GREAT ASTRONOMERS

by SIR ROBERT S. BALL D. Sc. LL. D. F. R. S. 

Lowndean Professor of Astronomy and Geometry in theUniversity of Cambridge

Author of "In Starry Realms" "In the High Heavens" etc. 

[PLATE: GREENWICH OBSERVATORY. ]

PREFACE. 

It has been my object in these pages to present the life of eachastronomer in such detail as to enable the reader to realise insome degree the man's character and surroundings; and I haveendeavoured to indicate as clearly as circumstances would permitthe main features of the discoveries by which he has become known. 

There are many types of astronomers--from the stargazer who merelywatches the heavens, to the abstract mathematician who merelyworks at his desk; it has, consequently, been necessary in thecase of some lives to adopt a very different treatment from thatwhich seemed suitable for others. 

While the work was in progress, some of the sketches appeared in"Good Words. " The chapter on Brinkley has been chiefly derived froman article on the "History of Dunsink Observatory, " which waspublished on the occasion of the tercentenary celebration of theUniversity of Dublin in 1892, and the life of Sir William RowanHamilton is taken, with a few alterations and omissions, from anarticle contributed to the "Quarterly Review" on Graves' life ofthe great mathematician. The remaining chapters now appear forthe first time. For many of the facts contained in the sketch ofthe late Professor Adams, I am indebted to the obituary noticewritten by my friend Dr. J. W. L. Glaisher, for the Royal AstronomicalSociety; while with regard to the late Sir George Airy, I have asimilar acknowledgment to make to Professor H. H. Turner. To myfriend Dr. Arthur A. Rambaut I owe my hearty thanks for hiskindness in aiding me in the revision of the work. 

R. S. B. The Observatory, Cambridge. October, 1895

CONTENTS. 

INTRODUCTION. 

PTOLEMY. 

COPERNICUS. 

TYCHO BRAHE. 

GALILEO. 

KEPLER. 

ISAAC NEWTON. 

FLAMSTEED. 

HALLEY. 

BRADLEY. 

WILLIAM HERSCHEL. 

LAPLACE. 

BRINKLEY. 

JOHN HERSCHEL. 

THE EARL OF ROSSE. 

AIRY. 

HAMILTON. 

LE VERRIER. 

ADAMS. 

LIST OF ILLUSTRATIONS. 

THE OBSERVATORY, GREENWICH. 

PTOLEMY. 

PTOLEMY'S PLANETARY SCHEME. 

PTOLEMY'S THEORY OF THE MOVEMENT OF MARS. 

THORN, FROM AN OLD PRINT. 

COPERNICUS. 

FRAUENBURG, FROM AN OLD PRINT. 

EXPLANATION OF PLANETARY MOVEMENTS. 

TYCHO BRAHE. 

TYCHO'S CROSS STAFF. 

TYCHO'S "NEW STAR" SEXTANT OF 1572. 

TYCHO'S TRIGONIC SEXTANT. 

TYCHO'S ASTRONOMIC SEXTANT. 

TYCHO'S EQUATORIAL ARMILLARY. 

THE GREAT AUGSBURG QUADRANT. 

TYCHO'S "NEW SCHEME OF THE TERRESTRIAL SYSTEM, " 1577. 

URANIBORG AND ITS GROUNDS. 

GROUND-PLAN OF THE OBSERVATORY. 

THE OBSERVATORY OF URANIBORG, ISLAND OF HVEN. 

EFFIGY ON TYCHO'S TOMB AT PRAGUE. By Permission of Messrs. A. & C. Black. 

TYCHO'S MURAL QUADRANT, URANIBORG. 

GALILEO'S PENDULUM. 

GALILEO. 

THE VILLA ARCETRI. 

FACSIMILE SKETCH OF LUNAR SURFACE BY GALILEO. 

CREST OF GALILEO'S FAMILY. 

KEPLER'S SYSTEM OF REGULAR SOLIDS. 

KEPLER. 

SYMBOLICAL REPRESENTATION OF THE PLANETARY SYSTEM. 

THE COMMEMORATION OF THE RUDOLPHINE TABLES. 

WOOLSTHORPE MANOR. 

TRINITY COLLEGE, CAMBRIDGE. 

DIAGRAM OF A SUNBEAM. 

ISAAC NEWTON. 

SIR ISAAC NEWTON'S LITTLE REFLECTOR. 

SIR ISAAC NEWTON'S SUN-DIAL. 

SIR ISAAC NEWTON'S TELESCOPE. 

SIR ISAAC NEWTON'S ASTROLABE. 

SIR ISAAC NEWTON'S SUN-DIAL IN THE ROYAL SOCIETY. 

FLAMSTEED'S HOUSE. 

FLAMSTEED. 

HALLEY. 

GREENWICH OBSERVATORY IN HALLEY'S TIME. 

7, NEW KING STREET, BATH. From a Photograph by John Poole, Bath. 

WILLIAM HERSCHEL. 

CAROLINE HERSCHEL. 

STREET VIEW, HERSCHEL HOUSE, SLOUGH. From a Photograph by Hill & Saunders, Eton. 

GARDEN VIEW, HERSCHEL HOUSE, SLOUGH. From a Photograph by Hill & Saunders, Eton. 

OBSERVATORY, HERSCHEL HOUSE, SLOUGH. From a Photograph by Hill & Saunders, Eton. 

THE 40-FOOT TELESCOPE, HERSCHEL HOUSE, SLOUGH. From a Photograph by Hill & Saunders, Eton. 

LAPLACE. 

THE OBSERVATORY, DUNSINK. From a Photograph by W. Lawrence, Dublin. 

ASTRONOMETER MADE BY SIR JOHN HERSCHEL. 

SIR JOHN HERSCHEL. 

NEBULA IN SOUTHERN HEMISPHERE. 

THE CLUSTER IN THE CENTAUR. 

OBSERVATORY AT FELDHAUSEN. 

GRANITE COLUMN AT FELDHAUSEN. 

THE EARL OF ROSSE. 

BIRR CASTLE. From a Photograph by W. Lawrence, Dublin. 

THE MALL, PARSONSTOWN. From a Photograph by W. Lawrence, Dublin. 

LORD ROSSE'S TELESCOPE. From a Photograph by W. Lawrence, Dublin. 

ROMAN CATHOLIC CHURCH, PARSONSTOWN. From a Photograph by W. Lawrence, Dublin. 

AIRY. From a Photograph by E. P. Adams, Greenwich. 

HAMILTON. 

ADAMS. 

THE OBSERVATORY, CAMBRIDGE. 

INTRODUCTION. 

Of all the natural sciences there is not one which offers suchsublime objects to the attention of the inquirer as does the scienceof astronomy. From the earliest ages the study of the stars hasexercised the same fascination as it possesses at the present day. Among the most primitive peoples, the movements of the sun, the moon, and the stars commanded attention from their supposed influence onhuman affairs. 

The practical utilities of astronomy were also obvious in primevaltimes. Maxims of extreme antiquity show how the avocations of thehusbandman are to be guided by the movements of the heavenly bodies. The positions of the stars indicated the time to plough, and the timeto sow. To the mariner who was seeking a way across the tracklessocean, the heavenly bodies offered the only reliable marks by whichhis path could be guided. There was, accordingly, a stimulus bothfrom intellectual curiosity and from practical necessity to followthe movements of the stars. Thus began a search for the causes ofthe ever-varying phenomena which the heavens display. 

Many of the earliest discoveries are indeed prehistoric. The greatdiurnal movement of the heavens, and the annual revolution of thesun, seem to have been known in times far more ancient than those towhich any human monuments can be referred. The acuteness of theearly observers enabled them to single out the more important of thewanderers which we now call planets. They saw that the star-likeobjects, Jupiter, Saturn, and Mars, with the more conspicuous Venus, constituted a class of bodies wholly distinct from the fixed starsamong which their movements lay, and to which they bear such asuperficial resemblance. But the penetration of the earlyastronomers went even further, for they recognized that Mercury alsobelongs to the same group, though this particular object is seen sorarely. It would seem that eclipses and other phenomena wereobserved at Babylon from a very remote period, while the most ancientrecords of celestial observations that we possess are to be found inthe Chinese annals. 

The study of astronomy, in the sense in which we understand the word, may be said to have commenced under the reign of the Ptolemies atAlexandria. The most famous name in the science of this period isthat of Hipparchus who lived and worked at Rhodes about the year160BC. It was his splendid investigations that first wrought theobserved facts into a coherent branch of knowledge. He recognizedthe primary obligation which lies on the student of the heavens tocompile as complete an inventory as possible of the objects which arethere to be found. Hipparchus accordingly commenced by undertaking, on a small scale, a task exactly similar to that on which modernastronomers, with all available appliances of meridian circles, andphotographic telescopes, are constantly engaged at the present day. He compiled a catalogue of the principal fixed stars, which is ofspecial value to astronomers, as being the earliest work of its kindwhich has been handed down. He also studied the movements of the sunand the moon, and framed theories to account for the incessantchanges which he saw in progress. He found a much more difficultproblem in his attempt to interpret satisfactorily the complicatedmovements of the planets. With the view of constructing a theorywhich should give some coherent account of the subject, he made manyobservations of the places of these wandering stars. How great werethe advances which Hipparchus accomplished may be appreciated if wereflect that, as a preliminary task to his more purely astronomicallabours, he had to invent that branch of mathematical science bywhich alone the problems he proposed could be solved. It was forthis purpose that he devised the indispensable method of calculationwhich we now know so well as trigonometry. Without the aid renderedby this beautiful art it would have been impossible for any reallyimportant advance in astronomical calculation to have been effected. 

But the discovery which shows, beyond all others, that Hipparchuspossessed one of the master-minds of all time was the detection ofthat remarkable celestial movement known as the precession of theequinoxes. The inquiry which conducted to this discovery involved amost profound investigation, especially when it is remembered that inthe days of Hipparchus the means of observation of the heavenlybodies were only of the rudest description, and the availableobservations of earlier dates were extremely scanty. We can but lookwith astonishment on the genius of the man who, in spite of suchdifficulties, was able to detect such a phenomenon as the precession, and to exhibit its actual magnitude. I shall endeavour to explainthe nature of this singular celestial movement, for it may be said tooffer the first instance in the history of science in which we findthat combination of accurate observation with skilful interpretation, of which, in the subsequent development of astronomy, we have so manysplendid examples. 

The word equinox implies the condition that the night is equal to theday. To a resident on the equator the night is no doubt equal to theday at all times in the year, but to one who lives on any other partof the earth, in either hemisphere, the night and the day are notgenerally equal. There is, however, one occasion in spring, andanother in autumn, on which the day and the night are each twelvehours at all places on the earth. When the night and day are equalin spring, the point which the sun occupies on the heavens is termedthe vernal equinox. There is similarly another point in which thesun is situated at the time of the autumnal equinox. In anyinvestigation of the celestial movements the positions of these twoequinoxes on the heavens are of primary importance, and Hipparchus, with the instinct of genius, perceived their significance, andcommenced to study them. It will be understood that we can alwaysdefine the position of a point on the sky with reference to thesurrounding stars. No doubt we do not see the stars near the sunwhen the sun is shining, but they are there nevertheless. Theingenuity of Hipparchus enabled him to determine the positions ofeach of the two equinoxes relatively to the stars which lie in itsimmediate vicinity. After examination of the celestial places ofthese points at different periods, he was led to the conclusion thateach equinox was moving relatively to the stars, though that movementwas so slow that twenty five thousand years would necessarily elapsebefore a complete circuit of the heavens was accomplished. Hipparchustraced out this phenomenon, and established it on an impregnablebasis, so that all astronomers have ever since recognised theprecession of the equinoxes as one of the fundamental facts ofastronomy. Not until nearly two thousand years after Hipparchus hadmade this splendid discovery was the explanation of its cause givenby Newton. 

From the days of Hipparchus down to the present hour the science ofastronomy has steadily grown. One great observer after another hasappeared from time to time, to reveal some new phenomenon with regardto the celestial bodies or their movements, while from time to timeone commanding intellect after another has arisen to explain the trueimport of the facts of observations. The history of astronomy thusbecomes inseparable from the history of the great men to whoselabours its development is due. 

In the ensuing chapters we have endeavoured to sketch the lives andthe work of the great philosophers, by whose labours the science ofastronomy has been created. We shall commence with Ptolemy, who, after the foundations of the science had been laid by Hipparchus, gave to astronomy the form in which it was taught throughout theMiddle Ages. We shall next see the mighty revolution in ourconceptions of the universe which are associated with the name ofCopernicus. We then pass to those periods illumined by the genius ofGalileo and Newton, and afterwards we shall trace the careers ofother more recent discoverers, by whose industry and genius theboundaries of human knowledge have been so greatly extended. Ourhistory will be brought down late enough to include some of theillustrious astronomers who laboured in the generation which has justpassed away. 

PTOLEMY. 

[PLATE: PTOLEMY. ]

The career of the famous man whose name stands at the head of thischapter is one of the most remarkable in the history of humanlearning. There may have been other discoverers who have done morefor science than ever Ptolemy accomplished, but there never has beenany other discoverer whose authority on the subject of the movementsof the heavenly bodies has held sway over the minds of men for solong a period as the fourteen centuries during which his opinionsreigned supreme. The doctrines he laid down in his famous book, "TheAlmagest, " prevailed throughout those ages. No substantial additionwas made in all that time to the undoubted truths which this workcontained. No important correction was made of the serious errorswith which Ptolemy's theories were contaminated. The authority ofPtolemy as to all things in the heavens, and as to a good many thingson the earth (for the same illustrious man was also a diligentgeographer), was invariably final. 

Though every child may now know more of the actual truths of thecelestial motions than ever Ptolemy knew, yet the fact that his workexercised such an astonishing effect on the human intellect for somesixty generations, shows that it must have been an extraordinaryproduction. We must look into the career of this wonderful man todiscover wherein lay the secret of that marvellous success which madehim the unchallenged instructor of the human race for such aprotracted period. 

Unfortunately, we know very little as to the personal history ofPtolemy. He was a native of Egypt, and though it has been sometimesconjectured that he belonged to the royal families of the same name, yet there is nothing to support such a belief. The name, Ptolemy, appears to have been a common one in Egypt in those days. The timeat which he lived is fixed by the fact that his first recordedobservation was made in 127 AD, and his last in 151 AD. When we addthat he seems to have lived in or near Alexandria, or to use his ownwords, "on the parallel of Alexandria, " we have said everything thatcan be said so far as his individuality is concerned. 

Ptolemy is, without doubt, the greatest figure in ancient astronomy. He gathered up the wisdom of the philosophers who had preceded him. He incorporated this with the results of his own observations, andillumined it with his theories. His speculations, even when theywere, as we now know, quite erroneous, had such an astonishingverisimilitude to the actual facts of nature that they commandeduniversal assent. Even in these modern days we not unfrequently findlovers of paradox who maintain that Ptolemy's doctrines not only seemtrue, but actually are true. 

In the absence of any accurate knowledge of the science of mechanics, philosophers in early times were forced to fall back on certainprinciples of more or less validity, which they derived from theirimagination as to what the natural fitness of things ought to be. There was no geometrical figure so simple and so symmetrical as acircle, and as it was apparent that the heavenly bodies pursuedtracks which were not straight lines, the conclusion obviouslyfollowed that their movements ought to be circular. There was noargument in favour of this notion, other than the merely imaginaryreflection that circular movement, and circular movement alone, was"perfect, " whatever "perfect" may have meant. It was furtherbelieved to be impossible that the heavenly bodies could have anyother movements save those which were perfect. Assuming this, itfollowed, in Ptolemy's opinion, and in that of those who came afterhim for fourteen centuries, that all the tracks of the heavenlybodies were in some way or other to be reduced to circles. 

Ptolemy succeeded in devising a scheme by which the apparent changesthat take place in the heavens could, so far as he knew them, beexplained by certain combinations of circular movement. This seemedto reconcile so completely the scheme of things celestial with thegeometrical instincts which pointed to the circle as the type ofperfect movement, that we can hardly wonder Ptolemy's theory met withthe astonishing success that attended it. We shall, therefore, setforth with sufficient detail the various steps of this famousdoctrine. 

Ptolemy commences with laying down the undoubted truth that the shapeof the earth is globular. The proofs which he gives of thisfundamental fact are quite satisfactory; they are indeed the sameproofs as we give today. There is, first of all, the well-knowncircumstance of which our books on geography remind us, that when anobject is viewed at a distance across the sea, the lower part of theobject appears cut off by the interposing curved mass of water. 

The sagacity of Ptolemy enabled him to adduce another argument, which, though not quite so obvious as that just mentioned, demonstrates the curvature of the earth in a very impressive mannerto anyone who will take the trouble to understand it. Ptolemymentions that travellers who went to the south reported, that, asthey did so, the appearance of the heavens at night underwent agradual change. Stars that they were familiar with in the northernskies gradually sank lower in the heavens. The constellation of theGreat Bear, which in our skies never sets during its revolution roundthe pole, did set and rise when a sufficient southern latitude hadbeen attained. On the other hand, constellations new to theinhabitants of northern climes were seen to rise above the southernhorizon. These circumstances would be quite incompatible with thesupposition that the earth was a flat surface. Had this been so, alittle reflection will show that no such changes in the apparentmovements of the stars would be the consequence of a voyage to thesouth. Ptolemy set forth with much insight the significance of thisreasoning, and even now, with the resources of modern discoveries tohelp us, we can hardly improve upon his arguments. 

Ptolemy, like a true philosopher disclosing a new truth to the world, illustrated and enforced his subject by a variety of happydemonstrations. I must add one of them, not only on account of itsstriking nature, but also because it exemplifies Ptolemy'sacuteness. If the earth were flat, said this ingenious reasoner, sunset must necessarily take place at the same instant, no matter inwhat country the observer may happen to be placed. Ptolemy, however, proved that the time of sunset did vary greatly as the observer'slongitude was altered. To us, of course, this is quite obvious;everybody knows that the hour of sunset may have been reached inGreat Britain while it is still noon on the western coast ofAmerica. Ptolemy had, however, few of those sources of knowledgewhich are now accessible. How was he to show that the sun actuallydid set earlier at Alexandria than it would in a city which lay ahundred miles to the west? There was no telegraph wire by whichastronomers at the two Places could communicate. There was nochronometer or watch which could be transported from place to place;there was not any other reliable contrivance for the keeping oftime. Ptolemy's ingenuity, however, pointed out a thoroughlysatisfactory method by which the times of sunset at two places couldbe compared. He was acquainted with the fact, which must indeed havebeen known from the very earliest times, that the illumination of themoon is derived entirely from the sun. He knew that an eclipse ofthe moon was due to the interposition of the earth which cuts off thelight of the sun. It was, therefore, plain that an eclipse of themoon must be a phenomenon which would begin at the same instant fromwhatever part of the earth the moon could be seen at the time. Ptolemy, therefore, brought together from various quarters the localtimes at which different observers had recorded the beginning of alunar eclipse. He found that the observers to the west made the timeearlier and earlier the further away their stations were fromAlexandria. On the other hand, the eastern observers set down thehour as later than that at which the phenomenon appeared atAlexandria. As these observers all recorded something which indeedappeared to them simultaneously, the only interpretation was, thatthe more easterly a place the later its time. Suppose there were anumber of observers along a parallel of latitude, and each noted thehour of sunset to be six o'clock, then, since the eastern times areearlier than western times, 6 p. M. At one station A will correspondto 5 p. M. At a station B sufficiently to the west. If, therefore, it is sunset to the observer at A, the hour of sunset will not yet bereached for the observer at B. This proves conclusively that thetime of sunset is not the same all over the earth. We have, however, already seen that the apparent time of sunset would be the same fromall stations if the earth were flat. When Ptolemy, therefore, demonstrated that the time of sunset was not the same at variousplaces, he showed conclusively that the earth was not flat. 

As the same arguments applied to all parts of the earth where Ptolemyhad either been himself, or from which he could gain the necessaryinformation, it followed that the earth, instead of being the flatplain, girdled with an illimitable ocean, as was generally supposed, must be in reality globular. This led at once to a startlingconsequence. It was obvious that there could be no supports of anykind by which this globe was sustained; it therefore followed thatthe mighty object must be simply poised in space. This is indeed anastonishing doctrine to anyone who relies on what merely seems theevidence of the senses, without giving to that evidence its dueintellectual interpretation. According to our ordinary experience, the very idea of an object poised without support in space, appearspreposterous. Would it not fall? we are immediately asked. Yes, doubtless it could not remain poised in any way in which we try theexperiment. We must, however, observe that there are no such ideasas upwards or downwards in relation to open space. To say that abody falls downwards, merely means that it tries to fall as nearly aspossible towards the centre of the earth. There is no one directionalong which a body will tend to move in space, in preference to anyother. This may be illustrated by the fact that a stone let fall atNew Zealand will, in its approach towards the earth's centre, beactually moving upwards as far as any locality in our hemisphere isconcerned. Why, then, argued Ptolemy, may not the earth remainpoised in space, for as all directions are equally upward or equallydownward, there seems no reason why the earth should require anysupport? By this reasoning he arrives at the fundamental conclusionthat the earth is a globular body freely lying in space, andsurrounded above, below, and on all sides by the glittering stars ofheaven. 

The perception of this sublime truth marks a notable epoch in thehistory of the gradual development of the human intellect. No doubt, other philosophers, in groping after knowledge, may have set forthcertain assertions that are more or less equivalent to thisfundamental truth. It is to Ptolemy we must give credit, however, not only for announcing this doctrine, but for demonstrating it byclear and logical argument. We cannot easily project our minds backto the conception of an intellectual state in which this truth wasunfamiliar. It may, however, be well imagined that, to one whothought the earth was a flat plain of indefinite extent, it would benothing less than an intellectual convulsion for him to be forced tobelieve that he stood upon a spherical earth, forming merely aparticle relatively to the immense sphere of the heavens. 

What Ptolemy saw in the movements of the stars led him to theconclusion that they were bright points attached to the inside of atremendous globe. The movements of this globe which carried thestars were only compatible with the supposition that the earthoccupied its centre. The imperceptible effect produced by a changein the locality of the observer on the apparent brightness of thestars made it plain that the dimensions of the terrestrial globe mustbe quite insignificant in comparison with those of the celestialsphere. The earth might, in fact, be regarded as a grain of sandwhile the stars lay upon a globe many yards in diameter. 

So tremendous was the revolution in human knowledge implied by thisdiscovery, that we can well imagine how Ptolemy, dazzled as it wereby the fame which had so justly accrued to him, failed to make onefurther step. Had he made that step, it would have emancipated thehuman intellect from the bondage of fourteen centuries of servitudeto a wholly monstrous notion of this earth's importance in the schemeof the heavens. The obvious fact that the sun, the moon, and thestars rose day by day, moved across the sky in a gloriousnever-ending procession, and duly set when their appointed courseshad been run, demanded some explanation. The circumstance that thefixed stars preserved their mutual distances from year to year, andfrom age to age, appeared to Ptolemy to prove that the sphere whichcontained those stars, and on whose surface they were believed by himto be fixed, revolved completely around the earth once every day. Hewould thus account for all the phenomena of rising and settingconsistently with the supposition that our globe was stationary. Probably this supposition must have appeared monstrous, even toPtolemy. He knew that the earth was a gigantic object, but, large asit may have been, he knew that it was only a particle in comparisonwith the celestial sphere, yet he apparently believed, and certainlysucceeded in persuading other men to believe, that the celestialsphere did actually perform these movements. 

Ptolemy was an excellent geometer. He knew that the rising and thesetting of the sun, the moon, and the myriad stars, could have beenaccounted for in a different way. If the earth turned rounduniformly once a day while poised at the centre of the sphere of theheavens, all the phenomena of rising and setting could be completelyexplained. This is, indeed, obvious after a moment's reflection. Consider yourself to be standing on the earth at the centre of theheavens. There are stars over your head, and half the contents ofthe heavens are visible, while the other half are below yourhorizon. As the earth turns round, the stars over your head willchange, and unless it should happen that you have taken up yourposition at either of the poles, new stars will pass into your view, and others will disappear, for at no time can you have more than halfof the whole sphere visible. The observer on the earth would, therefore, say that some stars were rising, and that some stars weresetting. We have, therefore, two totally distinct methods, each ofwhich would completely explain all the observed facts of the diurnalmovement. One of these suppositions requires that the celestialsphere, bearing with it the stars and other celestial bodies, turnsuniformly around an invisible axis, while the earth remainsstationary at the centre. The other supposition would be, that it isthe stupendous celestial sphere which remains stationary, while theearth at the centre rotates about the same axis as the celestialsphere did before, but in an opposite direction, and with a uniformvelocity which would enable it to complete one turn in twenty-fourhours. Ptolemy was mathematician enough to know that either of thesesuppositions would suffice for the explanation of the observedfacts. Indeed, the phenomena of the movements of the stars, so faras he could observe them, could not be called upon to pronounce whichof these views was true, and which was false. 

Ptolemy had, therefore, to resort for guidance to indirect lines ofreasoning. One of these suppositions must be true, and yet itappeared that the adoption of either was accompanied by a greatdifficulty. It is one of his chief merits to have demonstrated thatthe celestial sphere was so stupendous that the earth itself wasabsolutely insignificant in comparison therewith. If, then, thisstupendous sphere rotated once in twenty-four hours, the speed withwhich the movement of some of the stars must be executed would be soportentous as to seem well-nigh impossible. It would, therefore, seem much simpler on this ground to adopt the other alternative, andto suppose the diurnal movements were due to the rotation of theearth. Here Ptolemy saw, or at all events fancied he saw, objectionsof the weightiest description. The evidence of the senses appeareddirectly to controvert the supposition that this earth is anythingbut stationary. Ptolemy might, perhaps, have dismissed thisobjection on the ground that the testimony of the senses on such amatter should be entirely subordinated to the interpretation whichour intelligence would place upon the facts to which the sensesdeposed. Another objection, however, appeared to him to possess thegravest moment. It was argued that if the earth were rotating, thereis nothing to make the air participate in this motion, mankind wouldtherefore be swept from the earth by the furious blasts which wouldarise from the movement of the earth through an atmosphere at rest. Even if we could imagine that the air were carried round with theearth, the same would not apply, so thought Ptolemy, to any objectsuspended in the air. So long as a bird was perched on a tree, hemight very well be carried onward by the moving earth, but the momenthe took wing, the ground would slip from under him at a frightfulpace, so that when he dropped down again he would find himself at adistance perhaps ten times as great as that which a carrier-pigeon ora swallow could have traversed in the same time. Some vague delusionof this description seems even still to crop up occasionally. Iremember hearing of a proposition for balloon travelling of a veryremarkable kind. The voyager who wanted to reach any other place inthe same latitude was simply to ascend in a balloon, and wait theretill the rotation of the earth conveyed the locality which happenedto be his destination directly beneath him, whereupon he was to letout the gas and drop down! Ptolemy knew quite enough naturalphilosophy to be aware that such a proposal for locomotion would bean utter absurdity; he knew that there was no such relative shiftbetween the air and the earth as this motion would imply. Itappeared to him to be necessary that the air should lag behind, ifthe earth had been animated by a movement of rotation. In this hewas, as we know, entirely wrong. There were, however, in his days noaccurate notions on the subject of the laws of motion. 

Assiduous as Ptolemy may have been in the study of the heavenlybodies, it seems evident that he cannot have devoted much thought tothe phenomena of motion of terrestrial objects. Simple, indeed, arethe experiments which might have convinced a philosopher much lessacute than Ptolemy, that, if the earth did revolve, the air mustnecessarily accompany it. If a rider galloping on horseback tosses aball into the air, it drops again into his hand, just as it wouldhave done had he been remaining at rest during the ball's flight; theball in fact participates in the horizontal motion, so that though itreally describes a curve as any passer-by would observe, yet itappears to the rider himself merely to move up and down in a straightline. This fact, and many others similar to it, demonstrate clearlythat if the earth were endowed with a movement of rotation, theatmosphere surrounding it must participate in that movement. Ptolemydid not know this, and consequently he came to the conclusion thatthe earth did not rotate, and that, therefore, notwithstanding thetremendous improbability of so mighty an object as the celestialsphere spinning round once in every twenty-four hours, there was nocourse open except to believe that this very improbable thing didreally happen. Thus it came to pass that Ptolemy adopted as thecardinal doctrine of his system a stationary earth poised at thecentre of the celestial sphere, which stretched around on all sidesat a distance so vast that the diameter of the earth was aninappreciable point in comparison therewith. 

