a great sword, as if about tostrike. At the end of the point there were three stars. On both sides ofthe rays of this comet were seen a great number of axes, knives, blood-coloured swords, among which were a great number of hideous humanfaces, with beards and bristling hair. " Paré, it is true, was noastronomer; yet this shows the effect of the phenomenon, even upon a manof great learning, as undoubtedly he was. It should here be mentionedthat nothing very remarkable happened at or near the year 1528. 

Concerning the comet of 1680, the extraordinary story got about that, atRome, a hen had laid an egg on which appeared a representation of thecomet!

But the superstitions with regard to comets were now nearing their end. The last blow was given by Halley, who definitely proved that theyobeyed the laws of gravitation, and circulated around the sun as planetsdo; and further announced that the comet of 1682 had a period ofseventy-six years, which would cause it to reappear in the year 1759. Wehave seen how this prediction was duly verified. We have seen, too, howthis comet appeared again in 1835, and how it is due to return in theearly part of 1910. 

[26] With regard to the words "Isti mirant stella" in the figure, Mr. W. T. Lynn suggests that they may not, after all, be the grammaticallybad Latin which they appear, but that the legend is really "Istimirantur stellam, " the missing letters being supposed to be hidden bythe building and the comet. 

CHAPTER XXI

METEORS OR SHOOTING STARS

Any one who happens to gaze at the sky for a short time on a clear nightis pretty certain to be rewarded with a view of what is popularly knownas a "shooting star. " Such an object, however, is not a star at all, buthas received its appellation from an analogy; for the phenomenon givesto the inexperienced in these matters an impression as if one of themany points of light, which glitter in the vaulted heaven, had suddenlybecome loosened from its place, and was falling towards the earth. Inits passage across the sky the moving object leaves behind a trail oflight which usually lasts for a few moments. Shooting stars, or meteors, as they are technically termed, are for the most part very small bodies, perhaps no larger than peas or pebbles, which, dashing towards our earthfrom space beyond, are heated to a white heat, and reduced to powder bythe friction resulting from their rapid passage into our atmosphere. This they enter at various degrees of speed, in some cases so great as45 miles a second. The speed, of course, will depend greatly uponwhether the earth and the meteors are rushing towards each other, orwhether the latter are merely overtaking the earth. In the first ofthese cases the meteors will naturally collide with the atmosphere withgreat force; in the other case they will plainly come into it with muchless rapidity. As has been already stated, it is from observations ofsuch bodies that we are enabled to estimate, though very imperfectly, the height at which the air around our globe practically ceases, andthis height is imagined to be somewhere about 100 miles. Fortunate, indeed, is it for us that there is a goodly layer of atmosphere over ourheads, for, were this not so, these visitors from space would strikeupon the surface of our earth night and day, and render existence stillmore unendurable than many persons choose to consider it. To what abombardment must the moon be continually subject, destitute as she is ofsuch an atmospheric shield!

It is only in the moment of their dissolution that we really learnanything about meteors, for these bodies are much too small to be seenbefore they enter our atmosphere. The débris arising from theirdestruction is wafted over the earth, and, settling down eventually uponits surface, goes to augment the accumulation of that humble domesticcommodity which men call dust. This continual addition of materialtends, of course, to increase the mass of the earth, though the effectthus produced will be on an exceedingly small scale. 

The total number of meteors moving about in space must be practicallycountless. The number which actually dash into the earth's atmosphereduring each year is, indeed, very great. Professor Simon Newcomb, thewell-known American astronomer, has estimated that, of the latter, thoselarge enough to be seen with the naked eye cannot be in all less than146, 000, 000, 000 per annum. Ten times more numerous still are thought tobe those insignificant ones which are seen to pass like mere sparks oflight across the field of an observer's telescope. 

Until comparatively recent times, perhaps up to about a hundred yearsago, it was thought that meteors were purely terrestrial phenomena whichhad their origin in the upper regions of the air. It, however, began tobe noticed that at certain periods of the year these moving objectsappeared to come from definite areas of the sky. Considerations, therefore, respecting their observed velocities, directions, andaltitudes, gave rise to the theory that they are swarms of small bodiestravelling around the sun in elongated elliptical orbits, all along thelength of which they are scattered, and that the earth, in its annualrevolution, rushing through the midst of such swarms at the same epocheach year, naturally entangles many of them in its atmospheric net. 

The dates at which the earth is expected to pass through the principalmeteor-swarms are now pretty well known. These swarms are distinguishedfrom one another by the direction of the sky from which the meteors seemto arrive. Many of the swarms are so wide that the earth takes days, andeven weeks, to pass through them. In some of these swarms, or streams, as they are also called, the meteors are distributed with fair evennessalong the entire length of their orbits, so that the earth is greetedwith a somewhat similar shower at each yearly encounter. In others, thechief portions are bunched together, so that, in certain years, thedisplay is exceptional (see Fig. 20, p. 269). That part of the heavensfrom which a shower of meteors is seen to emanate is called the"radiant, " or radiant point, because the foreshortened view we get ofthe streaks of light makes it appear as if they radiated outwards fromthis point. In observations of these bodies the attention of astronomersis directed to registering the path and speed of each meteor, and toascertaining the position of the radiant. It is from data such as thesethat computations concerning the swarms and their orbits are made. 

[Illustration: FIG. 20. --Passage of the Earth through the thickestportion of a Meteor Swarm. The Earth and the Meteors are hererepresented as approaching each other from opposite directions. ]

For the present state of knowledge concerning meteors, astronomy islargely indebted to the researches of Mr. W. F. Denning, of Bristol, andof the late Professor A. S. Herschel. 

During the course of each year the earth encounters a goodly number ofmeteor-swarms. Three of these, giving rise to fine displays, are verywell known--the "Perseids, " or August Meteors, and the "Leonids" and"Bielids, " which appear in November. 

Of the above three the _Leonid_ display is by far the most important, and the high degree of attention paid to it has laid the foundation ofmeteoric astronomy in much the same way that the study of thefascinating corona has given such an impetus to our knowledge of thesun. The history of this shower of meteors may be traced back as far asA. D. 902, which was known as the "Year of the Stars. " It is related thatin that year, on the night of October 12th--the shower now comes about amonth later--whilst the Moorish King, Ibrahim Ben Ahmed, lay dyingbefore Cosenza, in Calabria, "a multitude of falling stars scatteredthemselves across the sky like rain, " and the beholders shuddered atwhat they considered a dread celestial portent. We have, however, littleknowledge of the subsequent history of the Leonids until 1698, sincewhich time the maximum shower has appeared with considerable regularityat intervals of about thirty-three years. But it was not until 1799 thatthey sprang into especial notice. On the 11th November in that year asplendid display was witnessed at Cumana, in South America, by thecelebrated travellers, Humboldt and Bonpland. Finer still, andsurpassing all displays of the kind ever seen, was that of November 12, 1833, when the meteors fell thick as snowflakes, 240, 000 being estimatedto have appeared during seven hours. Some of them were even so bright asto be seen in full daylight. The radiant from which the meteors seem todiverge was ascertained to be situated in the head of the constellationof the Lion, or "Sickle of Leo, " as it is popularly termed, whencetheir name--Leonids. It was from a discussion of the observations thenmade that the American astronomer, Olmsted, concluded that these meteorssprang upon us from interplanetary space, and were not, as had beenhitherto thought, born of our atmosphere. Later on, in 1837, Olbersformulated the theory that the bodies in question travelled around thesun in an elliptical orbit, and at the same time he established theperiodicity of the maximum shower. 

The periodic time of recurrence of this maximum, namely, aboutthirty-three years, led to eager expectancy as 1866 drew near. Hopeswere then fulfilled, and another splendid display took place, of whichSir Robert Ball, who observed it, has given a graphic description in his_Story of the Heavens_. The display was repeated upon a smaller scale inthe two following years. The Leonids were henceforth deemed to hold ananomalous position among meteor swarms. According to theory the earthcut through their orbit at about the same date each year, and so acertain number were then seen to issue from the radiant. But, inaddition, after intervals of thirty-three years, as has been seen, anexceptional display always took place; and this state of things was notlimited to one year alone, but was repeated at each meeting for aboutthree years running. The further assumption was, therefore, made thatthe swarm was much denser in one portion of the orbit thanelsewhere, [27] and that this congested part was drawn out to such anextent that the earth could pass through the crossing place duringseveral annual meetings, and still find it going by like a longprocession (see Fig. 20, p. 269). 

In accordance with this ascertained period of thirty-three years, therecurrence of the great Leonid shower was timed to take place on the15th of November 1899. But there was disappointment then, and thedisplays which occurred during the few years following were not of muchimportance. A good deal of comment was made at the time, and theorieswere accordingly put forward to account for the failure of the greatshower. The most probable explanation seems to be, that the attractionof one of the larger planets--Jupiter perhaps--has diverted the orbitsomewhat from its old position, and the earth does not in consequencecut through the swarm in the same manner as it used to do. 

The other November display alluded to takes place between the 23rd and27th of that month. It is called the _Andromedid_ Shower, because themeteors appear to issue from the direction of the constellation ofAndromeda, which at that period of the year is well overhead during theearly hours of the night. These meteors are also known by the name of_Bielids_, from a connection which the orbit assigned to them appears tohave with that of the well-known comet of Biela. 

M. Egenitis, Director of the Observatory of Athens, accords to theBielids a high antiquity. He traces the shower back to the days of theEmperor Justinian. Theophanes, the Chronicler of that epoch, writing ofthe famous revolt of Nika in the year A. D. 532, says:--"During the sameyear a great fall of stars came from the evening till the dawn. " M. Egenitis notes another early reference to these meteors in A. D. 752, during the reign of the Eastern Emperor, Constantine Copronymous. Writing of that year, Nicephorus, a Patriarch of Constantinople, has asfollows:--"All the stars appeared to be detached from the sky, and tofall upon the earth. "

The Bielids, however, do not seem to have attracted particular noticeuntil the nineteenth century. Attention first began to be riveted uponthem on account of their suspected connection with Biela's comet. Itappeared that the same orbit was shared both by that comet and theBielid swarm. It will be remembered that the comet in question was notseen after its appearance in 1852. Since that date, however, the Bielidshower has shown an increased activity; which was further noticed to beespecially great in those years in which the comet, had it stillexisted, would be due to pass near the earth. 

The third of these great showers to which allusion has above been made, namely, the _Perseids_, strikes the earth about the 10th of August; forwhich reason it is known on the Continent under the name of the "tearsof St. Lawrence, " the day in question being sacred to that Saint. Thisshower is traceable back many centuries, even as far as the year A. D. 811. The name given to these meteors, "Perseids, " arises from the factthat their radiant point is situated in the constellation of Perseus. This shower is, however, not by any means limited to the particularnight of August 10th, for meteors belonging to the swarm may be observedto fall in more or less varying quantities from about July 8th to August22nd. The Perseid meteors sometimes fall at the rate of about sixty perhour. They are noted for their great rapidity of motion, and theirtrails besides often persist for a minute or two before beingdisseminated. Unlike the other well-known showers, the radiants of whichare stationary, that of the Perseids shifts each night a little in aneasterly direction. 

The orbit of the Perseids cuts that of the earth almost perpendicularly. The bodies are generally supposed to be the result of the disintegrationof an ancient comet which travelled in the same orbit. Tuttle's Comet, which passed close to the earth in 1862, also belongs to this orbit; andits period of revolution is calculated to be 131 years. The Perseidsappear to be disseminated all along this great orbit, for we meet themin considerable quantities each year. The bodies in question are ingeneral particularly small. The swarm has, however, like most others, asomewhat denser portion, and through this the earth passed in 1848. The_aphelion_, or point where the far end of the orbit turns back againtowards the sun, is situated right away beyond the path of Neptune, at adistance of forty-eight times that of the earth from the sun. The cometof 1532 also belongs to the Perseid orbit. It revisited theneighbourhood of the earth in 1661, and should have returned in 1789. But we have no record of it in that year; for which omission the thenpolitically disturbed state of Europe may account. If not alreadydisintegrated, this comet is due to return in 1919. 

This supposed connection between comets and meteor-swarms must be alsoextended to the case of the Leonids. These meteors appear to travelalong the same track as Tempel's Comet of 1866. 

It is considered that the attractions of the various bodies of thesolar system upon a meteor swarm must eventually result in breaking upthe "bunched" portion, so that in time the individual meteors shouldbecome distributed along the whole length of the orbit. Upon thisassumption the Perseid swarm, in which the meteors are fairly wellscattered along its path, should be of greater age than the Leonid. Asto the Leonid swarm itself, Le Verrier held that it was first broughtinto the solar system in A. D. 126, having been captured from outer spaceby the gravitative action of the planet Uranus. 

The acknowledged theory of meteor swarms has naturally given rise to anidea, that the sunlight shining upon such a large collection ofparticles ought to render a swarm visible before its collision with theearth's atmosphere. Several attempts have therefore been made to searchfor approaching swarms by photography, but, so far, it appears withoutsuccess. It has also been proposed, by Mr. W. H. S. Monck, that the starsin those regions from which swarms are due, should be carefully watched, to see if their light exhibits such temporary diminutions as would belikely to arise from the momentary interposition of a cloud of movingparticles. 

Between ten and fifteen years ago it happened that several well-knownobservers, employed in telescopic examination of the sun and moon, reported that from time to time they had seen small dark bodies, sometimes singly, sometimes in numbers, in passage across the discs ofthe luminaries. It was concluded that these were meteors moving in spacebeyond the atmosphere of the earth. The bodies were called "darkmeteors, " to emphasise the fact that they were seen in their naturalcondition, and not in that momentary one in which they had hitherto beenalways seen; _i. E. _ when heated to white heat, and rapidly vaporised, inthe course of their passage through the upper regions of our air. This"discovery" gave promise of such assistance to meteor theories, thatcalculations were made from the directions in which they had been seento travel, and the speeds at which they had moved, in the hope that someinformation concerning their orbits might be revealed. But after a whilesome doubt began to be thrown upon their being really meteors, andeventually an Australian observer solved the mystery. He found that theywere merely tiny particles of dust, or of the black coating on the innerpart of the tube of the telescope, becoming detached from the sides ofthe eye-piece and falling across the field of view. He was led to thisconclusion by having noted that a gentle tapping of his instrumentproduced the "dark" bodies in great numbers! Thus the opportunity ofobserving meteors beyond our atmosphere had once more failed. 

_Meteorites_, also known as ærolites and fireballs, are usually placedin quite a separate category from meteors. They greatly exceed thelatter in size, are comparatively rare, and do not appear in any wayconnected with the various showers of meteors. The friction of theirpassage through the atmosphere causes them to shine with a great light;and if not shattered to pieces by internal explosions, they reach theground to bury themselves deep in it with a great rushing and noise. When found by uncivilised peoples, or savages, they are, on account oftheir celestial origin, usually regarded as objects of wonder and ofworship, and thus have arisen many mythological legends and deificationsof blackened stones. On the other hand, when they get into thepossession of the civilised, they are subjected to careful examinationsand tests in chemical laboratories. The bodies are, as a rule, composedof stone, in conjunction with iron, nickel, and such elements as existin abundance upon our earth; though occasionally specimens are foundwhich are practically pure metal. In the museums of the great capitalsof both Continents are to be seen some fine collections of meteorites. Several countries--Greenland and Mexico, for instance--contain in thesoil much meteoric iron, often in masses so large as to baffle allattempts at removal. Blocks of this kind have been known to furnish thenatives in their vicinity for many years with sources of workable iron. 

The largest meteorite in the world is one known as the Anighitometeorite. It was brought to the United States by the explorer Peary, who found it at Cape York in Greenland. He estimates its weight at from90 to 100 tons. One found in Mexico, called the Bacubirito, comes next, with an estimated weight of 27-1/2 tons. The third in size is theWillamette meteorite, found at Willamette in Oregon in 1902. It measures10 × 6-1/2 × 4-1/2 feet, and weighs about 15-1/2 tons. 

[27] The "gem" of the meteor ring, as it has been termed. 

CHAPTER XXII

THE STARS

In the foregoing chapters we have dealt at length with those celestialbodies whose nearness to us brings them into our especial notice. Theentire room, however, taken up by these bodies, is as a mere point inthe immensities of star-filled space. The sun, too, is but an ordinarystar; perhaps quite an insignificant one[28] in comparison with themajority of those which stud that background of sky against which theplanets are seen to perform their wandering courses. 

Dropping our earth and the solar system behind, let us go afield andexplore the depths of space. 

We have seen how, in very early times, men portioned out the great massof the so-called "fixed stars" into divisions known as constellations. The various arrangements, into which the brilliant points of light fellas a result of perspective, were noticed and roughly compared with suchforms as were familiar to men upon the earth. Imagination quickly saw inthem the semblances of heroes and of mighty fabled beasts; and, aroundthese monstrous shapes, legends were woven, which told how the greatdeeds done in the misty dawn of historical time had been enshrined bythe gods in the sky as an example and a memorial for men. Though thecenturies have long outlived such fantasies, yet the constellationfigures and their ancient names have been retained to this day, prettywell unaltered for want of any better arrangement. The Great and LittleBears, Cassiopeia, Perseus, and Andromeda, Orion and the rest, glitterin our night skies just as they did centuries and centuries ago. 

Many persons seem to despair of gaining any real knowledge of astronomy, merely because they are not versed in recognising the constellations. For instance, they will say:--"What is the use of my reading anythingabout the subject? Why, I believe I couldn't even point out the GreatBear, were I asked to do so!" But if such persons will only consider fora moment that what we call the Great Bear has no existence in fact, theyneed not be at all disheartened. Could we but view this familiarconstellation from a different position in space, we should perhaps bequite unable to recognise it. Mountain masses, for instance, when seenfrom new directions, are often unrecognisable. 

It took, as we have seen, a very long time for men to acknowledge theimmense distances of the stars from our earth. Their seemingunchangeableness of position was, as we have seen, largely responsiblefor the idea that the earth was immovable in space. It is a wonder thatthe Copernican system ever gained the day in the face of this apparentfixity of the stars. As time went on, it became indeed necessary toaccord to these objects an almost inconceivable distance, in order toaccount for the fact that they remained apparently quite undisplaced, notwithstanding the journey of millions of miles which the earth was nowacknowledged to make each year around the sun. In the face of thegradual and immense improvement in telescopes, this apparent immobilityof the stars was, however, not destined to last. The first ascertaineddisplacement of a star, namely that of 61 Cygni, noted by Bessel in theyear 1838, definitely proved to men the truth of the Copernican system. Since then some forty more stars have been found to show similar tinydisplacements. We are, therefore, in possession of the fact, that theactual distances of a few out of the great host can be calculated. 

To mention some of these. The nearest star to the earth, so far as weyet know, is Alpha Centauri, which is distant from us about 25 billionsof miles. The light from this star, travelling at the stupendous rate ofabout 186, 000 miles per second, takes about 4-1/4 years to reach ourearth, or, to speak astronomically, Alpha Centauri is about 4-1/4 "lightyears" distant from us. Sirius--the brightest star in the whole sky--isat twice this distance, _i. E. _ about 8-1/2 light years. Vega is about 30light years distant from us, Capella about 32, and Arcturus about 100. 

The displacements, consequent on the earth's movement, have, however, plainly nothing to say to any real movements on the part of the starsthemselves. The old idea was that the stars were absolutely fixed; hencearose the term "fixed stars"--a term which, though inaccurate, has notyet been entirely banished from the astronomical vocabulary. But carefulobservations extending over a number of years have shown slight changesof position among these bodies; and such alterations cannot be ascribedto the revolution of the earth in its orbit, for they appear to takeplace in every direction. These evidences of movement are known as"proper motions, " that is to say, actual motions in space proper to thestars themselves. Stars which are comparatively near to us show, as arule, greater proper motions than those which are farther off. It mustnot, however, be concluded that these proper motions are of any verynoticeable amounts. They are, as a matter of fact, merely upon the sameapparently minute scale as other changes in the heavens; and wouldlargely remain unnoticed were it not for the great precision of modernastronomical instruments. 

One of the swiftest moving of the stars is a star of the sixth magnitudein the constellation of the Great Bear; which is known as "1830Groombridge, " because this was the number assigned to it in a catalogueof stars made by an astronomer of that name. It is popularly known asthe "Runaway Star, " a name given to it by Professor Newcomb. Its speedis estimated to be at least 138 miles per second. It may be actuallymoving at a much greater rate, for it is possible that we see its pathsomewhat foreshortened. 

A still greater proper motion--the greatest, in fact, known--is that ofan eighth magnitude star in the southern hemisphere, in theconstellation of Pictor. Nothing, indeed, better shows the enormousdistance of the stars from us, and the consequent inability of even suchrapid movements to alter the appearance of the sky during the course ofages, than the fact that it would take more than two centuries for thestar in question to change its position in the sky by a space equal tothe apparent diameter of the moon; a statement which is equivalent tosaying that, were it possible to see this star with the naked eye, whichit is not, at least twenty-five years would have to elapse before onewould notice that it had changed its place at all!

Both the stars just mentioned are very faint. That in Pictor is, as hasbeen said, not visible to the naked eye. It appears besides to be a verysmall body, for Sir David Gill finds a parallax which makes it only asfar off from us as Sirius. The Groombridge star, too, is just about thelimit of ordinary visibility. It is, indeed, a curious fact that thefainter stars seem, on the average, to be moving more rapidly than thebrighter. 

Investigations into proper motions lead us to think that every one ofthe stars must be moving in space in some particular direction. To takea few of the best known. Sirius and Vega are both approaching our systemat a rate of about 10 miles per second, Arcturus at about 5 miles persecond, while Capella is receding from us at about 15 miles per second. Of the twin brethren, Castor and Pollux, Castor is moving away from usat about 4-1/2 miles per second, while Pollux is coming towards us atabout 33 miles per second. 