Ptolemy having thus deliberately rejected the doctrine of the earth'srotation, had to make certain other entirely erroneous suppositions. It was easily seen that each star required exactly the same periodfor the performance of a complete revolution of the heavens. Ptolemyknew that the stars were at enormous distances from the earth, thoughno doubt his notions on this point came very far short of what weknow to be the reality. If the stars had been at very varieddistances, then it would be so wildly improbable that they should allaccomplish their revolutions in the same time, that Ptolemy came tothe conclusion that they must be all at the same distance, that is, that they must be all on the surface of a sphere. This view, howevererroneous, was corroborated by the obvious fact that the stars in theconstellations preserved their relative places unaltered forcenturies. Thus it was that Ptolemy came to the conclusion that theywere all fixed on one spherical surface, though we are not informedas to the material of this marvellous setting which sustained thestars like jewels. 

Nor should we hastily pronounce this doctrine to be absurd. Thestars do appear to lie on the surface of a sphere, of which theobserver is at the centre; not only is this the aspect which theskies present to the untechnical observer, but it is the aspect inwhich the skies are presented to the most experienced astronomer ofmodern days. No doubt he knows well that the stars are at the mostvaried distances from him; he knows that certain stars are ten times, or a hundred times, or a thousand times, as far as other stars. Nevertheless, to his eye the stars appear on the surface of thesphere, it is on that surface that his measurements of the relativeplaces of the stars are made; indeed, it may be said that almost allthe accurate observations in the observatory relate to the places ofthe stars, not as they really are, but as they appear to be projectedon that celestial sphere whose conception we owe to the genius ofPtolemy. 

This great philosopher shows very ingeniously that the earth must beat the centre of the sphere. He proves that, unless this were thecase, each star would not appear to move with the absolute uniformitywhich does, as a matter of fact, characterise it. In all thesereasonings we cannot but have the most profound admiration for thegenius of Ptolemy, even though he had made an error so enormous inthe fundamental point of the stability of the earth. Another errorof a somewhat similar kind seemed to Ptolemy to be demonstrated. Hehad shown that the earth was an isolated object in space, and beingsuch was, of course, capable of movement. It could either be turnedround, or it could be moved from one place to another. We know thatPtolemy deliberately adopted the view that the earth did not turnround; he had then to investigate the other question, as to whetherthe earth was animated by any movement of translation. He came tothe conclusion that to attribute any motion to the earth would beincompatible with the truths at which he had already arrived. Theearth, argued Ptolemy, lies at the centre of the celestial sphere. If the earth were to be endowed with movement, it would not liealways at this point, it must, therefore, shift to some other part ofthe sphere. The movements of the stars, however, preclude thepossibility of this; and, therefore, the earth must be as devoid ofany movement of translation as it is devoid of rotation. Thus it wasthat Ptolemy convinced himself that the stability of the earth, as itappeared to the ordinary senses, had a rational philosophicalfoundation. 

Not unfrequently it is the lot of the philosophers to contend againstthe doctrines of the vulgar, but when it happens, as in the case ofPtolemy's researches, that the doctrines of the vulgar arecorroborated by philosophical investigation which bear the stamp ofthe highest authority, it is not to be wondered at that suchdoctrines should be deemed well-nigh impregnable. In this way wemay, perhaps, account for the remarkable fact that the theories ofPtolemy held unchallenged sway over the human intellect for the vastperiod already mentioned. 

Up to the present we have been speaking only of those primary motionsof the heavens, by which the whole sphere appeared to revolve onceevery twenty-four hours. We have now to discuss the remarkabletheories by which Ptolemy endeavoured to account for the monthlymovement of the moon, for the annual movement of the sun, and for theperiodic movements of the planets which had gained for them thetitles of the wandering stars. 

Possessed with the idea that these movements must be circular, ormust be capable, directly or indirectly, of being explained bycircular movements, it seemed obvious to Ptolemy, as indeed it haddone to previous astronomers, that the track of the moon through thestars was a circle of which the earth is the centre. A similarmovement with a yearly period must also be attributed to the sun, forthe changes in the positions of the constellations in accordance withthe progress of the seasons, placed it beyond doubt that the sun madea circuit of the celestial sphere, even though the bright light ofthe sun prevented the stars in its vicinity, from being seen indaylight. Thus the movements both of the sun and the moon, as wellas the diurnal rotation of the celestial sphere, seemed to justifythe notion that all celestial movements must be "perfect, " that is tosay, described uniformly in those circles which were the only perfectcurves. 

The simplest observations, however, show that the movements of theplanets cannot be explained in this simple fashion. Here thegeometrical genius of Ptolemy shone forth, and he devised a scheme bywhich the apparent wanderings of the planets could be accounted forwithout the introduction of aught save "perfect" movements. 

To understand his reasoning, let us first set forth clearly thosefacts of observation which require to be explained. I shall take, inparticular, two planets, Venus and Mars, as these illustrate, in themost striking manner, the peculiarities of the inner and the outerplanets respectively. The simplest observations would show thatVenus did not move round the heavens in the same fashion as the sunor the moon. Look at the evening star when brightest, as it appearsin the west after sunset. Instead of moving towards the east amongthe stars, like the sun or the moon, we find, week after week, thatVenus is drawing in towards the sun, until it is lost in thesunbeams. Then the planet emerges on the other side, not to be seenas an evening star, but as a morning star. In fact, it was plainthat in some ways Venus accompanied the sun in its annual movement. Now it is found advancing in front of the sun to a certain limiteddistance, and now it is lagging to an equal extent behind the sun. 

[FIG. 1. PTOLEMY'S PLANETARY SCHEME. ]

These movements were wholly incompatible with the supposition thatthe journeys of Venus were described by a single motion of the kindregarded as perfect. It was obvious that the movement was connectedin some strange manner with the revolution of the sun, and here wasthe ingenious method by which Ptolemy sought to render account ofit. Imagine a fixed arm to extend from the earth to the sun, asshown in the accompanying figure (Fig. 1), then this arm will moveround uniformly, in consequence of the sun's movement. At a point Pon this arm let a small circle be described. Venus is supposed torevolve uniformly in this small circle, while the circle itself iscarried round continuously by the movement of the sun. In this wayit was possible to account for the chief peculiarities in themovement of Venus. It will be seen that, in consequence of therevolution around P, the spectator on the earth will sometimes seeVenus on one side of the sun, and sometimes on the other side, sothat the planet always remains in the sun's vicinity. By properlyproportioning the movements, this little contrivance simulated thetransitions from the morning star to the evening star. Thus thechanges of Venus could be accounted for by a Combination of the"perfect" movement of P in the circle which it described uniformlyround the earth, combined with the "perfect" motion of Venus in thecircle which it described uniformly around the moving centre. 

In a precisely similar manner Ptolemy rendered an explanation of thefitful apparitions of Mercury. Now just on one side of the sun, andnow just on the other, this rarely-seen planet moved like Venus on acircle whereof the centre was also carried by the line joining thesun and the earth. The circle, however, in which Mercury actuallyrevolved had to be smaller than that of Venus, in order to accountfor the fact that Mercury lies always much closer to the sun than thebetter-known planet. 

[FIG. 2. PTOLEMY'S THEORY OF THE MOVEMENT OF MARS. ]

The explanation of the movement of an outer planet like Mars couldalso be deduced from the joint effect of two perfect motions. Thechanges through which Mars goes are, however, so different from themovements of Venus that quite a different disposition of the circlesis necessary. For consider the facts which characterise themovements of an outer planet such as Mars. In the first place, Marsaccomplishes an entire circuit of the heaven. In this respect, nodoubt, it may be said to resemble the sun or the moon. A littleattention will, however, show that there are extraordinaryirregularities in the movement of the planet. Generally speaking, itspeeds its way from west to east among the stars, but sometimes theattentive observer will note that the speed with which the planetadvances is slackening, and then it will seem to become stationary. Some days later the direction of the planet's movement will bereversed, and it will be found moving from the east towards thewest. At first it proceeds slowly and then quickens its pace, untila certain speed is attained, which afterwards declines until a secondstationary position is reached. After a due pause the originalmotion from west to east is resumed, and is continued until a similarcycle of changes again commences. Such movements as these wereobviously quite at variance with any perfect movement in a singlecircle round the earth. Here, again, the geometrical sagacity ofPtolemy provided him with the means of representing the apparentmovements of Mars, and, at the same time, restricting the explanationto those perfect movements which he deemed so essential. In Fig. 2we exhibit Ptolemy's theory as to the movement of Mars. We have, asbefore, the earth at the centre, and the sun describing its circularorbit around that centre. The path of Mars is to be taken asexterior to that of the sun. We are to suppose that at a pointmarked M there is a fictitious planet, which revolves around theearth uniformly, in a circle called the DEFERENT. This point M, which is thus animated by a perfect movement, is the centre of acircle which is carried onwards with M, and around the circumferenceof which Mars revolves uniformly. It is easy to show that thecombined effect of these two perfect movements is to produce exactlythat displacement of Mars in the heavens which observationdiscloses. In the position represented in the figure, Mars isobviously pursuing a course which will appear to the observer as amovement from west to east. When, however, the planet gets round tosuch a position as R, it is then moving from east to west inconsequence of its revolution in the moving circle, as indicated bythe arrowhead. On the other hand, the whole circle is carriedforward in the opposite direction. If the latter movement be lessrapid than the former, then we shall have the backward movement ofMars on the heavens which it was desired to explain. By a properadjustment of the relative lengths of these arms the movements of theplanet as actually observed could be completely accounted for. 

The other outer planets with which Ptolemy was acquainted, namely, Jupiter and Saturn, had movements of the same general character asthose of Mars. Ptolemy was equally successful in explaining themovements they performed by the supposition that each planet hadperfect rotation in a circle of its own, which circle itself hadperfect movement around the earth in the centre. 

It is somewhat strange that Ptolemy did not advance one step further, as by so doing he would have given great simplicity to his system. Hemight, for instance, have represented the movements of Venus equallywell by putting the centre of the moving circle at the sun itself, and correspondingly enlarging the circle in which Venus revolved. Hemight, too, have arranged that the several circles which the outerplanets traversed should also have had their centres at the sun. Theplanetary system would then have consisted of an earth fixed at thecentre, of a sun revolving uniformly around it, and of a system ofplanets each describing its own circle around a moving centre placedin the sun. Perhaps Ptolemy had not thought of this, or perhaps hemay have seen arguments against it. This important step was, however, taken by Tycho. He considered that all the planets revolvedaround the sun in circles, and that the sun itself, bearing all theseorbits, described a mighty circle around the earth. This pointhaving been reached, only one more step would have been necessary toreach the glorious truths that revealed the structure of the solarsystem. That last step was taken by Copernicus. 

COPERNICUS

[PLATE: THORN, FROM AN OLD PRINT. ]

The quaint town of Thorn, on the Vistula, was more than two centuriesold when Copernicus was born there on the 19th of February, 1473. Thesituation of this town on the frontier between Prussia and Poland, with the commodious waterway offered by the river, made it a place ofconsiderable trade. A view of the town, as it was at the time of thebirth of Copernicus, is here given. The walls, with theirwatch-towers, will be noted, and the strategic importance which thesituation of Thorn gave to it in the fifteenth century still belongsthereto, so much so that the German Government recently constitutedthe town a fortress of the first class. 

Copernicus, the astronomer, whose discoveries make him the greatpredecessor of Kepler and Newton, did not come from a noble family, as certain other early astronomers have done, for his father was atradesman. Chroniclers are, however, careful to tell us that one ofhis uncles was a bishop. We are not acquainted with any of thosedetails of his childhood or youth which are often of such interest inother cases where men have risen to exalted fame. It would appearthat the young Nicolaus, for such was his Christian name, receivedhis education at home until such time as he was deemed sufficientlyadvanced to be sent to the University at Cracow. The education thathe there obtained must have been in those days of a very primitivedescription, but Copernicus seems to have availed himself of it tothe utmost. He devoted himself more particularly to the study ofmedicine, with the view of adopting its practice as the profession ofhis life. The tendencies of the future astronomer were, however, revealed in the fact that he worked hard at mathematics, and, likeone of his illustrious successors, Galileo, the practice of the artof painting had for him a very great interest, and in it he obtainedsome measure of success. 

By the time he was twenty-seven years old, it would seem thatCopernicus had given up the notion of becoming a medicalpractitioner, and had resolved to devote himself to science. He wasengaged in teaching mathematics, and appears to have acquired somereputation. His growing fame attracted the notice of his uncle thebishop, at whose suggestion Copernicus took holy orders, and he waspresently appointed to a canonry in the cathedral of Frauenburg, nearthe mouth of the Vistula. 

To Frauenburg, accordingly, this man of varied gifts retired. Possessing somewhat of the ascetic spirit, he resolved to devote hislife to work of the most serious description. He eschewed allordinary society, restricting his intimacies to very grave andlearned companions, and refusing to engage in conversation of anyuseless kind. It would seem as if his gifts for painting werecondemned as frivolous; at all events, we do not learn that hecontinued to practise them. In addition to the discharge of histheological duties, his life was occupied partly in ministeringmedically to the wants of the poor, and partly with his researches inastronomy and mathematics. His equipment in the matter ofinstruments for the study of the heavens seems to have been of a verymeagre description. He arranged apertures in the walls of his houseat Allenstein, so that he could observe in some fashion the passageof the stars across the meridian. That he possessed some talent forpractical mechanics is proved by his construction of a contrivancefor raising water from a stream, for the use of the inhabitants ofFrauenburg. Relics of this machine are still to be seen. 

[PLATE: COPERNICUS. ]

The intellectual slumber of the Middle Ages was destined to beawakened by the revolutionary doctrines of Copernicus. It may benoted, as an interesting circumstance, that the time at which hediscovered the scheme of the solar system has coincided with aremarkable epoch in the world's history. The great astronomer hadjust reached manhood at the time when Columbus discovered the newworld. 

Before the publication of the researches of Copernicus, the orthodoxscientific creed averred that the earth was stationary, and that theapparent movements of the heavenly bodies were indeed realmovements. Ptolemy had laid down this doctrine 1, 400 years before. In his theory this huge error was associated with so much importanttruth, and the whole presented such a coherent scheme for theexplanation of the heavenly movements, that the Ptolemaic theory wasnot seriously questioned until the great work of Copernicusappeared. No doubt others, before Copernicus, had from time to timein some vague fashion surmised, with more or less plausibility, thatthe sun, and not the earth, was the centre about which the systemreally revolved. It is, however, one thing to state a scientificfact; it is quite another thing to be in possession of the train ofreasoning, founded on observation or experiment, by which that factmay be established. Pythagoras, it appears, had indeed told hisdisciples that it was the sun, and not the earth, which was thecentre of movement, but it does not seem at all certain thatPythagoras had any grounds which science could recognise for thebelief which is attributed to him. So far as information isavailable to us, it would seem that Pythagoras associated his schemeof things celestial with a number of preposterous notions in naturalphilosophy. He may certainly have made a correct statement as towhich was the most important body in the solar system, but hecertainly did not provide any rational demonstration of the fact. Copernicus, by a strict train of reasoning, convinced those who wouldlisten to him that the sun was the centre of the system. It isuseful for us to consider the arguments which he urged, and by whichhe effected that intellectual revolution which is always connectedwith his name. 

The first of the great discoveries which Copernicus made relates tothe rotation of the earth on its axis. That general diurnalmovement, by which the stars and all other celestial bodies appear tobe carried completely round the heavens once every twenty-four hours, had been accounted for by Ptolemy on the supposition that theapparent movements were the real movements. As we have already seen, Ptolemy himself felt the extraordinary difficulty involved in thesupposition that so stupendous a fabric as the celestial sphereshould spin in the way supposed. Such movements required that manyof the stars should travel with almost inconceivable velocity. Copernicus also saw that the daily rising and setting of the heavenlybodies could be accounted for either by the supposition that thecelestial sphere moved round and that the earth remained at rest, orby the supposition that the celestial sphere was at rest while theearth turned round in the opposite direction. He weighed thearguments on both sides as Ptolemy had done, and, as the result ofhis deliberations, Copernicus came to an opposite conclusion fromPtolemy. To Copernicus it appeared that the difficulties attendingthe supposition that the celestial sphere revolved, were vastlygreater than those which appeared so weighty to Ptolemy as to forcehim to deny the earth's rotation. 

Copernicus shows clearly how the observed phenomena could beaccounted for just as completely by a rotation of the earth as by arotation of the heavens. He alludes to the fact that, to those onboard a vessel which is moving through smooth water, the vesselitself appears to be at rest, while the objects on shore seem to bemoving past. If, therefore, the earth were rotating uniformly, wedwellers upon the earth, oblivious of our own movement, would wronglyattribute to the stars the displacement which was actually theconsequence of our own motion. 

Copernicus saw the futility of the arguments by which Ptolemy hadendeavoured to demonstrate that a revolution of the earth wasimpossible. It was plain to him that there was nothing whatever towarrant refusal to believe in the rotation of the earth. In hisclear-sightedness on this matter we have specially to admire thesagacity of Copernicus as a natural philosopher. It had been urgedthat, if the earth moved round, its motion would not be imparted tothe air, and that therefore the earth would be uninhabitable by theterrific winds which would be the result of our being carried throughthe air. Copernicus convinced himself that this deduction waspreposterous. He proved that the air must accompany the earth, justas his coat remains round him, notwithstanding the fact that he iswalking down the street. In this way he was able to show that all apriori objections to the earth's movements were absurd, and thereforehe was able to compare together the plausibilities of the two rivalschemes for explaining the diurnal movement. 

[PLATE: FRAUENBURG, FROM AN OLD PRINT. ]

Once the issue had been placed in this form, the result could not belong in doubt. Here is the question: Which is it more likely--thatthe earth, like a grain of sand at the centre of a mighty globe, should turn round once in twenty-four hours, or that the whole ofthat vast globe should complete a rotation in the opposite directionin the same time? Obviously, the former is far the more simplesupposition. But the case is really much stronger than this. Ptolemyhad supposed that all the stars were attached to the surface of asphere. He had no ground whatever for this supposition, except thatotherwise it would have been well-nigh impossible to have devised ascheme by which the rotation of the heavens around a fixed earthcould have been arranged. Copernicus, however, with the justinstinct of a philosopher, considered that the celestial sphere, however convenient from a geometrical point of view, as a means ofrepresenting apparent phenomena, could not actually have a materialexistence. In the first place, the existence of a material celestialsphere would require that all the myriad stars should be at exactlythe same distances from the earth. Of course, no one will say thatthis or any other arbitrary disposition of the stars is actuallyimpossible, but as there was no conceivable physical reason why thedistances of all the stars from the earth should be identical, itseemed in the very highest degree improbable that the stars should beso placed. 

Doubtless, also, Copernicus felt a considerable difficulty as to thenature of the materials from which Ptolemy's wonderful sphere was tobe constructed. Nor could a philosopher of his penetration havefailed to observe that, unless that sphere were infinitely large, there must have been space outside it, a consideration which wouldopen up other difficult questions. Whether infinite or not, it wasobvious that the celestial sphere must have a diameter at least manythousands of times as great as that of the earth. From theseconsiderations Copernicus deduced the important fact that the starsand the other celestial bodies must all be vast objects. He was thusenabled to put the question in such a form that it could hardlyreceive any answer but the correct one. Which is it more rational tosuppose, that the earth should turn round on its axis once intwenty-four hours, or that thousands of mighty stars should circleround the earth in the same time, many of them having to describecircles many thousands of times greater in circumference than thecircuit of the earth at the equator? The obvious answer pressed uponCopernicus with so much force that he was compelled to rejectPtolemy's theory of the stationary earth, and to attribute thediurnal rotation of the heavens to the revolution of the earth on itsaxis. 

Once this tremendous step had been taken, the great difficultieswhich beset the monstrous conception of the celestial spherevanished, for the stars need no longer be regarded as situated atequal distances from the earth. Copernicus saw that they might lieat the most varied degrees of remoteness, some being hundreds orthousands of times farther away than others. The complicatedstructure of the celestial sphere as a material object disappearedaltogether; it remained only as a geometrical conception, whereon wefind it convenient to indicate the places of the stars. Once theCopernican doctrine had been fully set forth, it was impossible foranyone, who had both the inclination and the capacity to understandit, to withhold acceptance of its truth. The doctrine of astationary earth had gone for ever. 

Copernicus having established a theory of the celestial movementswhich deliberately set aside the stability of the earth, it seemednatural that he should inquire whether the doctrine of a moving earthmight not remove the difficulties presented in other celestialphenomena. It had been universally admitted that the earth layunsupported in space. Copernicus had further shown that it possesseda movement of rotation. Its want of stability being thus recognised, it seemed reasonable to suppose that the earth might also have someother kinds of movements as well. In this, Copernicus essayed tosolve a problem far more difficult than that which had hithertooccupied his attention. It was a comparatively easy task to show howthe diurnal rising and setting could be accounted for by the rotationof the earth. It was a much more difficult undertaking todemonstrate that the planetary movements, which Ptolemy hadrepresented with so much success, could be completely explained bythe supposition that each of those planets revolved uniformly roundthe sun, and that the earth was also a planet, accomplishing acomplete circuit of the sun once in the course of a year. 

[PLATE: EXPLANATION OF PLANETARY MOVEMENTS. ]

It would be impossible in a sketch like the present to enter into anydetail as to the geometrical propositions on which this beautifulinvestigation of Copernicus depended. We can only mention a few ofthe leading principles. It may be laid down in general that, if anobserver is in movement, he will, if unconscious of the fact, attribute to the fixed objects around him a movement equal andopposite to that which he actually possesses. A passenger on acanal-boat sees the objects on the banks apparently moving backwardwith a speed equal to that by which he is himself advancingforwards. By an application of this principle, we can account forall the phenomena of the movements of the planets, which Ptolemy hadso ingeniously represented by his circles. Let us take, forinstance, the most characteristic feature in the irregularities ofthe outer planets. We have already remarked that Mars, thoughgenerally advancing from west to east among the stars, occasionallypauses, retraces his steps for awhile, again pauses, and then resumeshis ordinary onward progress. Copernicus showed clearly how thiseffect was produced by the real motion of the earth, combined withthe real motion of Mars. In the adjoining figure we represent aportion of the circular tracks in which the earth and Mars move inaccordance with the Copernican doctrine. I show particularly thecase where the earth comes directly between the planet and the sun, because it is on such occasions that the retrograde movement (for sothis backward movement of Mars is termed) is at its highest. Mars isthen advancing in the direction shown by the arrow-head, and theearth is also advancing in the same direction. We, on the earth, however, being unconscious of our own motion, attribute, by theprinciple I have already explained, an equal and opposite motion toMars. The visible effect upon the planet is, that Mars has twomovements, a real onward movement in one direction, and an apparentmovement in the opposite direction. If it so happened that the earthwas moving with the same speed as Mars, then the apparent movementwould exactly neutralise the real movement, and Mars would seem to beat rest relatively to the surrounding stars. Under the actualcircumstances represented, however, the earth is moving faster thanMars, and the consequence is, that the apparent movement of theplanet backwards exceeds the real movement forwards, the net resultbeing an apparent retrograde movement. 

With consummate skill, Copernicus showed how the applications of thesame principles could account for the characteristic movements of theplanets. His reasoning in due time bore down all opposition. Thesupreme importance of the earth in the system vanished. It had nowmerely to take rank as one of the planets. 

The same great astronomer now, for the first time, rendered somethinglike a rational account of the changes of the seasons. Nor didcertain of the more obscure astronomical phenomena escape hisattention. 

He delayed publishing his wonderful discoveries to the world until hewas quite an old man. He had a well-founded apprehension of thestorm of opposition which they would arouse. However, he yielded atlast to the entreaties of his friends, and his book was sent to thepress. But ere it made its appearance to the world, Copernicus wasseized by mortal illness. A copy of the book was brought to him onMay 23, 1543. We are told that he was able to see it and to touchit, but no more, and he died a few hours afterwards. He was buriedin that Cathedral of Frauenburg, with which his life had been soclosely associated. 

TYCHO BRAHE. 

The most picturesque figure in the history of astronomy isundoubtedly that of the famous old Danish astronomer whose namestands at the head of this chapter. Tycho Brahe was alike notablefor his astronomical genius and for the extraordinary vehemence of acharacter which was by no means perfect. His romantic career as aphilosopher, and his taste for splendour as a Danish noble, hisardent friendships and his furious quarrels, make him an idealsubject for a biographer, while the magnificent astronomical workwhich he accomplished, has given him imperishable fame. 

The history of Tycho Brahe has been admirably told by Dr. Dreyer, theaccomplished astronomer who now directs the observatory at Armagh, though himself a countryman of Tycho. Every student of the career ofthe great Dane must necessarily look on Dr. Dreyer's work as thechief authority on the subject. Tycho sprang from an illustriousstock. His family had flourished for centuries, both in Sweden andin Denmark, where his descendants are to be met with at the presentday. The astronomer's father was a privy councillor, and havingfilled important positions in the Danish government, he wasultimately promoted to be governor of Helsingborg Castle, where hespent the last years of his life. His illustrious son Tycho was bornin 1546, and was the second child and eldest boy in a family of ten. 

It appears that Otto, the father of Tycho, had a brother namedGeorge, who was childless. George, however, desired to adopt a boyon whom he could lavish his affection and to whom he could bequeathhis wealth. A somewhat singular arrangement was accordingly enteredinto by the brothers at the time when Otto was married. It wasagreed that the first son who might be born to Otto should beforthwith handed over by the parents to George to be reared andadopted by him. In due time little Tycho appeared, and wasimmediately claimed by George in pursuance of the compact. But itwas not unnatural that the parental instinct, which had been dormantwhen the agreement was made, should here interpose. Tycho's fatherand mother receded from the bargain, and refused to part with theirson. George thought he was badly treated. However, he took noviolent steps until a year later, when a brother was born to Tycho. The uncle then felt no scruple in asserting what he believed to behis rights by the simple process of stealing the first-born nephew, which the original bargain had promised him. After a little time itwould seem that the parents acquiesced in the loss, and thus it wasin Uncle George's home that the future astronomer passed hischildhood. 

When we read that Tycho was no more than thirteen years old at thetime he entered the University of Copenhagen, it might be at firstsupposed that even in his boyish years he must have exhibited some ofthose remarkable talents with which he was afterwards to astonish theworld. Such an inference should not, however, be drawn. The fact isthat in those days it was customary for students to enter theuniversities at a much earlier age than is now the case. Not, indeed, that the boys of thirteen knew more then than the boys ofthirteen know now. But the education imparted in the universities atthat time was of a much more rudimentary kind than that which weunderstand by university education at present. In illustration ofthis Dr. Dreyer tells us how, in the University of Wittenberg, one ofthe professors, in his opening address, was accustomed to point outthat even the processes of multiplication and division in arithmeticmight be learned by any student who possessed the necessarydiligence. 

It was the wish and the intention of his uncle that Tycho's educationshould be specially directed to those branches of rhetoric andphilosophy which were then supposed to be a necessary preparation forthe career of a statesman. Tycho, however, speedily made it plain tohis teachers that though he was an ardent student, yet the thingswhich interested him were the movements of the heavenly bodies andnot the subtleties of metaphysics. 

[PLATE: TYCHO BRAHE. ]

On the 21st October, 1560, an eclipse of the sun occurred, which waspartially visible at Copenhagen. Tycho, boy though he was, took theutmost interest in this event. His ardour and astonishment inconnection with the circumstance were chiefly excited by the factthat the time of the occurrence of the phenomenon could be predictedwith so much accuracy. Urged by his desire to understand the matterthoroughly, Tycho sought to procure some book which might explainwhat he so greatly wanted to know. In those days books of any kindwere but few and scarce, and scientific books were especiallyunattainable. It so happened, however, that a Latin version ofPtolemy's astronomical works had appeared a few years before theeclipse took place, and Tycho managed to buy a copy of this book, which was then the chief authority on celestial matters. Young asthe boy astronomer was, he studied hard, although perhaps not alwayssuccessfully, to understand Ptolemy, and to this day his copy of thegreat work, copiously annotated and marked by the schoolboy hand, ispreserved as one of the chief treasures in the library of theUniversity at Prague. 

After Tycho had studied for about three years at the University ofCopenhagen, his uncle thought it would be better to send him, as wasusual in those days, to complete his education by a course of studyin some foreign university. The uncle cherished the hope that inthis way the attention of the young astronomer might be withdrawnfrom the study of the stars and directed in what appeared to him amore useful way. Indeed, to the wise heads of those days, thepursuit of natural science seemed so much waste of good time whichmight otherwise be devoted to logic or rhetoric or some other branchof study more in vogue at that time. To assist in this attempt towean Tycho from his scientific tastes, his uncle chose as a tutor toaccompany him an intelligent and upright young man named Vedel, whowas four years senior to his pupil, and accordingly, in 1562, we findthe pair taking up their abode at the University of Leipzig. 

The tutor, however, soon found that he had undertaken a most hopelesstask. He could not succeed in imbuing Tycho with the slightest tastefor the study of the law or the other branches of knowledge whichwere then thought so desirable. The stars, and nothing but thestars, engrossed the attention of his pupil. We are told that allthe money he could obtain was spent secretly in buying astronomicalbooks and instruments. He learned the name of the stars from alittle globe, which he kept hidden from Vedel, and only ventured touse during the latter's absence. No little friction was at firstcaused by all this, but in after years a fast and enduring friendshipgrew up between Tycho and his tutor, each of whom learned to respectand to love the other. 

Before Tycho was seventeen he had commenced the difficult task ofcalculating the movements of the planets and the places which theyoccupied on the sky from time to time. He was not a little surprisedto find that the actual positions of the planets differed very widelyfrom those which were assigned to them by calculations from the bestexisting works of astronomers. With the insight of genius he sawthat the only true method of investigating the movements of theheavenly bodies would be to carry on a protracted series ofmeasurements of their places. This, which now seems to us soobvious, was then entirely new doctrine. Tycho at once commencedregular observations in such fashion as he could. His firstinstrument was, indeed, a very primitive one, consisting of a simplepair of compasses, which he used in this way. He placed his eye atthe hinge, and then opened the legs of the compass so that one legpointed to one star and the other leg to the other star. The compasswas then brought down to a divided circle, by which means the numberof degrees in the apparent angular distance of the two stars wasdetermined. 

His next advance in instrumental equipment was to provide himselfwith the contrivance known as the "cross-staff, " which he used toobserve the stars whenever opportunity offered. It must, of course, be remembered that in those days there were no telescopes. In theabsence of optical aid, such as lenses afford the modern observers, astronomers had to rely on mechanical appliances alone to measure theplaces of the stars. Of such appliances, perhaps the most ingeniouswas one known before Tycho's time, which we have represented in theadjoining figure. 

[PLATE: TYCHO'S CROSS STAFF. ]

Let us suppose that it be desired to measure the angle between twostars, then if the angle be not too large it can be determined in thefollowing manner. Let the rod AB be divided into inches and parts ofan inch, and let another rod, CD, slide up and down along AB in sucha way that the two always remain perpendicular to each other. "Sights, " like those on a rifle, are placed at A and C, and there isa pin at D. It will easily be seen that, by sliding the movable baralong the fixed one, it must always be possible when the stars arenot too far apart to bring the sights into such positions that onestar can be seen along DC and the other along DA. This having beenaccomplished, the length from A to the cross-bar is read off on thescale, and then, by means of a table previously prepared, the valueof the required angular distance is obtained. If the angle betweenthe two stars were greater than it would be possible to measure inthe way already described, then there was a provision by which thepin at D might be moved along CD into some other position, so as tobring the angular distance of the stars within the range of theinstrument. 

[PLATE: TYCHO'S "NEW STAR" SEXTANT OF 1572. (The arms, of walnut wood, are about 5 1/2 ft. Long. )]

No doubt the cross-staff is a very primitive contrivance, but whenhandled by one so skilful as Tycho it afforded results ofconsiderable accuracy. I would recommend any reader who may have ataste for such pursuits to construct a cross-staff for himself, andsee what measurements he can accomplish with its aid. 

To employ this little instrument Tycho had to evade the vigilance ofhis conscientious tutor, who felt it his duty to interdict all suchoccupations as being a frivolous waste of time. It was when Vedelwas asleep that Tycho managed to escape with his cross staff andmeasure the places of the heavenly bodies. Even at this early ageTycho used to conduct his observations on those thoroughly soundprinciples which lie at the foundation of all accurate modernastronomy. Recognising the inevitable errors of workmanship in hislittle instrument, he ascertained their amount and allowed for theirinfluence on the results which he deduced. This principle, employedby the boy with his cross-staff in 1564, is employed at the presentday by the Astronomer Royal at Greenwich with the most superbinstruments that the skill of modern opticians has been able toconstruct. 

[PLATE: TYCHO'S TRIGONIC SEXTANT. (The arms, AB and AC, are about 5 1/2 ft. Long. )]

After the death of his uncle, when Tycho was nineteen years of age, it appears that the young philosopher was no longer interfered within so far as the line which his studies were to take was concerned. Always of a somewhat restless temperament, we now find that heshifted his abode to the University of Rostock, where he speedilymade himself notable in connection with an eclipse of the moon on28th October, 1566. Like every other astronomer of those days, Tychohad always associated astronomy with astrology. He considered thatthe phenomena of the heavenly bodies always had some significance inconnection with human affairs. Tycho was also a poet, and in theunited capacity of poet, astrologer, and astronomer, he posted upsome verses in the college at Rostock announcing that the lunareclipse was a prognostication of the death of the great TurkishSultan, whose mighty deeds at that time filled men's minds. Presentlynews did arrive of the death of the Sultan, and Tycho was accordinglytriumphant; but a little later it appeared that the decease had takenplace BEFORE the eclipse, a circumstance which caused many a laugh atTycho's expense. 

[PLATE: TYCHO'S ASTRONOMIC SEXTANT. (Made of steel: the arms, AB, AC, measure 4 ft. )

PLATE: TYCHO'S EQUATORIAL ARMILLARY. (The meridian circle, E B C A D, made of solid steel, is nearly 6 ft. In diameter. )]

Tycho being of a somewhat turbulent disposition, it appears that, while at the University of Rostock, he had a serious quarrel withanother Danish nobleman. We are not told for certain what was thecause of the dispute. It does not, however, seem to have had anymore romantic origin than a difference of opinion as to which of themknew the more mathematics. They fought, as perhaps it was becomingfor two astronomers to fight, under the canopy of heaven in utterdarkness at the dead of night, and the duel was honourably terminatedwhen a slice was taken off Tycho's nose by the insinuating sword ofhis antagonist. For the repair of this injury the ingenuity of thegreat instrument-maker was here again useful, and he made asubstitute for his nose "with a composition of gold and silver. " Theimitation was so good that it is declared to have been quite equal tothe original. Dr. Lodge, however, pointedly observes that it doesnot appear whether this remark was made by a friend or an enemy. 

[PLATE: THE GREAT AUGSBURG QUADRANT. (Built of heart of oak; the radii about 19 ft. )

PLATE: TYCHO'S "NEW SCHEME OF THE TERRESTRIAL SYSTEM, " 1577. ]

The next few years Tycho spent in various places ardently pursuingsomewhat varied branches of scientific study. At one time we hear ofhim assisting an astronomical alderman, in the ancient city ofAugsburg, to erect a tremendous wooden machine--a quadrant of 19-feetradius--to be used in observing the heavens. At another time welearn that the King of Denmark had recognised the talents of hisillustrious subject, and promised to confer on him a pleasantsinecure in the shape of a canonry, which would assist him with themeans for indulging his scientific pursuits. Again we are told thatTycho is pursuing experiments in chemistry with the greatest energy, nor is this so incompatible as might at first be thought with hisdevotion to astronomy. In those early days of knowledge thedifferent sciences seemed bound together by mysterious bonds. Alchemists and astrologers taught that the several planets werecorrelated in some mysterious manner with the several metals. Itwas, therefore hardly surprising that Tycho should have included astudy of the properties of the metals in the programme of hisastronomical work. 

[PLATE: URANIBORG AND ITS GROUNDS. 

PLATE: GROUND-PLAN OF THE OBSERVATORY. ]

An event, however, occurred in 1572 which stimulated Tycho'sastronomical labours, and started him on his life's work. On the11th of November in that year, he was returning home to supper aftera day's work in his laboratory, when he happened to lift his face tothe sky, and there he beheld a brilliant new star. It was in theconstellation of Cassiopeia, and occupied a position in which therehad certainly been no bright star visible when his attention had lastbeen directed to that part of the heavens. Such a phenomenon was sostartling that he found it hard to trust the evidence of his senses. He thought he must be the subject of some hallucination. Hetherefore called to the servants who were accompanying him, and askedthem whether they, too, could see a brilliant object in the directionin which he pointed. They certainly could, and thus he becameconvinced that this marvellous object was no mere creation of thefancy, but a veritable celestial body--a new star of surpassingsplendour which had suddenly burst forth. In these days of carefulscrutiny of the heavens, we are accustomed to the occasional outbreakof new stars. It is not, however, believed that any new star whichhas ever appeared has displayed the same phenomenal brilliance as wasexhibited by the star of 1572. 

This object has a value in astronomy far greater than it might atfirst appear. It is true, in one sense, that Tycho discovered thenew star, but it is equally true, in a different sense, that it wasthe new star which discovered Tycho. Had it not been for thisopportune apparition, it is quite possible that Tycho might havefound a career in some direction less beneficial to science than thatwhich he ultimately pursued. 

[PLATE: THE OBSERVATORY OF URANIBORG, ISLAND OF HVEN. ]

When he reached his home on this memorable evening, Tycho immediatelyapplied his great quadrant to the measurement of the place of the newstar. His observations were specially directed to the determinationof the distance of the object. He rightly conjectured that if itwere very much nearer to us than the stars in its vicinity, thedistance of the brilliant body might be determined in a short time bythe apparent changes in its distance from the surrounding points. Itwas speedily demonstrated that the new star could not be as near asthe moon, by the simple fact that its apparent place, as comparedwith the stars in its neighbourhood, was not appreciably altered whenit was observed below the pole, and again above the pole at aninterval of twelve hours. Such observations were possible, inasmuchas the star was bright enough to be seen in full daylight. Tychothus showed conclusively that the body was so remote that thediameter of the earth bore an insignificant ratio to the star'sdistance. His success in this respect is the more noteworthy when wefind that many other observers, who studied the same object, came tothe erroneous conclusion that the new star was quite as near as themoon, or even much nearer. In fact, it may be said, that with regardto this object Tycho discovered everything which could possibly havebeen discovered in the days before telescopes were invented. He notonly proved that the star's distance was too great for measurement, but he showed that it had no proper motion on the heavens. Herecorded the successive changes in its brightness from week to week, as well as the fluctuations in hue with which the alterations inlustre were accompanied. 

It seems, nowadays, strange to find that such thoroughly scientificobservations of the new star as those which Tycho made, possessed, even in the eyes of the great astronomer himself, a profoundastrological significance. We learn from Dr. Dreyer that, in Tycho'sopinion, "the star was at first like Venus and Jupiter, and itseffects will therefore, first, be pleasant; but as it then becamelike Mars, there will next come a period of wars, seditions, captivity, and death of princes, and destruction of cities, togetherwith dryness and fiery meteors in the air, pestilence, and venomoussnakes. Lastly, the star became like Saturn, and thus will finallycome a time of want, death, imprisonment, and all kinds of sadthings!" Ideas of this kind were, however, universally entertained. It seemed, indeed, obvious to learned men of that period that such anapparition must forebode startling events. One of the chief theoriesthen held was, that just as the Star of Bethlehem announced the firstcoming of Christ, so the second coming, and the end of the world, washeralded by the new star of 1572. 

The researches of Tycho on this object were the occasion of his firstappearance as an author. The publication of his book was however, for some time delayed by the urgent remonstrances of his friends, whothought it was beneath the dignity of a nobleman to condescend towrite a book. Happily, Tycho determined to brave the opinion of hisorder; the book appeared, and was the first of a series of greatastronomical productions from the same pen. 

[PLATE: EFFIGY ON TYCHO'S TOMB AT PRAGUE. ]

The fame of the noble Dane being now widespread, the King of Denmarkentreated him to return to his native country, and to deliver acourse of lectures on astronomy in the University of Copenhagen. Withsome reluctance he consented, and his introductory oration has beenpreserved. He dwells, in fervent language, upon the beauty and theinterest of the celestial phenomena. He points out the imperativenecessity of continuous and systematic observation of the heavenlybodies in order to extend our knowledge. He appeals to the practicalutility of the science, for what civilised nation could exist withouthaving the means of measuring time? He sets forth how the study ofthese beautiful objects "exalts the mind from earthly and trivialthings to heavenly ones;" and then he winds up by assuring them that"a special use of astronomy is that it enables us to draw conclusionsfrom the movements in the celestial regions as to human fate. "

An interesting event, which occurred in 1572, distracted Tycho'sattention from astronomical matters. He fell in love. The younggirl on whom his affections were set appears to have sprung fromhumble origin. Here again his august family friends sought todissuade him from a match they thought unsuitable for a nobleman. But Tycho never gave way in anything. It is suggested that he didnot seek a wife among the highborn dames of his own rank from thedread that the demands of a fashionable lady would make too great aninroad on the time that he wished to devote to science. At allevents, Tycho's union seems to have been a happy one, and he had alarge family of children; none of whom, however, inherited theirfather's talents. 

[PLATE: TYCHO'S MURAL QUADRANT PICTURE, URANIBORG. ]

Tycho had many scientific friends in Germany, among whom his work washeld in high esteem. The treatment that he there met with seemed tohim so much more encouraging than that which he received in Denmarkthat he formed the notion of emigrating to Basle and making it hispermanent abode. A whisper of this intention was conveyed to thelarge-hearted King of Denmark, Frederick II. He wisely realised howgreat would be the fame which would accrue to his realm if he couldinduce Tycho to remain within Danish territory and carry on there thegreat work of his life. A resolution to make a splendid proposal toTycho was immediately formed. A noble youth was forthwith despatchedas a messenger, and ordered to travel day and night until he reachedTycho, whom he was to summon to the king. The astronomer was in bedon the morning Of 11th February, 1576, when the message wasdelivered. Tycho, of course, set off at once and had an audience ofthe king at Copenhagen. The astronomer explained that what he wantedwas the means to pursue his studies unmolested, whereupon the kingoffered him the Island of Hven, in the Sound near Elsinore. There hewould enjoy all the seclusion that he could desire. The king furtherpromised that he would provide the funds necessary for building ahouse and for founding the greatest observatory that had ever yetbeen reared for the study of the heavens. After due deliberation andconsultation with his friends, Tycho accepted the king's offer. Hewas forthwith granted a pension, and a deed was drawn up formallyassigning the Island of Hven to his use all the days of his life. 

The foundation of the famous castle of Uraniborg was laid on 30thAugust, 1576. The ceremony was a formal and imposing one, inaccordance with Tycho's ideas of splendour. A party of scientificfriends had assembled, and the time had been chosen so that theheavenly bodies were auspiciously placed. Libations of costly wineswere poured forth, and the stone was placed with due solemnity. Thepicturesque character of this wonderful temple for the study of thestars may be seen in the figures with which this chapter isillustrated. 

One of the most remarkable instruments that has ever been employed instudying the heavens was the mural quadrant which Tycho erected inone of the apartments of Uraniborg. By its means the altitudes ofthe celestial bodies could be observed with much greater accuracythan had been previously attainable. This wonderful contrivance isrepresented on the preceding page. It will be observed that thewalls of the room are adorned by pictures with a lavishness ofdecoration not usually to be found in scientific establishments. 

A few years later, when the fame of the observatory at Hven becamemore widely spread, a number of young men flocked to Tycho to studyunder his direction. He therefore built another observatory fortheir use in which the instruments were placed in subterranean roomsof which only the roofs appeared above the ground. There was awonderful poetical inscription over the entrance to this undergroundobservatory, expressing the astonishment of Urania at finding, evenin the interior of the earth, a cavern devoted to the study of theheavens. Tycho was indeed always fond of versifying, and he lost noopportunity of indulging this taste whenever an occasion presenteditself. 

Around the walls of the subterranean observatory were the pictures ofeight astronomers, each with a suitable inscription--one of these ofcourse represented Tycho himself, and beneath were written words tothe effect that posterity should judge of his work. The eighthpicture depicted an astronomer who has not yet come into existence. Tychonides was his name, and the inscription presses the modest hopethat when he does appear he will be worthy of his great predecessor. The vast expenses incurred in the erection and the maintenance ofthis strange establishment were defrayed by a succession of grantsfrom the royal purse. 

For twenty years Tycho laboured hard at Uraniborg in the pursuit ofscience. His work mainly consisted in the determination of theplaces of the moon, the planets, and the stars on the celestialsphere. The extraordinary pains taken by Tycho to have hisobservations as accurate as his instruments would permit, have justlyentitled him to the admiration of all succeeding astronomers. Hisisland home provided the means of recreation as well as a place forwork. He was surrounded by his family, troops of friends were notwanting, and a pet dwarf seems to have been an inmate of his curiousresidence. By way of change from his astronomical labours he usedfrequently to work with his students in his chemical laboratory. Itis not indeed known what particular problems in chemistry occupiedhis attention. We are told, however, that he engaged largely in theproduction of medicines, and as these appear to have been dispensedgratuitously there was no lack of patients. 

Tycho's imperious and grasping character frequently brought him intodifficulties, which seem to have increased with his advancing years. He had ill-treated one of his tenants on Hven, and an adversedecision by the courts seems to have greatly exasperated theastronomer. Serious changes also took place in his relations to thecourt at Copenhagen. When the young king was crowned in 1596, hereversed the policy of his predecessor with reference to Hven. Theliberal allowances to Tycho were one after another withdrawn, andfinally even his pension was stopped. Tycho accordingly abandonedHven in a tumult of rage and mortification. A few years later wefind him in Bohemia a prematurely aged man, and he died on the 24thOctober, 1601. 

GALILEO. 

Among the ranks of the great astronomers it would be difficult tofind one whose life presents more interesting features and remarkablevicissitudes than does that of Galileo. We may consider him as thepatient investigator and brilliant discoverer. We may consider himin his private relations, especially to his daughter, Sister MariaCeleste, a woman of very remarkable character; and we have also thepathetic drama at the close of Galileo's life, when the philosopherdrew down upon himself the thunders of the Inquisition. 

The materials for the sketch of this astonishing man are sufficientlyabundant. We make special use in this place of those charmingletters which his daughter wrote to him from her convent home. Morethan a hundred of these have been preserved, and it may well bedoubted whether any more beautiful and touching series of lettersaddressed to a parent by a dearly loved child have ever beenwritten. An admirable account of this correspondence is contained ina little book entitled "The Private Life of Galileo, " publishedanonymously by Messrs. Macmillan in 1870, and I have been muchindebted to the author of that volume for many of the facts containedin this chapter. 

Galileo was born at Pisa, on 18th February, 1564. He was the eldestson of Vincenzo de' Bonajuti de' Galilei, a Florentine noble. Notwithstanding his illustrious birth and descent, it would seem thatthe home in which the great philosopher's childhood was spent was animpoverished one. It was obvious at least that the young Galileowould have to be provided with some profession by which he might earna livelihood. From his father he derived both by inheritance and byprecept a keen taste for music, and it appears that he became anexcellent performer on the lute. He was also endowed withconsiderable artistic power, which he cultivated diligently. Indeed, it would seem that for some time the future astronomer entertainedthe idea of devoting himself to painting as a profession. Hisfather, however, decided that he should study medicine. Accordingly, we find that when Galileo was seventeen years of age, and had added aknowledge of Greek and Latin to his acquaintance with the fine arts, he was duly entered at the University of Pisa. 

Here the young philosopher obtained some inkling of mathematics, whereupon he became so much interested in this branch of science, that he begged to be allowed to study geometry. In compliance withhis request, his father permitted a tutor to be engaged for thispurpose; but he did so with reluctance, fearing that the attention ofthe young student might thus be withdrawn from that medical workwhich was regarded as his primary occupation. The event speedilyproved that these anxieties were not without some justification. Thepropositions of Euclid proved so engrossing to Galileo that it wasthought wise to avoid further distraction by terminating themathematical tutor's engagement. But it was too late for the desiredend to be attained. Galileo had now made such progress that he wasable to continue his geometrical studies by himself. Presently headvanced to that famous 47th proposition which won his livelyadmiration, and on he went until he had mastered the six books ofEuclid, which was a considerable achievement for those days. 

The diligence and brilliance of the young student at Pisa did not, however, bring him much credit with the University authorities. Inthose days the doctrines of Aristotle were regarded as the embodimentof all human wisdom in natural science as well as in everythingelse. It was regarded as the duty of every student to learnAristotle off by heart, and any disposition to doubt or even toquestion the doctrines of the venerated teacher was regarded asintolerable presumption. But young Galileo had the audacity to thinkfor himself about the laws of nature. He would not take anyassertion of fact on the authority of Aristotle when he had the meansof questioning nature directly as to its truth or falsehood. Histeachers thus came to regard him as a somewhat misguided youth, though they could not but respect the unflagging industry with whichhe amassed all the knowledge he could acquire. 

[PLATE: GALILEO'S PENDULUM. ]

We are so accustomed to the use of pendulums in our clocks thatperhaps we do not often realise that the introduction of this methodof regulating time-pieces was really a notable invention worthy thefame of the great astronomer to whom it was due. It appears thatsitting one day in the Cathedral of Pisa, Galileo's attention becameconcentrated on the swinging of a chandelier which hung from theceiling. It struck him as a significant point, that whether the arcthrough which the pendulum oscillated was a long one or a short one, the time occupied in each vibration was sensibly the same. Thissuggested to the thoughtful observer that a pendulum would afford themeans by which a time-keeper might be controlled, and accordinglyGalileo constructed for the first time a clock on this principle. Theimmediate object sought in this apparatus was to provide a means ofaiding physicians in counting the pulses of their patients. 

The talents of Galileo having at length extorted due recognition fromthe authorities, he was appointed, at the age of twenty-five, Professor of Mathematics at the University of Pisa. Then came thetime when he felt himself strong enough to throw down the gauntlet tothe adherents of the old philosophy. As a necessary part of hisdoctrine on the movement of bodies Aristotle had asserted that thetime occupied by a stone in falling depends upon its weight, so thatthe heavier the stone the less time would it require to fall from acertain height to the earth. It might have been thought that astatement so easily confuted by the simplest experiments could neverhave maintained its position in any accepted scheme of philosophy. But Aristotle had said it, and to anyone who ventured to express adoubt the ready sneer was forthcoming, "Do you think yourself acleverer man than Aristotle?" Galileo determined to demonstrate inthe most emphatic manner the absurdity of a doctrine which had forcenturies received the sanction of the learned. The summit of theLeaning Tower of Pisa offered a highly dramatic site for the greatexperiment. The youthful professor let fall from the overhanging topa large heavy body and a small light body simultaneously. Accordingto Aristotle the large body ought to have reached the ground muchsooner than the small one, but such was found not to be the case. Inthe sight of a large concourse of people the simple fact wasdemonstrated that the two bodies fell side by side, and reached theground at the same time. Thus the first great step was taken in theoverthrow of that preposterous system of unquestioning adhesion todogma, which had impeded the development of the knowledge of naturefor nearly two thousand years. 

This revolutionary attitude towards the ancient beliefs was notcalculated to render Galileo's relations with the Universityauthorities harmonious. He had also the misfortune to make enemiesin other quarters. Don Giovanni de Medici, who was then the Governorof the Port of Leghorn, had designed some contrivance by which heproposed to pump out a dock. But Galileo showed up the absurdity ofthis enterprise in such an aggressive manner that Don Giovanni tookmortal offence, nor was he mollified when the truths of Galileo'scriticisms were abundantly verified by the total failure of hisridiculous invention. In various ways Galileo was made to feel hisposition at Pisa so unpleasant that he was at length compelled toabandon his chair in the University. The active exertions of hisfriends, of whom Galileo was so fortunate as to have had throughouthis life an abundant supply, then secured his election to theProfessorship of Mathematics at Padua, whither he went in 1592. 

[PLATE: PORTRAIT OF GALILEO. ]

It was in this new position that Galileo entered on that marvellouscareer of investigation which was destined to revolutionize science. The zeal with which he discharged his professorial duties was indeedof the most unremitting character. He speedily drew such crowds tolisten to his discourses on Natural Philosophy that his lecture-roomwas filled to overflowing. He also received many private pupils inhis house for special instruction. Every moment that could be sparedfrom these labours was devoted to his private study and to hisincessant experiments. 

Like many another philosopher who has greatly extended our knowledgeof nature, Galileo had a remarkable aptitude for the invention ofinstruments designed for philosophical research. To facilitate hispractical work, we find that in 1599 he had engaged a skilled workmanwho was to live in his house, and thus be constantly at hand to trythe devices for ever springing from Galileo's fertile brain. Amongthe earliest of his inventions appears to have been the thermometer, which he constructed in 1602. No doubt this apparatus in itsprimitive form differed in some respects from the contrivance we callby the same name. Galileo at first employed water as the agent, bythe expansion of which the temperature was to be measured. Heafterwards saw the advantage of using spirits for the same purpose. It was not until about half a century later that mercury came to berecognised as the liquid most generally suitable for the thermometer. 

The time was now approaching when Galileo was to make that mightystep in the advancement of human knowledge which followed on theapplication of the telescope to astronomy. As to how his idea ofsuch an instrument originated, we had best let him tell us in his ownwords. The passage is given in a letter which he writes to hisbrother-in-law, Landucci. 

"I write now because I have a piece of news for you, though whetheryou will be glad or sorry to hear it I cannot say; for I have now nohope of returning to my own country, though the occurrence which hasdestroyed that hope has had results both useful and honourable. Youmust know, then, that two months ago there was a report spread herethat in Flanders some one had presented to Count Maurice of Nassau aglass manufactured in such a way as to make distant objects appearvery near, so that a man at the distance of two miles could beclearly seen. This seemed to me so marvellous that I began to thinkabout it. As it appeared to me to have a foundation in the Theory ofPerspective, I set about contriving how to make it, and at length Ifound out, and have succeeded so well that the one I have made is farsuperior to the Dutch telescope. It was reported in Venice that Ihad made one, and a week since I was commanded to show it to hisSerenity and to all the members of the senate, to their infiniteamazement. Many gentlemen and senators, even the oldest, haveascended at various times the highest bell-towers in Venice to spyout ships at sea making sail for the mouth of the harbour, and haveseen them clearly, though without my telescope they would have beeninvisible for more than two hours. The effect of this instrument isto show an object at a distance of say fifty miles, as if it were butfive miles. "

The remarkable properties of the telescope at once commandeduniversal attention among intellectual men. Galileo receivedapplications from several quarters for his new instrument, of whichit would seem that he manufactured a large number to be distributedas gifts to various illustrious personages. 