Much of our knowledge of proper motions has been obtained indirectly bymeans of the spectroscope, on the Doppler principle already treated of, by which we are enabled to ascertain whether a source from which lightis coming is approaching or receding. 

The sun being, after all, a mere star, it will appear only natural forit also to have a proper motion of its own. This is indeed the case; andit is rushing along in space at a rate of between ten and twelve milesper second, carrying with it its whole family of planets and satellites, of comets and meteors. The direction in which it is advancing is towardsa point in the constellation of Lyra, not far from its chief star Vega. This is shown by the fact that the stars about the region in questionappear to be opening out slightly, while those in the contrary portionof the sky appear similarly to be closing together. 

Sir William Herschel was the first to discover this motion of the sunthrough space; though in the idea that such a movement might take placehe seems to have been anticipated by Mayer in 1760, by Michell in 1767, and by Lalande in 1776. 

A suggestion has been made that our solar system, in its motion throughthe celestial spaces, may occasionally pass through regions whereabnormal magnetic conditions prevail, in consequence of whichdisturbances may manifest themselves throughout the system at the sameinstant. Thus the sun may be getting the credit of _producing_ what itmerely reacts to in common with the rest of its family. But thissuggestion, plausible though it may seem, will not explain why themagnetic disturbances experienced upon our earth show a certaindependence upon such purely local facts, as the period of the sun'srotation, for instance. 

One would very much like to know whether the movement of the sun isalong a straight line, or in an enormous orbit around some centre. Theidea has been put forward that it may be moving around the centre ofgravity of the whole visible stellar universe. Mädler, indeed, propounded the notion that Alcyone--the chief star in the group known asthe Pleiades--occupied this centre, and that everything revolved aroundit. He went even further to proclaim that here was the Place of theAlmighty, the Mansion of the Eternal! But Mädler's ideas upon this pointhave long been shelved. 

To return to the general question of the proper motion of stars. 

In several instances these motions appear to take place in groups, as ifcertain stars were in some way associated together. For example, a largenumber of the stars composing the Pleiades appear to be moving throughspace in the same direction. Also, of the seven stars composing thePlough, all but two--the star at the end of its "handle, " and that oneof the "pointers, " as they are called, which is the nearer to the polestar--have a common proper motion, _i. E. _ are moving in the samedirection and nearly at the same rate. 

Further still, the well-known Dutch astronomer, Professor Kapteyn, ofGroningen, has lately reached the astonishing conclusion that a greatpart of the visible universe is occupied by two vast streams of starstravelling in opposite directions. In both these great streams, theindividual bodies are found, besides, to be alike in design, alike inchemical constitution, and alike in the stage of their development. 

A fable related by the Persian astronomer, Al Sufi (tenth century, A. D. )shows well the changes in the face of the sky which proper motions arebound to produce after great lapses of time. According to this fable thestars Sirius and Procyon were the sisters of the star Canopus. Canopusmarried Rigel (another star, ) but, having murdered her, he fled towardsthe South Pole, fearing the anger of his sisters. The fable goes on torelate, among other things, that Sirius followed him across the MilkyWay. Mr. J. E. Gore, in commenting on the story, thinks that it may bebased upon a tradition of Sirius having been seen by the men of theStone Age on the opposite side of the Milky Way to that on which it nowis. 

Sirius is in that portion of the heavens _from_ which the sun isadvancing. Its proper motion is such that it is gaining upon the earthat the rate of about ten miles per second, and so it must overtake thesun after the lapse of great ages. Vega, on the other hand, is comingtowards us from that part of the sky _towards_ which the sun istravelling. It should be about half a million years before the sun andVega pass by one another. Those who have specially investigated thisquestion say that, as regards the probability of a near approach, it ismuch more likely that Vega will be then so far to one side of the sun, that her brightness will not be much greater than it is at this moment. 

Considerations like these call up the chances of stellar collisions. Such possibilities need not, however, give rise to alarm; for the stars, as a rule, are at such great distances from each other, that theprobability of relatively near approaches is slight. 

We thus see that the constellations do not in effect exist, and thatthere is in truth no real background to the sky. We find further thatthe stars are strewn through space at immense distances from each other, and are moving in various directions hither and thither. The sun, whichis merely one of them, is moving also in a certain direction, carryingthe solar system along with it. It seems, therefore, but natural tosuppose that many a star may be surrounded by some planetary system in away similar to ours, which accompanies it through space in the course ofits celestial journeyings. 

[28] Vega, for instance, shines one hundred times more brightly than thesun would do, were it to be removed to the distance at which that staris from us. 

CHAPTER XXIII

THE STARS--_continued_

The stars appear to us to be scattered about the sky without any orderlyarrangement. Further, they are of varying degrees of brightness; somebeing extremely brilliant, whilst others can but barely be seen. Thebrightness of a star may arise from either of two causes. On the onehand, the body may be really very bright in itself; on the other hand, it may be situated comparatively near to us. Sometimes, indeed, boththese circumstances may come into play together. 

Since variation in brightness is the most noticeable characteristic ofthe stars, men have agreed to class them in divisions called"magnitudes. " This term, it must be distinctly understood, is employedin such classification without any reference whatever to actual size, being merely taken to designate roughly the amount of light which wereceive from a star. The twenty brightest stars in the sky are usuallyclassed in the first magnitude. In descending the scale, each magnitudewill be noticed to contain, broadly speaking, three times as many starsas the one immediately above it. Thus the second magnitude contains 65, the third 190, the fourth 425, the fifth 1100, and the sixth 3200. Thelast of these magnitudes is about the limit of the stars which we areable to see with the naked eye. Adding, therefore, the above numberstogether, we find that, without the aid of the telescope, we cannot seemore than about 5000 stars in the entire sky--northern and southernhemispheres included. Quite a small telescope will, however, allow us tosee down to the ninth magnitude, so that the total number of starsvisible to us with such very moderate instrumental means will be wellover 100, 000. 

It must not, however, be supposed that the stars included within eachmagnitude are all of exactly the same brightness. In fact, it would bedifficult to say if there exist in the whole sky two stars which send usprecisely the same amount of light. In arranging the magnitudes, allthat was done was to make certain broad divisions, and to class withinthem such stars as were much on a par with regard to brightness. It mayhere be noted that a standard star of the first magnitude gives us aboutone hundred times as much light as a star of the sixth magnitude, andabout one million times as much as one of the sixteenth magnitude--whichis near the limit of what we can see with the very best telescope. 

Though the first twenty stars in the sky are popularly considered asbeing of the first magnitude, yet several of them are much brighter thanan average first magnitude star would be. For instance, Sirius--thebrightest star in the whole sky--is equal to about eleven firstmagnitude stars, like, say, Aldebaran. In consequence of suchdifferences, astronomers are agreed in classifying the brightest of themas _brighter_ than the standard first magnitude star. On this principleSirius would be about two and a half magnitudes _above_ the first. Thisnotation is usefully employed in making comparisons between the amountof light which we receive from the sun, and that which we get from anindividual star. Thus the sun will be about twenty-seven and a halfmagnitudes _above_ the first magnitude. The range, therefore, betweenthe light which we receive from the sun (considered merely as a verybright star) and the first magnitude stars is very much greater thanthat between the latter and the faintest star which can be seen with thetelescope, or even registered upon the photographic plate. 

To classify stars merely by their magnitudes, without some definite noteof their relative position in the sky, would be indeed of little avail. We must have some simple method of locating them in the memory, and theconstellations of the ancients here happily come to our aid. A systemcombining magnitudes with constellations was introduced by Bayer in1603, and is still adhered to. According to this the stars in eachconstellation, beginning with the brightest star, are designated by theletters of the Greek alphabet taken in their usual order. For example, in the constellation of Canis Major, or the Greater Dog, the brighteststar is the well-known Sirius, called by the ancients the "Dog Star";and this star, in accordance with Bayer's method, has received the Greekletter [a] (alpha), and is consequently known as Alpha CanisMajoris. [29] As soon as the Greek letters are used up in this way theRoman alphabet is brought into requisition, after which recourse is hadto ordinary numbers. 

Notwithstanding this convenient arrangement, some of the brighteststars are nearly always referred to by certain proper names given tothem in old times. For instance, it is more usual to speak of Sirius, Arcturus, Vega, Capella, Procyon, Aldebaran, Regulus, and so on, than of[a] Canis Majoris, [a] Boötis, [a] Lyræ, [a] Aurigæ, [a] Canis Minoris, [a] Tauri, [a] Leonis, &c. &c. 

In order that future generations might be able to ascertain what changeswere taking place in the face of the sky, astronomers have from time totime drawn up catalogues of stars. These lists have included stars of acertain degree of brightness, their positions in the sky being notedwith the utmost accuracy possible at the period. The earliest knowncatalogue of this kind was made, as we have seen, by the celebratedGreek astronomer, Hipparchus, about the year 125 B. C. It contained 1080stars. It was revised and brought up to date by Ptolemy in A. D. 150. Another celebrated list was that drawn up by the Persian astronomer, AlSufi, about the year A. D. 964. In it 1022 stars were noted down. Acatalogue of 1005 stars was made in 1580 by the famous Danishastronomer, Tycho Brahe. Among modern catalogues that of Argelander(1799-1875) contained as many as 324, 198 stars. It was extended bySchönfeld so as to include a portion of the Southern Hemisphere, inwhich way 133, 659 more stars were added. 

In recent years a project was placed on foot of making a photographicsurvey of the sky, the work to be portioned out among various nations. Agreat part of this work has already been brought to a conclusion. About15, 000, 000 stars will appear upon the plates; but, so far, it has beenproposed to catalogue only about a million and a quarter of thebrightest of them. This idea of surveying the face of the sky byphotography sprang indirectly from the fine photographs which Sir DavidGill took, when at the Cape of Good Hope, of the Comet of 1882. Theimmense number of star-images which had appeared upon his platessuggested the idea that photography could be very usefully employed toregister the relative positions of the stars. 

The arrangement of seven stars known as the "Plough" is perhaps the mostfamiliar configuration in the sky (see Plate XIX. , p. 292). In theUnited States it is called the "Dipper, " on account of its likeness tothe outline of a saucepan, or ladle. "Charles' Wain" was the old Englishname for it, and readers of Cæsar will recollect it under_Septentriones_, or the "Seven Stars, " a term which that writer uses asa synonym for the North. Though identified in most persons' minds with_Ursa Major_, or the Great Bear, the Plough is actually only a smallportion of that famous constellation. Six out of the seven stars whichgo to make up the well-known figure are of the second magnitude, whilethe remaining one, which is the middle star of the group, is of thethird. 

The Greek letters, as borne by the individual stars of the Plough, are aplain transgression of Bayer's method as above described, for they havecertainly not been allotted here in accordance with the proper order ofbrightness. For instance, the third magnitude star, just alluded to asbeing in the middle of the group, has been marked with the Greek letter[d] (Delta); and so is made to take rank _before_ the stars composingwhat is called the "handle" of the Plough, which are all of the secondmagnitude. Sir William Herschel long ago drew attention to the irregularmanner in which Bayer's system had been applied. It is, indeed, a greatpity that this notation was not originally worked out with greater careand correctness; for, were it only reliable, it would afford greatassistance to astronomers in judging of what changes in relativebrightness have taken place among the stars. 

Though we may speak of using the constellations as a method of findingour way about the sky, it is, however, to certain marked groupings inthem of the brighter stars that we look for our sign-posts. 

Most of the constellations contain a group or so of noticeable stars, whose accidental arrangement dimly recalls the outline of some familiargeometrical figure and thus arrests the attention. [30] For instance, inan almost exact line with the two front stars of the Plough, or"pointers" as they are called, [31] and at a distance about five times asfar away as the interval between them, there will be found a third starof the second magnitude. This is known as Polaris, or the Pole Star, forit very nearly occupies that point of the heaven towards which the northpole of the earth's axis is _at present_ directed (see Plate XIX. , p. 292). Thus during the apparently daily rotation of the heavens, thisstar looks always practically stationary. It will, no doubt, beremembered how Shakespeare has put into the mouth of Julius Cæsar thesememorable words:--

"But I am constant as the northern star, Of whose true-fix'd and resting qualityThere is no fellow in the firmament. "

[Illustration: PLATE XIX. THE SKY AROUND THE NORTH POLE

We see here the Plough, the Pole Star, Ursa Minor, Auriga, Cassiopeia'sChair, and Lyra. Also the Circle of Precession, along which the Polemakes a complete revolution in a period of 25, 868 years, and theTemporary Star discovered by Tycho Brahe in the year 1572. 

(Page 291)]

On account of the curvature of the earth's surface, the height at whichthe Pole Star is seen above the horizon at any place depends regularlyupon the latitude; that is to say, the distance of the place in questionfrom the equator. For instance, at the north pole of the earth, wherethe latitude is greatest, namely, 90°, the Pole Star will appeardirectly overhead; whereas in England, where the latitude is about 50°, it will be seen a little more than half way up the northern sky. At theequator, where the latitude is _nil_, the Pole Star will be on thehorizon due north. 

In consequence of its unique position, the Pole Star is of very greatservice in the study of the constellations. It is a kind of centrearound which to hang our celestial ideas--a starting point, so to speak, in our voyages about the sky. 

According to the constellation figures, the Pole Star is in _UrsaMinor_, or the Little Bear, and is situated at the end of the tail ofthat imaginary figure (see Plate XIX. , p. 292). The chief stars of thisconstellation form a group not unlike the Plough, except that the"handle" is turned in the contrary direction. The Americans, inconsequence, speak of it as the "Little Dipper. "

Before leaving this region of the sky, it will be well to draw attentionto the second magnitude star [z] in the Great Bear (Zeta Ursæ Majoris), which is the middle star in the "handle" of the Plough. This star isusually known as Mizar, a name given to it by the Arabians. A personwith good eyesight can see quite near to it a fifth magnitude star, known under the name of Alcor. We have here a very good example of thatdeception in the estimation of objects in the sky, which has beenalluded to in an earlier chapter. Alcor is indeed distant from Mizar byabout one-third the apparent diameter of the moon, yet no one wouldthink so!

On the other side of Polaris from the Plough, and at about an equalapparent distance, will be found a figure in the form of an irregular"W", made up of second and third magnitude stars. This is the well-known"Cassiopeia's Chair"--portion of the constellation of _Cassiopeia_ (seePlate XIX. , p. 292). 

On either side of the Pole Star, about midway between the Plough andCassiopeia's Chair, but a little further off from it than these, are theconstellations of _Auriga_ and _Lyra_ (see Plate XIX. , p. 292). Theformer constellation will be easily recognised, because its chieffeatures are a brilliant yellowish first magnitude star, with one of thesecond magnitude not far from it. The first magnitude star is Capella, the other is [b] Aurigæ. Lyra contains only one first magnitudestar--Vega, pale blue in colour. This star has a certain interest for usfrom the fact that, as a consequence of that slow shift of direction ofthe earth's axis known as Precession, it will be very near the northpole of the heavens in some 12, 000 years, and so will then be consideredthe pole star (see Plate XIX. , p. 292). The constellation of Lyraitself, it must also be borne in mind, occupies that region of theheavens towards which the solar system is travelling. 

The handle of the Plough points roughly towards the constellation of_Boötes_, in which is the brilliant first magnitude star Arcturus. Thisstar is of an orange tint. 

Between Boötes and Lyra lie the constellations of _Corona Borealis_ (orthe Northern Crown) and _Hercules_. The chief feature of CoronaBorealis, which is a small constellation, is a semicircle of six smallstars, the brightest of which is of the second magnitude. Theconstellation of Hercules is very extensive, but contains no starbrighter than the third magnitude. 

Near to Lyra, on the side away from Hercules, are the constellations of_Cygnus_ and _Aquila_. Of the two, the former is the nearer to the PoleStar, and will be recognised by an arrangement of stars widely set inthe form of a cross, or perhaps indeed more like the framework of aboy's kite. The position of Aquila will be found through the fact thatthree of its brightest stars are almost in a line and close together. The middle of these is Altair, a yellowish star of the first magnitude. 

At a little distance from Ursa Major, on the side away from the PoleStar, is the constellation of _Leo_, or the Lion. Its chief feature is aseries of seven stars, supposed to form the head of that animal. Thearrangement of these stars is, however, much more like a sickle, wherefore this portion of the constellation is usually known as the"Sickle of Leo. " At the end of the handle of the sickle is a white firstmagnitude star--Regulus. 

The reader will, no doubt, recollect that it is from a point in theSickle of Leo that the Leonid meteors appear to radiate. 

The star second in brightness in the constellation of Leo is known asDenebola. This star, now below the second magnitude, seems to have beenvery much brighter in the past. It is noted, indeed, as a brilliantfirst magnitude star by Al Sufi, that famous Persian astronomer wholived, as we have seen, in the tenth century. Ptolemy also notes it asof the first magnitude. 

In the neighbourhood of Auriga, and further than it from the Pole Star, are several remarkable constellations--Taurus, Orion, Gemini, CanisMinor, and Canis Major (see Plate XX. , p. 296). 

The first of these, _Taurus_ (or the Bull), contains two conspicuousstar groups--the Pleiades and the Hyades. The Pleiades are six or sevensmall stars quite close together, the majority of which are of thefourth magnitude. This group is sometimes occulted by the moon. The wayin which the stars composing it are arranged is somewhat similar to thatin the Plough, though of course on a scale ever so much smaller. Theimpression which the group itself gives to the casual glance is thusadmirably pictured in Tennyson's _Locksley Hall_:--

"Many a night I saw the Pleiads, rising through the mellow shade, Glitter like a swarm of fire-flies tangled in a silver braid. "

[Illustration: PLATE XX. ORION AND HIS NEIGHBOURS

We see here that magnificent region of the sky which contains thebrightest star of all--Sirius. Note also especially the Milky Way, thePleiades, the Hyades, and the "Belt" and "Sword" of Orion. 

(Page 296)]

The group of the Hyades occupies the "head" of the Bull, and is muchmore spread out than that of the Pleiades. It is composed besides ofbrighter stars, the brightest being one of the first magnitude, Aldebaran. This star is of a red colour, and is sometimes known as the"Eye of the Bull. "

The constellation of _Orion_ is easily recognised as an irregularquadrilateral formed of four bright stars, two of which, Betelgeux(reddish) and Rigel (brilliant white), are of the first magnitude. Inthe middle of the quadrilateral is a row of three second magnitudestars, known as the "Belt" of Orion. Jutting off from this is anotherrow of stars called the "Sword" of Orion. 

The constellation of _Gemini_, or the Twins, contains two brightstars--Castor and Pollux--close to each other. Pollux, though markedwith the Greek letter [b], is the brighter of the two, and nearly of thestandard first magnitude. 

Just further from the Pole than Gemini, is the constellation of _CanisMinor_, or the Lesser Dog. Its chief star is a white first magnitudeone--Procyon. 

Still further again from the Pole than Canis Minor is the constellationof _Canis Major_, or the Greater Dog. It contains the brightest star inthe whole sky, the first magnitude star Sirius, bluish-white in colour, also known as the "Dog Star. " This star is almost in line with the starsforming the Belt of Orion, and is not far from that constellation. 

Taken in the following order, the stars Capella, [b] Aurigæ, Castor, Pollux, Procyon, and Sirius, when they are all above the horizon at thesame time, form a beautiful curve stretching across the heaven. 

The groups of stars visible in the southern skies have by no means thesame fascination for us as those in the northern. The ancients were ingeneral unacquainted with the regions beyond the equator, and so theirscheme of constellations did not include the sky around the South Poleof the heavens. In modern times, however, this part of the celestialexpanse was also portioned out into constellations for the purpose ofeasy reference; but these groupings plainly lack that simplicity ofconception and legendary interest which are so characteristic of theolder ones. 

The brightest star in the southern skies is found in the constellationof _Argo_, and is known as Canopus. In brightness it comes next toSirius, and so is second in that respect in the entire heaven. It doesnot, however, rise above the English horizon. 

Of the other southern constellations, two call for especial notice, andthese adjoin each other. One is _Centaurus_ (or the Centaur), whichcontains the two first magnitude stars, [a] and [b] Centauri. The firstof these, Alpha Centauri, comes next in brightness to Canopus, and isnotable as being the nearest of all the stars to our earth. The otherconstellation is called _Crux_, and contains five stars set in the formof a rough cross, known as the "Southern Cross. " The brightest of these, [a] Crucis, is of the first magnitude. 

Owing to the Precession of the Equinoxes, which, as we have seen, gradually shifts the position of the Pole among the stars, certainconstellations used to be visible in ancient times in more northerlylatitudes than at present. For instance, some five thousand years agothe Southern Cross rose above the English horizon, and was just visiblein the latitude of London. It has, however, long ago even ceased to beseen in the South of Europe. The constellation of Crux happens to besituated in that remarkable region of the southern skies, in which arefound the stars Canopus and Alpha Centauri, and also the most brilliantportion of the Milky Way. It is believed to be to this grand celestialregion that allusion is made in the Book of Job (ix. 9), under the titleof the "Chambers of the South. " The "Cross" must have been still anotable feature in the sky of Palestine in the days when that ancientpoem was written. 

There is no star near enough to the southern pole of the heavens to earnthe distinction of South Polar Star. 

The Galaxy, or _Milky Way_ (see Plate XX. , p. 296), is a broad band ofdiffused light which is seen to stretch right around the sky. Thetelescope, however, shows it to be actually composed of a great host ofvery faint stars--too faint, indeed, to be separately distinguished withthe naked eye. Along a goodly stretch of its length it is cleft in two;while near the south pole of the heavens it is entirely cut across by adark streak. 

In this rapid survey of the face of the sky, we have not been able to domore than touch in the broadest manner upon some of the most noticeablestar groups and a few of the most remarkable stars. To go any further isnot a part of our purpose; our object being to deal with celestialbodies as they actually are, and not in those groupings under which theydisplay themselves to us as a mere result of perspective. 

[29] Attention must here be drawn to the fact that the name of theconstellation is always put in the genitive case. 