But it was reserved for Galileo himself to make that application ofthe instrument to the celestial bodies by which its peculiar powerswere to inaugurate the new era in astronomy. The first discoverythat was made in this direction appears to have been connected withthe number of the stars. Galileo saw to his amazement that throughhis little tube he could count ten times as many stars in the sky ashis unaided eye could detect. Here was, indeed, a surprise. We arenow so familiar with the elementary facts of astronomy that it is notalways easy to realise how the heavens were interpreted by theobservers in those ages prior to the invention of the telescope. Wecan hardly, indeed, suppose that Galileo, like the majority of thosewho ever thought of such matters, entertained the erroneous beliefthat the stars were on the surface of a sphere at equal distancesfrom the observer. No one would be likely to have retained hisbelief in such a doctrine when he saw how the number of visible starscould be increased tenfold by means of Galileo's telescope. It wouldhave been almost impossible to refuse to draw the inference that thestars thus brought into view were still more remote objects which thetelescope was able to reveal, just in the same way as it showedcertain ships to the astonished Venetians, when at the time theseships were beyond the reach of unaided vision. 

Galileo's celestial discoveries now succeeded each other rapidly. That beautiful Milky Way, which has for ages been the object ofadmiration to all lovers of nature, never disclosed its true natureto the eye of man till the astronomer of Padua turned on it his magictube. The splendid zone of silvery light was then displayed asstar-dust scattered over the black background of the sky. It wasobserved that though the individual stars were too small to be seenseverally without optical aid, yet such was their incredible numberthat the celestial radiance produced that luminosity with which everystargazer was so familiar. 

But the greatest discovery made by the telescope in these early days, perhaps, indeed, the greatest discovery that the telescope has everaccomplished, was the detection of the system of four satellitesrevolving around the great planet Jupiter. This phenomenon was sowholly unexpected by Galileo that, at first, he could hardly believehis eyes. However, the reality of the existence of a system of fourmoons attending the great planet was soon established beyond allquestion. Numbers of great personages crowded to Galileo to see forthemselves this beautiful miniature representing the sun with itssystem of revolving planets. 

Of course there were, as usual, a few incredulous people who refusedto believe the assertion that four more moving bodies had to be addedto the planetary system. They scoffed at the notion; they said thesatellites may have been in the telescope, but that they were not inthe sky. One sceptical philosopher is reported to have affirmed, that even if he saw the moons of Jupiter himself he would not believein them, as their existence was contrary to the principles ofcommon-sense!

There can be no doubt that a special significance attached to the newdiscovery at this particular epoch in the history of science. Itmust be remembered that in those days the doctrine of Copernicus, declaring that the sun, and not the earth, was the centre of thesystem, that the earth revolved on its axis once a day, and that itdescribed a mighty circle round the sun once a year, had onlyrecently been promulgated. This new view of the scheme of nature hadbeen encountered with the most furious opposition. It may possiblyhave been that Galileo himself had not felt quite confident in thesoundness of the Copernican theory, prior to the discovery of thesatellites of Jupiter. But when a picture was there exhibited inwhich a number of relatively small globes were shown to be revolvingaround a single large globe in the centre, it seemed impossible notto feel that the beautiful spectacle so displayed was an emblem ofthe relations of the planets to the sun. It was thus made manifestto Galileo that the Copernican theory of the planetary system must bethe true one. The momentous import of this opinion upon the futurewelfare of the great philosopher will presently appear. 

It would seem that Galileo regarded his residence at Padua as a stateof undesirable exile from his beloved Tuscany. He had always ayearning to go back to his own country and at last the desiredopportunity presented itself. For now that Galileo's fame had becomeso great, the Grand Duke of Tuscany desired to have the philosopherresident at Florence, in the belief that he would shed lustre on theDuke's dominions. Overtures were accordingly made to Galileo, andthe consequence was that in 1616 we find him residing at Florence, bearing the title of Mathematician and Philosopher to the Grand Duke. 

Two daughters, Polissena and Virginia, and one son, Vincenzo, hadbeen born to Galileo in Padua. It was the custom in those days thatas soon as the daughter of an Italian gentleman had grown up, herfuture career was somewhat summarily decided. Either a husband wasto be forthwith sought out, or she was to enter the convent with theobject of taking the veil as a professed nun. It was arranged thatthe two daughters of Galileo, while still scarcely more thanchildren, should both enter the Franciscan convent of St. Matthew, atArcetri. The elder daughter Polissena, took the name of Sister MariaCeleste, while Virginia became Sister Arcangela. The latter seems tohave been always delicate and subject to prolonged melancholy, andshe is of but little account in the narrative of the life ofGalileo. But Sister Maria Celeste, though never leaving the convent, managed to preserve a close intimacy with her beloved father. Thiswas maintained only partly by Galileo's visits, which were veryirregular and were, indeed, often suspended for long intervals. Buthis letters to this daughter were evidently frequent andaffectionate, especially in the latter part of his life. Mostunfortunately, however, all his letters have been lost. There aregrounds for believing that they were deliberately destroyed whenGalileo was seized by the Inquisition, lest they should have beenused as evidence against him, or lest they should have compromisedthe convent where they were received. But Sister Maria Celeste'sletters to her father have happily been preserved, and most touchingthese letters are. We can hardly read them without thinking how thesweet and gentle nun would have shrunk from the idea of theirpublication. 

Her loving little notes to her "dearest lord and father, " as she usedaffectionately to call Galileo, were almost invariably accompanied bysome gift, trifling it may be, but always the best the poor nun hadto bestow. The tender grace of these endearing communications wasall the more precious to him from the fact that the rest of Galileo'srelatives were of quite a worthless description. He alwaysacknowledged the ties of his kindred in the most generous way, buttheir follies and their vices, their selfishness and theirimportunities, were an incessant source of annoyance to him, almostto the last day of his life. 

On 19th December, 1625, Sister Maria Celeste writes:--

"I send two baked pears for these days of vigil. But as the greatesttreat of all, I send you a rose, which ought to please you extremely, seeing what a rarity it is at this season; and with the rose you mustaccept its thorns, which represent the bitter passion of our Lord, whilst the green leaves represent the hope we may entertain thatthrough the same sacred passion we, having passed through thedarkness of the short winter of our mortal life, may attain to thebrightness and felicity of an eternal spring in heaven. "

When the wife and children of Galileo's shiftless brother came totake up their abode in the philosopher's home, Sister Maria Celestefeels glad to think that her father has now some one who, howeverimperfectly, may fulfil the duty of looking after him. A gracefulnote on Christmas Eve accompanies her little gifts. She hopes that--

"In these holy days the peace of God may rest on him and all thehouse. The largest collar and sleeves I mean for Albertino, theother two for the two younger boys, the little dog for baby, and thecakes for everybody, except the spice-cakes, which are for you. Accept the good-will which would readily do much more. "

The extraordinary forbearance with which Galileo continually placedhis time, his purse, and his influence at the service of those whohad repeatedly proved themselves utterly unworthy of his countenance, is thus commented on by the good nun. --

"Now it seems to me, dearest lord and father, that your lordship iswalking in the right path, since you take hold of every occasion thatpresents itself to shower continual benefits on those who only repayyou with ingratitude. This is an action which is all the morevirtuous and perfect as it is the more difficult. "

When the plague was raging in the neighbourhood, the lovingdaughter's solicitude is thus shown:--

"I send you two pots of electuary as a preventive against theplague. The one without the label consists of dried figs, walnuts, rue, and salt, mixed together with honey. A piece of the size of awalnut to be taken in the morning, fasting, with a little Greekwine. "

The plague increasing still more, Sister Maria Celeste obtained withmuch difficulty, a small quantity of a renowned liqueur, made byAbbess Ursula, an exceptionally saintly nun. This she sends to herfather with the words:--

"I pray your lordship to have faith in this remedy. For if you haveso much faith in my poor miserable prayers, much more may you have inthose of such a holy person; indeed, through her merits you may feelsure of escaping all danger from the plague. "

Whether Galileo took the remedy we do not know, but at all eventshe escaped the plague. 

[PLATE: THE VILLA ARCETRI. Galileo's residence, where Milton visited him. ]

From Galileo's new home in Florence the telescope was again directedto the skies, and again did astounding discoveries reward theastronomer's labours. The great success which he had met with instudying Jupiter naturally led Galileo to look at Saturn. Here hesaw a spectacle which was sufficiently amazing, though he failed tointerpret it accurately. It was quite manifest that Saturn did notexhibit a simple circular disc like Jupiter, or like Mars. It seemedto Galileo as if the planet consisted of three bodies, a large globein the centre, and a smaller one on each side. The enigmaticalnature of the discovery led Galileo to announce it in an enigmaticalmanner. He published a string of letters which, when dulytransposed, made up a sentence which affirmed that the planet Saturnwas threefold. Of course we now know that this remarkable appearanceof the planet was due to the two projecting portions of the ring. With the feeble power of Galileo's telescope, these seemed merelylike small globes or appendages to the large central body. 

The last Of Galileo's great astronomical discoveries related to thelibration of the moon. I think that the detection of this phenomenonshows his acuteness of observation more remarkably than does any oneof his other achievements with the telescope. It is well known thatthe moon constantly keeps the same face turned towards the earth. When, however, careful measurements have been made with regard to thespots and marks on the lunar surface, it is found that there is aslight periodic variation which permits us to see now a little to theeast or to the west, now a little to the north or to the south ofthe average lunar disc. 

But the circumstances which make the career of Galileo so especiallyinteresting from the biographer's point of view, are hardly so muchthe triumphs that he won as the sufferings that he endured. Thesufferings and the triumphs were, however, closely connected, and itis fitting that we should give due consideration to what was perhapsthe greatest drama in the history of science. 

On the appearance of the immortal work of Copernicus, in which it wastaught that the earth rotated on its axis, and that the earth, likethe other planets, revolved round the sun, orthodoxy stood aghast. The Holy Roman Church submitted this treatise, which bore the name"De Revolutionibus Orbium Coelestium, " to the Congregation of theIndex. After due examination it was condemned as heretical in 1615. Galileo was suspected, on no doubt excellent grounds, of entertainingthe objectionable views of Copernicus. He was accordingly privatelysummoned before Cardinal Bellarmine on 26th February 1616, and dulyadmonished that he was on no account to teach or to defend theobnoxious doctrines. Galileo was much distressed by thisintimation. He felt it a serious matter to be deprived of theprivilege of discoursing with his friends about the Copernicansystem, and of instructing his disciples in the principles of thegreat theory of whose truth he was perfectly convinced. It painedhim, however, still more to think, devout Catholic as he was, thatsuch suspicions of his fervent allegiance to his Church should everhave existed, as were implied by the words and monitions of CardinalBellarmine. 

In 1616, Galileo had an interview with Pope Paul V. , who received thegreat astronomer very graciously, and walked up and down with him inconversation for three-quarters of an hour. Galileo complained tohis Holiness of the attempts made by his enemies to embarrass himwith the authorities of the Church, but the Pope bade him becomforted. His Holiness had himself no doubts of Galileo'sorthodoxy, and he assured him that the Congregation of the Indexshould give Galileo no further trouble so long as Paul V. Was in thechair of St. Peter. 

On the death of Paul V. In 1623, Maffeo Barberini was elected Pope, as Urban VIII. This new Pope, while a cardinal, had been an intimatefriend of Galileo's, and had indeed written Latin verses in praise ofthe great astronomer and his discoveries. It was therefore notunnatural for Galileo to think that the time had arrived when, withthe use of due circumspection, he might continue his studies and hiswritings, without fear of incurring the displeasure of the Church. Indeed, in 1624, one of Galileo's friends writing from Rome, urgesGalileo to visit the city again, and added that--

"Under the auspices of this most excellent, learned, and benignantPontiff, science must flourish. Your arrival will be welcome to hisHoliness. He asked me if you were coming, and when, and in short, heseems to love and esteem you more than ever. "

The visit was duly paid, and when Galileo returned to Florence, thePope wrote a letter from which the following is an extract, commanding the philosopher to the good offices of the youngFerdinand, who had shortly before succeeded his father in the GrandDuchy of Tuscany. 

"We find in Galileo not only literary distinction, but also the loveof piety, and he is also strong in those qualities by which thepontifical good-will is easily obtained. And now, when he has beenbrought to this city to congratulate us on our elevation, we havevery lovingly embraced him; nor can we suffer him to return to thecountry whither your liberality calls him, without an ample provisionof pontifical love. And that you may know how dear he is to us, wehave willed to give him this honourable testimonial of virtue andpiety. And we further signify that every benefit which you shallconfer upon him, imitating or even surpassing your father'sliberality, will conduce to our gratification. "

The favourable reception which had been accorded to him by Pope UrbanVIII. Seems to have led Galileo to expect that there might be somecorresponding change in the attitude of the Papal authorities on thegreat question of the stability of the earth. He accordinglyproceeded with the preparation of the chief work of his life, "TheDialogue of the two Systems. " It was submitted for inspection by theconstituted authorities. The Pope himself thought that, if a fewconditions which he laid down were duly complied with, there could beno objection to the publication of the work. In the first place, thetitle of the book was to be so carefully worded as to show plainlythat the Copernican doctrine was merely to be regarded as anhypothesis, and not as a scientific fact. Galileo was alsoinstructed to conclude the book with special arguments which had beensupplied by the Pope himself, and which appeared to his Holiness tobe quite conclusive against the new doctrine of Copernicus. 

Formal leave for the publication of the Dialogue was then given toGalileo by the Inquisitor General, and it was accordingly sent to thepress. It might be thought that the anxieties of the astronomerabout his book would then have terminated. As a matter of fact, theyhad not yet seriously begun. Riccardi, the Master of the SacredPalace, having suddenly had some further misgivings, sent to Galileofor the manuscript while the work was at the printer's, in order thatthe doctrine it implied might be once again examined. Apparently, Riccardi had come to the conclusion that he had not given the mattersufficient attention, when the authority to go to press had beenfirst and, perhaps, hastily given. Considerable delay in the issueof the book was the result of these further deliberations. At last, however, in June, 1632, Galileo's great work, "The Dialogue of thetwo Systems, " was produced for the instruction of the world, thoughthe occasion was fraught with ruin to the immortal author. 

[PLATE: FACSIMILE SKETCH OF LUNAR SURFACE BY GALILEO. ]

The book, on its publication, was received and read with the greatestavidity. But presently the Master of The Sacred Palace found reasonto regret that he had given his consent to its appearance. Heaccordingly issued a peremptory order to sequestrate every copy inItaly. This sudden change in the Papal attitude towards Galileoformed the subject of a strong remonstrance addressed to the Romanauthorities by the Grand Duke of Tuscany. The Pope himself seemed tohave become impressed all at once with the belief that the workcontained matter of an heretical description. The generalinterpretation put upon the book seems to have shown the authoritiesthat they had mistaken its true tendency, notwithstanding the factthat it had been examined again and again by theologians deputed forthe duty. To the communication from the Grand Duke the Pope returnedanswer, that he had decided to submit the book to a congregation of"learned, grave, and saintly men, " who would weigh every word in it. The views of his Holiness personally on the subject were expressed inhis belief that the Dialogue contained the most perverse matter thatcould come into a reader's hands. 

The Master of the Sacred Palace was greatly blamed by the authoritiesfor having given his sanction to its issue. He pleaded that the bookhad not been printed in the precise terms of the original manuscriptwhich had been submitted to him. It was also alleged that Galileohad not adhered to his promise of inserting properly the argumentswhich the Pope himself had given in support of the old and orthodoxview. One of these had, no doubt, been introduced, but, so far frommending Galileo's case, it had made matters really look worse for thepoor philosopher. The Pope's argument had been put into the mouth ofone of the characters in the Dialogue named "Simplicio. " Galileo'senemies maintained that by adopting such a method for the expressionof his Holiness's opinion, Galileo had intended to hold the Popehimself up to ridicule. Galileo's friends maintained that nothingcould have been farther from his intention. It seems, however, highly probable that the suspicions thus aroused had something to sayto the sudden change of front on the part of the Papal authorities. 

On 1st October, 1632, Galileo received an order to appear before theInquisition at Rome on the grave charge of heresy. Galileo, ofcourse, expressed his submission, but pleaded for a respite fromcompliance with the summons, on the ground of his advanced age andhis failing health. The Pope was, however, inexorable; he said thathe had warned Galileo of his danger while he was still his friend. The command could not be disobeyed. Galileo might perform thejourney as slowly as he pleased, but it was imperatively necessaryfor him to set forth and at once. 

On 20th January, 1633, Galileo started on his weary journey to Rome, in compliance with this peremptory summons. On 13th February he wasreceived as the guest of Niccolini, the Tuscan ambassador, who hadacted as his wise and ever-kind friend throughout the whole affair. It seemed plain that the Holy Office were inclined to treat Galileowith as much clemency and consideration as was consistent with thedetermination that the case against him should be proceeded with tothe end. The Pope intimated that in consequence of his respect forthe Grand Duke of Tuscany he should permit Galileo to enjoy theprivilege, quite unprecedented for a prisoner charged with heresy, ofremaining as an inmate in the ambassador's house. He ought, strictly, to have been placed in the dungeons of the Inquisition. When the examination of the accused had actually commenced, Galileowas confined, not, indeed, in the dungeons, but in comfortable roomsat the Holy Office. 

By the judicious and conciliatory language of submission whichNiccolini had urged Galileo to use before the Inquisitors, they wereso far satisfied that they interceded with the Pope for his release. During the remainder of the trial Galileo was accordingly permittedto go back to the ambassador's, where he was most heartily welcomed. Sister Maria Celeste, evidently thinking this meant that the wholecase was at an end, thus expresses herself:--

"The joy that your last dear letter brought me, and the having toread it over and over to the nuns, who made quite a jubilee onhearing its contents, put me into such an excited state that at lastI got a severe attack of headache. "

In his defence Galileo urged that he had already been acquitted in1616 by Cardinal Bellarmine, when a charge of heresy was broughtagainst him, and he contended that anything he might now have done, was no more than he had done on the preceding occasion, when theorthodoxy of his doctrines received solemn confirmation. TheInquisition seemed certainly inclined to clemency, but the Pope wasnot satisfied. Galileo was accordingly summoned again on the 21stJune. He was to be threatened with torture if he did not forthwithgive satisfactory explanations as to the reasons which led him towrite the Dialogue. In this proceeding the Pope assured the Tuscanambassador that he was treating Galileo with the utmost considerationpossible in consequence of his esteem and regard for the Grand Duke, whose servant Galileo was. It was, however, necessary that someexemplary punishment be meted out to the astronomer, inasmuch as bythe publication of the Dialogue he had distinctly disobeyed theinjunction of silence laid upon him by the decree of 1616. Nor wasit admissible for Galileo to plead that his book had been sanctionedby the Master of the Sacred College, to whose inspection it had beenagain and again submitted. It was held, that if the Master of theSacred College had been unaware of the solemn warning the philosopherhad already received sixteen years previously, it was the duty ofGalileo to have drawn his attention to that fact. 

On the 22nd June, 1633, Galileo was led to the great hall of theInquisition, and compelled to kneel before the cardinals thereassembled and hear his sentence. In a long document, mostelaborately drawn up, it is definitely charged against Galileo that, in publishing the Dialogue, he committed the essentially grave errorof treating the doctrine of the earth's motion as open todiscussion. Galileo knew, so the document affirmed, that the Churchhad emphatically pronounced this notion to be contrary to Holy Writ, and that for him to consider a doctrine so stigmatized as having anyshadow of probability in its favour was an act of disrespect to theauthority of the Church which could not be overlooked. It was alsocharged against Galileo that in his Dialogue he has put the strongestarguments into the mouth, not of those who supported the orthodoxdoctrine, but of those who held the theory as to the earth's motionwhich the Church had so deliberately condemned. 

After due consideration of the defence made by the prisoner, it wasthereupon decreed that he had rendered himself vehemently suspectedof heresy by the Holy Office, and in consequence had incurred all thecensures and penalties of the sacred canons, and other decreespromulgated against such persons. The graver portion of thesepunishments would be remitted, if Galileo would solemnly repudiatethe heresies referred to by an abjuration to be pronounced by him inthe terms laid down. 

At the same time it was necessary to mark, in some emphatic manner, the serious offence which had been committed, so that it might serveboth as a punishment to Galileo and as a warning to others. It wasaccordingly decreed that he should be condemned to imprisonment inthe Holy Office during the pleasure of the Papal authorities, andthat he should recite once a week for three years the sevenPenitential Psalms. 

Then followed that ever-memorable scene in the great hall of theInquisition, in which the aged and infirm Galileo, the inventor ofthe telescope and the famous astronomer, knelt down to abjure beforethe most eminent and reverend Lords Cardinal, Inquisitors Generalthroughout the Christian Republic against heretical depravity. Withhis hands on the Gospels, Galileo was made to curse and detest thefalse opinion that the sun was the centre of the universe andimmovable, and that the earth was not the centre of the same, andthat it moved. He swore that for the future he will never say norwrite such things as may bring him under suspicion, and that if hedoes so he submits to all the pains and penalties of the sacredcanons. This abjuration was subsequently read in Florence beforeGalileo's disciples, who had been specially summoned to attend. 

It has been noted that neither on the first occasion, in 1616, nor onthe second in 1633, did the reigning Pope sign the decrees concerningGalileo. The contention has accordingly been made that Paul V. AndUrban VIII. Are both alike vindicated from any technicalresponsibility for the attitude of the Romish Church towards theCopernican doctrines. The significance of this circumstance has beencommented on in connection with the doctrine of the infallibility ofthe Pope. 

We can judge of the anxiety felt by Sister Maria Celeste about herbeloved father during these terrible trials. The wife of theambassador Niccolini, Galileo's steadfast friend, most kindly wroteto give the nun whatever quieting assurances the case would permit. There is a renewed flow of these touching epistles from the daughterto her father. Thus she sends word--

"The news of your fresh trouble has pierced my soul with grief allthe more that it came quite unexpectedly. "

And again, on hearing that he had been permitted to leave Rome, she writes--

"I wish I could describe the rejoicing of all the mothers and sisterson hearing of your happy arrival at Siena. It was indeed mostextraordinary. On hearing the news the Mother Abbess and many of thenuns ran to me, embracing me and weeping for joy and tenderness. "

The sentence of imprisonment was at first interpreted leniently bythe Pope. Galileo was allowed to reside in qualified durance in thearchbishop's house at Siena. Evidently the greatest pain that heendured arose from the forced separation from that daughter, whom hehad at last learned to love with an affection almost comparable withthat she bore to him. She had often told him that she never had anypleasure equal to that with which she rendered any service to herfather. To her joy, she discovers that she can relieve him from thetask of reciting the seven Penitential Psalms which had been imposedas a Penance:--

"I began to do this a while ago, " she writes, "and it gives me muchpleasure. First, because I am persuaded that prayer in obedience toHoly Church must be efficacious; secondly, in order to save you thetrouble of remembering it. If I had been able to do more, mostwillingly would I have entered a straiter prison than the one I livein now, if by so doing I could have set you at liberty. "

[PLATE: CREST OF GALILEO'S FAMILY. ]

Sister Maria Celeste was gradually failing in health, but the greatprivilege was accorded to her of being able once again to embrace herbeloved lord and master. Galileo had, in fact, been permitted toreturn to his old home; but on the very day when he heard of hisdaughter's death came the final decree directing him to remain in hisown house in perpetual solitude. 

Amid the advancing infirmities of age, the isolation from friends, and the loss of his daughter, Galileo once again sought consolationin hard work. He commenced his famous dialogue on Motion. Gradually, however, his sight began to fail, and blindness was at last added tohis other troubles. On January 2nd, 1638, he writes to Diodati:--

"Alas, your dear friend and servant, Galileo, has been for the lastmonth perfectly blind, so that this heaven, this earth, this universewhich I by my marvellous discoveries and clear demonstrations haveenlarged a hundred thousand times beyond the belief of the wise menof bygone ages, henceforward is for me shrunk into such a small spaceas is filled by my own bodily sensations. "

But the end was approaching--the great philosopher, was attacked bylow fever, from which he died on the 8th January, 1643. 

KEPLER. 

While the illustrious astronomer, Tycho Brahe, lay on his death-bed, he had an interview which must ever rank as one of the importantincidents in the history of science. The life of Tycho had beenpassed, as we have seen, in the accumulation of vast stores ofcareful observations of the positions of the heavenly bodies. It wasnot given to him to deduce from his splendid work the results towhich they were destined to lead. It was reserved for anotherastronomer to distil, so to speak, from the volumes in which Tycho'sfigures were recorded, the great truths of the universe which thosefigures contained. Tycho felt that his work required an interpreter, and he recognised in the genius of a young man with whom he wasacquainted the agent by whom the world was to be taught some of thegreat truths of nature. To the bedside of the great Danishastronomer the youthful philosopher was summoned, and with his lastbreath Tycho besought of him to spare no labour in the performance ofthose calculations, by which alone the secrets of the movements ofthe heavens could be revealed. The solemn trust thus imposed wasduly accepted, and the man who accepted it bore the immortal name ofKepler. 

Kepler was born on the 27th December, 1571, at Weil, in the Duchy ofWurtemberg. It would seem that the circumstances of his childhoodmust have been singularly unhappy. His father, sprung from awell-connected family, was but a shiftless and idle adventurer; norwas the great astronomer much more fortunate in his other parent. Hismother was an ignorant and ill-tempered woman; indeed, theill-assorted union came to an abrupt end through the desertion of thewife by her husband when their eldest son John, the hero of ourpresent sketch, was eighteen years old. The childhood of this lad, destined for such fame, was still further embittered by thecircumstance that when he was four years old he had a severe attackof small-pox. Not only was his eyesight permanently injured, buteven his constitution appears to have been much weakened by thisterrible malady. 

It seems, however, that the bodily infirmities of young John Keplerwere the immediate cause of his attention being directed to thepursuit of knowledge. Had the boy been fitted like other boys forordinary manual work, there can be hardly any doubt that to manualwork his life must have been devoted. But, though his body wasfeeble, he soon gave indications of the possession of considerablemental power. It was accordingly thought that a suitable sphere forhis talents might be found in the Church which, in those days, wasalmost the only profession that afforded an opening for anintellectual career. We thus find that by the time John Kepler wasseventeen years old he had attained a sufficient standard ofknowledge to entitle him to admission on the foundation of theUniversity at Tubingen. 

In the course of his studies at this institution he seems to havedivided his attention equally between astronomy and divinity. It notunfrequently happens that when a man has attained considerableproficiency in two branches of knowledge he is not able to see veryclearly in which of the two pursuits his true vocation lies. Hisfriends and onlookers are often able to judge more wisely than hehimself can do as to which Of the two lines it would be better forhim to pursue. This incapacity for perceiving the path in whichgreatness awaited him, existed in the case of Kepler. Personally, heinclined to enter the ministry, in which a promising career seemedopen to him. He yielded, however, to friends, who evidently knew himbetter than he knew himself, and accepted in 1594, the importantProfessorship of astronomy which had been offered to him in theUniversity of Gratz. 

It is difficult for us in these modern days to realise the somewhatextraordinary duties which were expected from an astronomicalprofessor in the sixteenth century. He was, of course, required toemploy his knowledge of the heavens in the prediction of eclipses, and of the movements of the heavenly bodies generally. This seemsreasonable enough; but what we are not prepared to accept is theobligation which lay on the astronomers to predict the fates ofnations and the destinies of individuals. 

It must be remembered that it was the almost universal belief inthose days, that all the celestial spheres revolved in somemysterious fashion around the earth, which appeared by far the mostimportant body in the universe. It was imagined that the sun, themoon, and the stars indicated, in the vicissitudes of theirmovements, the careers of nations and of individuals. Such being thegenerally accepted notion, it seemed to follow that a professor whowas charged with the duty of expounding the movements of the heavenlybodies must necessarily be looked to for the purpose of decipheringthe celestial decrees regarding the fate of man which the heavenlyluminaries were designed to announce. 

Kepler threw himself with characteristic ardour into even thisfantastic phase of the labours of the astronomical professor; hediligently studied the rules of astrology, which the fancies ofantiquity had compiled. Believing sincerely as he did in theconnection between the aspect of the stars and the state of humanaffairs, he even thought that he perceived, in the events of his ownlife, a corroboration of the doctrine which affirmed the influence ofthe planets upon the fate of individuals. 