[30] The early peoples, as we have seen, appear to have been attractedby those groupings of the stars which reminded them in a way of thefigures of men and animals. We moderns, on the other hand, seek almostinstinctively for geometrical arrangements. This is, perhaps, symptomatic of the evolution of the race. In the growth of theindividual we find, for example, something analogous. A child, who hasbeen given pencil and paper, is almost certain to produce grotesquedrawings of men and animals; whereas the idle and half-consciousscribblings which a man may make upon his blotting-paper are usually ofa geometrical character. 

[31] Because the line joining them _points_ in the direction of the PoleStar. 

CHAPTER XXIV

SYSTEMS OF STARS

Many stars are seen comparatively close together. This may plainly arisefrom two reasons. Firstly, the stars may happen to be almost in the sameline of sight; that is to say, seen in nearly the same direction; andthough one star may be ever so much nearer to us than the other, theresult will give all the appearance of a related pair. A seemingarrangement of two stars in this way is known as a "double, " or doublestar; or, indeed, to be very precise, an "optical double. " Secondly, ina pair of stars, both bodies may be about the same distance from us, andactually connected as a system like, for instance, the moon and theearth. A pairing of stars in this way, though often casually alluded toas a double star, is properly termed a "binary, " or binary system. 

But collocations of stars are by no means limited to two. We find, indeed, all over the sky such arrangements in which there are three ormore stars; and these are technically known as "triple" or "multiple"stars respectively. Further, groups are found in which a great number ofstars are closely massed together, such a massing together of starsbeing known as a "cluster. "

The Pole Star (Polaris) is a double star, one of the components being ofa little below the second magnitude, and the other a little below theninth. They are so close together that they appear as one star to thenaked eye, but they may be seen separate with a moderately sizedtelescope. The brighter star is yellowish, and the faint one white. Thisbrighter star is found _by means of the spectroscope_ to be actuallycomposed of three stars so very close together that they cannot be seenseparately even with a telescope. It is thus a triple star, and thethree bodies of which it is composed are in circulation about eachother. Two of them are darker than the third. 

The method of detecting binary stars by means of the spectroscope is anapplication of Doppler's principle. It will, no doubt, be rememberedthat, according to the principle in question, we are enabled, fromcertain shiftings of the lines in the spectrum of a luminous body, toascertain whether that body is approaching us or receding from us. Nowthere are certain stars which always appear single even in the largesttelescopes, but when the spectroscope is directed to them a spectrum_with two sets of lines_ is seen. Such stars must, therefore, be double. Further, if the shiftings of the lines, in a spectrum like this, tell usthat the component stars are making small movements to and from us whichgo on continuously, we are therefore justified in concluding that theseare the orbital revolutions of a binary system greatly compressed bydistance. Such connected pairs of stars, since they cannot be seenseparately by means of any telescope, no matter how large, are known as"spectroscopic binaries. "

In observations of spectroscopic binaries we do not always get a doublespectrum. Indeed, if one of the components be below a certainmagnitude, its spectrum will not appear at all; and so we are left inthe strange uncertainty as to whether this component is merely faint oractually dark. It is, however, from the shiftings of the lines in thespectrum of the other component that we see that an orbital movement isgoing on, and are thus enabled to conclude that two bodies are hereconnected into a system, although one of these bodies resolutely refusesdirectly to reveal itself even to the all-conquering spectroscope. 

Mizar, that star in the handle of the Plough to which we have alreadydrawn attention, will be found with a small telescope to be a finedouble, one of the components being white and the other greenish. Actually, however, as the American astronomer, Professor F. R. Moulton, points out, these stars are so far from each other that if we could betransferred to one of them we should see the other merely as an ordinarybright star. The spectroscope shows that the brighter of these stars isagain a binary system of two huge suns, the components revolving aroundeach other in a period of about twenty days. This discovery made byProfessor E. C. Pickering, the _first_ of the kind by means of thespectroscope, was announced in 1889 from the Harvard Observatory in theUnited States. 

A star close to Vega, known as [e] (Epsilon) Lyræ (see Plate XIX. , p. 292), is a double, the components of which may be seen separately withthe naked eye by persons with very keen eyesight. If this star, however, be viewed with the telescope, the two companions will be seen far apart;and it will be noticed that each of them is again a double. 

By means of the spectroscope Capella is shown to be really composed oftwo stars (one about twice as bright as the other) situated very closetogether and forming a binary system. Sirius is also a binary system;but it is what is called a "visual" one, for its component stars may be_seen_ separately in very large telescopes. Its double, or ratherbinary, nature, was discovered in 1862 by the celebrated optician AlvanG. Clark, while in the act of testing the 18-inch refracting telescope, then just constructed by his firm, and now at the Dearborn Observatory, Illinois, U. S. A. The companion is only of the tenth magnitude, andrevolves around Sirius in a period of about fifty years, at a meandistance equal to about that of Uranus from the sun. Seen from Sirius, it would shine only something like our full moon. It must beself-luminous and not a mere planet; for Mr. Gore has shown that if itshone only by the light reflected from Sirius, it would be quiteinvisible even in the Great Yerkes Telescope. 

Procyon is also a binary, its companion having been discovered byProfessor J. M. Schaeberle at the Lick Observatory in 1896. The period ofrevolution in this system is about forty years. Observations by Mr. T. Lewis of Greenwich seem, however, to point to the companion being asmall nebula rather than a star. 

The star [ê] (Eta) Cassiopeiæ (see Plate XIX. , p. 292), is easily seenas a fine double in telescopes of moderate size. It is a binary system, the component bodies revolving around their common centre of gravity ina period of about two hundred years. This system is comparatively nearto us, _i. E. _ about nine light years, or a little further off thanSirius. 

In a small telescope the star Castor will be found double, thecomponents, one of which is brighter than the other, forming a binarysystem. The fainter of these was found by Belopolsky, with thespectroscope, to be composed of a system of two stars, one bright andthe other either dark or not so bright, revolving around each other in aperiod of about three days. The brighter component of Castor is also aspectroscopic binary, with a period of about nine days; so that thewhole of what we see with the naked eye as Castor, is in reality aremarkable system of four stars in mutual orbital movement. 

Alpha Centauri--the nearest star to the earth--is a visual binary, thecomponent bodies revolving around each other in a period of abouteighty-one years. The extent of this system is about the same as that ofSirius. Viewed from each other, the bodies would shine only like our sunas seen from Neptune. 

Among the numerous binary stars the orbits of some fifty have beensatisfactorily determined. Many double stars, for which this has not yetbeen done, are, however, believed to be, without doubt, binary. In somecases a parallax has been found; so that we are enabled to estimate inmiles the actual extent of such systems, and the masses of the bodies interms of the sun's mass. 

Most of the spectroscopic binaries appear to be upon a smaller scalethan the telescopic ones. Some are, indeed, comparatively speaking, quite small. For instance, the component stars forming [b] Aurigæ areabout eight million miles apart, while in [z] Geminorum, the distancebetween the bodies is only a little more than a million miles. 

Spectroscopic binaries are probably very numerous. Professor W. W. Campbell, Director of the Lick Observatory, estimates, for instance, that, out of about every half-a-dozen stars, one is a spectroscopicbinary. 

It is only in the case of binary systems that we can discover the massesof stars at all. These are ascertained from their movements with regardto each other under the influence of their mutual gravitativeattractions. In the case of simple stars we have clearly nothing of thekind to judge by; though, if we can obtain a parallax, we may hazard aguess from their brightness. 

Binary stars were incidentally discovered by Sir William Herschel. Inhis researches to get a stellar parallax he had selected a number ofdouble stars for test purposes, on the assumption that, if one of such apair were much nearer than the other, it might show a displacement withregard to its neighbour as a direct consequence of the earth's orbitalmovement around the sun. He, however, failed entirely to obtain anyparallaxes, the triumph in this being, as we have seen, reserved forBessel. But in some of the double stars which he had selected, he foundcertain alterations in the relative positions of the bodies, whichplainly were not a consequence of the earth's motion, but showed ratherthat there was an actual circling movement of the bodies themselvesunder their mutual attractions. It is to be noted that the existence ofsuch connected pairs had been foretold as probable by the Rev. JohnMichell, who lived a short time before Herschel. 

The researches into binary systems--both those which can be seen withthe eye and those which can be observed by means of the spectroscope, ought to impress upon us very forcibly the wide sway of the law ofgravitation. 

Of star clusters about 100 are known, and such systems often containseveral thousand stars. They usually cover an area of sky somewhatsmaller than the moon appears to fill. In most clusters the stars arevery faint, and, as a rule, are between the twelfth and sixteenthmagnitudes. It is difficult to say whether these are actually smallbodies, or whether their faintness is due merely to their great distancefrom us, since they are much too far off to show any appreciableparallactic displacement. Mr. Gore, however, thinks there is goodevidence to show that the stars in clusters are really close, and thatthe clusters themselves fill a comparatively small space. 

One of the finest examples of a cluster is the great globular one, inthe constellation of Hercules, discovered by Halley in 1714. It containsover 5000 stars, and upon a clear, dark night is visible to the nakedeye as a patch of light. In the telescope, however, it is a wonderfulobject. There are also fine clusters in the constellations of Auriga, Pegasus, and Canes Venatici. In the southern heavens there are somemagnificent examples of globular clusters. This hemisphere seems, indeed, to be richer in such objects than the northern. For instance, there is a great one in the constellation of the Centaur, containingsome 6000 stars (see Plate XXI. , p. 306). 

[Illustration: PLATE XXI. THE GREAT GLOBULAR CLUSTER IN THE SOUTHERNCONSTELLATION OF CENTAURUS

From a photograph taken at the Cape Observatory, on May 24th, 1903. Timeof exposure, 1 hour. 

(Page 306)]

Certain remarkable groups of stars, of a nature similar to clusters, though not containing such faint or densely packed stars as those wehave just alluded to, call for a mention in this connection. The bestexample of such star groups are the Pleiades and the Hyades (see PlateXX. , p. 296), Coma Berenices, and Præsepe (or the Beehive), thelast-named being in the constellation of Cancer. 

Stars which alter in their brightness are called _Variable Stars_, or"variables. " The first star whose variability attracted attention isthat known as Omicron Ceti, namely, the star marked with the Greekletter [o] (Omicron) in the constellation of Cetus, or the Whale, aconstellation situated not far from Taurus. This star, the variabilityof which was discovered by Fabricius in 1596, is also known as Mira, orthe "Wonderful, " on account of the extraordinary manner in which itslight varies from time to time. The star known by the name of Algol, [32]popularly called the "Demon Star"--whose astronomical designation is [b](Beta) Persei, or the star second in brightness in the constellation ofPerseus--was discovered by Goodricke, in the year 1783, to be a variablestar. In the following year [b] Lyræ, the star in Lyra next in order ofbrightness after Vega, was also found by the same observer to be avariable. It may be of interest to the reader to know that Goodricke wasdeaf and dumb, and that he died in 1786 at the early age of twenty-oneyears!

It was not, however, until the close of the nineteenth century that muchattention was paid to variable stars. Now several hundreds of these areknown, thanks chiefly to the observations of, amongst others, ProfessorS. C. Chandler of Boston, U. S. A. , Mr. John Ellard Gore of Dublin, and Dr. A. W. Roberts of South Africa. This branch of astronomy has not, indeed, attracted as much popular attention as it deserves, no doubt because thenature of the work required does not call for the glamour of anobservatory or a large telescope. 

The chief discoveries with regard to variable stars have been made bythe naked eye, or with a small binocular. The amount of variation isestimated by a comparison with other stars. As in many other branches ofastronomy, photography is now employed in this quest with markedsuccess; and lately many variable stars have been found to exist inclusters and nebulæ. 

It was at one time considered that a variable star was in allprobability a body, a portion of whose surface had been relativelydarkened in some manner akin to that in which sun spots mar the face ofthe sun; and that when its axial rotation brought the less illuminatedportions in turn towards us, we witnessed a consequent diminution in thestar's general brightness. Herschel, indeed, inclined to thisexplanation, for his belief was that all the stars bore spots like thoseof the sun. It appears preferably thought nowadays that disturbancestake place periodically in the atmosphere or surroundings of certainstars, perhaps through the escape of imprisoned gases, and that this maybe a fruitful cause of changes of brilliancy. The theory in questionwill, however, apparently account for only one class of variable star, namely, that of which Mira Ceti is the best-known example. The scale onwhich it varies in brightness is very great, for it changes from thesecond to the ninth magnitude. For the other leading type of variablestar, Algol, of which mention has already been made, is the bestinstance. The shortness of the period in which the changes of brightnessin such stars go their round, is the chief characteristic of this latterclass. The period of Algol is a little under three days. This star whenat its brightest is of about the second magnitude, and when least brightis reduced to below the third magnitude; from which it follows that itslight, when at the minimum, is only about one-third of what it is whenat the maximum. It seems definitely proved by means of the spectroscopethat variables of this kind are merely binary stars, too close to beseparated by the telescope, which, as a consequence of their orbitschancing to be edgewise towards us, eclipse each other in turn timeafter time. If, for instance, both components of such a pair are bright, then when one of them is right behind the other, we will not, of course, get the same amount of light as when they are side by side. If, on theother hand, one of the components happens to be dark or less luminousand the other bright, the manner in which the light of the bright starwill be diminished when the darker star crosses its face should easilybe understood. It is to the second of these types that Algol is supposedto belong. The Algol system appears to be composed of a body about asbroad as our sun, which regularly eclipses a brighter body which has adiameter about half as great again. 

Since the companion of Algol is often spoken of as a _dark_ body, itwere well here to point out that we have no evidence at all that it isentirely devoid of light. We have already found, in dealing withspectroscopic binaries, that when one of the component stars is below acertain magnitude[33] its spectrum will not be seen; so one is left inthe glorious uncertainty as to whether the body in question isabsolutely dark, or darkish, or faint, or indeed only just out of rangeof the spectroscope. 

It is thought probable by good authorities that the companion of Algolis not quite dark, but has some inherent light of its own. It is, ofcourse, much too near Algol to be seen with the largest telescope. Thereis in fact a distance of only from two to three millions of milesbetween the bodies, from which Mr. Gore infers that they would probablyremain unseparated even in the largest telescope which could ever beconstructed by man. 

The number of known variables of the Algol type is, so far, small; notmuch indeed over thirty. In some of them the components are believed torevolve touching each other, or nearly so. An extreme example of this isfound in the remarkable star V. Puppis, an Algol variable of thesouthern hemisphere. Both its components are bright, and the period oflight variation is about one and a half days. Dr. A. W. Roberts findsthat the bodies are revolving around each other in actual contact. 

_Temporary stars_ are stars which have suddenly blazed out in regions ofthe sky where no star was previously seen, and have faded away more orless gradually. 

It was the appearance of such a star, in the year 134 B. C. , whichprompted Hipparchus to make his celebrated catalogue, with the object ofleaving a record by which future observers could note celestial changes. In 1572 another star of this kind flashed out in the constellation ofCassiopeia (see Plate XIX. , p. 292), and was detected by Tycho Brahe. Itbecame as bright as the planet Venus, and eventually was visible in theday-time. Two years later, however, it disappeared, and has never sincebeen seen. In 1604 Kepler recorded a similar star in the constellationof Ophiuchus which grew to be as bright as Jupiter. It also lasted forabout two years, and then faded away, leaving no trace behind. It israrely, however, that temporary stars attain to such a brilliance; andso possibly in former times a number of them may have appeared, but nothave risen to a sufficient magnitude to attract attention. Even now, unless such a star becomes clearly visible to the naked eye, it runs agood chance of not being detected. A curious point, worth noting, withregard to temporary stars is that the majority of them have appeared inthe Milky Way. 

These sudden visitations have in our day received the name of _Novæ_;that is to say, "New" Stars. Two, in recent years, attracted a good dealof attention. The first of these, known as Nova Aurigæ, or the New Starin the constellation of Auriga, was discovered by Dr. T. D. Anderson atEdinburgh in January 1892. At its greatest brightness it attained toabout the fourth magnitude. By April it had sunk to the twelfth, butduring August it recovered to the ninth magnitude. After this lastflare-up it gradually faded away. 

The startling suddenness with which temporary stars usually spring intobeing is the groundwork upon which theories to account for their originhave been erected. That numbers of dark stars, extinguished suns, so tospeak, may exist in space, there is a strong suspicion; and it is justpossible that we have an instance of one dark stellar body in thecompanion of Algol. That such dark stars might be in rapid motion isreasonable to assume from the already known movements of bright stars. Two dark bodies might, indeed, collide together, or a collision mighttake place between a dark star and a star too faint to be seen even withthe most powerful telescope. The conflagration produced by the impactwould thus appear where nothing had been seen previously. Again, asimilar effect might be produced by a dark body, or a star too faint tobe seen, being heated to incandescence by plunging in its course througha nebulous mass of matter, of which there are many examples lying aboutin space. 

The last explanation, which is strongly reminiscent of what takes placein shooting stars, appears more probable than the collision theory. Theflare-up of new stars continues, indeed, only for a comparatively shorttime; whereas a collision between two bodies would, on the other hand, produce an enormous nebula which might take even millions of years tocool down. We have, indeed, no record of any such sudden appearance of alasting nebula. 

The other temporary star, known as Nova Persei, or the new star in theconstellation of Perseus, was discovered early in the morning ofFebruary 22, 1901, also by Dr. Anderson. A day later it had grown to bebrighter than Capella. Photographs which had been taken, some three daysprevious to its discovery, of the very region of the sky in which it hadburst forth, were carefully examined, and it was not found in these. Atthe end of two days after its discovery Nova Persei had lost one-thirdof its light. During the ensuing six months it passed through a seriesof remarkable fluctuations, varying in brightness between the third andfifth magnitudes. In the month of August it was seen to be surrounded byluminous matter in the form of a nebula, which appeared to be graduallyspreading to some distance around. Taking into consideration the greatway off at which all this was taking place, it looked as if the new starhad ejected matter which was travelling outward with a velocityequivalent to that of light. The remarkable theory was, however, putforward by Professor Kapteyn and the late Dr. W. E. Wilson that theremight be after all no actual transmission of matter; but that perhapsthe real explanation was the gradual _illumination_ of hithertoinvisible nebulous matter, as a consequence of the flare-up which hadtaken place about six months before. It was, therefore, imagined thatsome dark body moving through space at a very rapid rate had plungedthrough a mass of invisible nebulous matter, and had consequently becomeheated to incandescence in its passage, very much like what happens to ameteor when moving through our atmosphere. The illumination thus set uptemporarily in one point, being transmitted through the nebulous wastesaround with the ordinary velocity of light, had gradually rendered thissurrounding matter visible. On the assumptions required to fit in withsuch a theory, it was shown that Nova Persei must be at a distance fromwhich light would take about three hundred years in coming to us. Theactual outburst of illumination, which gave rise to this temporary star, would therefore have taken place about the beginning of the reign ofJames I. 

Some recent investigations with regard to Nova Persei have, however, greatly narrowed down the above estimate of its distance from us. Forinstance, Bergstrand proposes a distance of about ninety-nine lightyears; while the conclusions of Mr. F. W. Very would bring it stillnearer, _i. E. _ about sixty-five light years. 

The last celestial objects with which we have here to deal are the_Nebulæ_. These are masses of diffused shining matter scattered here andthere through the depths of space. Nebulæ are of several kinds, and havebeen classified under the various headings of Spiral, Planetary, Ring, and Irregular. 

A typical _spiral_ nebula is composed of a disc-shaped central portion, with long curved arms projecting from opposite sides of it, which givean impression of rapid rotatory movement. 

The discovery of spiral nebulæ was made by Lord Rosse with his great6-foot reflector. Two good examples of these objects will be found inUrsa Major, while there is another fine one in Canes Venatici (see PlateXXII. , p. 314), a constellation which lies between Ursa Major andBoötes. But the finest spiral of all, perhaps the most remarkable nebulaknown to us, is the Great Nebula in the constellation of Andromeda, (seePlate XXIII. , p. 316)--a constellation just further from the pole thanCassiopeia. When the moon is absent and the night clear this nebula canbe easily seen with the naked eye as a small patch of hazy light. It isreferred to by Al Sufi. 

[Illustration: PLATE XXII. SPIRAL NEBULA IN THE CONSTELLATION OF CANESVENATICI

From a photograph by the late Dr. W. E. Wilson, D. Sc. , F. R. S. 

(Page 314)]

Spiral nebulæ are white in colour, whereas the other kinds of nebulahave a greenish tinge. They are also by far the most numerous; and thelate Professor Keeler, who considered this the normal type of nebula, estimated that there were at least 120, 000 of such spirals within thereach of the Crossley reflector of the Lick Observatory. ProfessorPerrine has indeed lately raised this estimate to half a million, andthinks that with more sensitive photographic plates and longer exposuresthe number of spirals would exceed a million. The majority of theseobjects are very small, and appear to be distributed over the sky in afairly uniform manner. 

_Planetary_ nebulæ are small faint roundish objects which, when seen inthe telescope, recall the appearance of a planet, hence their name. Oneof these nebulæ, known astronomically as G. C. 4373, has recently beenfound to be rushing through space towards the earth at a rate of betweenthirty and forty miles per second. It seems strange, indeed, that anygaseous mass should move at such a speed!

What are known as _ring_ nebulæ were until recently believed to form aspecial class. These objects have the appearance of mere rings ofnebulous matter. Much doubt has, however, been thrown upon their beingrings at all; and the best authorities regard them merely as spiralnebulæ, of which we happen to get a foreshortened view. Very fewexamples are known, the most famous being one in the constellation ofLyra, usually known as the Annular Nebula in Lyra. This object is soremote from us as to be entirely invisible to the naked eye. It containsa star of the fifteenth magnitude near to its centre. From photographstaken with the Crossley reflector, Professor Schaeberle finds in thisnebula evidences of spiral structure. It may here be mentioned that theGreat Nebula in Andromeda, which has now turned out to be a spiral, hadin earlier photographs the appearance of a ring. 

There also exist nebulæ of _irregular_ form, the most notable being theGreat Nebula in the constellation of Orion (see Plate XXIV. , p. 318). Itis situated in the centre of the "Sword" of Orion (see Plate XX. , p. 296). In large telescopes it appears as a magnificent object, and inactual dimensions it must be much on the same scale as the AndromedaNebula. The spectroscope tells us that it is a mass of glowing gas. 

The Trifid Nebula, situated in the constellation of Sagittarius, is anobject of very strange shape. Three dark clefts radiate from its centre, giving it an appearance as if it had been torn into shreds. 

The Dumb-bell Nebula, a celebrated object, so called from its likenessto a dumb-bell, turns out, from recent photographs taken by ProfessorSchaeberle, which bring additional detail into view, to be after all agreat spiral. 

There is a nest, or rather a cluster of nebulæ in the constellation ofComa Berenices; over a hundred of these objects being here gathered intoa space of sky about the size of our full moon. 

[Illustration: PLATE XXIII. THE GREAT NEBULA IN THE CONSTELLATION OFANDROMEDA

From a photograph taken at the Yerkes Observatory. 

(Page 314)]

The spectroscope informs us that spiral nebulæ are composed ofpartially-cooled matter. Their colour, as we have seen, is white. Nebulæof a greenish tint are, on the other hand, found to be entirely in agaseous condition. Just as the solar corona contains an unknown element, which for the time being has been called "Coronium, " so do the gaseousnebulæ give evidence of the presence of another unknown element. To thisSir William Huggins has given the provisional name of "Nebulium. "

The _Magellanic Clouds_ are two patches of nebulous-looking light, moreor less circular in form, which are situated in the southern hemisphereof the sky. They bear a certain resemblance to portions of the MilkyWay, but are, however, not connected with it. They have received theirname from the celebrated navigator, Magellan, who seems to have been oneof the first persons to draw attention to them. "Nubeculæ" is anothername by which they are known, the larger cloud being styled _nubeculamajor_ and the smaller one _nubecula minor_. They contain within themstars, clusters, and gaseous nebulæ. No parallax has yet been found forany object which forms part of the nubeculæ, so it is very difficult toestimate at what distance from us they may lie. They are, however, considered to be well within our stellar universe. 

Having thus brought to a conclusion our all too brief review of thestars and the nebulæ--of the leading objects in fine which the celestialspaces have revealed to man--we will close this chapter with a recentsummation by Sir David Gill of the relations which appear to obtainbetween these various bodies. "Huggins's spectroscope, " he says, "hasshown that many nebulæ are not stars at all; that many well-condensednebulæ, as well as vast patches of nebulous light in the sky, are butinchoate masses of luminous gas. Evidence upon evidence has accumulatedto show that such nebulæ consist of the matter out of which stars(_i. E. _ suns) have been and are being evolved. The different types ofstar spectra form such a complete and gradual sequence (from simplespectra resembling those of nebulæ onwards through types of graduallyincreasing complexity) as to suggest that we have before us, written inthe cryptograms of these spectra, the complete story of the evolution ofsuns from the inchoate nebula onwards to the most active sun (like ourown), and then downward to the almost heatless and invisible ball. Theperiod during which human life has existed upon our globe is probablytoo short--even if our first parents had begun the work--to affordobservational proof of such a cycle of change in any particular star;but the fact of such evolution, with the evidence before us, can hardlybe doubted. "[34]

[32] The name Al gûl, meaning the Demon, was what the old Arabianastronomers called it, which looks very much as if they had alreadynoticed its rapid fluctuations in brightness. 

[33] Mr. Gore thinks that the companion of Algol may be a star of thesixth magnitude. 

[34] Presidential Address to the British Association for the Advancementof Science (Leicester, 1907), by Sir David Gill, K. C. B. , LL. D. , F. R. S. , &c. &c. 

[Illustration: PLATE XXIV. THE GREAT NEBULA IN THE CONSTELLATION OFORION

From a photograph taken at the Yerkes Observatory. 

(Page 316)]

CHAPTER XXV

THE STELLAR UNIVERSE

The stars appear fairly evenly distributed all around us, except in oneportion of the sky where they seem very crowded, and so give one animpression of being very distant. This portion, known as the Milky Way, stretches, as we have already said, in the form of a broad band rightround the entire heavens. In those regions of the sky most distant fromthe Milky Way the stars appear to be thinly sown, but become more andmore closely massed together as the Milky Way is approached. 

This apparent distribution of the stars in space has given rise to atheory which was much favoured by Sir William Herschel, and which isusually credited to him, although it was really suggested by one ThomasWright of Durham in 1750; that is to say, some thirty years or morebefore Herschel propounded it. According to this, which is known as the"Disc" or "Grindstone" Theory, the stars are considered as arranged inspace somewhat in the form of a thick disc, or grindstone, close to the_central_ parts of which our solar system is situated. [35] Thus weshould see a greater number of stars when we looked out through the_length_ of such a disc in any direction, than when we looked outthrough its _breadth_. This theory was, for a time, supposed to accountquite reasonably for the Milky Way, and for the gradual increase in thenumber of stars in its vicinity. 

It is quite impossible to verify directly such a theory, for we know theactual distance of only about forty-three stars. We are unable, therefore, definitely to assure ourselves whether, as the grindstonetheory presupposes, the stellar universe actually reaches out very muchfurther from us in the direction of the Milky Way than in the otherparts of the sky. The theory is clearly founded upon the suppositionthat the stars are more or less equal in size, and are scattered throughspace at fairly regular distances from each other. 

Brightness, therefore, had been taken as implying nearness to us, andfaintness great distance. But we know to-day that this is not the case, and that the stars around us are, on the other hand, of various degreesof brightness and of all orders of size. Some of the faint stars--forinstance, the galloping star in Pictor--are indeed nearer to us thanmany of the brighter ones. Sirius, on the other hand, is twice as faroff from us as [a] Centauri, and yet it is very much brighter; whileCanopus, which in brightness is second only to Sirius out of the wholesky, is too far off for its distance to be ascertained! It must beremembered that no parallax had yet been found for any star in the daysof Herschel, and so his estimations of stellar distances werenecessarily of a very circumstantial kind. He did not, however, continuealways to build upon such uncertain ground; but, after some furtherexamination of the Milky Way, he gave up his idea that the stars wereequally disposed in space, and eventually abandoned the grindstonetheory. 

Since we have no means of satisfactorily testing the matter, throughfinding out the various distances from us at which the stars are reallyplaced, one might just as well go to the other extreme, and assume thatthe thickening of stars in the region of the Milky Way is not an effectof perspective at all, but that the stars in that part of the sky areactually more crowded together than elsewhere--a thing which astronomersnow believe to be the case. Looked at in this way, the shape of thestellar universe might be that of a globe-shaped aggregation of stars, in which the individuals are set at fairly regular distances from eachother; the whole being closely encircled by a belt of densely packedstars. It must, however, be allowed that the gradual increase in thenumber of stars towards the Milky Way appears a strong argument infavour of the grindstone theory; yet the belt theory, as above detailed, seems to meet with more acceptance. 

There is, in fact, one marked circumstance which is remarkably difficultof explanation by means of the grindstone theory. This is the existenceof vacant spaces--holes, so to speak, in the groundwork of the MilkyWay. For instance, there is a cleft running for a good distance alongits length, and there is also a starless gap in its southern portion. Itseems rather improbable that such a great number of stars could havearranged themselves so conveniently, as to give us a clear view rightout into empty space through such a system in its greatest thickness;as if, in fact, holes had been bored, and clefts made, from the boundaryof the disc clean up to where our solar system lies. Sir John Herschellong ago drew attention to this point very forcibly. It is plain thatsuch vacant spaces can, on the other hand, be more simply explained asmere holes in a belt; and the best authorities maintain that theappearance of the Milky Way confirms a view of this kind. 

Whichever theory be indeed the correct one, it appears at any rate thatthe stars do not stretch out in every direction to an infinite distance;but that _the stellar system is of limited extent_, and has in fact aboundary. 

In the first place, Science has no grounds for supposing that light isin any way absorbed or destroyed merely by its passage through the"ether, " that imponderable medium which is believed to transmit theluminous radiations through space. This of course is tantamount tosaying that all the direct light from all the stars should reach us, excepting that little which is absorbed in its passage through our ownatmosphere. If stars, and stars, and stars existed in every directionoutwards without end, it can be proved mathematically that in suchcircumstances there could not remain the tiniest space in the skywithout a star to fill it, and that therefore the heavens would alwaysblaze with light, and the night would be as bright as the noonday. [36]How very far indeed this is from being the case, may be gathered from anestimate which has been made of the general amount of light which wereceive from the stars. According to this estimate the sky is consideredas more or less dark, the combined illumination sent to us by all thestars being only about the one-hundreth part of what we get from thefull moon. [37]

Secondly, it has been suggested that although light may not suffer anyextinction or diminution from the ether itself, still a great deal ofillumination may be prevented from reaching us through myriads ofextinguished suns, or dark meteoric matter lying about in space. Theidea of such extinguished suns, dark stars in fact, seems however to bemerely founded upon the sole instance of the invisible companion ofAlgol; but, as we have seen, there is no proof whatever that it is adark body. Again, some astronomers have thought that the dark holes inthe Milky Way, "Coal Sacks, " as they are called, are due to masses ofcool, or partially cooled matter, which cuts off the light of the starsbeyond. The most remarkable of these holes is one in the neighbourhoodof the Southern Cross, known as the "Coal Sack in Crux. " But Mr. Gorethinks that the cause of the holes is to be sought for rather in whatSir William Herschel termed "clustering power, " _i. E. _ a tendency on thepart of stars to accumulate in certain places, thus leaving othersvacant; and the fact that globular and other clusters are to be foundvery near to such holes certainly seems corroborative of this theory. Insumming up the whole question, Professor Newcomb maintains that theredoes not appear any evidence of the light from the Milky Way stars, which are apparently the furthest bodies we see, being intercepted bydark bodies or dark matter. As far as our telescopes can penetrate, heholds that we see the stars _just as they are_. 

Also, if there did exist an infinite number of stars, one would expectto find evidence in some direction of an overpoweringly greatforce, --the centre of gravity of all these bodies. 

It is noticed, too, that although the stars increase in number withdecrease in magnitude, so that as we descend in the scale we find threetimes as many stars in each magnitude as in the one immediately aboveit, yet this progression does not go on after a while. There is, infact, a rapid falling off in numbers below the twelfth magnitude; whichlooks as if, at a certain distance from us, the stellar universe werebeginning to _thin out_. 

Again, it is estimated, by Mr. Gore and others, that only about 100millions of stars are to be seen in the whole of the sky with the bestoptical aids. This shows well the limited extent of the stellar system, for the number is not really great. For instance, there are from fifteento sixteen times as many persons alive upon the earth at this moment!

Last of all, there appears to be strong photographic evidence that oursidereal system is limited in extent. Two photographs taken by the lateDr. Isaac Roberts of a region rich in stellar objects in theconstellation of Cygnus, clearly show what has been so eloquently calledthe "darkness behind the stars. " One of these photographs was taken in1895, and the other in 1898. On both occasions the state of theatmosphere was practically the same, and the sensitiveness of the filmswas of the same degree. The exposure in the first case was only onehour; in the second it was about two hours and a half. And yet bothphotographs show _exactly the same stars, even down to the faintest_. From this one would gather that the region in question, which is one ofthe most thickly star-strewn in the Milky Way, is _penetrable rightthrough_ with the means at our command. Dr. Roberts himself incommenting upon the matter drew attention to the fact, that manyastronomers seemed to have tacitly adopted the assumption that the starsextend indefinitely through space. 

From considerations such as these the foremost astronomical authoritiesof our time consider themselves justified in believing that thecollection of stars around us is _finite_; and that although our besttelescopes may not yet be powerful enough to penetrate to the finalstars, still the rapid decrease in numbers as space is sounded withincreasing telescopic power, points strongly to the conclusion that theboundaries of the stellar system may not lie very far beyond theuttermost to which we can at present see. 

Is it possible then to make an estimate of the extent of this stellarsystem?

Whatever estimates we may attempt to form cannot however be regarded asat all exact, for we know the actual distances of such a very few onlyof the nearest of the stars. But our knowledge of the distances even ofthese few, permits us to assume that the stars close around us may besituated, on an average, at about eight light-years from each other; andthat this holds good of the stellar spaces, with the exception of theencircling girdle of the Milky Way, where the stars seem actually to bemore closely packed together. This girdle further appears to contain thegreater number of the stars. Arguing along these lines, ProfessorNewcomb reaches the conclusion that the farthest stellar bodies which wesee are situated at about between 3000 and 4000 light-years from us. 

Starting our inquiry from another direction, we can try to form anestimate by considering the question of proper motions. 

It will be noticed that such motions do not depend entirely upon theactual speed of the stars themselves, but that some of the apparentmovement arises indirectly from the speed of our own sun. The part in aproper motion which can be ascribed to the movement of our solar systemthrough space is clearly a displacement in the nature of a parallax--SirWilliam Herschel called it "_Systematic_ Parallax"; so that knowing thedistance which we move over in a certain lapse of time, we are able tohazard a guess at the distances of a good many of the stars. An inquiryupon such lines must needs be very rough, and is plainly based upon theassumption that the stars whose distances we attempt to estimate aremoving at an average speed much like that of our own sun, and that theyare not "runaway stars" of the 1830 Groombridge order. Be that as itmay, the results arrived at by Professor Newcomb from this method ofreasoning are curiously enough very much on a par with those founded onthe few parallaxes which we are really certain about; with the exceptionthat they point to somewhat closer intervals between the individualstars, and so tend to narrow down our previous estimate of the extent ofthe stellar system. 

Thus far we get, and no farther. Our solar system appears to liesomewhere near the centre of a great collection of stars, separated eachone from the other, on an average, by some 40 billions of miles; thewhole being arranged in the form of a mighty globular cluster. Lightfrom the nearest of these stars takes some four years to come to us. Ittakes about 1000 times as long to reach us from the confines of thesystem. This globe of stars is wrapt around closely by a stellar girdle, the individual stars in which are set together more densely than thosein the globe itself. The entire arrangement appears to be constructedupon a very regular plan. Here and there, as Professor Newcomb pointsout, the aspect of the heavens differs in small detail; but generally itmay be laid down that the opposite portions of the sky, whether in theMilky Way itself, or in those regions distant from it, show a markeddegree of symmetry. The proper motions of stars in correspondingportions of the sky reveal the same kind of harmony, a harmony which mayeven be extended to the various colours of the stars. The stellarsystem, which we see disposed all around us, appears in fine to bear allthe marks of an _organised whole_. 

The older astronomers, to take Sir William Herschel as an example, supposed some of the nebulæ to be distant "universes. " Sir William wasled to this conclusion by the idea he had formed that, when histelescopes failed to show the separate stars of which he imagined theseobjects to be composed, he must put down the failure to their stupendousdistance from us. For instance, he thought the Orion Nebula, which isnow known to be made up of glowing gas, to be an external stellarsystem. Later on, however, he changed his mind upon this point, and cameto the conclusion that "shining fluid" would better account both forthis nebula, and for others which his telescopes had failed to separateinto component stars. 

The old ideas with regard to external systems and distant universes havebeen shelved as a consequence of recent research. All known clusters andnebulæ are now firmly believed to lie _within_ our stellar system. 

This view of the universe of stars as a sort of island in theimmensities, does not, however, give us the least idea about the actualextent of space itself. Whether what is called space is really infinite, that is to say, stretches out unendingly in every direction, or whetherit has eventually a boundary somewhere, are alike questions which thehuman mind seems utterly unable to picture to itself. 

[35] The Ptolemaic idea dies hard!

[36] Even the Milky Way itself is far from being a blaze of light, whichshows that the stars composing it do not extend outwards indefinitely. 

[37] Mr. Gore has recently made some remarkable deductions, with regardto the amount of light which we get from the stars. He considers thatmost of this light comes from stars below the sixth magnitude; andconsequently, if all the stars visible to the naked eye were to beblotted out, the glow of the night sky would remain practically the sameas it is at present. Going to the other end of the scale, he thinks alsothat the combined light which we get from all the stars below theseventeenth magnitude is so very small, that it may be neglected in suchan estimation. He finds, indeed, that if there are stars so low as thetwentieth magnitude, one hundred millions of them would only be equal inbrightness to a single first-magnitude star like Vega. On the otherhand, it is possible that the light of the sky at night is not entirelydue to starlight, but that some of it may be caused by phosphorescentglow. 

CHAPTER XXVI

THE STELLAR UNIVERSE--_continued_

It is very interesting to consider the proper motions of stars withreference to such an isolated stellar system as has been pictured in theprevious chapter. These proper motions are so minute as a rule, that weare quite unable to determine whether the stars which show them aremoving along in straight lines, or in orbits of immense extent. Itwould, in fact, take thousands of years of careful observation todetermine whether the paths in question showed any degree of curving. Inthe case of the more distant stars, the accurate observations which havebeen conducted during the last hundred years have not so far revealedany proper motions with regard to them; but one cannot escape theconclusion that these stars move as the others do. 

If space outside our stellar system is infinite in extent, and if allthe stars within that system are moving unchecked in every conceivabledirection, the result must happen that after immense ages these starswill have drawn apart to such a distance from each other, that thesystem will have entirely disintegrated, and will cease to exist as aconnected whole. Eventually, indeed, as Professor Newcomb points out, the stars will have separated so far from each other that each will beleft by itself in the midst of a black and starless sky. If, however, acertain proportion of stars have a speed sufficiently slow, they willtend under mutual attraction to be brought to rest by collisions, orforced to move in orbits around each other. But those stars which moveat excessive speeds, such, for instance, as 1830 Groombridge, or thestar in the southern constellation of Pictor, seem utterly incapable ofbeing held back in their courses by even the entire gravitative force ofour stellar system acting as a whole. These stars must, therefore, moveeventually right through the system and pass out again into the emptyspaces beyond. Add to this; certain investigations, made into the speedof 1830 Groombridge, furnish a remarkable result. It is calculated, indeed, that had this star been _falling through infinite space forever_, pulled towards us by the combined gravitative force of our entiresystem of stars, it could not have gathered up anything like the speedwith which it is at present moving. No force, therefore, which we canconjure out of our visible universe, seems powerful enough either tohave impressed upon this runaway star the motion which it now has, or tostay it in its wild course. What an astounding condition of things!

Speculations like this call up a suspicion that there may yet existother universes, other centres of force, notwithstanding the apparentsolitude of our stellar system in space. It will be recollected that theidea of this isolation is founded upon such facts as, that the heavensdo not blaze with light, and that the stars gradually appear to thin outas we penetrate the system with increasing telescopic power. Butperchance there is something which hinders us from seeing out into spacebeyond our cluster of stars; which prevents light, in fact, fromreaching us from other possible systems scattered through the depthsbeyond. It has, indeed, been suggested by Mr. Gore[38] that thelight-transmitting ether may be after all merely a kind of "atmosphere"of the stars; and that it may, therefore, thin off and cease a littlebeyond the confines of our stellar system, just as the air thins off andpractically ceases at a comparatively short distance from the earth. Aclashing together of solid bodies outside our atmosphere could plainlysend us no sound, for there is no air extending the whole way to bear toour ears the vibrations thus set up; so light emitted from any bodylying beyond our system of stars, would not be able to come to us if theether, whose function it is to convey the rays of light, ceased at ornear the confines of that system. 

Perchance we have in this suggestion the key to the mystery of how oursun and the other stellar bodies maintain their functions of temperatureand illumination. The radiations of heat and light arriving at thelimits of this ether, and unable to pass any further, may be thrown backagain into the system in some altered form of energy. 

But these, at best, are mere airy and fascinating speculations. We have, indeed, no evidence whatever that the luminiferous ether ceases at theboundary of the stellar system. If, therefore, it extends outwardsinfinitely in every direction, and if it has no absorbing or weakeningeffect on the vibrations which it transmits, we cannot escape from theconclusion that practically all the rays of light ever emitted by allthe stars must chase one another eternally through the never-endingabysses of space. 

[38] _Planetary and Stellar Studies_, by John Ellard Gore, F. R. A. S. , M. R. I. A. , London, 1888. 

CHAPTER XXVII

THE BEGINNING OF THINGS

LAPLACE'S NEBULAR HYPOTHESIS

Dwelling upon the fact that all the motions of revolution and rotationin the solar system, as known in his day, took place in the samedirection and nearly in the same plane, the great French astronomer, Laplace, about the year 1796, put forward a theory to account for theorigin and evolution of that system. He conceived that it had come intobeing as a result of the gradual contraction, through cooling, of anintensely heated gaseous lens-shaped mass, which had originally occupiedits place, and had extended outwards beyond the orbit of the furthestplanet. He did not, however, attempt to explain how such a mass mighthave originated! He went on to suppose that this mass, _in some manner_, perhaps by mutual gravitation among its parts, had acquired a motion ofrotation in the same direction as the planets now revolve. As thisnebulous mass parted with its heat by radiation, it contracted towardsthe centre. Becoming smaller and smaller, it was obliged to rotatefaster and faster in order to preserve its equilibrium. Meanwhile, inthe course of contraction, rings of matter became separated from thenucleus of the mass, and were left behind at various intervals. Theserings were swept up into subordinate masses similar to the originalnebula. These subordinate masses also contracted in the same manner, leaving rings behind them which, in turn, were swept up to formsatellites. Saturn's ring was considered, by Laplace, as the onlyportion of the system left which still showed traces of thisevolutionary process. It is even probable that it may have suggested thewhole of the idea to him. 

Laplace was, however, not the first philosopher who had speculated alongthese lines concerning the origin of the world. 