[PLATE: KEPLER'S SYSTEM OF REGULAR SOLIDS. ]

But quite independently of astrology there seem to have been manyother delusions current among the philosophers of Kepler's time. Itis now almost incomprehensible how the ablest men of a few centuriesago should have entertained such preposterous notions, as they did, with respect to the system of the universe. As an instance of whatis here referred to, we may cite the extraordinary notion which, under the designation of a discovery, first brought Kepler intofame. Geometers had long known that there were five, but no morethan five, regular solid figures. There is, for instance, the cubewith six sides, which is, of course, the most familiar of thesesolids. Besides the cube there are other figures of four, eight, twelve, and twenty sides respectively. It also happened that therewere five planets, but no more than five, known to the ancients, namely, Mercury, Venus, Mars, Jupiter, and Saturn. To Kepler'slively imaginations this coincidence suggested the idea that the fiveregular solids corresponded to the five planets, and a number offancied numerical relations were adduced on the subject. Theabsurdity of this doctrine is obvious enough, especially when weobserve that, as is now well known, there are two large planets, anda host of small planets, over and above the magical number of theregular solids. In Kepler's time, however, this doctrine was so farfrom being regarded as absurd, that its announcement was hailed as agreat intellectual triumph. Kepler was at once regarded withfavour. It seems, indeed, to have been the circumstance whichbrought him into correspondence with Tycho Brahe. By its means alsohe became known to Galileo. 

The career of a scientific professor in those early days appearsgenerally to have been marked by rather more striking vicissitudesthan usually befall a professor in a modern university. Kepler was aProtestant, and as such he had been appointed to his professorship atGratz. A change, however, having taken place in the religious beliefentertained by the ruling powers of the University, the Protestantprofessors were expelled. It seems that special influence havingbeen exerted in Kepler's case on account of his exceptional eminence, he was recalled to Gratz and reinstated in the tenure of his chair. But his pupils had vanished, so that the great astronomer was glad toaccept a post offered him by Tycho Brahe in the observatory which thelatter had recently established near Prague. 

On Tycho's death, which occurred soon after, an opening presenteditself which gave Kepler the opportunity his genius demanded. He wasappointed to succeed Tycho in the position of imperial mathematician. But a far more important point, both for Kepler and for science, was that to him was confided the use of Tycho's observations. It was, indeed, by the discussion of Tycho's results that Kepler was enabledto make the discoveries which form such an important part ofastronomical history. 

Kepler must also be remembered as one of the first great astronomerswho ever had the privilege of viewing celestial bodies through atelescope. It was in 1610 that he first held in his hands one ofthose little instruments which had been so recently applied to theheavens by Galileo. It should, however, be borne in mind that theepoch-making achievements of Kepler did not arise from any telescopicobservations that he made, or, indeed, that any one else made. Theywere all elaborately deduced from Tycho's measurements of thepositions of the planets, obtained with his great instruments, whichwere unprovided with telescopic assistance. 

To realise the tremendous advance which science received fromKepler's great work, it is to be understood that all the astronomerswho laboured before him at the difficult subject of the celestialmotions, took it for granted that the planets must revolve incircles. If it did not appear that a planet moved in a fixed circle, then the ready answer was provided by Ptolemy's theory that thecircle in which the planet did move was itself in motion, so that itscentre described another circle. 

When Kepler had before him that wonderful series of observations ofthe planet, Mars, which had been accumulated by the extraordinaryskill of Tycho, he proved, after much labour, that the movements ofthe planet refused to be represented in a circular form. Nor wouldit do to suppose that Mars revolved in one circle, the centre ofwhich revolved in another circle. On no such supposition could themovements of the planets be made to tally with those which Tycho hadactually observed. This led to the astonishing discovery of the trueform of a planet's orbit. For the first time in the history ofastronomy the principle was laid down that the movement of a planetcould not be represented by a circle, nor even by combinations ofcircles, but that it could be represented by an elliptic path. Inthis path the sun is situated at one of those two points in theellipse which are known as its foci. 

[PLATE: KEPLER. ]

Very simple apparatus is needed for the drawing of one of thoseellipses which Kepler has shown to possess such astonishingastronomical significance. Two pins are stuck through a sheet ofpaper on a board, the point of a pencil is inserted in a loop ofstring which passes over the pins, and as the pencil is moved roundin such a way as to keep the string stretched, that beautiful curveknown as the ellipse is delineated, while the positions of the pinsindicate the two foci of the curve. If the length of the loop ofstring is unchanged then the nearer the pins are together, thegreater will be the resemblance between the ellipse and the circle, whereas the more the pins are separated the more elongated does theellipse become. The orbit of a great planet is, in general, one ofthose ellipses which approaches a nearly circular form. Itfortunately happens, however, that the orbit of Mars makes a widerdeparture from the circular form than any of the other importantplanets. It is, doubtless, to this circumstance that we mustattribute the astonishing success of Kepler in detecting the trueshape of a planetary orbit. Tycho's observations would not have beensufficiently accurate to have exhibited the elliptic nature of aplanetary orbit which, like that of Venus, differed very little froma circle. 

The more we ponder on this memorable achievement the more strikingwill it appear. It must be remembered that in these days we know ofthe physical necessity which requires that a planet shall revolve inan ellipse and not in any other curve. But Kepler had no suchknowledge. Even to the last hour of his life he remained inignorance of the existence of any natural cause which ordained thatplanets should follow those particular curves which geometers know sowell. Kepler's assignment of the ellipse as the true form of theplanetary orbit is to be regarded as a brilliant guess, the truth ofwhich Tycho's observations enabled him to verify. Kepler alsosucceeded in pointing out the law according to which the velocity ofa planet at different points of its path could be accuratelyspecified. Here, again, we have to admire the sagacity with whichthis marvellously acute astronomer guessed the deep truth of nature. In this case also he was quite unprovided with any reason forexpecting from physical principles that such a law as he discoveredmust be obeyed. It is quite true that Kepler had some slightknowledge of the existence of what we now know as gravitation. Hehad even enunciated the remarkable doctrine that the ebb and flow ofthe tide must be attributed to the attraction of the moon on thewaters of the earth. He does not, however, appear to have had anyanticipation of those wonderful discoveries which Newton was destinedto make a little later, in which he demonstrated that the lawsdetected by Kepler's marvellous acumen were necessary consequences ofthe principle of universal gravitation. 

[PLATE: SYMBOLICAL REPRESENTATION OF THE PLANETARY SYSTEM. ]

To appreciate the relations of Kepler and Tycho it is necessary tonote the very different way in which these illustrious astronomersviewed the system of the heavens. It should be observed thatCopernicus had already expounded the true system, which located thesun at the centre of the planetary system. But in the days of TychoBrahe this doctrine had not as yet commanded universal assent. Infact, the great observer himself did not accept the new views ofCopernicus. It appeared to Tycho that the earth not only appeared tobe the centre of things celestial, but that it actually was thecentre. It is, indeed, not a little remarkable that a student of theheavens so accurate as Tycho should have deliberately rejected theCopernican doctrine in favour of the system which now seems sopreposterous. Throughout his great career, Tycho steadily observedthe places of the sun, the moon, and the planets, and as steadilymaintained that all those bodies revolved around the earth fixed inthe centre. Kepler, however, had the advantage of belonging to thenew school. He utilised the observations of Tycho in developing thegreat Copernican theory whose teaching Tycho stoutly resisted. 

Perhaps a chapter in modern science may illustrate the intellectualrelation of these great men. The revolution produced by Copernicusin the doctrine of the heavens has often been likened to therevolution which the Darwinian theory produced in the views held bybiologists as to life on this earth. The Darwinian theory did not atfirst command universal assent even among those naturalists whoselives had been devoted with the greatest success to the study oforganisms. Take, for instance, that great naturalist, ProfessorOwen, by whose labours vast extension has been given to our knowledgeof the fossil animals which dwelt on the earth in past ages. Now, though Owens researches were intimately connected with the greatlabours of Darwin, and afforded the latter material for hisepoch-making generalization, yet Owen deliberately refused to acceptthe new doctrines. Like Tycho, he kept on rigidly accumulating hisfacts under the influence of a set of ideas as to the origin ofliving forms which are now universally admitted to be erroneous. If, therefore, we liken Darwin to Copernicus, and Owen to Tycho, we mayliken the biologists of the present day to Kepler, who interpretedthe results of accurate observation upon sound theoreticalprinciples. 

In reading the works of Kepler in the light of our modern knowledgewe are often struck by the extent to which his perception of thesublimest truths in nature was associated with the most extravaganterrors and absurdities. But, of course, it must be remembered thathe wrote in an age in which even the rudiments of science, as we nowunderstand it, were almost entirely unknown. 

It may well be doubted whether any joy experienced by mortals is moregenuine than that which rewards the successful searcher after naturaltruths. Every science-worker, be his efforts ever so humble, will beable to sympathise with the enthusiastic delight of Kepler when atlast, after years of toil, the glorious light broke forth, and thatwhich he considered to be the greatest of his astonishing laws firstdawned upon him. Kepler rightly judged that the number of days whicha planet required to perform its voyage round the sun must beconnected in some manner with the distance from the planet to thesun; that is to say, with the radius of the planet's orbit, inasmuchas we may for our present object regard the planet's orbit ascircular. 

Here, again, in his search for the unknown law, Kepler had noaccurate dynamical principles to guide his steps. Of course, we nowknow not only what the connection between the planet's distance andthe planet's periodic time actually is, but we also know that it is anecessary consequence of the law of universal gravitation. Kepler, it is true, was not without certain surmises on the subject, but theywere of the most fanciful description. His notions of the planets, accurate as they were in certain important respects, were mixed upwith vague ideas as to the properties of metals and the geometricalrelations of the regular solids. Above all, his reasoning waspenetrated by the supposed astrological influences of the stars andtheir significant relation to human fate. Under the influence ofsuch a farrago of notions, Kepler resolved to make all sorts oftrials in his search for the connection between the distance of aplanet from the sun and the time in which the revolution of thatplanet was accomplished. 

It was quite easily demonstrated that the greater the distance of theplanet from the sun the longer was the time required for itsjourney. It might have been thought that the time would be directlyproportional to the distance. It was, however, easy to show thatthis supposition did not agree with the fact. Finding that thissimple relation would not do, Kepler undertook a vast series ofcalculations to find out the true method of expressing theconnection. At last, after many vain attempts, he found, to hisindescribable joy, that the square of the time in which a planetrevolves around the sun was proportional to the cube of the averagedistance of the planet from that body. 

The extraordinary way in which Kepler's views on celestial matterswere associated with the wildest speculations, is well illustrated inthe work in which he propounded his splendid discovery just referredto. The announcement of the law connecting the distances of theplanets from the sun with their periodic times, was then mixed upwith a preposterous conception about the properties of the differentplanets. They were supposed to be associated with some profoundmusic of the spheres inaudible to human ears, and performed only forthe benefit of that being whose soul formed the animating spirit ofthe sun. 

Kepler was also the first astronomer who ever ventured to predict theoccurrence of that remarkable phenomenon, the transit of a planet infront of the sun's disc. He published, in 1629, a notice to thecurious in things celestial, in which he announced that both of theplanets, Mercury and Venus, were to make a transit across the sun onspecified days in the winter of 1631. The transit of Mercury wasduly observed by Gassendi, and the transit of Venus also took place, though, as we now know, the circumstances were such that it was notpossible for the phenomenon to be witnessed by any Europeanastronomer. 

In addition to Kepler's discoveries already mentioned, with which hisname will be for ever associated, his claim on the gratitude ofastronomers chiefly depends on the publication of his famousRudolphine tables. In this remarkable work means are provided forfinding the places of the planets with far greater accuracy than hadpreviously been attainable. 

Kepler, it must be always remembered, was not an astronomicalobserver. It was his function to deal with the observations made byTycho, and, from close study and comparison of the results, to workout the movements of the heavenly bodies. It was, in fact, Tycho whoprovided as it were the raw material, while it was the genius ofKepler which wrought that material into a beautiful and serviceableform. For more than a century the Rudolphine tables were regarded asa standard astronomical work. In these days we are accustomed tofind the movements of the heavenly bodies set forth with alldesirable exactitude in the NAUTICAL ALMANACK, and the similarpublication issued by foreign Governments. Let it be remembered thatit was Kepler who first imparted the proper impulse in thisdirection. 

[PLATE: THE COMMEMORATION OF THE RUDOLPHINE TABLES. ]

When Kepler was twenty-six he married an heiress from Styria, who, though only twenty-three years old, had already had some experiencein matrimony. Her first husband had died; and it was after hersecond husband had divorced her that she received the addresses ofKepler. It will not be surprising to hear that his domestic affairsdo not appear to have been particularly happy, and his wife died in1611. Two years later, undeterred by the want of success in hisfirst venture, he sought a second partner, and he evidentlydetermined not to make a mistake this time. Indeed, the methodicalmanner in which he made his choice of the lady to whom he shouldpropose has been duly set forth by him and preserved for ouredification. With some self-assurance he asserts that there were nofewer than eleven spinsters desirous of sharing his joys andsorrows. He has carefully estimated and recorded the merits anddemerits of each of these would-be brides. The result of hisdeliberations was that he awarded himself to an orphan girl, destitute even of a portion. Success attended his choice, and hissecond marriage seems to have proved a much more suitable union thanhis first. He had five children by the first wife and seven by thesecond. 

The years of Kepler's middle life were sorely distracted by a troublewhich, though not uncommon in those days, is one which we find itdifficult to realise at the present time. His mother, CatherineKepler, had attained undesirable notoriety by the suspicion that shewas guilty of witchcraft. Years were spent in legal investigations, and it was only after unceasing exertions on the part of theastronomer for upwards of a twelvemonth that he was finally able toprocure her acquittal and release from prison. 

It is interesting for us to note that at one time there was aproposal that Kepler should forsake his native country and adoptEngland as a home. It arose in this wise. The great man wasdistressed throughout the greater part of his life by pecuniaryanxieties. Finding him in a strait of this description, the Englishambassador in Venice, Sir Henry Wotton, in the year 1620, besoughtKepler to come over to England, where he assured him that he wouldobtain a favourable reception, and where, he was able to add, Kepler's great scientific work was already highly esteemed. But hisefforts were unavailing; Kepler would not leave his own country. Hewas then forty-nine years of age, and doubtless a home in a foreignland, where people spoke a strange tongue, had not sufficientattraction for him, even when accompanied with the substantialinducements which the ambassador was able to offer. Had Kepleraccepted this invitation, he would, in transferring his home toEngland, have anticipated the similar change which took place in thecareer of another great astronomer two centuries later. It will beremembered that Herschel, in his younger days, did transfer himselfto England, and thus gave to England the imperishable fame ofassociation with his triumphs. 

The publication of the Rudolphine tables of the celestial movementsentailed much expense. A considerable part of this was defrayed bythe Government at Venice but the balance occasioned no little troubleand anxiety to Kepler. No doubt the authorities of those days wereeven less Willing to spend money on scientific matters than are theGovernments of more recent times. For several years the imperialTreasury was importuned to relieve him from his anxieties. Theeffects of so much worry, and of the long journeys which wereinvolved, at last broke down Kepler's health completely. As we havealready mentioned, he had never been strong from infancy, and hefinally succumbed to a fever in November, 1630, at the age offifty-nine. He was interred at St. Peter's Church at Ratisbon. 

Though Kepler had not those personal characteristics which have madehis great predecessor, Tycho Brahe, such a romantic figure, yet apicturesque element in Kepler's character is not wanting. It was, however, of an intellectual kind. His imagination, as well as hisreasoning faculties, always worked together. He was incessantlyprompted by the most extraordinary speculations. The great majorityof them were in a high degree wild and chimerical, but every now andthen one of his fancies struck right to the heart of nature, and animmortal truth was brought to light. 

I remember visiting the observatory of one of our greatest modernastronomers, and in a large desk he showed me a multitude ofphotographs which he had attempted but which had not been successful, and then he showed me the few and rare pictures which had succeeded, and by which important truths had been revealed. With a felicity ofexpression which I have often since thought of, he alluded to thecontents of the desk as the "chips. " They were useless, but theywere necessary incidents in the truly successful work. So it is inall great and good work. Even the most skilful man of sciencepursues many a wrong scent. Time after time he goes off on sometrack that plays him false. The greater the man's genius andintellectual resource, the more numerous will be the ventures whichhe makes, and the great majority of those ventures are certain to befruitless. They are in fact, the "chips. " In Kepler's case thechips were numerous enough. They were of the most extraordinaryvariety and structure. But every now and then a sublime discoverywas made of such a character as to make us regard even the mostfantastic of Kepler's chips with the greatest veneration and respect. 

ISAAC NEWTON. 

It was just a year after the death of Galileo, that an infant cameinto the world who was christened Isaac Newton. Even the great fameof Galileo himself must be relegated to a second place in comparisonwith that of the philosopher who first expounded the true theory ofthe universe. 

Isaac Newton was born on the 25th of December (old style), 1642, atWoolsthorpe, in Lincolnshire, about a half-mile from Colsterworth, and eight miles south of Grantham. His father, Mr. Isaac Newton, haddied a few months after his marriage to Harriet Ayscough, thedaughter of Mr. James Ayscough, of Market Overton, in Rutlandshire. The little Isaac was at first so excessively frail and weakly thathis life was despaired of. The watchful mother, however, tended herdelicate child with such success that he seems to have thriven betterthan might have been expected from the circumstances of his infancy, and he ultimately acquired a frame strong enough to outlast theordinary span of human life. 

For three years they continued to live at Woolsthorpe, the widow'smeans of livelihood being supplemented by the income from anothersmall estate at Sewstern, in a neighbouring part of Leicestershire. 

[PLATE: WOOLSTHORPE MANOR. Showing solar dial made by Newton when a boy. ]

In 1645, Mrs. Newton took as a second husband the Rev. BarnabasSmith, and on moving to her new home, about a mile from Woolsthorpe, she entrusted little Isaac to her mother, Mrs. Ayscough. In duetime we find that the boy was sent to the public school at Grantham, the name of the master being Stokes. For the purpose of being nearhis work, the embryo philosopher was boarded at the house of Mr. Clark, an apothecary at Grantham. We learn from Newton himself thatat first he had a very low place in the class lists of the school, and was by no means one of those model school-boys who find favour inthe eyes of the school-master by attention to Latin grammar. Isaac'sfirst incentive to diligent study seems to have been derived from thecircumstance that he was severely kicked by one of the boys who wasabove him in the class. This indignity had the effect of stimulatingyoung Newton's activity to such an extent that he not only attainedthe desired object of passing over the head of the boy who hadmaltreated him, but continued to rise until he became the head of theschool. 

The play-hours of the great philosopher were devoted to pursuits verydifferent from those of most school-boys. His chief amusement wasfound in making mechanical toys and various ingenious contrivances. He watched day by day with great interest the workmen engaged inconstructing a windmill in the neighbourhood of the school, theresult of which was that the boy made a working model of the windmilland of its machinery, which seems to have been much admired, asindicating his aptitude for mechanics. We are told that Isaac alsoindulged in somewhat higher flights of mechanical enterprise. Heconstructed a carriage, the wheels of which were to be driven by thehands of the occupant, while the first philosophical instrument hemade was a clock, which was actuated by water. He also devoted muchattention to the construction of paper kites, and his skill in thisrespect was highly appreciated by his schoolfellows. Like a truephilosopher, even at this stage he experimented on the best methodsof attaching the string, and on the proportions which the tail oughtto have. He also made lanthorns of paper to provide himself withlight as he walked to school in the dark winter mornings. 

The only love affair in Newton's life appears to have commenced whilehe was still of tender years. The incidents are thus described inBrewster's "Life of Newton, " a work to which I am much indebted inthis chapter. 

"In the house where he lodged there were some female inmates, inwhose company he appears to have taken much pleasure. One of these, a Miss Storey, sister to Dr. Storey, a physician at Buckminster, nearColsterworth, was two or three years younger than Newton and to greatpersonal attractions she seems to have added more than the usualallotment of female talent. The society of this young lady and hercompanions was always preferred to that of his own school-fellows, and it was one of his most agreeable occupations to construct forthem little tables and cupboards, and other utensils for holdingtheir dolls and their trinkets. He had lived nearly six years in thesame house with Miss Storey, and there is reason to believe thattheir youthful friendship gradually rose to a higher passion; but thesmallness of her portion, and the inadequacy of his own fortune, appear to have prevented the consummation of their happiness. MissStorey was afterwards twice married, and under the name of Mrs. Vincent, Dr. Stukeley visited her at Grantham in 1727, at the age ofeighty-two, and obtained from her many particulars respecting theearly history of our author. Newton's esteem for her continuedunabated during his life. He regularly visited her when he went toLincolnshire, and never failed to relieve her from little pecuniarydifficulties which seem to have beset her family. "

The schoolboy at Grantham was only fourteen years of age when hismother became a widow for the second time. She then returned to theold family home at Woolsthorpe, bringing with her the three childrenof her second marriage. Her means appear to have been somewhatscanty, and it was consequently thought necessary to recall Isaacfrom the school. His recently-born industry had been such that hehad already made good progress in his studies, and his mother hopedthat he would now lay aside his books, and those silent meditationsto which, even at this early age, he had become addicted. It wasexpected that, instead of such pursuits, which were deemed quiteuseless, the boy would enter busily into the duties of the farm andthe details of a country life. But before long it became manifestthat the study of nature and the pursuit of knowledge had such afascination for the youth that he could give little attention toaught else. It was plain that he would make but an indifferentfarmer. He greatly preferred experimenting on his water-wheels tolooking after labourers, while he found that working at mathematicsbehind a hedge was much more interesting than chaffering about theprice of bullocks in the market place. Fortunately for humanity hismother, like a wise woman, determined to let her boy's genius havethe scope which it required. He was accordingly sent back toGrantham school, with the object of being trained in the knowledgewhich would fit him for entering the University of Cambridge. 

[PLATE: TRINITY COLLEGE, CAMBRIDGE. Showing Newton's rooms; on the leads of the gateway he placedhis telescope. ]

It was the 5th of June, 1660, when Isaac Newton, a youth of eighteen, was enrolled as an undergraduate of Trinity College, Cambridge. Little did those who sent him there dream that this boy was destinedto be the most illustrious student who ever entered the portals ofthat great seat of learning. Little could the youth himself haveforeseen that the rooms near the gateway which he occupied wouldacquire a celebrity from the fact that he dwelt in them, or that theante-chapel of his college was in good time to be adorned by thatnoble statue, which is regarded as one of the chief art treasures ofCambridge University, both on account of its intrinsic beauty and thefact that it commemorates the fame of her most distinguished alumnus, Isaac Newton, the immortal astronomer. Indeed, his advent at theUniversity seemed to have been by no means auspicious or brilliant. His birth was, as we have seen, comparatively obscure, and though hehad already given indication of his capacity for reflecting onphilosophical matters, yet he seems to have been but ill-equippedwith the routine knowledge which youths are generally expected totake with them to the Universities. 

From the outset of his college career, Newton's attention seems tohave been mainly directed to mathematics. Here he began to giveevidence of that marvellous insight into the deep secrets of naturewhich more than a century later led so dispassionate a judge asLaplace to pronounce Newton's immortal work as pre-eminent above allthe productions of the human intellect. But though Newton was one ofthe very greatest mathematicians that ever lived, he was never amathematician for the mere sake of mathematics. He employed hismathematics as an instrument for discovering the laws of nature. Hisindustry and genius soon brought him under the notice of theUniversity authorities. It is stated in the University records thathe obtained a Scholarship in 1664. Two years later we find thatNewton, as well as many residents in the University, had to leaveCambridge temporarily on account of the breaking out of the plague. The philosopher retired for a season to his old home at Woolsthorpe, and there he remained until he was appointed a Fellow of TrinityCollege, Cambridge, in 1667. From this time onwards, Newton'sreputation as a mathematician and as a natural philosopher steadilyadvanced, so that in 1669, while still but twenty-seven years of age, he was appointed to the distinguished position of Lucasian Professorof Mathematics at Cambridge. Here he found the opportunity tocontinue and develop that marvellous career of discovery which formedhis life's work. 

The earliest of Newton's great achievements in natural philosophy washis detection of the composite character of light. That a beam ofordinary sunlight is, in fact, a mixture of a very great number ofdifferent-coloured lights, is a doctrine now familiar to every onewho has the slightest education in physical science. We must, however, remember that this discovery was really a tremendous advancein knowledge at the time when Newton announced it. 

[PLATE: DIAGRAM OF A SUNBEAM. ]

We here give the little diagram originally drawn by Newton, toexplain the experiment by which he first learned the composition oflight. A sunbeam is admitted into a darkened room through anopening, H, in a shutter. This beam when not interfered with willtravel in a straight line to the screen, and there reproduce a brightspot of the same shape as the hole in the shutter. If, however, aprism of glass, A B C, be introduced so that the beam traverse it, then it will be seen at once that the light is deflected from itsoriginal track. There is, however, a further and most importantchange which takes place. The spot of light is not alone removed toanother part of the screen, but it becomes spread out into a longband beautifully coloured, and exhibiting the hues of the rainbow. Atthe top are the violet rays, and then in descending order we have theindigo, blue, green, yellow, orange, and red. 

The circumstance in this phenomenon which appears to haveparticularly arrested Newton's attention, was the elongation whichthe luminous spot underwent in consequence of its passage through theprism. When the prism was absent the spot was nearly circular, butwhen the prism was introduced the spot was about five times as longas it was broad. To ascertain the explanation of this was the firstproblem to be solved. It seemed natural to suppose that it might bedue to the thickness of the glass in the prism which the lighttraversed, or to the angle of incidence at which the light fell uponthe prism. He found, however, upon careful trial, that the phenomenoncould not be thus accounted for. It was not until after much patientlabour that the true explanation dawned upon him. He discovered thatthough the beam of white light looks so pure and so simple, yet inreality it is composed of differently coloured lights blendedtogether. These are, of course, indistinguishable in the compoundbeam, but they are separated or disentangled, so to speak, by theaction of the prism. The rays at the blue end of the spectrum aremore powerfully deflected by the action of the glass than are therays at the red end. Thus, the rays variously coloured red, orange, yellow, green, blue, indigo, violet, are each conducted to adifferent part of the screen. In this way the prism has the effectof exhibiting the constitution of the composite beam of light. 

To us this now seems quite obvious, but Newton did not adopt ithastily. With characteristic caution he verified the explanation bymany different experiments, all of which confirmed his discovery. Oneof these may be mentioned. He made a hole in the screen at that parton which the violet rays fell. Thus a violet ray was allowed to passthrough, all the rest of the light being intercepted, and on thisbeam so isolated he was able to try further experiments. Forinstance, when he interposed another prism in its path, he found, ashe expected, that it was again deflected, and he measured the amountof the deflection. Again he tried the same experiment with one ofthe red rays from the opposite end of the coloured band. He allowedit to pass through the same aperture in the screen, and he tested theamount by which the second prism was capable of producing deflection. He thus found, as he had expected to find, that the second prism wasmore efficacious in bending the violet rays than in bending the redrays. Thus he confirmed the fact that the various hues of therainbow were each bent by a prism to a different extent, violet beingacted upon the most, and red the least. 

[PLATE: ISAAC NEWTON. ]

Not only did Newton decompose a white beam into its constituentcolours, but conversely by interposing a second prism with its angleturned upwards, he reunited the different colours, and thusreproduced the original beam of white light. In several other waysalso he illustrated his famous proposition, which then seemed sostartling, that white light was the result of a mixture of all huesof the rainbow. By combining painters' colours in the rightproportion he did not indeed succeed in producing a mixture whichwould ordinarily be called white, but he obtained a grey pigment. Some of this he put on the floor of his room for comparison with apiece of white paper. He allowed a beam of bright sunlight to fallupon the paper and the mixed colours side by side, and a friend hecalled in for his opinion pronounced that under these circumstancesthe mixed colours looked the whiter of the two. 

By repeated demonstrations Newton thus established his greatdiscovery of the composite character of light. He at once perceivedthat his researches had an important bearing upon the principlesinvolved in the construction of a telescope. Those who employed thetelescope for looking at the stars, had been long aware of theimperfections which prevented all the various rays from beingconducted to the same focus. But this imperfection had hitherto beenerroneously accounted for. It had been supposed that the reason whysuccess had not been attained in the construction of a refractingtelescope was due to the fact that the object glass, made as it thenwas of a single piece, had not been properly shaped. Mathematicianshad abundantly demonstrated that a single lens, if properly figured, must conduct all rays of light to the same focus, provided all raysexperienced equal refraction in passing through the glass. UntilNewton's discovery of the composition of white light, it had beentaken for granted that the several rays in a white beam were equallyrefrangible. No doubt if this had been the case, a perfect telescopecould have been produced by properly shaping the object glass. Butwhen Newton had demonstrated that light was by no means so simple ashad been supposed, it became obvious that a satisfactory refractingtelescope was an impossibility when only a single object lens wasemployed, however carefully that lens might have been wrought. Suchan objective might, no doubt, be made to conduct any one group ofrays of a particular shade to the same focus, but the rays of othercolours in the beam of white light must necessarily travel somewhatastray. In this way Newton accounted for a great part of thedifficulties which had hitherto beset the attempts to construct aperfect refracting telescope. 

We now know how these difficulties can be, to a great extent, overcome, by employing for the objective a composite lens made of twopieces of glass possessing different qualities. To these achromaticobject glasses, as they are called, the great development ofastronomical knowledge, since Newton's time, is due. But it must beremarked that, although the theoretical possibility of constructingan achromatic lens was investigated by Newton, he certainly came tothe conclusion that the difficulty could not be removed by employinga composite objective, with two different kinds of glass. In thishis marvellous sagacity in the interpretation of nature seems foronce to have deserted him. We can, however, hardly regret thatNewton failed to discover the achromatic objective, when we observethat it was in consequence of his deeming an achromatic objective tobe impossible that he was led to the invention of the reflectingtelescope. Finding, as he believed, that the defects of thetelescope could not be remedied by any application of the principleof refraction he was led to look in quite a different direction forthe improvement of the tool on which the advancement of astronomydepended. The REFRACTION of light depended as he had found, upon thecolour of the light. The laws of REFLECTION were, however, quiteindependent of the colour. Whether rays be red or green, blue oryellow, they are all reflected in precisely the same manner from amirror. Accordingly, Newton perceived that if he could construct atelescope the action of which depended upon reflection, instead ofupon refraction, the difficulty which had hitherto proved aninsuperable obstacle to the improvement of the instrument would beevaded. 

[PLATE: SIR ISAAC NEWTON'S LITTLE REFLECTOR. ]

For this purpose Newton fashioned a concave mirror from a mixture ofcopper and tin, a combination which gives a surface with almost thelustre of silver. When the light of a star fell upon the surface, animage of the star was produced in the focus of this mirror, and thenthis image was examined by a magnifying eye-piece. Such is theprinciple of the famous reflecting telescope which bears the name ofNewton. The little reflector which he constructed, represented inthe adjoining figure, is still preserved as one of the treasures ofthe Royal Society. The telescope tube had the very modest dimensionof one inch in diameter. It was, however, the precursor of a wholeseries of magnificent instruments, each outstripping the other inmagnitude, until at last the culminating point was attained in 1845, by the construction of Lord Rosse's mammoth reflector of six feet inaperture. 

Newton's discovery of the composition of light led to an embitteredcontroversy, which caused no little worry to the great Philosopher. Some of those who attacked him enjoyed considerable and, it must beadmitted, even well-merited repute in the ranks of science. Theyalleged, however, that the elongation of the coloured band whichNewton had noticed was due to this, to that, or to the other--toanything, in fact, rather than to the true cause which Newtonassigned. With characteristic patience and love of truth, Newtonsteadily replied to each such attack. He showed most completely howutterly his adversaries had misunderstood the subject, and how slightindeed was their acquaintance with the natural phenomenon inquestion. In reply to each point raised, he was ever able to citefresh experiments and adduce fresh illustrations, until at last hisopponents retired worsted from the combat. 

It has been often a matter for surprise that Newton, throughout hiswhole career, should have taken so much trouble to expose the errorsof those who attacked his views. He used even to do this when itplainly appeared that his adversaries did not understand the subjectthey were discussing. A philosopher might have said, "I know I amright, and whether others think I am right or not may be a matter ofconcern to them, but it is certainly not a matter about which I needtrouble. If after having been told the truth they elect to remain inerror, so much the worse for them; my time can be better employedthan in seeking to put such people right. " This, however, was notNewton's method. He spent much valuable time in overthrowingobjections which were often of a very futile description. Indeed, hesuffered a great deal of annoyance from the persistency, and in somecases one might almost say from the rancour, of the attacks whichwere made upon him. Unfortunately for himself, he did not possessthat capacity for sublime indifference to what men may say, which isoften the happy possession of intellects greatly inferior to his. 

The subject of optics still continuing to engross Newton's attention, he followed up his researches into the structure of the sunbeam bymany other valuable investigations in connection with light. Everyone has noticed the beautiful colours manifested in a soap-bubble. Here was a subject which not unnaturally attracted the attention ofone who had expounded the colours of the spectrum with such success. He perceived that similar hues were produced by other thin plates oftransparent material besides soap-bubbles, and his ingenuity wassufficient to devise a method by which the thicknesses of thedifferent films could be measured. We can hardly, indeed, say that alike success attended his interpretation of these phenomena to thatwhich had been so conspicuous in his explanation of the spectrum. Itimplies no disparagement to the sublime genius of Newton to admitthat the doctrines he put forth as to the causes of the colours inthe soap-bubbles can be no longer accepted. We must remember thatNewton was a pioneer in accounting for the physical properties oflight. The facts that he established are indeed unquestionable, butthe explanations which he was led to offer of some of them are seento be untenable in the fuller light of our present knowledge. 

[PLATE: SIR ISAAC NEWTON'S SUN-DIAL. ]

Had Newton done nothing beyond making his wonderful discoveries inlight, his fame would have gone down to posterity as one of thegreatest of Nature's interpreters. But it was reserved for him toaccomplish other discoveries, which have pushed even his analysis ofthe sunbeam into the background; it is he who has expounded thesystem of the universe by the discovery of the law of universalgravitation. 

The age had indeed become ripe for the advent of the genius ofNewton. Kepler had discovered with marvellous penetration the lawswhich govern the movements of the planets around the sun, and invarious directions it had been more or less vaguely felt that theexplanation of Kepler's laws, as well as of many other phenomena, must be sought for in connection with the attractive power ofmatter. But the mathematical analysis which alone could deal withthis subject was wanting; it had to be created by Newton. 

At Woolsthorpe, in the year 1666, Newton's attention appears to havebeen concentrated upon the subject of gravitation. Whatever may bethe extent to which we accept the more or less mythical story as tohow the fall of an apple first directed the attention of thephilosopher to the fact that gravitation must extend through space, it seems, at all events, certain that this is an excellentillustration of the line of reasoning which he followed. He arguedin this way. The earth attracts the apple; it would do so, no matterhow high might be the tree from which that apple fell. It would thenseem to follow that this power which resides in the earth by which itcan draw all external bodies towards it, extends far beyond thealtitude of the loftiest tree. Indeed, we seem to find no limit toit. At the greatest elevation that has ever been attained, theattractive power of the earth is still exerted, and though we cannotby any actual experiment reach an altitude more than a few milesabove the earth, yet it is certain that gravitation would extend toelevations far greater. It is plain, thought Newton, that an applelet fall from a point a hundred miles above this earth's surface, would be drawn down by the attraction, and would continually gatherfresh velocity until it reached the ground. From a hundred miles itwas natural to think of what would happen at a thousand miles, or athundreds of thousands of miles. No doubt the intensity of theattraction becomes weaker with every increase in the altitude, butthat action would still exist to some extent, however lofty might bethe elevation which had been attained. 

It then occurred to Newton, that though the moon is at a distance oftwo hundred and forty thousand miles from the earth, yet theattractive power of the earth must extend to the moon. He wasparticularly led to think of the moon in this connection, not onlybecause the moon is so much closer to the earth than are any othercelestial bodies, but also because the moon is an appendage to theearth, always revolving around it. The moon is certainly attractedto the earth, and yet the moon does not fall down; how is this to beaccounted for? The explanation was to be found in the character ofthe moon's present motion. If the moon were left for a moment atrest, there can be no doubt that the attraction of the earth wouldbegin to draw the lunar globe in towards our globe. In the course ofa few days our satellite would come down on the earth with a mostfearful crash. This catastrophe is averted by the circumstance thatthe moon has a movement of revolution around the earth. Newton wasable to calculate from the known laws of mechanics, which he hadhimself been mainly instrumental in discovering, what the attractivepower of the earth must be, so that the moon shall move precisely aswe find it to move. It then appeared that the very power which makesan apple fall at the earth's surface is the power which guides themoon in its orbit. 

[PLATE: SIR ISAAC NEWTON'S TELESCOPE. ]

Once this step had been taken, the whole scheme of the universe mightalmost be said to have become unrolled before the eye of thephilosopher. It was natural to suppose that just as the moon wasguided and controlled by the attraction of the earth, so the earthitself, in the course of its great annual progress, should be guidedand controlled by the supreme attractive power of the sun. If thiswere so with regard to the earth, then it would be impossible todoubt that in the same way the movements of the planets could beexplained to be consequences of solar attraction. 

It was at this point that the great laws of Kepler became especiallysignificant. Kepler had shown how each of the planets revolves in anellipse around the sun, which is situated on one of the foci. Thisdiscovery had been arrived at from the interpretation ofobservations. Kepler had himself assigned no reason why the orbit ofa planet should be an ellipse rather than any other of the infinitenumber of closed curves which might be traced around the sun. Keplerhad also shown, and here again he was merely deducing the resultsfrom observation, that when the movements of two planets werecompared together, the squares of the periodic times in which eachplanet revolved were proportional to the cubes of their meandistances from the sun. This also Kepler merely knew to be true as afact, he gave no demonstration of the reason why nature should haveadopted this particular relation between the distance and theperiodic time rather than any other. Then, too, there was the law bywhich Kepler with unparalleled ingenuity, explained the way in whichthe velocity of a planet varies at the different points of its track, when he showed how the line drawn from the sun to the planetdescribed equal areas around the sun in equal times. These were thematerials with which Newton set to work. He proposed to infer fromthese the actual laws regulating the force by which the sun guidesthe planets. Here it was that his sublime mathematical genius cameinto play. Step by step Newton advanced until he had completelyaccounted for all the phenomena. 

In the first place, he showed that as the planet describes equalareas in equal times about the sun, the attractive force which thesun exerts upon it must necessarily be directed in a straight linetowards the sun itself. He also demonstrated the converse truth, that whatever be the nature of the force which emanated from a sun, yet so long as that force was directed through the sun's centre, anybody which revolved around it must describe equal areas in equaltimes, and this it must do, whatever be the actual character of thelaw according to which the intensity of the force varies at differentparts of the planet's journey. Thus the first advance was taken inthe exposition of the scheme of the universe. 

The next step was to determine the law according to which the forcethus proved to reside in the sun varied with the distance of theplanet. Newton presently showed by a most superb effort ofmathematical reasoning, that if the orbit of a planet were an ellipseand if the sun were at one of the foci of that ellipse, the intensityof the attractive force must vary inversely as the square of theplanet's distance. If the law had any other expression than theinverse square of the distance, then the orbit which the planet mustfollow would not be an ellipse; or if an ellipse, it would, at allevents, not have the sun in the focus. Hence he was able to showfrom Kepler's laws alone that the force which guided the planets wasan attractive power emanating from the sun, and that the intensity ofthis attractive power varied with the inverse square of the distancebetween the two bodies. 

These circumstances being known, it was then easy to show that thelast of Kepler's three laws must necessarily follow. If a number ofplanets were revolving around the sun, then supposing the materialsof all these bodies were equally affected by gravitation, it can bedemonstrated that the square of the periodic time in which eachplanet completes its orbit is proportional to the cube of thegreatest diameter in that orbit. 

[PLATE: SIR ISAAC NEWTON'S ASTROLABE. ]

These superb discoveries were, however, but the starting point fromwhich Newton entered on a series of researches, which disclosed manyof the profoundest secrets in the scheme of celestial mechanics. Hisnatural insight showed that not only large masses like the sun andthe earth, and the moon, attract each other, but that every particlein the universe must attract every other particle with a force whichvaries inversely as the square of the distance between them. If, forexample, the two particles were placed twice as far apart, then theintensity of the force which sought to bring them together would bereduced to one-fourth. If two particles, originally ten milesasunder, attracted each other with a certain force, then, when thedistance was reduced to one mile, the intensity of the attractionbetween the two particles would be increased one-hundred-fold. Thisfertile principle extends throughout the whole of nature. In somecases, however, the calculation of its effect upon the actualproblems of nature would be hardly possible, were it not for anotherdiscovery which Newton's genius enabled him to accomplish. In thecase of two globes like the earth and the moon, we must remember thatwe are dealing not with particles, but with two mighty masses ofmatter, each composed of innumerable myriads of particles. Everyparticle in the earth does attract every particle in the moon with aforce which varies inversely as the square of their distance. Thecalculation of such attractions is rendered feasible by the followingprinciple. Assuming that the earth consists of materialssymmetrically arranged in shells of varying densities, we may then, in calculating its attraction, regard the whole mass of the globe asconcentrated at its centre. Similarly we may regard the moon asconcentrated at the centre of its mass. In this way the earth andthe moon can both be regarded as particles in point of size, eachparticle having, however, the entire mass of the correspondingglobe. The attraction of one particle for another is a much moresimple matter to investigate than the attraction of the myriaddifferent points of the earth upon the myriad different points of themoon. 

Many great discoveries now crowded in upon Newton. He first of allgave the explanation of the tides that ebb and flow around ourshores. Even in the earliest times the tides had been shown to berelated to the moon. It was noticed that the tides were speciallyhigh during full moon or during new moon, and this circumstanceobviously pointed to the existence of some connection between themoon and these movements of the water, though as to what thatconnection was no one had any accurate conception until Newtonannounced the law of gravitation. Newton then made it plain that therise and fall of the water was simply a consequence of the attractivepower which the moon exerted upon the oceans lying upon our globe. Heshowed also that to a certain extent the sun produces tides, and hewas able to explain how it was that when the sun and the moon bothconspire, the joint result was to produce especially high tides, which we call "spring tides"; whereas if the solar tide was low, while the lunar tide was high, then we had the phenomenon of "neap"tides. 

But perhaps the most signal of Newton's applications of the law ofgravitation was connected with certain irregularities in themovements of the moon. In its orbit round the earth our satelliteis, of course, mainly guided by the great attraction of our globe. Ifthere were no other body in the universe, then the centre of the moonmust necessarily perform an ellipse, and the centre of the earthwould lie in the focus of that ellipse. Nature, however, does notallow the movements to possess the simplicity which this arrangementwould imply, for the sun is present as a source of disturbance. Thesun attracts the moon, and the sun attracts the earth, but indifferent degrees, and the consequence is that the moon's movementwith regard to the earth is seriously affected by the influence ofthe sun. It is not allowed to move exactly in an ellipse, nor is theearth exactly in the focus. How great was Newton's achievement inthe solution of this problem will be appreciated if we realise thathe not only had to determine from the law of gravitation the natureof the disturbance of the moon, but he had actually to construct themathematical tools by which alone such calculations could beeffected. 

The resources of Newton's genius seemed, however, to prove equal toalmost any demand that could be made upon it. He saw that eachplanet must disturb the other, and in that way he was able to rendera satisfactory account of certain phenomena which had perplexed allpreceding investigators. That mysterious movement by which the poleof the earth sways about among the stars had been long an unsolvedenigma, but Newton showed that the moon grasped with its attractionthe protuberant mass at the equatorial regions of the earth, and thustilted the earth's axis in a way that accounted for the phenomenonwhich had been known but had never been explained for two thousandyears. All these discoveries were brought together in that immortalwork, Newton's "Principia. "

Down to the year 1687, when the "Principia" was published, Newton hadlived the life of a recluse at Cambridge, being entirely occupiedwith those transcendent researches to which we have referred. But inthat year he issued from his seclusion under circumstances ofconsiderable historical interest. King James the Second attempted aninvasion of the rights and privileges of the University of Cambridgeby issuing a command that Father Francis, a Benedictine monk, shouldbe received as a Master of Arts in the University, without having takenthe oaths of allegiance and supremacy. With this arbitrary commandthe University sternly refused to comply. The Vice-Chancellor wasaccordingly summoned to answer for an act of contempt to the authorityof the Crown. Newton was one of nine delegates who were chosen todefend the independence of the University before the High Court. They were able to show that Charles the Second, who had issued aMANDAMUS under somewhat similar circumstances, had been induced afterdue consideration to withdraw it. This argument appeared satisfactory, and the University gained their case. Newton's next step in publiclife was his election, by a narrow majority, as member for theUniversity, and during the years 1688 and 1689, he seems to haveattended to his parliamentary duties with considerable regularity. 

An incident which happened in 1692 was apparently the cause ofconsiderable disturbance in Newton's equanimity, if not in hishealth. He had gone to early morning chapel, leaving a lightedcandle among his papers on his desk. Tradition asserts that hislittle dog "Diamond" upset the candle; at all events, when Newtoncame back he found that many valuable papers had perished in aconflagration. The loss of these manuscripts seems to have had aserious effect. Indeed, it has been asserted that the distressreduced Newton to a state of mental aberration for a considerabletime. This has, apparently, not been confirmed, but there is nodoubt that he experienced considerable disquiet, for in writing onSeptember 13th, 1693, to Mr. Pepys, he says:

"I am extremely troubled at the embroilment I am in, and haveneither ate nor slept well this twelvemonth, nor have my formerconsistency of mind. "

Notwithstanding the fame which Newton had achieved, by thepublication of his, "Principia, " and by all his researches, the Statehad not as yet taken any notice whatever of the most illustrious manof science that this or any other country has ever produced. Many ofhis friends had exerted themselves to procure him some permanentappointment, but without success. It happened, however, that Mr. Montagu, who had sat with Newton in Parliament, was appointedChancellor of the Exchequer in 1694. Ambitious of distinction in hisnew office, Mr. Montagu addressed himself to the improvement of thecurrent coin, which was then in a very debased condition. Itfortunately happened that an opportunity occurred of appointing a newofficial in the Mint; and Mr. Montagu on the 19th of March, 1695, wrote to offer Mr. Newton the position of warden. The salary was tobe five or six hundred a year, and the business would not requiremore attendance than Newton could spare. The Lucasian professoraccepted this post, and forthwith entered upon his new duties. 

The knowledge of physics which Newton had acquired by his experimentswas of much use in connection with his duties at the Mint. Hecarried out the re-coinage with great skill in the course of twoyears, and as a reward for his exertions, he was appointed, in 1697, to the Mastership of the Mint, with a salary between 1, 200 Pounds and1, 500 Pounds per annum. In 1701, his duties at the Mint being soengrossing, he resigned his Lucasian professorship at Cambridge, andat the same time he had to surrender his fellowship at TrinityCollege. This closed his connection with the University ofCambridge. It should, however, be remarked that at a somewhatearlier stage in his career he was very nearly being appointed to anoffice which might have enabled the University to retain the greatphilosopher within its precincts. Some of his friends had almostsucceeded in securing his nomination to the Provostship of King'sCollege, Cambridge; the appointment, however, fell through, inasmuchas the statute could not be evaded, which required that the Provostof King's College should be in holy orders. 

In those days it was often the custom for illustrious mathematicians, when they had discovered a solution for some new and strikingproblem, to publish that problem as a challenge to the world, whilewithholding their own solution. A famous instance of this is foundin what is known as the Brachistochrone problem, which was solved byJohn Bernouilli. The nature of this problem may be mentioned. Itwas to find the shape of the curve along which a body would slidedown from one point (A) to another point (B) in the shortest time. Itmight at first be thought that the straight line from A to B, as itis undoubtedly the shortest distance between the points, would alsobe the path of quickest descent; but this is not so. There is acurved line, down which a bead, let us say, would run on a smoothwire from A to B in a shorter time than the same bead would requireto run down the straight wire. Bernouilli's problem was to find outwhat that curve must be. Newton solved it correctly; he showed thatthe curve was a part of what is termed a cycloid--that is to say, acurve like that which is described by a point on the rim of acarriage-wheel as the wheel runs along the ground. Such was Newton'sgeometrical insight that he was able to transmit a solution of theproblem on the day after he had received it, to the President of theRoyal Society. 

In 1703 Newton, whose world wide fame was now established, waselected President of the Royal Society. Year after year he wasre-elected to this distinguished position, and his tenure, whichlasted twenty-five years, only terminated with his life. It was indischarge of his duties as President of the Royal Society that Newtonwas brought into contact with Prince George of Denmark. In April, 1705, the Queen paid a visit to Cambridge as the guest of Dr. Bentley, the then Master of Trinity, and in a court held at TrinityLodge on April 15th, 1705, the honour of knighthood was conferredupon the discoverer of gravitation. 

Urged by illustrious friends, who sought the promotion of knowledge, Newton gave his attention to the publication of a new edition of the"Principia. " His duties at the Mint, however, added to the supremeduty of carrying on his original investigations, left him but littletime for the more ordinary task of the revision. He was accordinglyinduced to associate with himself for this purpose a distinguishedyoung mathematician, Roger Coates, a Fellow of Trinity College, Cambridge, who had recently been appointed Plumian Professor ofAstronomy. On July 27th, 1713, Newton, by this time a favourite atCourt, waited on the Queen, and presented her with a copy of the newedition of the "Principia. "

Throughout his life Newton appears to have been greatly interested intheological studies, and he specially devoted his attention to thesubject of prophecy. He left behind him a manuscript on theprophecies of Daniel and the Apocalypse of St. John, and he alsowrote various theological papers. Many other subjects had from timeto time engaged his attention. He studied the laws of heat; heexperimented in pursuit of the dreams of the Alchymist; while thephilosopher who had revealed the mechanism of the heavens foundoccasional relaxation in trying to interpret hieroglyphics. In thelast few years of his life he bore with fortitude a painful ailment, and on Monday, March 20th, 1727, he died in the eighty-fifth year ofhis age. On Tuesday, March 28th, he was buried in Westminster Abbey. 

Though Newton lived long enough to receive the honour that hisastonishing discoveries so justly merited, and though for many yearsof his life his renown was much greater than that of any of hiscontemporaries, yet it is not too much to say that, in the yearswhich have since elapsed, Newton's fame has been ever steadilyadvancing, so that it never stood higher than it does at this moment. 

We hardly know whether to admire more the sublime discoveries atwhich he arrived, or the extraordinary character of the intellectualprocesses by which those discoveries were reached. Viewed fromeither standpoint, Newton's "Principia" is incomparably the greatestwork on science that has ever yet been produced. 

[PLATE: SIR ISAAC NEWTON'S SUN-DIAL IN THE ROYAL SOCIETY. ]

FLAMSTEED. 

Among the manuscripts preserved at Greenwich Observatory are certaindocuments in which Flamsteed gives an account of his own life. Wemay commence our sketch by quoting the following passage from thisautobiography:--"To keep myself from idleness, and to recreatemyself, I have intended here to give some account of my life, in myyouth, before the actions thereof, and the providences of Godtherein, be too far passed out of my memory; and to observe theaccidents of all my years, and inclinations of my mind, thatwhosoever may light upon these papers may see I was not so whollytaken up, either with my father's business or my mathematics, butthat I both admitted and found time for other as weightyconsiderations. "

The chief interest which attaches to the name of Flamsteed arisesfrom the fact that he was the first of the illustrious series ofAstronomers Royal who have presided over Greenwich Observatory. Inthat capacity Flamsteed was able to render material assistance toNewton by providing him with the observations which his lunar theoryrequired. 

John Flamsteed was born at Denby, in Derbyshire, on the 19th ofAugust, 1646. His mother died when he was three years old, and thesecond wife, whom his father took three years later, only lived untilFlamsteed was eight, there being also two younger sisters. In hisboyhood the future astronomer tells us that he was very fond of thoseromances which affect boy's imagination, but as he writes, "At twelveyears of age I left all the wild ones and betook myself to read thebetter sort of them, which, though they were not probable, yetcarried no seeming impossibility in the picturing. " By the timeFlamsteed was fifteen years old he had embarked in still more seriouswork, for he had read Plutarch's "Lives, " Tacitus' "Roman History, "and many other books of a similar description. In 1661 he became illwith some serious rheumatic affection, which obliged him to bewithdrawn from school. It was then for the first time that hereceived the rudiments of a scientific education. He had, however, attained his sixteenth year before he made any progress inarithmetic. He tells us how his father taught him "the doctrine offractions, " and "the golden rule of three"--lessons which he seemedto have learned easily and quickly. One of the books which he readat this time directed his attention to astronomical instruments, andhe was thus led to construct for himself a quadrant, by which hecould take some simple astronomical observations. He furthercalculated a table to give the sun's altitudes at different hours, and thus displayed those tastes for practical astronomy which helived to develop so greatly. It appears that these scientificstudies were discountenanced by his father, who designed that his sonshould follow a business career. Flamsteed's natural inclination, however, forced him to prosecute astronomical work, notwithstandingthe impediments that lay in his path. Unfortunately, hisconstitutional delicacy seems to have increased, and he had justcompleted his eighteenth year, "when, " to use his own words, "thewinter came on and thrust me again into the chimney, whence the heatand the dryness of the preceding summer had happily once beforewithdrawn me. But, it not being a fit season for physic, it wasthought fit to let me alone this winter, and try the skill of anotherphysician on me in the spring. "

It appears that at this time a quack named Valentine Greatrackes, wasreputed to have effected most astonishing cures in Ireland merely bythe stroke of his hands, without the application of any medicinewhatever. Flamsteed's father, despairing of any remedy for his sonfrom the legitimate branch of the profession, despatched him toIreland on August 26th, 1665, he being then, as recorded withastronomical accuracy, "nineteen years, six days, and eleven hoursold. " The young astronomer, accompanied by a friend, arrived on aTuesday at Liverpool but the wind not being favourable, they remainedthere till the following Friday, when a shift of the wind to the easttook place. They embarked accordingly on a vessel called the SUPPLYat noon, and on Saturday night came in sight of Dublin. Ere theycould land, however, they were nearly being wrecked on LambayIsland. This peril safely passed, there was a long delay forquarantine before they were at last allowed on shore. On Thursday, September 6th, they set out from Dublin, where they had beensojourning at the "Ship" Hotel, in Dame Street, towards Assaune, where Greatrackes received his patients. 

[PLATE: FLAMSTEED'S HOUSE. ]

Flamsteed gives an interesting account of his travels in Ireland. They dined at Naas on the first day, and on September 8th theyreached Carlow, a town which is described as one of the fairest theysaw on their journey. By Sunday morning, September 10th, having losttheir way several times, they reached Castleton, called commonly FourMile Waters. Flamsteed inquired of the host in the inn where theymight find a church, but was told that the minister lived twelvemiles away, and that they had no sermon except when he came toreceive his tithes once a year, and a woman added that "they hadplenty enough of everything necessary except the word of God. " Thetravellers accordingly went on to Cappoquin, which lies up the riverBlackwater, on the road to Lismore, eight miles from Youghal. Thencethey immediately started on foot to Assaune. About a mile fromCappoquin, and entering into the house of Mr. Greatrackes, they sawhim touch several patients, "whereof some were nearly cured, otherswere on the mending hand, and some on whom his strokes had noeffect. " Flamsteed was touched by the famous quack on the afternoonof September 11th, but we are hardly surprised to hear his remarkthat "he found not his disease to stir. " Next morning the astronomercame again to see Mr. Greatrackes, who had "a kind of majestical yetaffable presence, and a composed carriage. " Even after the thirdtouching had been submitted to, no benefit seems to have beenderived. We must, however record, to the credit of Mr. Greatrackes, that he refused to accept any payment from Flamsteed, because he wasa stranger. 