Nearly fifty years before, in 1750 to be exact, Thomas Wright, ofDurham, had put forward a theory to account for the origin of the wholesidereal universe. In his theory, however, the birth of our solar systemwas treated merely as an incident. Shortly afterwards the subject wastaken up by the famous German philosopher, Kant, who dealt with thequestion in a still more ambitious manner, and endeavoured to account indetail for the origin of the solar system as well as of the siderealuniverse. Something of the trend of such theories may be gathered fromthe remarkable lines in Tennyson's _Princess_:--

"This world was once a fluid haze of light, Till toward the centre set the starry tides, And eddied into suns, that wheeling castThe planets. "

The theory, as worked out by Kant, was, however, at the best merely a_tour de force_ of philosophy. Laplace's conception was much lessambitious, for it did not attempt to explain the origin of the entireuniverse, but only of the solar system. Being thus reasonably limited inits scope, it more easily obtained credence. The arguments of Laplacewere further founded upon a mathematical basis. The great place whichhe occupied among the astronomers of that time caused his theory toexert a preponderating influence on scientific thought during thecentury which followed. 

A modification of Laplace's theory is the Meteoritic Hypothesis of SirNorman Lockyer. According to the views of that astronomer, the materialof which the original nebula was composed is presumed to have been inthe meteoric, rather than in the gaseous, state. Sir Norman Lockyerholds, indeed, that nebulæ are, in reality, vast swarms of meteors, andthe light they emit results from continual collisions between theconstituent particles. The French astronomer, Faye, also proposed tomodify Laplace's theory by assuming that the nebula broke up into ringsall at once, and not in detail, as Laplace had wished to suppose. 

The hypothesis of Laplace fits in remarkably well with the theory putforward in later times by Helmholtz, that the heat of the sun is kept upby the continual contraction of its mass. It could thus have onlycontracted to its present size from one very much larger. 

Plausible, however, as Laplace's great hypothesis appears on thesurface, closer examination shows several vital objections, a few ofthose set forth by Professor Moulton being here enumerated--

Although Laplace held that the orbits of the planets were sufficientlynear to being in the one plane to support his views, yet laterinvestigators consider that their very deviations from this plane are astrong argument against the hypothesis. 

Again, it is thought that if the theory were the correct explanation, the various orbits of the planets would be much more nearly circularthan they are. 

It is also thought that such interlaced paths, as those in which theasteroids and the little planet Eros move, are most unlikely to havebeen produced as a result of Laplace's nebula. 

Further, while each of the rings was sweeping up its matter into a bodyof respectable dimensions, its gravitative power would have been for thetime being so weak, through being thus spread out, that any lighterelements, as, for instance, those of the gaseous order, would haveescaped into space in accordance with the principles of the kinetictheory. 

_The idea that rings would at all be left behind at certain intervalsduring the contraction of the nebula is, perhaps, one of the weakestpoints in Laplace's hypothesis. _

Mathematical investigation does not go to show that the rings, presumingthey could be left behind during the contraction of the mass, would haveaggregated into planetary bodies. Indeed, it rather points to thereverse. 

Lastly, such a discovery as that the ninth satellite of Saturn revolvesin a _retrograde_ direction--that is to say, in a direction contrary tothe other revolutions and rotations in our solar system--appearsdirectly to contradict the hypothesis. 

Although Laplace's hypothesis seems to break down under the keencriticism to which it has been subjected, yet astronomers have notrelinquished the idea that our solar system has probably had its originfrom a nebulous mass. But the apparent failure of the Laplacian theoryis emphasised by the fact, that _not a single example of a nebula, inthe course of breaking up into concentric rings, is known to exist inthe entire heaven_. Indeed, as we saw in Chapter XXIV. , there seems tobe no reliable example of even a "ring" nebula at all. Mr. Gore haspointed this out very succinctly in his recently published work, _Astronomical Essays_, where he says:--"To any one who still persists inmaintaining the hypothesis of ring formation in nebulæ, it may be saidthat the whole heavens are against him. "

The conclusions of Keeler already alluded to, that the spiral is thenormal type of nebula, has led during the past few years to a new theoryby the American astronomers, Professors Chamberlin and Moulton. In thedetailed account of it which they have set forth, they show that thoseanomalies which were stumbling-blocks to Laplace's theory do notcontradict theirs. To deal at length with this theory, to which the nameof "Planetesimal Hypothesis" has been given, would not be possible in abook of this kind. But it may be of interest to mention that the authorsof the theory in question remount the stream of time still further thandid Laplace, and seek to explain the _origin_ of the spiral nebulæthemselves in the following manner:--

Having begun by assuming that the stars are moving apparently in everydirection with great velocities, they proceed to point out that sooneror later, although the lapse of time may be extraordinarily long, collisions or near approaches between stars are bound to occur. In thecase of collisions the chances are against the bodies striking togethercentrally, it being very much more likely that they will hit each otherrather towards the side. The nebulous mass formed as a result of thedisintegration of the bodies through their furious impact would thuscome into being with a spinning movement, and a spiral would ensue. Again, the stars may not actually collide, but merely approach near toeach other. If very close, the interaction of gravitation will give riseto intense strains, or tides, which will entirely disintegrate thebodies, and a spiral nebula will similarly result. As happens upon ourearth, two such tides would rise opposite to each other; and, consequently, it is a noticeable fact that spiral nebulæ have almostinvariably two opposite branches (see Plate XXII. , p 314). Even if notso close, the gravitational strains set up would produce tremendouseruptions of matter; and in this case, a spiral movement would also begenerated. On such an assumption the various bodies of the solar systemmay be regarded as having been ejected from parent masses. 

The acceptance of the Planetesimal Hypothesis in the place of theHypothesis of Laplace will not, as we have seen, by any means do awaywith the probability that our solar system, and similar systems, haveoriginated from a nebulous mass. On the contrary it puts that idea on afirmer footing than before. The spiral nebulæ which we see in theheavens are on a vast scale, and may represent the formation of stellarsystems and globular clusters. Our solar system may have arisen from asmall spiral. 

We will close these speculations concerning the origin of things with ashort sketch of certain investigations made in recent years by SirGeorge H. Darwin, of Cambridge University, into the question of theprobable birth of our moon. He comes to the conclusion that at leastfifty-four millions of years ago the earth and moon formed one body, which had a diameter of a little over 8000 miles. This body rotated onan axis in about five hours, namely, about five times as fast as it doesat present. The rapidity of the rotation caused such a tremendous strainthat the mass was in a condition of, what is called, unstableequilibrium; very little more, in fact, being required to rend itasunder. The gravitational pull of the sun, which, as we have alreadyseen, is in part the cause of our ordinary tides, supplied this extrastrain, and a portion of the mass consequently broke off, which recededgradually from the rest and became what we now know as the moon. SirGeorge Darwin holds that the gravitational action of the sun will intime succeed in also disturbing the present apparent harmony of theearth-moon system, and will eventually bring the moon back towards theearth, so that after the lapse of great ages they will re-unite onceagain. 

In support of this theory of the terrestrial origin of the moon, Professor W. H. Pickering has put forward a bold hypothesis that oursatellite had its origin in the great basin of the Pacific. This oceanis roughly circular, and contains no large land masses, except theAustralian Continent. He supposes that, prior to the moon's birth, ourglobe was already covered with a slight crust. In the tearing away ofthat portion which was afterwards destined to become the moon theremaining area of the crust was rent in twain by the shock; and thuswere formed the two great continental masses of the Old and New Worlds. These masses floated apart across the fiery ocean, and at last settledin the positions which they now occupy. In this way Professor Pickeringexplains the remarkable parallelism which exists between the oppositeshores of the Atlantic. The fact of this parallelism had, however, beennoticed before; as, for example, by the late Rev. S. J. Johnson, in hisbook _Eclipses, Past and Future_, where we find the following passage:--

"If we look at our maps we shall see the parts of one Continent that jutout agree with the indented portions of another. The prominent coast ofAfrica would fit in the opposite opening between North and SouthAmerica, and so in numerous other instances. A general rending asunderof the World would seem to have taken place when the foundations of thegreat deep were broken up. "

Although Professor Pickering's theory is to a certain degree anticipatedin the above words, still he has worked out the idea much more fully, and given it an additional fascination by connecting it with the birthof the moon. He points out, in fact, that there is a remarkablesimilarity between the lunar volcanoes and those in the immediateneighbourhood of the Pacific Ocean. He goes even further to suggest thatAustralia is another portion of the primal crust which was detached outof the region now occupied by the Indian Ocean, where it was originallyconnected with the south of India or the east of Africa. 

Certain objections to the theory have been put forward, one of which isthat the parallelism noticed between the opposite shores of the Atlanticis almost too perfect to have remained through some sixty millions ofyears down to our own day, in the face of all those geological movementsof upheaval and submergence, which are perpetually at work upon ourglobe. Professor Pickering, however, replies to this objection bystating that many geologists believe that the main divisions of land andwater on the earth are permanent, and that the geological alterationswhich have taken place since these were formed have been merely of atemporary and superficial nature. 

CHAPTER XXVIII

THE END OF THINGS

We have been trying to picture the beginning of things. We will now tryto picture the end. 

In attempting this, we find that our theories must of necessity belimited to the earth, or at most to the solar system. The time-honouredexpression "End of the World" really applies to very little beyond theend of our own earth. To the people of past ages it, of course, meantvery much more. For them, as we have seen, the earth was the centre ofeverything; and the heavens and all around were merely a kind of minoraccompaniment, created, as they no doubt thought, for their especialbenefit. In the ancient view, therefore, the beginning of the earthmeant the beginning of the universe, and the end of the earth theextinction of all things. The belief, too, was general that this endwould be accomplished through fire. In the modern view, however, thebirth and death of the earth, or indeed of the solar system, might passas incidents almost unnoticed in space. They would be but mere links inthe chain of cosmic happenings. 

A number of theories have been forward from time to time prognosticatingthe end of the earth, and consequently of human life. We will concludewith a recital of a few of them, though which, if any, is the true one, the Last Men alone can know. 

Just as a living creature may at any moment die in the fulness ofstrength through sudden malady or accident, or, on the other hand, maymeet with death as a mere consequence of old age, so may our globe bedestroyed by some sudden cataclysm, or end in slow processes of decay. Barring accidents, therefore, it would seem probable that the growingcold of the earth, or the gradual extinction of the sun, should aftermany millions of years close the chapter of life, as we know it. On theformer of these suppositions, the decrease of temperature on our globemight perhaps be accelerated by the thinning of the atmosphere, throughthe slow escape into space of its constituent gases, or their gradualchemical combination with the materials of the earth. The subterraneanheat entirely radiated away, there would no longer remain any of thosevolcanic elevating forces which so far have counteracted the slowwearing down of the land surface of our planet, and thus what waterremained would in time wash over all. If this preceded the growing coldof the sun, certain strange evolutions of marine forms of life would bethe last to endure, but these, too, would have to go in the end. 

Should, however, the actual process be the reverse of this, and the suncool down the quicker, then man would, as a consequence of hisscientific knowledge, tend in all probability to outlive the other formsof terrestrial life. In such a vista we can picture the regions of theearth towards the north and south becoming gradually more and moreuninhabitable through cold, and human beings withdrawing before theslow march of the icy boundary, until the only regions capable ofhabitation would lie within the tropics. In such a struggle between manand destiny science would be pressed to the uttermost, in the devisingof means to counteract the slow diminution of the solar heat and thegradual disappearance of air and water. By that time the axial rotationof our globe might possibly have been slowed down to such an extent thatone side alone of its surface would be turned ever towards the fastdying sun. And the mind's eye can picture the last survivors of thehuman race, huddled together for warmth in a glass-house somewhere onthe equator, waiting for the end to come. 

The mere idea of the decay and death of the solar system almost bringsto one a cold shudder. All that sun's light and heat, which means somuch to us, entirely a thing of the past. A dark, cold ball rushingalong in space, accompanied by several dark, cold balls circlingceaselessly around it. One of these a mere cemetery, in which therewould be no longer any recollection of the mighty empires, the loves andhates, and all that teeming play of life which we call History. Tombstones of men and of deeds, whirling along forgotten in the darknessand silence. _Sic transit gloria mundi. _

In that brilliant flight of scientific fancy, the _Time Machine_, Mr. H. G. Wells has pictured the closing years of the earth in some suchlong-drawn agony as this. He has given us a vision of a desolate beachby a salt and almost motionless sea. Foul monsters of crab-like formcrawl slowly about, beneath a huge hull of sun, red and fixed in thesky. The rocks around are partly coated with an intensely greenvegetation, like the lichen in caves, or the plants which grow in aperpetual twilight. And the air is now of an exceeding thinness. 

He dips still further into the future, and thus predicts the final formof life:--

"I saw again the moving thing upon the shoal--there was no mistake nowthat it was a moving thing--against the red water of the sea. It was around thing, the size of a football perhaps, or it may be bigger, andtentacles trailed down from it; it seemed black against the welteringblood-red water, and it was hopping fitfully about. "

What a description of the "Heir of all the Ages!"

To picture the end of our world as the result of a cataclysm of somekind, is, on the other hand, a form of speculation as intensely dramaticas that with which we have just been dealing is unutterably sad. 

It is not so many years ago, for instance, that men feared a suddencatastrophe from the possible collision of a comet with our earth. Theunreasoning terror with which the ancients were wont to regard thesemysterious visitants to our skies had, indeed, been replaced by anapprehension of quite another kind. For instance, as we have seen, theannouncement in 1832 that Biela's Comet, then visible, would cut throughthe orbit of the earth on a certain date threw many persons into averitable panic. They did not stop to find out the real facts of thecase, namely, that, at the time mentioned, the earth would be nearly amonth's journey from the point indicated!

It is, indeed, very difficult to say what form of damage the earthwould suffer from such a collision. In 1861 it passed, as we have seen, through the tail of the comet without any noticeable result. But thehead of a comet, on the other hand, may, for aught we know, containwithin it elements of peril for us. A collision with this part might, for instance, result in a violent bombardment of meteors. But thesemeteors could not be bodies of any great size, for the masses of cometsare so very minute that one can hardly suppose them to contain any largeor dense constituent portions. 

The danger, however, from a comet's head might after all be a danger toour atmosphere. It might precipitate, into the air, gases which wouldasphyxiate us or cause a general conflagration. It is scarcely necessaryto point out that dire results would follow upon any interference withthe balance of our atmosphere. For instance, the well-known Frenchastronomer, M. Camille Flammarion, [39] has imagined the absorption ofthe nitrogen of the air in this way; and has gone on to picture men andanimals reduced to breathing only oxygen, first becoming excited, thenmad, and finally ending in a perfect saturnalia of delirium. 

Lastly, though we have no proof that stars eventually become dark andcold, for human time has so far been all too short to give us even thesmallest evidence as to whether heat and light are diminishing in ourown sun, yet it seems natural to suppose that such bodies must at lastcease their functions, like everything else which we know of. We may, therefore, reasonably presume that there are dark bodies scattered inthe depths of space. We have, indeed, a suspicion of at least one, though perhaps it partakes rather of a planetary nature, namely, that"dark" body which continually eclipses Algol, and so causes thetemporary diminution of its light. As the sun rushes towards theconstellation of Lyra such an extinguished sun may chance to find itselfin his path; just as a derelict hulk may loom up out of the darknessright beneath the bows of a vessel sailing the great ocean. 

Unfortunately a collision between the sun and a body of this kind couldnot occur with such merciful suddenness. A tedious warning of itsapproach would be given from that region of the heavens whither oursystem is known to be tending. As the dark object would become visibleonly when sufficiently near our sun to be in some degree illuminated byhis rays, it might run the chance at first of being mistaken for a newplanet. If such a body were as large, for instance, as our own sun, itshould, according to Mr. Gore's calculations, reveal itself to thetelescope some fifteen years before the great catastrophe. Steadily itsdisc would appear to enlarge, so that, about nine years after itsdiscovery, it would become visible to the naked eye. At length thedoomed inhabitants of the earth, paralysed with terror, would see theirrelentless enemy shining like a second moon in the northern skies. Rapidly increasing in apparent size, as the gravitational attractions ofthe solar orb and of itself interacted more powerfully with diminishingdistance, it would at last draw quickly in towards the sun and disappearin the glare. 

It is impossible for us to conceive anything more terrible than theseclosing days, for no menace of catastrophe which we can picture couldbear within it such a certainty of fulfilment. It appears, therefore, useless to speculate on the probable actions of men in their nowterrestrial prison. Hope, which so far had buoyed them up in the direstcalamities, would here have no place. Humanity, in the fulness of itsstrength, would await a wholesale execution from which there could be nochance at all of a reprieve. Observations of the approaching body wouldhave enabled astronomers to calculate its path with great exactness, andto predict the instant and character of the impact. Eight minutes afterthe moment allotted for the collision the resulting tide of flame wouldsurge across the earth's orbit, and our globe would quickly pass away invapour. 

And what then?

A nebula, no doubt; and after untold ages the formation possibly from itof a new system, rising phoenix-like from the vast crematorium andfilling the place of the old one. A new central sun, perhaps, with itsattendant retinue of planets and satellites. And teeming life, perchance, appearing once more in the fulness of time, when temperaturein one or other of these bodies had fallen within certain limits, andother predisposing conditions had supervened. 

 "The world's great age begins anew, The golden years return, The earth doth like a snake renew Her winter weeds outworn: Heaven smiles, and faiths and empires gleam Like wrecks of a dissolving dream. 

 A brighter Hellas rears its mountains From waves serener far; A new Peneus rolls his fountains Against the morning star; Where fairer Tempes bloom, there sleep Young Cyclads on a sunnier deep. 

 A loftier Argo cleaves the main, Fraught with a later prize; Another Orpheus sings again, And loves, and weeps, and dies; A new Ulysses leaves once more Calypso for his native shore. 

 * * * * *

Oh cease! must hate and death return? Cease! must men kill and die?Cease! drain not to its dregs the urn Of bitter prophecy!The world is weary of the past, --Oh might it die or rest at last!"

[39] See his work, _La Fin du Monde_, wherein the various ways by whichour world may come to an end are dealt with at length, and in aprofoundly interesting manner. 

INDEX

 Achromatic telescope, 115, 116

 Adams, 24, 236, 243

 Aerial telescopes, 110, 111

 Agathocles, Eclipse of, 85

 Agrippa, Camillus, 44

 Ahaz, dial of, 85

 Air, 166

 Airy, Sir G. B. , 92

 Al gûl, 307

 Al Sufi, 284, 290, 296, 315

 Alcor, 294

 Alcyone, 284

 Aldebaran, 103, 288, 290, 297

 Algol, 307, 309-310, 312, 323, 347

 Alpha, Centauri, 52-53, 280, 298-299, 304, 320

 Alpha Crucis, 298

 Alps, Lunar, 200

 Altair, 295

 Altitude of objects in sky, 196

 Aluminium, 145

 Amos viii. 9, 85

 Anderson, T. D. , 311-312

 Andromeda (constellation), 279, 314; Great Nebula in, 314, 316

 Andromedid meteors, 272

 Anglo-Saxon Chronicle, 87-88

 Anighito meteorite, 277

 Annular eclipse, 65-68, 80, 92, 99

 Annular Nebula in Lyra, 315-316

 Annulus, 68

 Ansæ, 242-243

 Anticipation in discovery, 236-237

 Apennines, Lunar, 200

 Aphelion, 274

 Apparent enlargement of celestial objects, 192-196

 Apparent size of celestial objects deceptive, 196, 294

 Apparent sizes of sun and moon, variations in, 67, 80, 178

 Aquila (constellation), 295

 Arabian astronomers, 107, 307

 Arago, 92, 257

 Arc, degrees minutes and seconds of, 60

 Arcturus, 280, 282, 290, 295

 Argelander, 290

 Argo (constellation), 298

 Aristarchus of Samos, 171

 Aristarchus (lunar crater), 205

 Aristophanes, 101

 Aristotle, 161, 173, 185

 Arrhenius 222, 253-254

 Assyrian tablet, 84

 Asteroidal zone, analogy of, to Saturn's rings, 238

 Asteroids (or minor planets), 30-31, 225-228, 336; discovery of the, 23, 244; Wolf's method of discovering, 226-227

 Astrology, 56

 _Astronomical Essays_, 63, 337

 Astronomical Society, Royal, 144

 _Astronomy, Manual of_, 166

 Atlantic Ocean, parallelism of opposite shores, 340-341

 Atlas, the Titan, 18

 Atmosphere, absorption by earth's, 129-130; ascertainment of, by spectroscope, 124-125, 212; height of earth's, 167, 267; of asteroids, 226; of earth, 129, 130, 166-169, 218, 222, 267, 346; of Mars, 156, 212, 216; of Mercury, 156; of moon, 70-71, 156, 201-203; of Jupiter, 231; of planets, 125; of Saturn's rings, 239