Finding it useless to protract his stay any longer, Flamsteed and hisfriend set out on their return to Dublin. In the course of hisjourney he seems to have been much impressed with Clonmel, which hedescribes as an "exceedingly pleasantly seated town. " But in thosedays a journey to Ireland was so serious an enterprise that whenFlamsteed did arrive safely back at Derby after an absence of amonth, he adds, "For God's providence in this journey, His name bepraised, Amen. "

As to the expected benefits to his health from the expedition we mayquote his own words: "In the winter following I was indifferenthearty, and my disease was not so violent as it used to be at thattime formerly. But whether through God's mercy I received thisthrough Mr. Greatrackes' touch, or my journey and vomiting at sea, Iam uncertain; but, by some circumstances, I guess that I received abenefit from both. "

It is evident that by this time Flamsteed's interest in allastronomical matters had greatly increased. He studied theconstruction of sun-dials, he formed a catalogue of seventy of thefixed stars, with their places on the heavens, and he computed thecircumstances of the solar eclipse which was to happen on June 22nd, 1666. It is interesting to note that even in those days thedoctrines of the astrologers still found a considerable degree ofcredence, and Flamsteed spent a good deal of his time in astrologicalstudies and computations. He investigated the methods of casting anativity, but a suspicion, or, indeed, rather more than a suspicion, seems to have crossed his mind as to the value of these astrologicalpredictions, for he says in fine, "I found astrology to givegenerally strong conjectural hints, not perfect declarations. "

All this time, however, the future Astronomer Royal was steadilyadvancing in astronomical inquiries of a recondite nature. He hadinvestigated the obliquity of the ecliptic with extreme care, so faras the circumstances of astronomical observation would at that timepermit. He had also sought to discover the sun's distance from theearth in so far as it could be obtained by determining when the moonwas exactly half illuminated, and he had measured, with muchaccuracy, the length of the tropical year. It will thus be seenthat, even at the age of twenty, Flamsteed had made marked progress, considering how much his time had been interfered with by ill-health. 

Other branches of astronomy began also to claim his attention. Welearn that in 1669 and 1670 he compared the planets Jupiter and Marswith certain fixed stars near which they passed. His instrumentalmeans, though very imperfect, were still sufficient to enable him tomeasure the intervals on the celestial sphere between the planets andthe stars. As the places of the stars were known, Flamsteed was thusable to obtain the places of the planets. This is substantially theway in which astronomers of the present day still proceed when theydesire to determine the places of the planets, inasmuch as, directlyor indirectly those places are always obtained relatively to thefixed stars. By his observations at this early period, Flamsteedwas, it is true, not able to obtain any great degree of accuracy; hesucceeded, however, in proving that the tables by which the places ofthe planets were ordinarily given were not to be relied upon. 

[PLATE: FLAMSTEED. ]

Flamsteed's labours in astronomy and in the allied branches ofscience were now becoming generally known, and he gradually came tocorrespond with many distinguished men of learning. One of the firstoccasions which brought the talents of the young astronomer into famewas the publication of some calculations concerning certainastronomical phenomena which were to happen in the year 1670. In themonthly revolution of the moon its disc passes over those stars whichlie along its track. The disappearance of a star by theinterposition of the moon is called an "occultation. " Owing to thefact that our satellite is comparatively near us, the position whichthe moon appears to occupy on the heavens varies from different partsof the earth, it consequently happens that a star which would beocculted to an observer in one locality, would often not be occultedto an observer who was situated elsewhere. Even when an occultationis visible from both places, the times at which the star disappearsfrom view will, generally speaking, be different. Much calculationis therefore necessary to decide the circumstances under which theoccultations of stars may be visible from any particular station. Having a taste for such computations, Flamsteed calculated theoccultations which were to happen in the year 1670, it being the casethat several remarkable stars would be passed over by the moon duringthis year. Of course at the present time, we find such informationduly set forth in the NAUTICAL ALMANAC, but a couple of centuries agothere was no such source of astronomical knowledge as is now to befound in that invaluable publication, which astronomers andnavigators know so well. Flamsteed accordingly sent the results ofhis work to the President of the Royal Society. The paper whichcontained them was received very favourably, and at once broughtFlamsteed into notice among the most eminent members of thatillustrious body, one of whom, Mr. Collins, became through life hisfaithful friend and constant correspondent. Flamsteed's father wasnaturally gratified with the remarkable notice which his son wasreceiving from the great and learned; accordingly he desired him togo to London, that he might make the personal acquaintance of thosescientific friends whom he had only known by correspondencepreviously. Flamsteed was indeed glad to avail himself of thisopportunity. Thus he became acquainted with Dr. Barrow, andespecially with Newton, who was then Lucasian Professor ofMathematics at Cambridge. It seems to have been in consequence ofthis visit to London that Flamsteed entered himself as a member ofJesus College, Cambridge. We have but little information as to hisUniversity career, but at all events he took his degree of M. A. OnJune 5th, 1674. 

Up to this time it would seem that Flamsteed had been engaged, to acertain extent, in the business carried on by his father. It is truethat he does not give any explicit details, yet there are frequentreferences to journeys which he had to take on business matters. Butthe time now approached when Flamsteed was to start on an independentcareer, and it appears that he took his degree in Cambridge with theobject of entering into holy orders, so that he might settle in asmall living near Derby, which was in the gift of a friend of hisfather, and would be at the disposal of the young astronomer. Thisscheme was, however, not carried out, but Flamsteed does not tell uswhy it failed, his only remark being, that "the good providence ofGod that had designed me for another station ordered it otherwise. "

Sir Jonas Moore, one of the influential friends whom Flamsteed'stalents had attracted, seems to have procured for him the position ofking's astronomer, with a salary of 100 pounds per annum. A largersalary appears to have been designed at first for this office, whichwas now being newly created, but as Flamsteed was resolved on takingholy orders, a lesser salary was in his case deemed sufficient. Thebuilding of the observatory, in which the first Astronomer Royal wasto be installed, seems to have been brought about, or, at all events, its progress was accelerated, in a somewhat curious manner. 

A Frenchman, named Le Sieur de S. Pierre, came over to London topromulgate a scheme for discovering longitudes, then a question ofmuch importance. He brought with him introductions to distinguishedpeople, and his mission attracted a great deal of attention. Theproposals which he made came under Flamsteed's notice, who pointedout that the Frenchman's projects were quite inapplicable in thepresent state of astronomical science, inasmuch as the places of thestars were not known with the degree of accuracy which would benecessary if such methods were to be rendered available. Flamsteedthen goes on to say:--"I heard no more of the Frenchman after this;but was told that my letters had been shown King Charles. He wasstartled at the assertion of the fixed stars' places being false inthe catalogue, and said, with some vehemence, he must have them anewobserved, examined, and corrected, for the use of his seamen. "

The first question to be settled was the site for the newobservatory. Hyde Park and Chelsea College were both mentioned assuitable localities, but, at Sir Christopher Wren's suggestion, Greenwich Hill was finally resolved upon. The king made a grant offive hundred pounds of money. He gave bricks from Tilbury Fort, while materials, in the shape of wood, iron, and lead, were availablefrom a gatehouse demolished in the Tower. The king also promisedwhatever further material aid might be shown to be necessary. Thefirst stone of the Royal Observatory was laid on August 10th, 1675, and within a few years a building was erected in which the art ofmodern practical astronomy was to be created. Flamsteed strove withextraordinary diligence, and in spite of many difficulties, to obtaina due provision of astronomical instruments, and to arrange for thecarrying on of his observations. Notwithstanding the king'spromises, the astronomer was, however, but scantily provided withmeans, and he had no assistants to help him in his work. It followsthat all the observations, as well as the reductions, and, indeed, all the incidental work of the observatory, had to be carried on byhimself alone. Flamsteed, as we have seen, had, however, manystaunch friends. Sir Jonas Moore in particular at all times renderedhim most valuable assistance, and encouraged him by the warm sympathyand keen interest which he showed in astronomy. The work of thefirst Astronomer Royal was frequently interrupted by recurrentattacks of the complaints to which we have already referred. He sayshimself that "his distempers stick so close that that he cannotremove them, " and he lost much time by prostration from headaches, aswell as from more serious affections. 

The year 1678 found him in the full tide of work in his observatory. He was specially engaged on the problem of the earth's motion, whichhe sought to derive from observations of the sun and of Venus. Butthis, as well as many other astronomical researches which heundertook, were only subsidiary to that which he made the main taskof his life, namely, the formation of a catalogue of fixed stars. Atthe time when Flamsteed commenced his career, the only availablecatalogue of fixed stars was that of Tycho Brahe. This work had beenpublished at the commencement of the seventeenth century, and itcontained about a thousand stars. The positions assigned to thesestars, though obtained with wonderful skill, considering the manydifficulties under which Tycho laboured, were quite inaccurate whenjudged by our modern standards. Tycho's instruments were necessarilymost rudely divided, and he had, of course, no telescopes to aid him. Consequently it was merely by a process of sighting that he couldobtain the places of the stars. It must further be remembered thatTycho had no clocks, and no micrometers. He had, indeed, but littlecorrect knowledge of the motions of the heavenly bodies to guidehim. To determine the longitudes of a few principal stars heconceived the ingenious idea of measuring by day the position ofVenus with respect to the sun, an observation which the exceptionalbrightness of this planet rendered possible without telescopic aid, and then by night he observed the position of Venus with regard tothe stars. 

It has been well remarked by Mr. Baily, in his introduction to the"British Catalogue of Stars, " that "Flamsteed's observations, by afortunate combination of circumstances, commenced a new and abrilliant era. It happened that, at that period, the powerful mindof Newton was directed to this subject; a friendly intercourse thenexisted between these two distinguished characters; and thus thefirst observations that could lay any claim to accuracy were at oncebrought in aid of those deep researches in which our illustriousgeometer was then engaged. The first edition of the `Principia'bears testimony to the assistance afforded by Flamsteed to Newton inthese inquiries; although the former considers that theacknowledgment is not so ample as it ought to have been. "

Although Flamsteed's observations can hardly be said to possess theaccuracy of those made in more recent times, when instruments so muchsuperior to his have been available, yet they possess an interest ofa special kind from their very antiquity. This circumstance rendersthem of particular importance to the astronomer, inasmuch as they arecalculated to throw light on the proper motions of the stars. Flamsteed's work may, indeed, be regarded as the origin of allsubsequent catalogues, and the nomenclature which he adopted, thoughin some respects it can hardly be said to be very defensible, is, nevertheless, that which has been adopted by all subsequentastronomers. There were also a great many errors, as might beexpected in a work of such extent, composed almost entirely ofnumerical detail. Many of these errors have been corrected by Bailyhimself, the assiduous editor of "Flamsteed's Life and Works, " forFlamsteed was so harassed from various causes in the latter part ofhis life, and was so subject to infirmities all through his career, that he was unable to revise his computations with the care thatwould have been necessary. Indeed, he observed many additional starswhich he never included in the British Catalogue. It is, as Bailywell remarks, "rather a matter of astonishment that he accomplishedso much, considering his slender means, his weak frame, and thevexations which he constantly experienced. "

Flamsteed had the misfortune, in the latter part of his life, tobecome estranged from his most eminent scientific contemporaries. Hehad supplied Newton with places of the moon, at the urgentsolicitation of the author of the "Principia, " in order that thelunar theory should be carefully compared with observation. ButFlamsteed appears to have thought that in Newton's further requestfor similar information, he appeared to be demanding as a right thatwhich Flamsteed considered he was only called upon to render as afavour. A considerable dispute grew out of this matter, and thereare many letters and documents, bearing on the difficulties whichsubsequently arose, that are not, perhaps, very creditable to eitherparty. 

Notwithstanding his feeble constitution, Flamsteed lived to the ageof seventy-three, his death occurring on the last day of the year1719. 

HALLEY. 

Isaac Newton was just fourteen years of age when the birth of EdmundHalley, who was destined in after years to become Newton's warmlyattached friend, and one of his most illustrious scientificcontemporaries, took place. There can be little doubt that the fameas an astronomer which Halley ultimately acquired, great as itcertainly was, would have been even greater still had it not beensomewhat impaired by the misfortune that he had to shine in the samesky as that which was illumined by the unparalleled genius of Newton. 

Edmund Halley was born at Haggerston, in the Parish of St. Leonard's, Shoreditch, on October 29th, 1656. His father, who bore the samename as his famous son, was a soap-boiler in Winchester Street, London, and he had conducted his business with such success that heaccumulated an ample fortune. I have been unable to obtain more thana very few particulars with respect to the early life of the futureastronomer. It would, however, appear that from boyhood he showedconsiderable aptitude for the acquisition of various kinds oflearning, and he also had some capacity for mechanical invention. Halley seems to have received a sound education at St. Paul's School, then under the care of Dr. Thomas Gale. 

Here, the young philosopher rapidly distanced his competitors in thevarious branches of ordinary school instruction. His superioritywas, however, most conspicuous in mathematical studies, and, as anatural development of such tastes, we learn that by the time he hadleft school he had already made good progress in astronomy. At theage of seventeen he was entered as a commoner at Queen's College, Oxford, and the reputation that he brought with him to the Universitymay be inferred from the remark of the writer of "AthenaeOxonienses, " that Halley came to Oxford "with skill in Latin, Greek, and Hebrew, and such a knowledge of geometry as to make a completedial. " Though his studies were thus of a somewhat multifariousnature, yet it is plain that from the first his most favouritepursuit was astronomy. His earliest efforts in practical observationwere connected with an eclipse which he observed from his father'shouse in Winchester Street. It also appears that he had studiedtheoretical branches of astronomy so far as to be conversant with theapplication of mathematics to somewhat abstruse problems. 

Up to the time of Kepler, philosophers had assumed almost as an axiomthat the heavenly bodies must revolve in circles and that the motionof the planet around the orbit which it described must be uniform. Wehave already seen how that great philosopher, after very perseveringlabour, succeeded in proving that the orbits of the planets were notcircles, but that they were ellipses of small eccentricity. Keplerwas, however, unable to shake himself free from the prevailing notionthat the angular motion of the planet ought to be of a uniformcharacter around some point. He had indeed proved that the motionround the focus of the ellipse in which the sun lies is not of thisdescription. One of his most important discoveries even related tothe fact that at some parts of its orbit a planet swings around thesun with greater angular velocity than at others. But it so happensthat in elliptic tracks which differ but little from circles, as isthe case with all the more important planetary orbits, the motionround the empty focus of the ellipse is very nearly uniform. Itseemed natural to assume, that this was exactly the case, in whichevent each of the two foci of the ellipse would have had a specialsignificance in relation to the movement of the planet. The youthfulHalley, however, demonstrated that so far as the empty focus wasconcerned, the movement of the planet around it, though so nearlyuniform, was still not exactly so, and at the age of nineteen, hepublished a treatise on the subject which at once placed him in theforemost rank amongst theoretical astronomers. 

But Halley had no intention of being merely an astronomer with hispen. He longed to engage in the practical work of observing. He sawthat the progress of exact astronomy must depend largely on thedetermination of the positions of the stars with all attainableaccuracy. He accordingly determined to take up this branch of work, which had been so successfully initiated by Tycho Brahe. 

At the present day, astronomers of the great national observatoriesare assiduously engaged in the determination of the places of thestars. A knowledge of the exact positions of these bodies is indeedof the most fundamental importance, not alone for the purposes ofscientific astronomy, but also for navigation and for extensiveoperations of surveying in which accuracy is desired. The fact thatHalley determined to concentrate himself on this work shows clearlythe scientific acumen of the young astronomer. 

Halley, however, found that Hevelius, at Dantzig, and Flamsteed, theAstronomer Royal at Greenwich, were both engaged on work of thischaracter. He accordingly determined to direct his energies in a waythat he thought would be more useful to science. He resigned to thetwo astronomers whom I have named the investigation of the stars inthe northern hemisphere, and he sought for himself a field hithertoalmost entirely unworked. He determined to go to the southernhemisphere, there to measure and survey those stars which wereinvisible in Europe, so that his work should supplement the laboursof the northern astronomers, and that the joint result of his laboursand of theirs might be a complete survey of the most important starson the surface of the heavens. 

In these days, after so many ardent students everywhere have devotedthemselves to the study of Nature, it seems difficult for a beginnerto find a virgin territory in which to commence his explorations. Halley may, however, be said to have enjoyed the privilege ofcommencing to work in a magnificent region, the contents of whichwere previously almost entirely unknown. Indeed none of the starswhich were so situated as to have been invisible from Tycho Brahe'sobservatory at Uraniborg, in Denmark, could be said to have beenproperly observed. There was, no doubt, a rumour that a Dutchman hadobserved southern stars from the island of Sumatra, and certain starswere indicated in the southern heavens on a celestial globe. Onexamination, however, Halley found that no reliance could be placedon the results which had been obtained, so that practically the fieldbefore him may be said to have been unworked. 

At the age of twenty, without having even waited to take that degreeat the university which the authorities would have been glad toconfer on so promising an undergraduate, this ardent student ofNature sought his father's permission to go to the southernhemisphere for the purpose of studying the stars which lie around thesouthern pole. His father possessed the necessary means, and he hadlikewise the sagacity to encourage the young astronomer. He wasindeed most anxious to make every thing as easy as possible for sohopeful a son. He provided him with an allowance of 300 pounds ayear, which was regarded as a very munificent provision in thosedays. Halley was also furnished with letters of recommendation fromKing Charles II. , as well as from the directors of the East IndiaCompany. He accordingly set sail with his instruments in the year1676, in one of the East India Company's ships, for the island of St. Helena, which he had selected as the scene of his labours. 

[PLATE: HALLEY. ]

After an uneventful voyage of three months, the astronomer landed onSt. Helena, with his sextant of five and a half feet radius, and atelescope 24 feet long, and forthwith plunged with ardour into hisinvestigation of the southern skies. He met, however, with one veryconsiderable disappointment. The climate of this island had beenrepresented to him as most favourable for astronomical observation;but instead of the pure blue skies he had been led to expect, hefound that they were almost always more or less clouded, and thatrain was frequent, so that his observations were very muchinterrupted. On this account he only remained at St. Helena for asingle year, having, during that time, and in spite of manydifficulties, accomplished a piece of work which earned for him thetitle of "our southern Tycho. " Thus did Halley establish his fame asan astronomer on the same lonely rock in mid-Atlantic, which nearly acentury and a-half later became the scene of Napoleon's imprisonment, when his star, in which he believed so firmly, had irretrievably set. 

On his return to England, Halley prepared a map which showed theresult of his labours, and he presented it to the king, in 1677. Like his great predecessor Tycho, Halley did not altogether disdainthe arts of the courtier, for he endeavoured to squeeze a newconstellation into the group around the southern pole which he styled"The Royal Oak, " adding a description to the effect that theincidents of which "The Royal Oak" was a symbol were of sufficientimportance to be inscribed on the surface of the heavens. 

There is reason to think that Charles II. Duly appreciated thescientific renown which one of his subjects had achieved, and it wasprobably through the influence of the king that Halley was made aMaster of Arts at Oxford on November 18th, 1678. Special referencewas made on the occasion to his observations at St. Helena, asevidence of unusual attainments in mathematics and astronomy. Thisdegree was no small honour to such a young man, who, as we have seen, quitted his university before he had the opportunity of graduating inthe ordinary manner. 

On November 30th, in the same year, the astronomer received a furtherdistinction in being elected a Fellow of the Royal Society. Fromthis time forward he took a most active part in the affairs of theSociety, and the numerous papers which he read before it form a veryvaluable part of that notable series of volumes known as the"Philosophical Transactions. " He was subsequently elected to theimportant office of secretary to the Royal Society, and he dischargedthe duties of his post until his appointment to Greenwichnecessitated his resignation. 

Within a year of Halley's election as a Fellow of the Royal Society, he was chosen by the Society to represent them in a discussion whichhad arisen with Hevelius. The nature of this discussion, or ratherthe fact that any discussion should have been necessary, may seemstrange to modern astronomers, for the point is one on which it wouldnow seem impossible for there to be any difference of opinion. Wemust, however, remember that the days of Halley were, comparativelyspeaking, the days of infancy as regards the art of astronomicalobservation, and issues that now seem obvious were often, in thoseearly times, the occasions of grave and anxious consideration. Theparticular question on which Halley had to represent the RoyalSociety may be simply stated. When Tycho Brahe made his memorableinvestigations into the places of the stars, he had no telescopes tohelp him. The famous instruments at Uraniborg were merely providedwith sights, by which the telescope was pointed to a star on the sameprinciple as a rifle is sighted for a target. Shortly after Tycho'stime, Galileo invented the telescope. Of course every one admittedat once the extraordinary advantages which the telescope had tooffer, so far as the mere question of the visibility of objects wasconcerned. But the bearing of Galileo's invention upon what we maydescribe as the measuring part of astronomy was not so immediatelyobvious. If a star be visible to the unaided eye, we can determineits place by such instruments as those which Tycho used, in which notelescope is employed. We can, however, also avail ourselves of aninstrument in which we view the star not directly but through theintervention of the telescope. Can the place of the star bedetermined more accurately by the latter method than it can when thetelescope is dispensed with? With our present knowledge, of course, there is no doubt about the answer; every one conversant withinstruments knows that we can determine the place of a star far moreaccurately with the telescope than is possible by any mere sightingapparatus. In fact an observer would be as likely to make an errorof a minute with the sighting apparatus in Tycho's instrument, as hewould be to make an error of a second with the modern telescope, or, to express the matter somewhat differently, we may say, speakingquite generally, that the telescopic method of determining the placesof the stars does not lead to errors more than one-sixtieth part asgreat as which are unavoidable when we make use of Tycho's method. 

But though this is so apparent to the modern astronomer, it was notat all apparent in the days of Halley, and accordingly he was sentoff to discuss the question with the Continental astronomers. Hevelius, as the representative of the older method, which Tycho hademployed with such success, maintained that an instrument could bepointed more accurately at a star by the use of sights than by theuse of a telescope, and vigorously disputed the claims put forward bythose who believed that the latter method was the more suitable. OnMay 14th, 1679, Halley started for Dantzig, and the energeticcharacter of the man may be judged from the fact that on the verynight of his arrival he commenced to make the necessaryobservations. In those days astronomical telescopes had onlyobtained a fractional part of the perfection possessed by theinstruments in our modern observatories, and therefore it may not besurprising that the results of the trial were not immediatelyconclusive. Halley appears to have devoted much time to theinvestigation; indeed, he remained at Dantzig for more than atwelvemonth. On his return to England, he spoke highly of the skillwhich Hevelius exhibited in the use of his antiquated methods, butHalley was nevertheless too sagacious an observer to be shaken in hispreference for the telescopic method of observation. 

The next year we find our young astronomer starting for a Continentaltour, and we, who complain if the Channel passage lasts more than anhour or two, may note Halley's remark in writing to Hooke on June15th, 1680: "Having fallen in with bad weather we took forty hours inthe journey from Dover to Calais. " The scientific distinction whichhe had already attained was such that he was received in Paris withmarked attention. A great deal of his time seems to have been passedin the Paris observatory, where Cassini, the presiding genius, himself an astronomer of well-deserved repute, had extended a heartywelcome to his English visitor. They made observations together ofthe place of the splendid comet which was then attracting universalattention, and Halley found the work thus done of much use when hesubsequently came to investigate the path pursued by this body. Halley was wise enough to spare no pains to derive all possibleadvantages from his intercourse with the distinguished savants of theFrench capital. In the further progress of his tour he visited theprincipal cities of the Continent, leaving behind him everywhere thememory of an amiable disposition and of a rare intelligence. 

After Halley's return to England, in 1682, he married a young ladynamed Mary Tooke, with whom he lived happily, till her deathfifty-five years later. On his marriage, he took up his abode inIslington, where he erected his instruments and recommenced hisobservations. 

It has often been the good fortune of astronomers to render practicalservices to humanity by their investigations, and Halley'sachievements in this respect deserve to be noted. A few years afterhe had settled in England, he published an important paper on thevariation of the magnetic compass, for so the departure of the needlefrom the true north is termed. This subject had indeed early engagedhis attention, and he continued to feel much interest in it up to theend of his life. With respect to his labours in this direction, SirJohn Herschel says: "To Halley we owe the first appreciation of thereal complexity of the subject of magnetism. It is wonderful indeed, and a striking proof of the penetration and sagacity of thisextraordinary man, that with his means of information he should havebeen able to draw such conclusions, and to take so large andcomprehensive a view of the subject as he appears to have done. " In1692, Halley explained his theory of terrestrial magnetism, andbegged captains of ships to take observations of the variations ofthe compass in all parts of the world, and to communicate them to theRoyal Society, "in order that all the facts may be readily availableto those who are hereafter to complete this difficult and complicatedsubject. "

The extent to which Halley was in advance of his contemporaries, inthe study of terrestrial magnetism, may be judged from the fact thatthe subject was scarcely touched after his time till the year 1811. The interest which he felt in it was not of a merely theoreticalkind, nor was it one which could be cultivated in an easy-chair. Likeall true investigators, he longed to submit his theory to the test ofexperiment, and for that purpose Halley determined to observe themagnetic variation for himself. He procured from King William III. The command of a vessel called the "Paramour Pink, " with which hestarted for the South Seas in 1694. This particular enterprise wasnot, however, successful; for, on crossing the line, some of his menfell sick and one of his lieutenants mutinied, so that he was obligedto return the following year with his mission unaccomplished. Thegovernment cashiered the lieutenant, and Halley having procured asecond smaller vessel to accompany the "Paramour Pink, " started oncemore in September, 1699. He traversed the Atlantic to the 52nddegree of southern latitude, beyond which his further advance wasstopped. "In these latitudes, " he writes to say, "we fell in withgreat islands of ice of so incredible height and magnitude, that Iscarce dare write my thoughts of it. "

On his return in 1700, Halley published a general chart, showing thevariation of the compass at the different places which he hadvisited. On these charts he set down lines connecting thoselocalities at which the magnetic variation was identical. He thusset an example of the graphic representation of large masses ofcomplex facts, in such a manner as to appeal at once to the eye, amethod of which we make many applications in the present day. 

But probably the greatest service which Halley ever rendered to humanknowledge was the share in which he took in bringing Newton's"Principia" before the world. In fact, as Dr. Glaisher, writing in1888, has truly remarked, "but for Halley the 'Principia' would nothave existed. "

It was a visit from Halley in the year 1684 which seems to have firstsuggested to Newton the idea of publishing the results of hisinvestigations on gravitation. Halley, and other scientificcontemporaries, had no doubt some faint glimmering of the great truthwhich only Newton's genius was able fully to reveal. Halley hadindeed shown how, on the assumptions that the planets move incircular orbits round the sun, and that the squares of their periodictimes are proportional to the cubes of their mean distances, it maybe proved that the force acting on each planet must vary inversely asthe square of its distance from the sun. Since, however, each of theplanets actually moves in an ellipse, and therefore, at continuallyvarying distances from the sun, it becomes a much more difficultmatter to account mathematically for the body's motions on thesupposition that the attractive force varies inversely as the squareof the distance. This was the question with which Halley foundhimself confronted, but which his mathematical abilities were notadequate to solve. It would seem that both Hooke and Sir ChristopherWren were interested in the same problem; in fact, the former claimedto have arrived at a solution, but declined to make known hisresults, giving as an excuse his desire that others having tried andfailed might learn to value his achievements all the more. Halley, however, confessed that his attempts at the solution wereunsuccessful, and Wren, in order to encourage the other twophilosophers to pursue the inquiry, offered to present a book offorty shillings value to either of them who should in the space oftwo months bring him a convincing proof of it. Such was the valuewhich Sir Christopher set on the Law of Gravitation, upon which thewhole fabric of modern astronomy may be said to stand. 