 "Atmosphere" of the stars, 331

 Atmospheric layer and "glass-house" compared, 167, 203

 August Meteors (Perseids), 270

 Auriga (constellation), 294-296, 306, 311; New Star in, 311

 Aurigæ, [b] (Beta), 294, 297, 304

 Aurora Borealis, 141, 143, 259

 Australia, suggested origin of, 340

 Axis, 29-30; of earth, 163, 180; small movement of earth's, 180-181

 Babylonian tablet, 84

 Babylonian idea of the moon, 185

 Bacon, Roger, 108

 Bacubirito meteorite, 277

 Bagdad, 107

 Baily, Francis, 92

 "Baily's Beads, " 69, 70, 91-92, 154

 Bailly (lunar crater), 199

 Ball, Sir Robert, 271

 Barnard, E. E. , 31, 224, 232-234, 237, 258

 "Bay of Rainbows, " 197

 Bayer's classification of stars, 289, 291-292

 Bayeux Tapestry, 263

 Bear, Great (constellation). _See_ Ursa Major; Little, _see_ Ursa Minor

 Beehive (Præsepe), 307

 Beer, 206

 Belopolsky, 304

 "Belt" of Orion, 297

 Belt theory of Milky Way, 321

 Belts of Jupiter, 230

 Bergstrand, 314

 Berlin star chart, 244

 Bessel, 173, 280, 305

 Beta ([b]) Lyræ, 307

 Beta ([b]) Persei. _See_ Algol

 Betelgeux, 297

 Bible, eclipses in, 85

 Biela's Comet, 256-257, 272-273, 345

 Bielids, 270, 272-273

 Billion, 51-52

 Binary stars, spectroscopic, 301-306, 309; visual, 300, 303-306

 "Black Drop, " 152-154

 "Black Hour, " 89

 "Black Saturday, " 89

 Blood, moon in eclipse like, 102

 Blue (rays of light), 121, 130

 Bode's Law, 22-23, 244-245

 Bolometer, 127

 Bond, G. P. , 236, 257

 Bonpland, 270

 Boötes (constellation), 295, 314

 Bradley, 111

 Brahe, Tycho, 290, 311

 Brédikhine's theory of comets' tails, 253-254, 256

 Bright eclipses of moon, 65, 102

 British Association for the Advancement of Science, 318

 _British Astronomical Association, Journal of_, 194

 British Museum, 84

 Bull (constellation). _See_ Taurus; "Eye" of the, 297; "Head" of the, 297

 Burgos, 98

 Busch, 93

 Cæsar, Julius, 85, 110, 180, 259, 262, 291, 293

 Calcium, 138, 145

 Callisto, 233-234

 Cambridge, 24, 91, 119, 243

 Campbell, 305

 Canali, 214

 "Canals" of Mars, 214-222, 224-225

 Cancer (constellation), 307

 Canes Venatici (constellation), 306, 314

 Canis Major (constellation), 289, 296-297; Minor, 296-297

 Canopus, 285, 298-299, 320

 Capella, 280, 282, 290, 294, 297, 303, 313

 Carbon, 145

 Carbon dioxide. _See_ Carbonic acid gas

 Carbonic acid gas, 166, 213, 221-222

 Carnegie Institution, Solar Observatory of, 118

 Cassegrainian telescope, 114, 118

 Cassini, J. D. , 236, 240

 "Cassini's Division" in Saturn's ring, 236, 238

 Cassiopeia (constellation), 279, 294, 311, 314

 Cassiopeiæ, [ê] (Eta), 303

 Cassiopeia's Chair, 294

 Cassius, Dion, 86

 Castor, 282, 297, 304

 Catalogues of stars, 106, 290-291, 311

 Centaur. _See_ Centaurus

 Centaurus (constellation), 298, 306

 Centre of gravity, 42, 283-284, 324

 Ceres, diameter of, 30, 225

 Ceti, Omicron (or Mira), 307-308

 Cetus, or the Whale (constellation), 307

 Chaldean astronomers, 74, 76

 Challis, 243-244

 Chamberlin, 337

 "Chambers of the South, " 299

 Chandler, 308

 Charles V. , 261

 "Charles' Wain, " 291

 Chemical rays, 127

 Chinese and eclipses, 83

 Chloride of sodium, 122

 Chlorine, 122, 145

 Christ, Birth of, 102

 Christian Era, first recorded solar eclipse in, 85

 Chromatic aberration, 110

 Chromosphere, 71-72, 93-94, 130-132, 138-139

 Circle, 171-173

 Clark, Alvan, & Sons, 117-118, 303

 Claudius, Emperor, 86

 Clavius (lunar crater), 199

 Clerk Maxwell, 237

 "Clouds" (of Aristophanes), 101

 Clustering power, 325

 Clusters of stars, 300, 306, 314, 328

 Coal Sacks. _See_ Holes in Milky Way

 Coelostat, 119

 Coggia's Comet, 254

 Colour, production of, in telescopes, 109-111, 115, 121

 Collision of comet with earth, 345-346; of dark star with sun, 346-348; of stars, 285, 312

 Columbus, 103

 Coma Berenices (constellation), 307, 316

 Comet, first discovery of by photography, 258; first orbit calculated, 255; first photograph of, 257-258; furthest distance seen, 258; passage of among satellites of Jupiter, 250; passage of earth and moon through tail of, 257, 346

 Comet of 1000 A. D. , 262; 1066, 262-264; 1680, 255, 265; 1811, 254-255; 1861, 254, 257, 346; 1881, 257-258; 1882, 251, 258, 291; 1889, 258; 1907, 258

 Comets, 27-28, 58, Chaps. XIX. And XX. , 345-346; ancient view of, 259-261; captured, 251-253; Chinese records of, 83-84; composition of, 252; contrasted with planets, 247; families of, 251-252, 256; meteor swarms and, 274; revealed by solar eclipses, 95-96; tails of, 141, 182, 248, 252-254

 Common, telescopes of Dr. A. A. , 118

 Conjunction, 209

 Constellations, 105, 278-279, 285, 289

 Contraction theory of sun's heat, 128-129, 335

 Cook, Captain, 154

 Cooke, 118

 Copernican system, 20, 107, 149, 170-173, 279, 280

 Copernicus, 20, 108, 149, 158, 170-172, 236

 Copernicus (lunar crater), 200, 204

 Copper, 145

 Corder, H. , 144

 Corona, 70-72, 90, 92-97, 132, 140-141, 270; earliest drawing of, 91; earliest employment of term, 90; earliest mention of, 86; earliest photograph of, 93; illumination given by, 71; possible change in shape of during eclipse, 96-98; structure of, 142-143; variations in shape of, 141

 Corona Borealis (constellation), 295

 Coronal matter, 142; streamers, 95-96, 141-143

 Coronium, 133, 142, 317

 Cotes, 91

 Coudé, equatorial, 119

 Cowell, P. H. , 255, 264

 Crabtree, 152

 Crape ring of Saturn, 236-237

 Craterlets on Mars, 220

 Craters (ring-mountains) on moon, 197-205, 214, 340; suggested origin of, 203-204, 214

 Crawford, Earl of, 94

 Crecy, supposed eclipse at battle of, 88-89

 Crescent moon, 183, 185

 Crommelin, A. C. D. , 255, 264

 Crossley Reflector, 118, 315-316

 Crown glass, 115

 Crucifixion, darkness of, 86

 Crucis, [a] (Alpha), 298

 Crux, or "Southern Cross" (constellation), 298-299, 323

 Cycle, sunspot, 136-137, 141, 143-144

 Cygni, 61, 173, 280

 Cygnus, or the Swan (constellation), 295, 325

 Daniel's Comet of 1897, 258

 Danzig, 111

 Dark Ages, 102, 107, 260

 Dark eclipses of moon, 65, 102-103

 Dark matter in space, 323

 Dark meteors, 275-276

 Dark stars, 309-310, 312, 323, 346-347

 "Darkness behind the stars, " 325

 Darwin, Sir G. H. , 339

 Davis, 94

 Dawes, 236

 Dearborn Observatory, 303

 Death from fright at eclipse, 73

 Debonnaire, Louis le, 88, 261

 Deimos, 223

 Deity, symbol of the, 87

 "Demon star. " _See_ Algol

 Denebola, 296

 Denning, W. F. , 269

 Densities of sun and planets, 39

 Density, 38

 Deslandres, 140

 Diameters of sun and planets, 31

 Disappearance of moon in lunar eclipse, 65, 102-103

 Disc, 60

 "Disc" theory. _See_ "Grindstone" theory

 Discoveries, independent, 236

 Discovery, anticipation in, 236-237; indirect methods of, 120

 "Dipper, " the, 291; the "Little, " 294

 Distance of a celestial body, how ascertained, 56-58; of sun from earth, how determined, 151, 211

 Distances of planets from sun, 47

 Distances of sun and moon, relative, 68

 Dog, the Greater. _See_ Canis Major; the Lesser, _see_ Canis Minor

 "Dog Star, " 289, 297

 Dollond, John, 115-116

 Donati's Comet, 254, 257

 Doppler's method, 125, 136, 282, 301-302

 Dorpat, 117

 Double canals of Mars, 214-215, 218-220

 Double planet, earth and moon a, 189

 Double stars, 300

 Douglass, 233

 "Dreams, Lake of, " 197

 Dumb-bell Nebula, 316

 Earth, 20, 22, 31, 39, 48, 64, Chap. XV. , 267; cooling of, 343; diameter of, 31; interior of, 166; mean distance of from sun, 47; rigidity of, 181; rotation of, 30, 33, 161-165, 170; shape of, 165; "tail" to, 182

 "Earthlight, " or "Earthshine, " 186

 Earth's axis, Precessional movement of, 175-177, 295, 298-299

 Earth's shadow, circular shape of, 64, 160

 Eclipse, 61

 Eclipse knowledge, delay of, 74

 Eclipse party, work of, 73

 Eclipse of sun, advance of shadow in total, 69; animal and plant life during, 71; earliest record of total, 84; description of total, 69-73; duration of total, 69, 72; importance of total, 68

 Eclipses, ascertainment of dates of past, 74; experience a necessity in solar, 73-74; of moon, 63-65, Chap. IX. , 203; photography in, 93; prediction of future, 74; recurrence of, 74-80

 Eclipses of sun, 25, 65-74, Chap. VIII. , 201-202, 234; 1612 A. D. , 90; 1715, 88, 91; 1724, 88, 91; 1836, 92; 1842, 92-93; 1851, 81, 93; 1868, 93; 1870, 94; 1871, 94; 1878, 95; 1882, 95; 1883, 95-96; 1893, 95-96; 1896, 96, 99; 1898, 96, 98; 1900, 97; 1905, 75-76, 80-81, 97-98; 1907, 98; 1908, 98; 1914, 99; 1927, 92, 99-100

 _Eclipses, Past and Future_, 340

 Egenitis, 272

 Electric furnace, 128

 Electric light, spectrum of, 122

 Elements composing sun, 144-145

 Ellipses, 32, 66, 172-173, 177-178

 Elliptic orbit, 66, 177

 Ellipticity, 32

 Elongation, Eastern, 147, 149; Western, 147, 149

 Encke's Comet, 253, 256

 "End of the World, " 342

 England, solar eclipses visible in, 87-88, 91-92

 Epsilon, ([e]) Lyræ, 302

 Equator, 48

 Equatorial telescope, 226

 Equinoxes. _See_ Precession of

 Eros, 210-211, 223, 226-227; discovery of, 24, 210, 227; importance of, 211; orbit of, 32, 37, 210, 336

 Eruptive prominences, 139

 _Esclistre_, 89

 Ether, 322-323, 331-332

 Europa, 233, 235

 Evans, J. E. , 219

 Evening star, 149-150, 241

 Everest, Mount, 200

 Evershed, 182

 Eye-piece, 110

 Fabricius, 307

 Faculæ, 136, 143

 Fauth, 205

 Faye, 335

 _Fin du Monde_, 346

 First quarter, 183

 "Fixed stars, " 280

 Flagstaff, 215-216, 220

 Flammarion, Camille, 346

 Flamsteed, 90

 "Flash spectrum, " 137

 "Flat, " 112

 Flint glass, 115

 Focus, 66, 177

 "Forty-foot Telescope, " 115

 Foster, 102

 Fraunhofer, 117

 French Academy of Sciences, 115

 Froissart, 89

 "Full moon" of Laplace, 190

 Galaxy. _See_ Milky Way. 

 Galilean telescope, 109

 Galileo, 55, 109, 172, 197, 206, 232-235, 242

 Galle, 24, 211, 244

 Ganymede, 233-234

 Gas light, spectrum of, 122

 Gegenschein, 181-182

 "Gem" of meteor ring, 271

 Gemini, or the Twins (constellation), 22, 296-297

 Geminorum, [z] (Zeta), 304

 Geometrical groupings of stars, 292

 "Giant" planet, 230, 238-239

 Gibbous, 183, 185

 Gill, Sir David, 211, 258, 291, 317-318

 Gold, 145

 Goodricke, 307

 Gore, J. E. , 63, 285, 303, 307-308, 310, 323-324, 331, 337, 347

 Granulated structure of photosphere, 134

 Gravitation (or gravity), 39, 41-45, 128, 306

 Greek ideas, 18, 158, 161-162, 171, 186, 197

 Green (rays of light), 121

 Greenwich Observatory, 143-144, 232, 255, 303

 Gregorian telescope, 113-114

 Grimaldi (lunar crater), 199

 "Grindstone" theory, 319-322

 "Groombridge, 1830, " 281-282, 326, 330

 Groups of stars, 306-307

 Grubb, Sir Howard, 118

 _Gulliver's Travels_, 224

 Hale, G. E. , 119, 140

 Half moon, 183, 185

 Hall, Asaph, 223

 Hall, Chester Moor, 115

 Halley, Edmund, 91, 255, 264-265, 306

 Halley's Comet, 255, 264-265

 Haraden Hill, 91

 Harvard, 118, 302

 Harvest moon, 190-192

 Hawaii, 221

 Heat rays, 127

 Heidelberg, 226, 232

 Height of lunar mountains, how determined, 201

 Height of objects in sky, estimation of, 196

 Helium, 138, 145, 182

 Helmholtz, 128, 335

 Hercules (constellation), 295

 Herod the Great, 101-102

 Herodotus, 84

 Herschel, A. S. , 269

 Herschel, Sir John, 92, 322

 Herschel, Sir William, 22, 36, 114-115, 204, 213, 235, 283, 292, 308, 319-320, 326-328

 Herschelian telescope, 114, 119

 Hesper, 109

 Hesperus, 150

 Hevelius, 111

 Hezekiah, 85

 Hi, 83

 Hindoos, 18

 Hipparchus, 106, 177, 290, 311

 Ho, 83

 Holes in Milky Way, 321-323

 Holmes, Oliver Wendell, 213

 Homer, 223

 Horace, Odes of, 106

 Horizon, 159

 Horizontal eclipse, 169

 Horrox, 44, 151-152

 Hour Glass Sea, 212

 Huggins, Sir William, 94, 125, 317

 Humboldt, 270

 "Hunter's moon, " 192

 Huyghens, 111-112, 240, 242-243

 Hyades, 296-297, 307

 Hydrocarbon gas, 254

 Hydrogen, 94, 131, 138, 140, 144, 156, 182, 254

 Ibrahim ben Ahmed, 270

 Ice-layer theory: Mars, 219; moon, 205, 219

 Illusion theory of Martian canals, 219

 Imbrium, Mare, 197

 Inclination of orbits, 36-37

 Indigo (rays of light), 121

 Inferior conjunction, 147, 149

 Inferior planets, 20, 22, Chap. XIV. , 229

 Instruments, pre-telescopic, 106-107, 172

 International photographic survey of sky, 290-291

 Intra-Mercurial planet, 25-26

 _Introduction to Astronomy_, 31

 Inverted view in astronomical telescope, 116-117

 Io, 233-234

 Iridum, Sinus, 197

 Iron, 145, 254

 _Is Mars Habitable?_ 221

 Jansen, 108

 Janssen, 94, 236, 258

 Japetus, 240

 Jessenius, 89

 Job, Book of, 299

 Johnson, S. J. , 103, 340

 Josephus, 101, 262

 Juno, 225

 Jupiter, 20, 22-23, 31, 34, 37, 42, 227-228, 230-236, 241, 272, 311; comet family of, 251-253, 256; discovery of eighth satellite, 26, 232; eclipse of, by satellite, 234; without satellites, 234-235

 Jupiter, satellites of, 26, 62, 108, 189, 232-235; their eclipses, 234-235; their occultations, 62, 234; their transits, 62, 234

 Kant, 334

 Kapteyn, 284, 313

 Keeler, 315, 337

 Kelvin, Lord, 129

 Kepler, 44, 152, 172, 237, 242, 245, 253, 311

 Kinetic theory, 156, 202, 212, 226, 231, 239, 336

 King, L. W. , 84

 _Knowledge_, 87

 Labrador, 97

 Lacus Somniorum, 197

 "Lake of Dreams, " 197

 Lalande, 244, 283

 Lampland, 215, 219

 Langley, 95, 127

 Laplace, 190, 333

 Laputa, 224

 Le Maire, 115

 Le Verrier, 24, 236, 243-244, 275

 Lead, 145

 Leibnitz Mountains (lunar), 200

 Leo (constellation), 270, 295-296

 Leonids, 270-272, 274-275

 Lescarbault, 25

 Lewis, T. , 303

 Lexell's Comet, 250

 Lick Observatory, 31, 98, 117-118, 215, 232, 303, 305, 315; Great Telescope of, 117, 215, 237

 "Life" of an eclipse of the moon, 80; of the sun, 77-78

 Life on Mars, Lowell's views, 217-218; Pickering's, 221; Wallace's, 221-223

 Light, no extinction of, 322-324; rays of, 127; velocity of, 52, 235-236; white, 121

 "Light year, " 53, 280

 Lindsay, Lord, 94

 Linné (lunar crater), 205

 Liouville, 190

 Lippershey, 108

 Liquid-filled lenses, 116

 _Locksley Hall_, 296; _Sixty Years After_, 109

 Lockyer, Sir Norman, 73, 94, 236, 335

 Loewy, 119, 206

 London, eclipses visible at, 87-88, 91-92

 Longfellow, 88

 Lowell Observatory, 215, 219, 233-234

 Lowell, Percival, 155, 212-213, 215-221

 Lucifer, 150

 Lynn, W. T. , 219, 263

 Lyra (constellation), 177, 283, 294-295, 307, 315, 347

 Mädler, 206, 284

 Magellanic Clouds, 317

 Magnetism, disturbances of terrestrial, 143, 283

 Magnitudes of stars, 287-289

 Major planets, 229-230

 "Man in the Moon, " 197

 _Manual of Astronomy_, 166

 Maps of the moon, 206

 Mare Imbrium, 197

 Mare Serenitatis, 205

 Mars, 20, 22-23, 31-32, 34, 37, 109, 155, 210-225, 234; compared with earth and moon, 221, 225; polar caps of, 212-214, 216; satellites of, 26, 223-224; temperature of, 213, 216, 221-222

 Mass, 38; of a star, how determined, 305

 Masses of celestial bodies, how ascertained, 42; of earth and moon compared, 42; of sun and planets compared, 39

 Maunder, E. W. , 87, 143, 219

 Maunder, Mrs. , E. W. , 96, 144

 Maxwell, Clerk, 237

 Mayer, Tobias, 206, 283

 McClean, F. K. , 98

 Mean distance, 46

 "Medicean Stars, " 232

 Mediterranean, eclipse tracks across, 94, 97

 Melbourne telescope, 118

 Melotte, P. , 232

 Mercator's Projection, 80-81

 Mercury (the metal), 145

 Mercury (the planet), 20, 22, 25-26, 31-32, 34, 37, Chap. XIV. ; markings on, 156; possible planets within orbit of, 25-26; transit of, 62, 151, 154

 Metals in sun, 145

 Meteor swarms, 268-269, 271, 274-275

 Meteors, 28, 56, 167, 259, Chap. XXI. 

 Meteors beyond earth's atmosphere, 275-276

 Meteorites, 276-277

 Meteoritic Hypothesis, 335

 Metius, Jacob, 108

 Michell, 283, 305

 Middle Ages, 102, 260, 264

 Middleburgh, 108

 Milky Way (or Galaxy), 285, 299, 311, 317, 319-327; penetration of, by photography, 325

 Million, 47, 51-52

 Minor planets. _See_ Asteroids. 

 Mira Ceti, 307-308

 "Mirk Monday, " 89

 Mirror (speculum), 111, 116

 Mizar, 294, 302

 Monck, W. H. S. , 275

 Mongol Emperors of India, 107

 Moon, 26, Chap. XVI. ; appearance of, in lunar eclipse, 65, 102-103; diameter of, 189; distance of, how ascertained, 58; distance of, from earth, 48; full, 63, 86, 149, 184, 189, 190, 206; mass of, 200, 202; mountains on, 197-205; how their height is determined, 201; movement of, 40-42; new, 86, 149, 183, 185; origin of, 339-341; plane of orbit of, 63; possible changes on, 204-205, 221; "seas" of, 197, 206; smallest detail visible on, 207; volume of, 200

 Morning star, 149-150, 241

 Moulton, F. R. , 31, 118, 128, 302, 335, 337

 Moye, 154

 Multiple stars, 300

 Musa-ben-Shakir, 44

 Mythology, 105

 Neap-tides, 179

 Nebulæ, 314-318, 328, 335, 345; evolution of stars from, 317-318

 Nebular Hypothesis of Laplace, 333-338

 Nebular hypotheses, Chap. XXVII. 

 Nebulium, 317

 Neison, 206

 Neptune, 20, 25, 31, 34, 37, 243-246, 249, 252, 274, 304; discovery of, 23-24, 94, 210, 236, 243-244; Lalande and, 244; possible planets beyond, 25, 252; satellite of, 26, 245; "year" in, 35-36

 "New" (or temporary), stars, 310-314

 Newcomb, Simon, 181, 267, 281, 324, 326-327, 329

 Newton, Sir Isaac, 40, 44, 91, 111-113, 115, 165, 172, 237, 255

 Newtonian telescope, 112, 114, 116, 119

 Nineveh Eclipse, 84-85

 Nitrogen, 145, 156, 166, 346

 Northern Crown, 295

 Nova Aurigæ, 311

 Nova Persei, 312-314

 Novæ. _See_ New (or temporary) stars

 Nubeculæ, 317

 "Oases" of Mars, 216, 220

 Object-glass, 109

 Oblate spheroid, 165

 Occultation, 61-62, 202, 296

 _Olaf, Saga of King_, 88

 Olbers, 227, 253, 256, 271

 "Old moon in new moon's arms, " 185

 Olmsted, 271

 Omicron (or "Mira") Ceti, 307-308

 Opposition, 209

 "Optick tube, " 108-109, 232

 Orange (rays of light), 121

 Orbit of moon, plane of, 63

 Orbits, 32, 36-37, 66, 150, 157

 Oriental astronomy, 107

 Orion (constellation), 195, 279, 296-297, 316; Great Nebula in, 316, 328

 Oxford, 139

 Oxygen, 145, 156, 166, 346

 Pacific Ocean, origin of moon in, 339

 Palitzch, 255

 Pallas, 225, 227

 Parallax, 57, 173, 280, 305, 320, 326

 Paré, Ambrose, 264-265

 Peal, S. E. , 205

 Peary, 277

 Pegasus (constellation), 306

 Penumbra of sunspot, 135

 Perennial full moon of Laplace, 190

 Pericles, 84

 Perrine, C. D. , 232-233, 315

 Perseids, 270, 273-275

 Perseus (constellation), 273, 279, 307, 312

 Phases of an inferior planet, 149, 160; of the moon, 149, 160, 183-185

 Phlegon, Eclipse of, 85-86

 Phobos, 223

 Phoebe, retrograde motion of, 240, 250, 336

 Phosphorescent glow in sky, 323

 Phosphorus (Venus), 150

 Photographic survey of sky, international, 290-291

 Photosphere, 130-131, 134

 Piazzi, 23

 Pickering, E. C. , 302

 Pickering, W. H. , 199, 205-206, 220-221, 240, 339-341

 Pictor, "runaway star" in constellation of, 281-282, 320, 330

 Plane of orbit, 36, 150

 Planetary nebulæ, 245, 315

 _Planetary and Stellar Studies_, 331

 Planetesimal hypothesis, 337-338

 Planetoids. _See_ Asteroids

 Planets, classification of, 229; contrasted with comets, 247; in Ptolemaic scheme, 171; relative distances of, from sun, 31-32

 Plato (lunar crater), 198

 Pleiades, 284, 296-297, 307

 Pliny, 169, 260

 Plough, 284, 291-296, 302

 Plutarch, 86, 89, 169, 181

 "Pointers, " 292

 Polaris. _See_ Pole Star

 Pole of earth, Precessional movement of, 176-177, 295, 298-299

 Pole Star, 33, 163, 177, 292-296, 300-301

 Poles, 30, 163-164; of earth, speed of point at, 164

 Pollux, 282, 297

 Posidonius, 186

 Powell, Sir George Baden, 96

 Præsepe (the Beehive), 307

 Precession of the Equinoxes, 177, 295, 298-299

 Pre-telescopic notions, 55

 Primaries, 26

 _Princess, The_ (Tennyson), 334

 Princeton Observatory, 258

 Prism, 121

 Prismatic colours, 111, 121

 Procyon, 284, 290, 297, 303

 Prominences, Solar, 72, 93, 131, 139-140, 143; first observation of, with spectroscope, 94, 140, 236

 Proper motions of stars, 126, 281-285, 326, 329-330

 Ptolemæus (lunar crater), 198-199, 204

 Ptolemaic idea, 319; system, 18, 19, 158, 171-172

 Ptolemy, 18, 101, 171, 290, 296

 Puiseux, P. , 206

 Pulkowa telescope, 117

 Puppis, V. , 310

 Quiescent prominences, 139

 Radcliffe Observer, 139

 "Radiant, " or radiant point, 269

 Radiation from sun, 130, 134

 Radium, 129, 138

 Rainbow, 121

 "Rainbows, Bay of, " 197

 Rambaut, A. , 139

 Ramsay, Sir William, 138

 Rays (on moon), 204

 Recurrence of eclipses, 74-80

 Red (rays of light), 121, 125, 127, 130

 Red Spot, the Great, 230

 Reflecting telescope, 111-116; future of, 119

 Reflector. _See_ Reflecting telescope

 Refracting and reflecting telescopes contrasted, 118

 Refracting telescope, 109-111, 115-117; limits to size of, 119-120

 Refraction, 121, 168-169

 Refractor, _See_ Refracting telescope

 Regulus, 290, 296

 Retrograde motion of Phoebe, 240, 250, 336

 "Reversing Layer, " 94, 130, 132, 137-138

 Revival of learning, 107

 Revolution, 30; of earth around sun, 170-173; periods of sun and planets, 35

 Riccioli, 198

 Rice-grain structure of photosphere, 134

 Rigel, 285, 297

 Rills (on moon), 204

 Ring-mountains of moon. _See_ Craters

 "Ring" nebulæ, 315, 337

 "Ring with wings, " 87

 Rings of Saturn, 108, 236-239, 241-243, 334

 Ritchey, G. W. , 118

 Roberts, A. W. , 308, 310

 Roberts, Isaac, 325

 "Roche's limit, " 238

 Roemer, 235

 Roman history, eclipses in, 85-86

 Romulus, 85

 Röntgen, 120

 Rosse, great telescope of Lord, 117, 314

 Rotation, 30; of earth, 33, 161-165, 170; of sun, 34, 125, 135-136, 231; periods of sun and planets, 35

 Royal Society of London, 90-91, 111

 Rubicon, Passage of the, 85

 "Runaway" stars, 281, 326, 330

 Sagittarius (constellation), 316

 Salt, spectrum of table, 122

 Samarcand, 107

 "Saros, " Chaldean, 76-78, 84

 Satellites, 26-27, 37

 Saturn, 20, 22, 34, 37, 108, 236-243, 258; comet family of, 252; a puzzle to the early telescope observers, 241-243; retrograde motion of satellite Phoebe, 240, 250, 336; ring system of, 241; satellites of, 36, 239-240; shadows of planet on rings and of rings on planet, 237

 Schaeberle, 95-96, 303, 316

 Schiaparelli, 155, 214, 223

 Schickhard (lunar crater), 199

 Schmidt, 206

 Schönfeld, 290

 Schuster, 95

 Schwabe, 136

 Scotland, solar eclipses visible in, 89-90, 92

 Sea of Serenity, 205

 "Sea of Showers, " 197

 "Seas" of moon, 197, 206

 Seasons on earth, 174-175; on Mars, 211

 Secondary bodies, 26

 Seneca, 95, 260

 _Septentriones_, 291

 Serenitatis, Mare, 205

 "Seven Stars, " 291

 "Shadow Bands, " 69

 Shadow of earth, circular shape of, 62-64

 Shadows on moon, inky blackness of, 202

 Shakespeare, 259, 293

 Sheepshanks Telescope, 119

 "Shining fluid" of Sir W. Herschel, 328

 "Shooting Stars. " _See_ Meteors

 Short (of Edinburgh), 114

 "Showers, Sea of, " 197

 Sickle of Leo, 270-271, 296

 Siderostat, 118

 Silver, 145

 Silvered mirrors for reflecting telescopes, 116

 Sinus Iridum, 197

 Sirius, 280, 282, 284-285, 288-290, 297, 303-304, 320; companion of, 303; stellar magnitude of, 289

 Size of celestial bodies, how ascertained, 59

 Skeleton telescopes, 110

 Sky, international photographic survey of, 290-291; light of the, 323

 Slipher, E. C. , 213, 222

 Smithsonian Institution of Washington, 98

 Snow on Mars, 213

 Sodium, 122, 124, 254

 Sohag, 95

 Solar system, 20-21, 29-31; centre of gravity of, 42; decay and death of, 344

 Somniorum, Lacus, 197

 Sound, 125, 166, 331

 South pole of heavens, 163, 285, 298-299

 Southern constellations, 298-299

 Southern Cross. _See_ Crux

 Space, 328

 Spain, early astronomy in, 107; eclipse tracks across 93, 97-98

 Spectroheliograph, 140

 Spectroscope, 120, 122, 124-125, 144-145, 212, 231; prominences first observed with, 94, 140, 236

 Spectrum of chromosphere, 132-133; of corona, 133; of photosphere, 132; of reversing layer, 132, 137; solar, 122-123, 127, 132

 Speculum, 111, 116; metal, 112

 Spherical bodies, 29

 Spherical shape of earth, proofs of, 158-161

 Spherical shapes of sun, planets, and satellites, 160

 Spiral nebulæ, 314-316, 337-338

 Spring balance, 166

 Spring tides, 192

 Spy-glass, 108

 "Square of the distance, " 43-44

 Stannyan, Captain, 90

 Star, mass of, how determined, 305; parallax of, first ascertained, 173, 280

 Stars, the, 20, 124, 126, 278 _et seq. _; brightness of, 287, 320; distances between, 326-327; distances of some, 173, 280, 320; diminution of, below twelfth magnitude, 324; evolution of, from nebulæ, 317-318; faintest magnitude of, 288; number of those visible altogether, 324; number of those visible to naked eye, 288

 "Steam cracks, " 221

 Steinheil, 118

 Stellar system, estimated extent of, 325-327; an organised whole, 327; limited extent of, 322-328, 330; possible disintegration of, 329

 Stiklastad, eclipse of, 88

 Stone Age, 285

 Stoney, G. J. , 202, 222

 Stonyhurst Observatory, 100

 _Story of the Heavens_, 271

 Streams of stars, Kapteyn's two, 284

 Stroobant, 196

 Stukeley, 91

 Sulphur, 145

 Summer, 175, 178

 Sun, Chaps XII. And XIII. ; as a star, 124, 278, 289; as seen from Neptune, 246, 304; chemical composition of, 144-145; distance of, how ascertained, 151, 211; equator of, 135-136, 139; gravitation at surface of, 129, 138-139; growing cold of, 343-344; mean distance of, from earth, 47, 211; motion of, through space, 282-286, 326; not a solid body, 136; poles of, 136; radiations from, 130; revolution of earth around, 170-173; stellar magnitude of, 288-289; variation in distance of, 66, 178

 Sunspots, 34, 125, 134-137, 140-141, 143-144, 308; influence of earth on, 144

 Suns and possible systems, 50, 286

 Superior conjunction, 147-149

 Superior planets, 22, 146, 209-210, 229

 Swan (constellation). _See_ Cygnus

 Swift, Dean, 224

 "Sword" of Orion, 297, 316

 Syrtis Major. _See_ Hour Glass Sea

 "_Systematic_ Parallax, " 326

 Systems, other possible, 50, 286

 Tails of comets, 182

 Tamerlane, 107

 Taurus (constellation), 103, 296-297, 307

 "Tears of St. Lawrence, " 273

 Tebbutt's Comet, 257-258

 Telescope, 33, 55, 107-108, 149; first eclipse of moon seen through, 104; of sun, 90

 Telescopes, direct view reflecting, 114; gigantic, 111; great constructors of, 117-118; great modern, 117-118

 Tempel's Comet, 274

 Temperature on moon, 203; of sun, 128

 Temporary (or new) stars, 310-314

 Tennyson, Lord, 109, 296, 334

 Terrestrial planets, 229-230

 Terrestrial telescope, 117

 Thales, Eclipse of, 84

 Themis, 240

 "Tidal drag, " 180, 188, 208, 344

 Tide areas, 179-180

 Tides, 178-180, 338-339

 _Time Machine_, 344

 Tin, 145

 Titan, 240

 Titius, 245

 Total phase, 71-72

 Totality, 72; track of, 66

 Trail of a minor planet, 226-227

 Transit, 62, 150-154; of Mercury, 62, 151, 154; of Venus, 62, 151-152, 154, 211

 Trifid Nebula, 316

 Triple stars, 300

 Tubeless telescopes, 110-111, 243

 Tubes used by ancients, 110

 Tuttle's Comet, 274

 Twilight, 167, 202

 Twinkling of stars, 168

 Twins (constellation). _See_ Gemini

 Tycho Brahe, 290, 311

 Tycho (lunar crater), 204

 Ulugh Beigh, 107

 Umbra of sunspot, 134-135

 Universe, early ideas concerning, 17-18, 158, 177, 342

 Universes, possibility of other, 330-331

 Uranus, 22-24, 31, 210, 243, 245, 275; comet family of, 252; discovery of, 22, 210, 243; rotation period of 34, 245; satellites of, 26, 245; "year" in, 35-36

 Ursa Major (constellation), 279, 281, 291, 295, 314; minor, 177, 279, 293-294

 Ursæ Majoris, ([z]) Zeta. _See_ Mizar

 Variable stars, 307-310

 Variations in apparent sizes of sun and moon, 67, 80, 178

 Vault, shape of the celestial, 194-196

 Vega, 177, 278, 280, 282-283, 285, 290, 294, 302, 307, 323

 Vegetation on Mars, 221, 217-218; on moon, 205

 Venus, 20, 22, 31, 71, 90, 108-109, 111, Chap. XIV. , 246, 311; rotation period of, 34, 155

 Very, F. W. , 314

 Vesta, 225, 227

 Violet (rays of light), 121-122, 125

 Virgil, 19

 Volcanic theory of lunar craters, 203-204, 214

 Volume, 38

 Volumes of sun and planets compared, 38-39

 "Vulcan, " 25

 Wallace, A. R. , on Mars, 220-223

 Water, lack of, on moon, 201-202

 Water vapour, 202, 213, 222

 Wargentin, 103

 Warner and Swasey Co. , 117

 Weather, moon and, 206-207

 Weathering, 202

 Webb, Rev. T. W. , 204

 Weight, 43, 165-166

 Wells, H. G. , 344

 Whale (constellation). _See_ Cetus

 Whewell, 190

 Willamette meteorite, 277

 Wilson, Mount, 118

 Wilson, W. E. , 313

 "Winged circle" (or "disc"), 87

 Winter, 175, 178

 Witt, 227

 Wolf, Max, 226-227, 232

 Wright, Thomas, 319, 334

 Wybord, 89

 Xenophon, 101

 Year, 35

 "Year" in Uranus and Neptune, 35-36

 Year, number of eclipses in a, 68

 "Year of the Stars, " 270

 Yellow (rays of light), 121-122, 124

 Yerkes Telescope Great, 117, 303

 Young, 94, 137, 166

 Zenith, 174

 Zinc, 145

 Zodiacal light, 181

 Zone of asteroids, 30-31, 227

THE END

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ABBOTT, Rev. E. A. , D. D. 

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BICKERSTETH, Rev. E. 

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CARTWRIGHT, JULIA. 

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CHORAL SERVICE-BOOK FOR PARISH CHURCHES, THE. 

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For other books by Professor CHURCH see Complete Catalogue. 

CLARK, J. W. , M. A. 

 Cambridge. With a coloured Frontispiece and many other Illustrations by A. BRUNET-DEBAINES and H. TOUSSAINT &c. Extra crown 8vo, 6s. ; also crown 8vo, cloth, 2s. Nett; leather, 3s. ; special leather, in box, 5s. Nett. 

CODY, Rev. H. A. 

 An Apostle of the North. The Biography of the late Bishop BOMPAS, First Bishop of Athabasca, and with an Introduction by the ARCHBISHOP of RUPERTS-LAND. With 42 Illustrations. Demy 8vo, 7s. 6d. Nett. 5s. Nett. 

CORBIN, T. W. 

 Engineering of To-day. With Seventy-three Illustrations and Diagrams. Extra crown 8vo, 5s. Nett. 

 Mechanical Inventions of To-Day. Ex. Crown 8vo; with Ninety-four Illustrations, 5s. Nett. 

CORNISH, C. J. 

 Animals of To-day: Their Life and Conversation. With Illustrations from Photographs by C. REID of Wishaw. Crown 8vo, 6s. 

 The Isle of Wight. Illustrated. Super-royal 8vo, sewed, 2s. 6d. Nett; half-linen, 3s. 6d. Nett; also a new edition, small 4to, cloth, 2s. ; leather, 3s. And 5s. 

 Life at the Zoo. Notes and Traditions of the Regent's Park Gardens. Illustrated from Photographs by GAMBIER BOLTON. Fifth Edition. Crown 8vo, 6s. 

 The Naturalist on the Thames. Many Illustrations. Demy 8vo, 7s. 6d. 

 The New Forest. Super-royal 8vo, sewed, 2s. 6d. Nett; half-linen, 3s. 6d. Nett; also new edition, small 4to, cloth, 2s. ; leather, 3s. Nett; and special velvet leather, each copy in a box, 5s. 

 The New Forest and the Isle of Wight. With Eight Plates and many other Illustrations. Super-royal 8vo, 7s. 6d. Nett. 

 Nights with an Old Gunner, and other Studies of Wild Life. With Sixteen Illustrations by LANCELOT SPEED, CHARLES WHYMPER, and from Photographs. Crown 8vo, 6s. 

 * * * * *

THE CROWN LIBRARY

A series of notable copyright books issued in uniform binding. Extracrown 8vo. With many illustrations, 5s. Nett. 

_JUST ISSUED. SECOND AND CHEAPER EDITION. _

SWANN, A. J. 

 Fighting the Slave Hunters in Central Africa. A Record of Twenty-six Years of Travel and Adventure round the Great Lakes, and of the overthrow of Tip-pu-Tib, Rumaliza, and other great Slave Traders. With 45 Illustrations and a Map, 5s. Nett. 

_RECENTLY ISSUED. _

GRUBB, W. BARBROOKE. 

 An Unknown People in an Unknown Land. An Account of the Life and Customs of the Lengua Indians of the Paraguayan Chaco, with Adventures and Experiences met with during Twenty Years' Pioneering and Exploration amongst them. With Twenty-four Illustrations and a Map. Extra crown 8vo, 5s. Nett. 

FRASER, Sir A. H. L. , K. C. S. I. , M. A. , LL. D. , Litt. D. , ex-Lieutenant-Governor of Bengal. 

 Among Indian Rajahs and Ryots. A Civil Servants' Recollections and Impressions of Thirty-seven Years of Work and Sport in the Central Provinces and Bengal. Third Edition, 5s. Nett. 

CODY, Rev. H. A. 

 An Apostle of the North. The Story of Bishop Bompas's Life amongst the Red Indians & Eskimo. Third Edition, 5s. Nett. 

PENNELL, T. L. , M. D. , B. Sc. 

 Among the Wild Tribes of the Afghan Frontier. A Record of Sixteen Years' close intercourse with the natives of Afghanistan and the North-West Frontier. Introduction by EARL ROBERTS. Extra crown 8vo. Twenty-six Illustrations and Map. Fifth Edition, 5s. Net. 

 * * * * *

CUST, LIONEL. 

 The Engravings of Albert Dürer. Illustrated. Super-royal 8vo, half-linen, 3s. 6d. Nett. 

 Paintings and Drawings of Albert Dürer. Illustrated. Super-royal 8vo, sewed, 3s. 6d. Nett. 

 Albrecht Dürer. A Study of his Life and Work. With Eight Copper Plates and many other Illustrations. Super-royal 8vo, 7s. 6d. 

DAVENPORT, CYRIL. 

 Cameos. With examples in Colour and many other Illustrations. Super-royal 8vo, sewed, 5s. Nett; cloth, 7s. Nett. 

 Royal English Bookbindings. With Coloured Plates and many other Illustrations. Super-royal 8vo, sewed, 3s. 6d. ; cloth, 4s. 6d. 

DAVIES, RANDALL, F. S. A. 

 English Society of the Eighteenth Century in Contemporary Art. With Four Coloured and many other Illustrations. Super royal 8vo, sewed, 5s. Nett; cloth, 7s. Nett. 

DAWSON, Rev. E. C. 

 The Life of Bishop Hannington. Crown 8vo, paper boards, 2s. 6d. ; or with Map and Illustrations, cloth, 3s. 6d. 

DESTRÉE, O. G. 

 The Renaissance of Sculpture in Belgium. Illustrated. Super-royal 8vo, sewed, 2s. 6d. Nett; half-linen, 3s. 6d. Nett. 

DOLMAGE, CECIL G. , M. A. , D. C. L. , LL. D. , F. R. A. S. 

 Astronomy of To-Day. A popular account in non-technical language. With Forty-six Illustrations and Diagrams. Extra crown 8vo, 5s. Nett. 

DOMVILLE-FIFE, CHARLES W. 

 Submarine Engineering of To-Day. Extra crown 8vo, 5s. Nett. 

ELZEVIR LIBRARY, THE. 

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Volume I. Fancy & Humour of Lamb. " II. Wit & Imagination of Disraeli. " III. Vignettes from Oliver Goldsmith. " IV. Wit & Sagacity of Dr. Johnson. " V. Insight & Imagination of John Ruskin. " VI. Vignettes of London Life from Dickens. " VII. XVIIIth Century Vignettes from Thackeray. " VIII. Vignettes of Country Life from Dickens. " IX. Wisdom & Humour of Carlyle. 

"Decidedly natty and original in get-up. "--_The Saturday Review. _

EVANS, WILLMOTT, M. D. 

 Medical Science of To-Day. Ex. Crn. 8vo; 24 Illustrations, 5s. Nett. 

WILMOT, EARDLEY, Rear-Admiral S. 

 Our Fleet To-day and its Development during the last Half Century. With many Illustrations. Crown 8vo, 5s. 

EVENTS OF OUR OWN TIMES

Crown 8vo. With Illustrations, 5s. Each. 

 The War in the Crimea. By General Sir E. HAMLEY, K. C. B. 

 The Indian Mutiny. By Colonel MALLESON, C. S. I. 

 The Afghan Wars, 1839-42, and 1878-80. By ARCHIBALD FORBES. 

 Our Fleet To-Day and its Development during the last Half-Century. By Rear-Admiral S. EARDLEY WILMOT. 

 The Refounding of the German Empire. By Colonel MALLESON, C. S. I. 

 The Liberation of Italy. By the Countess MARTINENGO CESARESCO. 

 Great Britain in Modern Africa. By EDGAR SANDERSON, M. A. 

 The War in the Peninsula. By A. INNES SHAND. 

FLETCHER, W. Y. 

 Bookbinding in France. Coloured Plates. Super-royal, sewed, 2s. 6d. Nett; half-linen, 3s. 6d. Nett. 

FORBES, ARCHIBALD. 

 The Afghan Wars of 1839-1842 and 1878-1880. With Four Portraits on Copper, and Maps and Plans. Crown 8vo, 5s. 

FRASER, Sir ANDREW H. L. 

 Among Indian Rajahs and Ryots. With 34 Illustrations and a Map. Demy 8vo, 18s. Nett. Third and Cheaper Edition, 5s. Nett. 

FRASER, DONALD. 

 Winning a Primitive People. Illustrated. Extra crown 8vo, 5s. Nett. 

FRIPP, Sir ALFRED D. , K. C. V. O. , & R. THOMPSON, F. R. C. S. 

 Human Anatomy for Art Students. Profusely Illustrated with Photographs and Drawings by INNES FRIPP, A. R. C. A. Square extra crown 8vo, 7s. 6d. Nett. 

FROBENIUS, LEO. 

 The Childhood of Man. A Popular Account of the Lives and Thoughts of Primitive Races. Translated by Prof. A. H. KEANE, LL. D. With 416 Illustrations. Demy 8vo, 16s. Nett. 

FRY, ROGER. 

 Discourses Delivered to the Students of the Royal Academy by Sir Joshua Reynolds. With an Introduction and Notes by ROGER FRY. With Thirty-three Illustrations. Square Crown 8vo, 7s. 6d. Nett. 

GARDNER, J. STARKIE. 

 Armour in England. With Eight Coloured Plates and many other Illustrations. Super-royal 8vo, sewed, 3s. 6d. Nett. 

 Foreign Armour in England. With Eight Coloured Plates and many other Illustrations. Super-royal 8vo, sewed, 3s. 6d. Nett. 

 Armour in England. With Sixteen Coloured Plates and many other Illustrations. The two parts in one volume. Super-royal 8vo, cloth, gilt top, 9s. Nett. 

GARNETT, R. , LL. D. 

 Richmond on Thames. Illustrated. Super-royal 8vo, sewed, 3s. 6d. Nett. 

GIBERNE, AGNES. 

 Beside the Waters of Comfort. Crown 8vo, 3s. 6d. 

GIBSON, CHARLES R. , F. R. S. E. 

 Electricity of To-Day. Its Works and Mysteries described in non-technical language. With 30 Illustrations. Extra crown 8vo, 5s. Nett. 

 Scientific Ideas of To-day. A Popular Account in non-technical language of the Nature of Matter, Electricity, Light, Heat, &c. , &c. With 25 Illustrations. Extra crown 8vo, 5s. Nett. 

 How Telegraphs and Telephones Work. With many Illustrations. Crown 8vo, 1s. 6d. Nett. 

 The Autobiography of an Electron. With 8 Illustrations. Long 8vo, 3s. 6d. Nett. 

 Wireless Telegraphy. With many Illustrations. Ex. Crn. 8vo, 2s. Nett. 

GODLEY, A. D. 

 Socrates and Athenian Society in his Day. Crown 8vo, 4s. 6d. 

 Aspects of Modern Oxford. With many Illustrations. Crown 8vo, cloth, 2s. Nett; lambskin, 3s. Nett; velvet leather, in box, 5s. Nett. 

GOLDEN RECITER (_See_ JAMES, Prof. CAIRNS. )

GOMES, EDWIN H. , M. A. 

 Seventeen Years among the Sea Dyaks of Borneo. With 40 Illustrations and a Map. Demy 8vo, 16s. Nett. 

GRAHAME, GEORGE. 

 Claude Lorrain. Illustrated. Super-royal 8vo, 2s. 6d. Nett; half-linen, 3s. 6d. Nett. 

GRIFFITH, M. E. HUME. 

 Behind the Veil in Persia and Turkish Arabia. An Account of an Englishwoman's Eight Years' Residence amongst the Women of the East. With 37 Illustrations and a Map. Demy 8vo, 16s. Nett. 

GRINDON, LEO. 

 Lancashire. Brief Historical and Descriptive Notes. With many Illustrations. Crown 8vo, 6s. 

GRUBB, W. BARBROOKE (Pioneer and Explorer of the Chaco). 

 An Unknown People in an Unknown Land. With Sixty Illustrations and a Map. Demy 8vo, 16s. Nett. Third and Cheaper Edition, 5s. 

 A Church in the Wilds. Illustrated. Extra crown 8vo, 5s. Nett. 

HADOW, W. H. 

 A Croatian Composer. Notes toward the Study of Joseph Haydn. Crown 8vo, 2s. 6d. Nett. 

 Studies in Modern Music. First Series. Berlioz, Schumann, Wagner. With an Essay on Music and Musical Criticism. With Five Portraits. Crown 8vo, 7s. 6d. 

 Studies in Modern Music. Second Series. Chopin, Dvoràk, Brahms. With an Essay on Musical Form. With Four Portraits. Crown 8vo, 7s. 6d. 

HAMERTON, P. G. 

 The Etchings of Rembrandt, and Dutch Etchers of the Seventeenth Century. By P. G. HAMERTON and LAURENCE BINTON. With Eight Copper Plates and many other Illustrations. Super-royal 8vo, 7s. 6d. Nett. 

 The Mount. Narrative of a Visit to the Site of a Gaulish City on Mount Beuvray. With a Description of the neighbouring City of Autun. Crown 8vo, 3s. 6d. 

 Round my House. Notes on Rural Life in Peace and War. Crown 8vo, with Illustrations, 2s. 6d. Nett. Cheaper edition, 2s. Nett. 

 Paris. Illustrated. New edition. Cloth, 2s. Nett; leather, 3s. Nett in special leather, full gilt, in box, 5s. Nett. 

HAMLEY, Gen. Sir E. 

 The War in the Crimea. With Copper Plates and other Illustrations. Crown 8vo, 5s. 

HANOUM ZEYNEB (Heroine of Pierre Loti's Novel "Les Désenchantées. ")

 A Turkish Woman's European Impressions. Edited by GRACE ELLISON. With a portrait by AUGUSTE RODIN and 23 other Illustrations from photographs. Crown 8vo, 6s. Nett. 

HARTLEY, C. GASQUOINE. 

 Things Seen in Spain. With Fifty Illustrations. Small 4to, cloth, 2s. ; leather, 3s. ; velvet leather in a box, 5s. Nett. 

HAYWOOD, Capt. A. H. W. 

 Through Timbuctu & Across the Great Sahara. Demy 8vo, with 41 Illustrations and a Map. 16s. Nett. 

HENDERSON, Major PERCY E. 

 A British Officer in the Balkans. Through Dalmatia, Montenegro, Turkey in Austria, Magyarland, Bosnia and Herzegovina. With 50 Illustrations and a Map. Gilt top. Demy 8vo, 16s. Nett. 

HERBERT, GEORGE. 

 The Temple. Sacred Poems and Ejaculations. The Text reprinted from the First Edition. With Seventy-six Illustrations after ALBERT DÜRER, HOLBRIN, and other Masters. Crown 8vo, cloth, 2s. Nett; leather, 3s. Nett. ; velvet leather in box, 5s. Nett. 

HOLLAND, CLIVE. 

 Things Seen in Japan. With Fifty beautiful illustrations of Japanese life in Town and Country. Small 4to, cloth, 2s. Nett; leather, 3s. Nett; velvet leather, in box, 5s. Nett. 

HUTCHINSON, Rev. H. N. 

 The Story of the Hills. A Popular Account of Mountains and How They were Made. With many Illustrations. Crown 8vo, 5s. 

HUTTON, C. A. 

 Greek Terracotta Statuettes. With a Preface by A. S. MURRAY, LL. D. With Seventeen Examples printed in Colour and Thirty-six printed in Monochrome. 5s. Nett; or cloth, 7s. Nett. 

HUTTON, SAMUEL KING, M. B. 

 Among the Eskimos of Labrador. Demy 8vo; with Forty-seven Illustrations and a Map. 16s. Nett. 

JAMES, CAIRNS. 

 The Golden Reciter. With an Introduction by CAIRNS JAMES, Professor of Elocution at the Royal Academy of Music, &c. With Selections from Rudyard Kipling, Thomas Hardy, R. L. Stevenson, Seton Merriman, H. G. Wells, Christina Rossetti, Anthony Hope, Austin Dobson, Maurice Hewlett, Conan Doyle, &c. &c. Extra crown 8vo, 704 pp. Cloth, 3s. 6d. , and thin paper edition in cloth with gilt edges, 5s. 

 "A more admirable book of its kind could not well be desired. "--_Liverpool Courier. _

 The Golden Humorous Reciter. Edited, and with a Practical Introduction, by CAIRNS JAMES, Professor of Elocution at the Royal College of Music and the Guildhall School of Music. A volume of Recitations and Readings selected from the writings of F. Anstey, J. M. Barrie, S. R. Crockett, Jerome K. Jerome, Barry Pain, A. W. Pinero, Owen Seaman, G. B. Shaw, &c. &c. Extra crown 8vo, over 700 pages, cloth, 3s. 6d. ; also a thin paper edition, with gilt edges, 5s. 

 * * * * *

THE ILLUMINATED SERIES

NEW BINDING. 

 Bound in antique leather with metal clasps. With illuminated frontispiece and title-page, and other illuminated pages. Finely printed at the Ballantyne Press, Edinburgh. Crown 8vo. Each copy in a box, 10s. 6d. Nett. Also in real classic vellum. Each copy in a box. 10s. 6d. Nett. 

 The Confessions of S. Augustine. 

 Of the Imitation of Christ. By THOMAS À KEMPIS. 

 The Sacred Seasons. By the BISHOP OF DURHAM. Also cloth, 6s. And 7s. 6d. Nett. 

JOY, BEDFORD. 

 A Synopsis of Roman History. Crown 8vo, 2s. 

KEANE, Prof. A. H. (_See_ FROBENIUS. )

LANG, ANDREW. 

 Oxford. New Edition. With 50 Illustrations by J. H. LORIMER, R. S. A. , T. HAMILTON CRAWFORD, R. S. W. , J. PENNELL, A. BRUNET-DEBAINES, A. TOUSSAINT, and R. KENT THOMAS. Extra crown 8vo, 6s. Also Pocket Edition, 2s. Nett; leather, 3s. Nett. Special yapp leather, full gilt, in box, 5s. Nett. 

LEE, Sir SIDNEY. 

 Stratford-on-Avon. From the Earliest Times to the Death of Shakespeare. New revised edition, with additional Illustrations. Extra crown 8vo, 6s. Pocket Edition, 2s. Nett; leather, 3s. Nett; and in special yapp leather, full gilt, in box, 5s. Nett. 

LEFROY, W. CHAMBERS. 

 The Ruined Abbeys of Yorkshire. With many Illustrations by A. BRUNET-DEBAINES and H. TOUSSAINT. Cr. 8vo, Cloth, 2s. Nett; leather, 3s. & 5s. Nett. 

LEYLAND, JOHN. 

 The Peak of Derbyshire. With Map, Etchings, and other Illustrations by HERBERT RAILTON and ALFRED DAWSON. New Edition, Crown 8vo, Cloth, 2s. ; leather, 3s. ; velvet leather, in a box, 5s. Nett. 

LOFTIE, Rev. W. J. 

 The Inns of Court and Chancery. With many Illustrations, chiefly by HERBERT RAILTON. Crown 8vo, cloth, 2s. Nett; leather, 3s. Nett and 5s. Nett. 

 Westminster Abbey. With Seventy-four Illustrations, chiefly by HERBERT RAILTON. Crown 8vo, 2s. 6d. Extra crown 8vo, 6s. 

 Whitehall. With many Illustrations. Super-royal 8vo, sewed, 2s. 6d. Nett; half-linen, 3s. 6d. Nett. 

MACKENZIE, Rev. W. B. 

 Married Life and the Dwellings of the Righteous. 3s. 6d. 

MALLESON, Colonel G. B. , C. S. I. 

 The Indian Mutiny. With Copper Plates and other Illustrations. Crown 8vo, 5s. 

 The Refounding of the German Empire. With Portrait and Plans. Crown 8vo, 5s. 

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MINIATURE LIBRARY OF DEVOTION

 Little Volumes of Short Extracts from the Christian Fathers. With Decorative Title-page and Photogravure Frontispiece. 32mo, cloth extra, each 1s. Nett; leather, each 1s. 6d. Nett. Also Three Volumes in leather in case, 4s. 6d. Nett. White vellum with gilt edges, each volume in a box, 2s. Nett. 

1. Saint Augustine. 2. Jeremy Taylor. 3. Saint Chrysostom. 4. Bishop Andrewes. 5. John Keble. 6. Thomas à Kempis. 7. Canon Liddon. 8. Fénelon. 9. William Law. 

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MINIATURE PORTFOLIO MONOGRAPHS

 A New Edition in 16mo. Most of the Volumes have been carefully revised by the Authors. Each Volume profusely Illustrated. Cloth, 2s. Nett; leather, 3s. Nett; velvet leather, in box, 5s. Nett. 

 Peter Paul Rubens. By R. A. M. STEVENSON. 

 Japanese Wood Engravings. By Professor W. ANDERSON. 

 Josiah Wedgwood. By Sir A. H. CHURCH, F. R. S. , Professor of Chemistry, Royal Academy of Arts. New & Revised Edition. 

 D. G. Rossetti. By F. G. STEPHENS, One of the Seven Members of the Pre-Raphaelite Brotherhood. 

 The Early Work of Raphael. By JULIA CARTWRIGHT (Mrs. Ady). 

 Fair Women in Painting and Poetry. By WILLIAM SHARP (Fiona Macleod). 

 Antoine Watteau. By CLAUDE PHILLIPS, Keeper of the Wallace Collection. 

 Raphael in Rome. By JULIA CARTWRIGHT (Mrs. Ady). 

 The New Forest. By C. J. CORNISH, Author of "Life of the Zoo, " &c. 

 The Isle of Wight. By C. J. CORNISH. 

 Gainsborough. By Sir WALTER ARMSTRONG, Keeper of the National Gallery of Ireland. 

THE LIBRARY OF MISSIONS

Illustrated. Extra Crown, 5s. 8vo, nett. 

 A Church in the Wilds. The Remarkable Story of the Establishment of the South American Mission amongst the hitherto Savage and Intractable Natives of the Paraguayan Chaco. By W. BARBROOKE GRUBB. 

 Winning a Primitive People. Sixteen Years' Work among the Warlike Tribe of the Ngoni and the Senga and Tumbuka Peoples of Central Africa. By the Rev. DONALD FRASER. 

 * * * * *

MITFORD, MARY RUSSELL. 

 Country Stories. With 68 Illustrations by GEORGE MORROW. Crown 8vo, cloth, gilt top, 2s. Nett; also in leather, 3s. Nett; and leather yapp, full gilt in box, 5s. Nett. 

MOULE, Archdeacon A. E. 

 New China and Old. Notes on the Country and People made during a Residence of Thirty Years. With Thirty Illustrations. New Edition, Revised. Crown 8vo, 5s. 

MOULE, Right Rev. H. C. G. , D. D. (Bishop of Durham). 

 The Sacred Seasons. Readings for the Sundays and Holy Days of the Christian Year. Printed in red and black throughout, and illuminated with specially drawn initial letters and ornaments, and with 12 illuminated pages printed in three colours and gold after illuminations in manuscripts at the British Museum. Extra crown 8vo, 6s. Nett; also white cloth, in box, 7s. 6d. Nett; antique leather with clasps, 10s. 6d. Nett. 

 At the Holy Communion. Helps for Preparation and Reception. Cloth, 1s. ; leather, 2s. Nett; calf, 4s. 6d. 

 Christ's Witness to the Life to Come. Crown 8vo, 3s. 6d. 

 Grace and Godliness. Studies in the Epistle to the Ephesians. Crown 8vo, 2s. 6d. 

 In the House of the Pilgrimage. Hymns and Sacred Songs. 2s. 6d. 

 Imitations and Translations. Crown 8vo, 2s. 6d. Nett. 

 Jesus and the Resurrection. Expository Studies on St. John xx. And xxi. Third Edition, 2s. 6d. 

 Lord's Supper, The. By BISHOP RIDLEY. Edited with Notes and a Life by the BISHOP OF DURHAM. Crown 8vo, 5s. 

 Our Prayer Book. Short Chapters on the Book of Common Prayer. 16mo, 1s. ; leather, 2s. Nett. 

 Pledges of His Love, The. Thoughts on the Holy Communion. 16mo, 1s. ; leather, 2s. Nett. 

 Prayers for the Home. A Month's Cycle of Morning and Evening Family Worship, with some Occasional Prayers. Crown 8vo, 3s. 6d. 

 Prayers and Promises. Messages from the Holy Scriptures. 16mo, 1s. ; leather, 2s. Nett; calf, 4s. 6d. 

 The Secret of the Presence, and other Sermons. Crown 8vo, 3s. 6d. 

 Temptation and Escape. Short Chapters for Beginners in the Christian Life. 16mo, 1s. ; leather, 2s. Nett. 

 Thoughts on Christian Sanctity. 16mo, cloth, 1s. ; leather, 2s. Nett. 

 Thoughts on Secret Prayer. 16mo, cloth, 1s. ; leather, 2s. Nett; calf, 4s. 6d. 

 Thoughts on the Spiritual Life. 16mo, cloth, 1s. ; leather, 2s. Nett. 

 Thoughts on Union with Christ. 16mo, cloth, 1s. ; leather, 2s. Nett. 

MURRAY, A. S. , LL. D. 

 Greek Bronzes. With Four Copper Plates and many other Illustrations. Super-royal 8vo, sewed, 3s. 6d. Nett; cloth, 4s. 6d. Nett. 

 Greek Bronzes, by Dr. MURRAY, and Greek Terracotta Statuettes, by C. A. HUTTON. With Four Photogravures, Eight Coloured Plates, and Seventy-seven other Illustrations. In one Volume. Super-royal 8vo, cloth, 10s. 6d. Nett. 

NETTLESHIP, J. T. 

 Morland, George. With Six Copper Plates and Thirty other Illustrations. Super-royal 8vo, sewed, 5s. Nett; cloth, 6s. Nett. 

THE NEW ART LIBRARY

EDITED BY M. H. SPIELMANN, F. S. A. , & P. G. KONODY. 

 "The admirable New Art Library. . . . Thoroughly practical. "--_The Connoisseur. _

THE PRACTICE AND SCIENCE OF DRAWING. 

 By HAROLD SPEED, Associé de la Société Nationale des Beaux-Arts; Member of the Society of Portrait Painters; Professor of Drawing at the Goldsmiths' College, &c. With Ninety-six Illustrations and Diagrams. Square ex. Crn. 8vo, 6s. Nett. 

THE PRACTICE OF OIL PAINTING AND DRAWING. 

 By SOLOMON J. SOLOMON, R. A. With Eighty Illustrations. 6s. Nett. 

HUMAN ANATOMY FOR ART STUDENTS. 

 By Sir ALFRED DOWNING FRIPP, K. C. V. O. , Lecturer upon Anatomy at Guy's Hospital, London, and RALPH THOMPSON, Ch. M. , F. R. C. S. , with a chapter on Comparative Anatomy, and Drawings by HARRY DIXON. With One hundred and fifty-nine Photographs and Drawings. Square extra crown 8vo, 7s. 6d. Nett. 

MODELLING AND SCULPTURE. 

 By ALBERT TOFT, A. R. C. A. , M. S. B. S. With 119 Photographs and Drawings. Square extra crown 8vo, 6s. Nett. 

 * * * * *

PAGE, J. Ll. WARDEN. 

 Exmoor, An Exploration of. With Maps, Etchings, and other Illustrations. Cheap Edition, 3s. 6d. 

PENNELL, T. L. , M. D. , B. Sc. , F. R. C. S. 

 Among the Wild Tribes of the Afghan Frontier. A Record of Sixteen Years' Close Intercourse with the Natives of the Indian Marches. With an Introduction by Field-Marshal LORD ROBERTS, V. C. Extra Crown 8vo. With 26 Illustrations and a Map. 5s. Nett. Fourth and Cheaper Edition. 

 Things Seen in Northern India. With 50 Illustrations. 2s. , 3s. , 5s. Nett. 

PHILLIPS, CLAUDE. 

 The Earlier Work of Titian. With many Illustrations. Super-royal 8vo, sewed, 3s. 6d. Nett; cloth, 4s. 6d. Nett. 

 The Later Work of Titian. With many Illustrations. Super-royal 8vo, sewed, 3s. 6d. Nett; cloth, 4s. 6d. Nett. 

 Titian, a Study of his Life and Work. With Eight Copper Plates and many other Illustrations. Super-royal 8vo, 9s. Nett. 

 The Picture Gallery of Charles I. With many Illustrations. Super-royal 8vo, sewed, 3s. 6d. Nett; cloth, 4s. 6d. Nett. 

 Frederick Walker. Sup. -roy. 8vo, sewed, 2s. 6d. Nett; half-linen, 3s. 6d. Nett. 

 Antoine Watteau. Sup. -roy. 8vo, sewed, 2s. 6d. Nett; half-linen, 3s. 6d. Nett; also small 4to, cloth, 2s. Nett; and 3s. And 5s. Nett in leather. 

POLLARD, A. W. 

 Italian Book Illustrations. Super-royal 8vo, sewed, 2s. 6d. Nett; half-linen, 3s. 6d. Nett. 

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PORTFOLIO MONOGRAPHS ON ARTISTIC SUBJECTS

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 Many of the Volumes are issued in two forms and at various nett prices. Where two prices are given, the first is that of the paper cover edition; the second that of the cloth. When only one price is given, the Volume is bound in paper only. 

ANDERSON, Prof. W. 

 Japanese Wood Engravings. 2s. 6d. And 3s. 6d. 

ARMSTRONG, Sir WALTER. 

 The Art of Velazquez. 3s. 6d. 

 The Life of Velazquez. 3s. 6d. 

 The Peel Collection and the Dutch School of Painting. 5s. And 7s. 

 Thomas Gainsborough. Half-linen, 3s. 6d. 

 W. Q. Orchardson. 2s. 6d. And 3s. 6d. 

BEDFORD, W. K. R. 

 Malta. 2s. 6d. 

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