Finding himself unequal to the task, Halley went down to Cambridge tosee Newton on the subject, and was delighted to learn that the greatmathematician had already completed the investigation. He showedHalley that the motions of all the planets could be completelyaccounted for on the hypothesis of a force of attraction directedtowards the sun, which varies inversely as the square of the distancefrom that body. 

Halley had the genius to perceive the tremendous importance ofNewton's researches, and he ceased not to urge upon the recluse manof science the necessity for giving his new discoveries publication. He paid another visit to Cambridge with the object of learning morewith regard to the mathematical methods which had already conductedNewton to such sublime truths, and he again encouraged the latterboth to pursue his investigations, and to give some account of themto the world. In December of the same year Halley had thegratification of announcing to the Royal Society that Newton hadpromised to send that body a paper containing his researches onGravitation. 

It seems that at this epoch the finances of the Royal Society were ata very low ebb. This impecuniosity was due to the fact that a bookby Willoughby, entitled "De Historia Piscium, " had been recentlyprinted by the society at great expense. In fact, the coffers wereso low that they had some difficulty in paying the salaries of theirpermanent officials. It appears that the public did not care aboutthe history of fishes, or at all events the volume did not meet withthe ready demand which was expected for it. Indeed, it has beenrecorded that when Halley had undertaken to measure the length of adegree of the earth's surface, at the request of the Royal Society, it was ordered that his expenses be defrayed either in 50 poundssterling, or in fifty books of fishes. Thus it happened that On June2nd, the Council, after due consideration of ways and means inconnection with the issue of the Principia, "ordered that Halleyshould undertake the business of looking after the book and printingit at his own charge, " which he engaged to do. 

It was, as we have elsewhere mentioned, characteristic of Newton thathe detested controversies, and he was, in fact, inclined to suppressthe third book of the "Principia" altogether rather than have anyconflict with Hooke with respect to the discoveries thereenunciated. He also thought of changing the name of the work to DeMotu Corporum Libri Duo, but upon second thoughts, he retained theoriginal title, remarking, as he wrote to Halley, "It will help thesale of the book, which I ought not to diminish, now it is yours, " asentence which shows conclusively, if further proof were necessary, that Halley had assumed the responsibility of its publication. 

Halley spared no pains in pushing forward the publication of hisillustrious friend's great work, so that in the same year he was in aposition to present a complete copy to King James II. , with a properdiscourse of his own. Halley also wrote a set of Latin hexameters inpraise of Newton's genius, which he printed at the beginning of thework. The last line of this specimen of Halley's poetic muse may bethus rendered: "Nor mortals nearer may approach the gods. "

The intimate friendship between the two greatest astronomers of thetime continued without interruption till the death of Newton. Ithas, indeed, been alleged that some serious cause of estrangementarose between them. There is, however, no satisfactory ground forthis statement; indeed, it may be regarded as effectually disposed ofby the fact that, in the year 1727, Halley took up the defence of hisfriend, and wrote two learned papers in support of Newton's "Systemof Chronology, " which had been seriously attacked by a certainecclesiastic. It is quite evident to any one who has studied thesepapers that Halley's friendship for Newton was as ardent as ever. 

The generous zeal with which Halley adopted and defended thedoctrines of Newton with regard to the movements of the celestialbodies was presently rewarded by a brilliant discovery, which hasmore than any of his other researches rendered his name a familiarone to astronomers. Newton, having explained the movement of theplanets, was naturally led to turn his attention to comets. Heperceived that their journeyings could be completely accounted for asconsequences of the attraction of the sun, and he laid down theprinciples by which the orbit of a comet could be determined, provided that observations of its positions were obtained at threedifferent dates. The importance of these principles was by no onemore quickly recognised than by Halley, who saw at once that itprovided the means of detecting something like order in the movementsof these strange wanderers. The doctrine of Gravitation seemed toshow that just as the planets revolved around the sun in ellipses, soalso must the comets. The orbit, however, in the case of the comet, is so extremely elongated that the very small part of the ellipticpath within which the comet is both near enough and bright enough tobe seen from the earth, is indistinguishable from a parabola. Applying these principles, Halley thought it would be instructive tostudy the movements of certain bright comets, concerning whichreliable observations could be obtained. At the expense of muchlabour, he laid down the paths pursued by twenty-four of thesebodies, which had appeared between the years 1337 and 1698. Amongstthem he noticed three, which followed tracks so closely resemblingeach other, that he was led to conclude the so called three cometscould only have been three different appearances of the same body. The first of these occurred in 1531, the second was seen by Kepler in1607, and the third by Halley himself in 1682. These dates suggestedthat the observed phenomena might be due to the successive returns ofone and the same comet after intervals of seventy-five or seventy-sixyears. On the further examination of ancient records, Halley foundthat a comet had been seen in the year 1456, a date, it will beobserved, seventy-five years before 1531. Another had been observedseventy-six years earlier than 1456, viz. , in 1380, and anotherseventy-five years before that, in 1305. 

As Halley thus found that a comet had been recorded on severaloccasions at intervals of seventy-five or seventy-six years, he wasled to the conclusion that these several apparitions related to oneand the same object, which was an obedient vassal of the sun, performing an eccentric journey round that luminary in a period ofseventy-five or seventy-six years. To realise the importance of thisdiscovery, it should be remembered that before Halley's time a comet, if not regarded merely as a sign of divine displeasure, or as an omenof intending disaster, had at least been regarded as a chance visitorto the solar system, arriving no one knew whence, and going no oneknew whither. 

A supreme test remained to be applied to Halley's theory. Thequestion arose as to the date at which this comet would be seenagain. We must observe that the question was complicated by the factthat the body, in the course of its voyage around the sun, wasexposed to the incessant disturbing action produced by the attractionof the several planets. The comet therefore, does not describe asimple ellipse as it would do if the attraction of the sun were theonly force by which its movement were controlled. Each of theplanets solicits the comet to depart from its track, and though theamount of these attractions may be insignificant in comparison withthe supreme controlling force of the sun, yet the departure from theellipse is quite sufficient to produce appreciable irregularities inthe comet's movement. At the time when Halley lived, no meansexisted of calculating with precision the effect of the disturbance acomet might experience from the action of the different planets. Halley exhibited his usual astronomical sagacity in deciding thatJupiter would retard the return of the comet to some extent. Had itnot been for this disturbance the comet would apparently have beendue in 1757 or early in 1758. But the attraction of the great planetwould cause delay, so that Halley assigned, for the date of itsre-appearance, either the end of 1758 or the beginning of 1759. Halley knew that he could not himself live to witness the fulfilmentof his prediction, but he says: "If it should return, according toour predictions, about the year 1758, impartial posterity will notrefuse to acknowledge that this was first discovered by anEnglishman. " This was, indeed, a remarkable prediction of an eventto occur fifty-three years after it had been uttered. The way inwhich it was fulfilled forms one of the most striking episodes in thehistory of astronomy. The comet was first seen on Christmas Day, 1758, and passed through its nearest point to the sun on March 13th, 1759. Halley had then been lying in his grave for seventeen years, yet the verification of his prophecy reflects a glory on his namewhich will cause it to live for ever in the annals of astronomy. Thecomet paid a subsequent visit in 1835, and its next appearance is dueabout 1910. 

Halley next entered upon a labour which, if less striking to theimagination than his discoveries with regard to comets, is still ofinestimable value in astronomy. He undertook a series ofinvestigations with the object of improving our knowledge of themovements of the planets. This task was practically finished in1719, though the results of it were not published until after hisdeath in 1749. In the course of it he was led to investigate closelythe motion of Venus, and thus he came to recognise for the first timethe peculiar importance which attaches to the phenomenon of thetransit of this planet across the sun. Halley saw that the transit, which was to take place in the year 1761, would afford a favourableopportunity for determining the distance of the sun, and thuslearning the scale of the solar system. He predicted thecircumstances of the phenomenon with an astonishing degree ofaccuracy, considering his means of information, and it isunquestionably to the exertions of Halley in urging the importance ofthe matter upon astronomers that we owe the unexampled degree ofinterest taken in the event, and the energy which scientific menexhibited in observing it. The illustrious astronomer had no hope ofbeing himself a witness of the event, for it could not happen tillmany years after his death. This did not, however, diminish hisanxiety to impress upon those who would then be alive, the importanceof the occurrence, nor did it lead him to neglect anything whichmight contribute to the success of the observations. As we now know, Halley rather over-estimated the value of the transit of Venus, as ameans of determining the solar distance. The fact is that thecircumstances are such that the observation of the time of contactbetween the edge of the planet and the edge of the sun cannot be madewith the accuracy which he had expected. 

In 1691, Halley became a candidate for the Savilian Professorship ofAstronomy at Oxford. He was not, however, successful, for hiscandidature was opposed by Flamsteed, the Astronomer Royal of thetime, and another was appointed. He received some consolation forthis particular disappointment by the fact that, in 1696, owing toNewton's friendly influence, he was appointed deputy Controller ofthe Mint at Chester, an office which he did not retain for long, asit was abolished two years later. At last, in 1703, he received whathe had before vainly sought, and he was appointed to the Savilianchair. 

His observations of the eclipse of the sun, which occurred in 1715, added greatly to Halley's reputation. This phenomenon excitedspecial attention, inasmuch as it was the first total eclipse of thesun which had been visible in London since the year 1140. Halleyundertook the necessary calculations, and predicted the variouscircumstances with a far higher degree of precision than the officialannouncement. He himself observed the phenomenon from the RoyalSociety's rooms, and he minutely describes the outer atmosphere ofthe sun, now known as the corona; without, however, offering anopinion as to whether it was a solar or a lunar appendage. 

At last Halley was called to the dignified office which he of all menwas most competent to fill. On February 9th, 1720, he was appointedAstronomer Royal in succession to Flamsteed. He found things at theRoyal Observatory in a most unsatisfactory state. Indeed, there wereno instruments, nor anything else that was movable; for such things, being the property of Flamsteed, had been removed by his widow, andthough Halley attempted to purchase from that lady some of theinstruments which his predecessor had employed, the unhappy personaldifferences which had existed between him and Flamsteed, and which, as we have already seen, prevented his election as Savilian Professorof Astronomy, proved a bar to the negotiation. Greenwich Observatorywore a very different appearance in those days, from that which themodern visitor, who is fortunate enough to gain admission, may nowbehold. Not only did Halley find it bereft of instruments, we learnbesides that he had no assistants, and was obliged to transact thewhole business of the establishment single-handed. 

In 1721, however, he obtained a grant of 500 pounds from the Board ofOrdnance, and accordingly a transit instrument was erected in thesame year. Some time afterwards he procured an eight-foot quadrant, and with these instruments, at the age of sixty-four, he commenced aseries of observations on the moon. He intended, if his life wasspared, to continue his observations for a period of eighteen years, this being, as astronomers know, a very important cycle in connectionwith lunar movements. The special object of this vast undertakingwas to improve the theory of the moon's motion, so that it mightserve more accurately to determine longitudes at sea. Thisself-imposed task Halley lived to carry to a successful termination, and the tables deduced from his observations, and published after hisdeath, were adopted almost universally by astronomers, those of theFrench nation being the only exception. 

Throughout his life Halley had been singularly free from illness ofevery kind, but in 1737 he had a stroke of paralysis. Notwithstandingthis, however, he worked diligently at his telescope till 1739, afterwhich his health began rapidly to give way. He died on January 14th, 1742, in the eighty-sixth year of his age, retaining his mentalfaculties to the end. He was buried in the cemetery of the church ofLee in Kent, in the same grave as his wife, who had died five yearspreviously. We are informed by Admiral Smyth that Pond, a laterAstronomer Royal, was afterwards laid in the same tomb. 

Halley's disposition seems to have been generous and candid, andwholly free from anything like jealousy or rancour. In person he wasrather above the middle height, and slight in build; his complexionwas fair, and he is said to have always spoken, as well as acted, with uncommon sprightliness. In the eloge pronounced upon him at theParis Academie Des Sciences, of which Halley had been made a memberin 1719 it was said, "he possessed all the qualifications which werenecessary to please princes who were desirous of instruction, with agreat extent of knowledge and a constant presence of mind; hisanswers were ready, and at the same time pertinent, judicious, politeand sincere. "

[PLATE: GREENWICH OBSERVATORY IN HALLEY'S TIME. ]

Thus we find that Peter the Great was one of his most ardentadmirers. He consulted the astronomer on matters connected withshipbuilding, and invited him to his own table. But Halley possessednobler qualifications than the capacity of pleasing Princes. He wasable to excite and to retain the love and admiration of his equals. This was due to the warmth of his attachments, the unselfishness ofhis devotion to his friends, and to a vein of gaiety and good-humourwhich pervaded all his conversation. 

BRADLEY. 

James Bradley was descended from an ancient family in the county ofDurham. He was born in 1692 or 1693, at Sherbourne, inGloucestershire, and was educated in the Grammar School atNorthleach. From thence he proceeded in due course to Oxford, wherehe was admitted a commoner at Balliol College, on March 15th, 1711. Much of his time, while an undergraduate, was passed in Essex withhis maternal uncle, the Rev. James Pound, who was a well-known man ofscience and a diligent observer of the stars. It was doubtless byintercourse with his uncle that young Bradley became so expert in theuse of astronomical instruments, but the immortal discoveries hesubsequently made show him to have been a born astronomer. 

The first exhibition of Bradley's practical skill seems to becontained in two observations which he made in 1717 and 1718. Theyhave been published by Halley, whose acuteness had led him toperceive the extraordinary scientific talents of the youngastronomer. Another illustration of the sagacity which Bradleymanifested, even at the very commencement of his astronomical career, is contained in a remark of Halley's, who says: "Dr. Pound and hisnephew, Mr. Bradley, did, myself being present, in the lastopposition of the sun and Mars this way demonstrate the extrememinuteness of the sun's parallax, and that it was not more thantwelve seconds nor less than nine seconds. " To make the significanceof this plain, it should be observed that the determination of thesun's parallax is equivalent to the determination of the distancefrom the earth to the sun. At the time of which we are now writing, this very important unit of celestial measurement was only veryimperfectly known, and the observations of Pound and Bradley may beinterpreted to mean that, from their observations, they had come tothe conclusion that the distance from the earth to the sun must bemore than 94 millions of miles, and less than 125 millions. We now, of course, know that they were not exactly right, for the truedistance of the sun is about 93 millions of miles. We cannot, however, but think that it was a very remarkable approach for theveteran astronomer and his brilliant nephew to make towards thedetermination of a magnitude which did not become accurately knowntill fifty years later. 

Among the earliest parts of astronomical work to which Bradley'sattention was directed, were the eclipses of Jupiter's satellites. These phenomena are specially attractive inasmuch as they can be soreadily observed, and Bradley found it extremely interesting tocalculate the times at which the eclipses should take place, and thento compare his observations with the predicted times. From thesuccess that he met with in this work, and from his other labours, Bradley's reputation as an astronomer increased so greatly that onNovember the 6th, 1718, he was elected a Fellow of the Royal Society. 

Up to this time the astronomical investigations of Bradley had beenmore those of an amateur than of a professional astronomer, and as itdid not at first seem likely that scientific work would lead to anypermanent provision, it became necessary for the youthful astronomerto choose a profession. It had been all along intended that heshould enter the Church, though for some reason which is not told us, he did not take orders as soon as his age would have entitled him todo so. In 1719, however, the Bishop of Hereford offered Bradley theVicarage of Bridstow, near Ross, in Monmouthshire, and on July 25th, 1720, he having then taken priest's orders, was duly instituted inhis vicarage. In the beginning of the next year, Bradley had someaddition to his income from the proceeds of a Welsh living, which, being a sinecure, he was able to hold with his appointment atBridstow. It appears, however, that his clerical occupations werenot very exacting in their demands upon his time, for he was stillable to pay long and often-repeated visits to his uncle atWandsworth, who, being himself a clergyman, seems to have receivedoccasional assistance in his ministerial duties from his astronomicalnephew. 

The time, however, soon arrived when Bradley was able to make achoice between continuing to exercise his profession as a divine, ordevoting himself to a scientific career. The Savilian Professorshipof Astronomy in the University of Oxford became vacant by the deathof Dr. John Keill. The statutes forbade that the Savilian Professorshould also hold a clerical appointment, and Mr. Pound wouldcertainly have been elected to the professorship had he consented tosurrender his preferments in the Church. But Pound was unwilling tosacrifice his clerical position, and though two or three othercandidates appeared in the field, yet the talents of Bradley were soconspicuous that he was duly elected, his willingness to resign theclerical profession having been first ascertained. 

There can be no doubt that, with such influential friends as Bradleypossessed, he would have made great advances had he adhered to hisprofession as a divine. Bishop Hoadly, indeed, with other marks offavour, had already made the astronomer his chaplain. The engrossingnature of Bradley's interest in astronomy decided him, however, tosacrifice all other prospects in comparison with the opening affordedby the Savilian Professorship. It was not that Bradley found himselfdevoid of interest in clerical matters, but he felt that the truescope for such abilities as he possessed would be better found in thedischarge of the scientific duties of the Oxford chair than in thespiritual charge of a parish. On April the 26th, 1722, Bradley readhis inaugural lecture in that new position on which he was destinedto confer such lustre. 

It must, of course, be remembered that in those early days the art ofconstructing the astronomical telescope was very imperfectlyunderstood. The only known method for getting over the peculiardifficulties presented in the construction of the refractingtelescope, was to have it of the most portentous length. In fact, Bradley made several of his observations with an instrument of twohundred and twelve feet focus. In such a case, no tube could beused, and the object glass was merely fixed at the top of a highpole. Notwithstanding the inconvenience and awkwardness of such aninstrument, Bradley by its means succeeded in making many carefulmeasurements. He observed, for example, the transit of Mercury overthe sun's disc, on October 9th, 1723; he also observed the dimensionsof the planet Venus, while a comet which Halley discovered on Octoberthe 9th, 1723, was assiduously observed at Wanstead up to the middleof the ensuing month. The first of Bradley's remarkablecontributions to the "Philosophical Transactions" relates to thiscomet, and the extraordinary amount of work that he went through inconnection therewith may be seen from an examination of his book ofCalculations which is still extant. 

The time was now approaching when Bradley was to make the first ofthose two great discoveries by which his name has acquired a lustrethat has placed him in the very foremost rank of astronomicaldiscoverers. As has been often the case in the history of science, the first of these great successes was attained while he was pursuinga research intended for a wholly different purpose. It had long beenrecognised that as the earth describes a vast orbit, nearly twohundred million miles in diameter, in its annual journey round thesun, the apparent places of the stars should alter, to some extent, in correspondence with the changes in the earth's position. Thenearer the star the greater the shift in its apparent place on theheavens, which must arise from the fact that it was seen fromdifferent positions in the earth's orbit. It had been pointed outthat these apparent changes in the places of the stars, due to themovement of the earth, would provide the means of measuring thedistances of the stars. As, however, these distances are enormouslygreat in comparison with the orbit which the earth describes aroundthe sun, the attempt to determine the distances of the stars by theshift in their positions had hitherto proved ineffectual. Bradleydetermined to enter on this research once again; he thought that byusing instruments of greater power, and by making measurements ofincreased delicacy, he would be able to perceive and to measuredisplacements which had proved so small as to elude the skill of theother astronomers who had previously made efforts in the samedirection. In order to simplify the investigation as much aspossible, Bradley devoted his attention to one particular star, BetaDraconis, which happened to pass near his zenith. The object ofchoosing a star in this position was to avoid the difficulties whichwould be introduced by refraction had the star occupied any otherplace in the heavens than that directly overhead. 

We are still able to identify the very spot on which the telescopestood which was used in this memorable research. It was erected atthe house then occupied by Molyneux, on the western extremity of KewGreen. The focal length was 24 feet 3 inches, and the eye-glass was3 and a half feet above the ground floor. The instrument was firstset up on November 26th, 1725. If there had be any appreciabledisturbance in the place of Beta Draconis in consequence of themovement of the earth around the sun, the star must appear to havethe smallest latitude when in conjunction with the sun, and thegreatest when in opposition. The star passed the meridian at noon inDecember, and its position was particularly noticed by Molyneux onthe third of that month. Any perceptible displacement byparallax--for so the apparent change in position, due to the earth'smotion, is called--would would have made the star shift towards thenorth. Bradley, however, when observing it on the 17th, wassurprised to find that the apparent place of the star, so far fromshifting towards the north, as they had perhaps hoped it would, wasfound to lie a little more to the south than when it was observedbefore. He took extreme care to be sure that there was no mistake inhis observation, and, true astronomer as he was, he scrutinized withthe utmost minuteness all the circumstances of the adjustment of hisinstruments. Still the star went to the south, and it continued soadvancing in the same direction until the following March, by whichtime it had moved no less than twenty seconds south from the placewhich it occupied when the first observation was made. After a briefpause, in which no apparent movement was perceptible, the star by themiddle of April appeared to be returning to the north. Early in Juneit reached the same distance from the zenith which it had inDecember. By September the star was as much as thirty-nine secondsmore to the north than it had been in March, then it returned towardsthe south, regaining in December the same situation which it hadoccupied twelve months before. 

This movement of the star being directly opposite to the movementswhich would have been the consequence of parallax, seemed to showthat even if the star had any parallax its effects upon the apparentplace were entirely masked by a much larger motion of a totallydifferent description. Various attempts were made to account for thephenomenon, but they were not successful. Bradley accordinglydetermined to investigate the whole subject in a more thoroughmanner. One of his objects was to try whether the same movementswhich he had observed in one star were in any similar degreepossessed by other stars. For this purpose he set up a newinstrument at Wanstead, and there he commenced a most diligentscrutiny of the apparent places of several stars which passed atdifferent distances from the zenith. He found in the course of thisresearch that other stars exhibited movements of a similardescription to those which had already proved so perplexing. For along time the cause of these apparent movements seemed a mystery. Atlast, however, the explanation of these remarkable phenomena dawnedupon him, and his great discovery was made. 

One day when Bradley was out sailing he happened to remark that everytime the boat was laid on a different tack the vane at the top of theboat's mast shifted a little, as if there had been a slight change inthe direction of the wind. After he had noticed this three or fourtimes he made a remark to the sailors to the effect that it was verystrange the wind should always happen to change just at the momentwhen the boat was going about. The sailors, however, said there hadbeen no change in the wind, but that the alteration in the vane wasdue to the fact that the boat's course had been altered. In fact, the position of the vane was determined both by the course of theboat and the direction of the wind, and if either of these werealtered there would be a corresponding change in the direction of thevane. This meant, of course, that the observer in the boat which wasmoving along would feel the wind coming from a point different fromthat in which the wind appeared to be blowing when the boat was atrest, or when it was sailing in some different direction. Bradley'ssagacity saw in this observation the clue to the Difficulty which hadso long troubled him. 

It had been discovered before the time of Bradley that the passage oflight through space is not an instantaneous phenomenon. Lightrequires time for its journey. Galileo surmised that the sun mayhave reached the horizon before we see it there, and it was indeedsufficiently obvious that a physical action, like the transmission oflight, could hardly take place without requiring some lapse of time. The speed with which light actually travelled was, however, so rapidthat its determination eluded all the means of experimenting whichwere available in those days. The penetration of Roemer hadpreviously detected irregularities in the observed times of theeclipses of Jupiter's satellites, which were undoubtedly due to theinterval which light required for stretching across theinterplanetary spaces. Bradley argued that as light can only travelwith a certain speed, it may in a measure be regarded like the wind, which he noticed in the boat. If the observer were at rest, that isto say, if the earth were a stationary object, the direction in whichthe light actually does come would be different from that in which itappears to come when the earth is in motion. It is true that theearth travels but eighteen miles a second, while the velocity withwhich light is borne along attains to as much as 180, 000 miles asecond. The velocity of light is thus ten thousand times greaterthan the speed of the earth. But even though the wind blew tenthousand times faster than the speed with which the boat was sailingthere would still be some change, though no doubt a very smallchange, in the position of the vane when the boat was in progressfrom the position it would have if the boat were at rest. Ittherefore occurred to this most acute of astronomers that when thetelescope was pointed towards a star so as to place it apparently inthe centre of the field of view, yet it was not generally the trueposition of the star. It was not, in fact, the position in which thestar would have been observed had the earth been at rest. Providedwith this suggestion, he explained the apparent movements of thestars by the principle known as the "aberration of light. " Everycircumstance was accounted for as a consequence of the relativemovements of the earth and of the light from the star. Thisbeautiful discovery not only established in the most forcible mannerthe nature of the movement of light; not only did it illustrate thetruth of the Copernican theory which asserted that the earth revolvedaround the sun, but it was also of the utmost importance in theimprovement of practical astronomy. Every observer now knows that, generally speaking, the position which the star appears to have isnot exactly the position in which the star does actually lie. Theobserver is, however, able, by the application of the principleswhich Bradley so clearly laid down, to apply to an observation thecorrection which is necessary to obtain from it the true place inwhich the object is actually situated. This memorable achievement atonce conferred on Bradley the highest astronomical fame. He testedhis discovery in every way, but only to confirm its truth in the mostcomplete manner. 

Halley, the Astronomer Royal, died on the 14th, January, 1742, andBradley was immediately pointed out as his successor. He wasaccordingly appointed Astronomer Royal in February, 1742. On firsttaking up his abode at Greenwich he was unable to conduct hisobservations owing to the wretched condition in which he found theinstruments. He devoted himself, however, assiduously to theirrepair, and his first transit observation is recorded on the 25thJuly, 1742. He worked with such energy that on one day it appearsthat 255 transit observations were taken by himself alone, and inSeptember, 1747, he had completed the series of observations whichestablished his second great discovery, the nutation of the earth'saxis. The way in which he was led to the detection of the nutationis strikingly illustrative of the extreme care with which Bradleyconducted his observations. He found that in the course of atwelvemonth, when the star had completed the movement which was dueto aberration, it did not return exactly to the same position whichit had previously occupied. At first he thought this must be due tosome instrumental error, but after closer examination and repeatedstudy of the effect as manifested by many different stars, he came tothe conclusion that its origin must be sought in some quite differentsource. The fact is that a certain change takes place in theapparent position of the stars which is not due to the movement ofthe star itself, but is rather to be attributed to changes in thepoints from which the star's positions are measured. 

We may explain the matter in this way. As the earth is not a sphere, but has protuberant parts at the equator, the attraction of the moonexercises on those protuberant parts a pulling effect whichcontinually changes the direction of the earth's axis, andconsequently the position of the pole must be in a state of incessantfluctuation. The pole to which the earth's axis points on the skyis, therefore, slowly changing. At present it happens to lie nearthe Pole Star, but it will not always remain there. It describes acircle around the pole of the Ecliptic, requiring about 25, 000 yearsfor a complete circuit. In the course of its progress the pole willgradually pass now near one star and now near another, so that manystars will in the lapse of ages discharge the various functions whichthe present Pole Star does for us. In about 12, 000 years, forinstance, the pole will have come near the bright star, Vega. Thismovement of the pole had been known for ages. But what Bradleydiscovered was that the pole, instead of describing an uniformmovement as had been previously supposed, followed a sinuous coursenow on one side and now on the other of its mean place. This hetraced to the fluctuations of the moon's orbit, which undergoes acontinuous change in a period of nineteen years. Thus the efficiencywith which the moon acts on the protuberant mass of the earth varies, and thus the pole is caused to oscillate. 

This subtle discovery, if perhaps in some ways less impressive thanBradley's earlier achievements of the detection of the aberration oflight, is regarded by astronomers as testifying even in a higherdegree to his astonishing care and skill as an observer, and justlyentitles him to a unique place among the astronomers whosediscoveries have been effected by consummate practical skill in theuse of astronomical instruments. 

Of Bradley's private or domestic life there is but little to tell. In1744, soon after he became Astronomer Royal, he married a daughter ofSamuel Peach, of Chalford, in Gloucestershire. There was but onechild, a daughter, who became the wife of her cousin, Rev. SamuelPeach, rector of Compton, Beauchamp, in Berkshire. 

Bradley's last two years of life were clouded by a melancholydepression of spirits, due to an apprehension that he should survivehis rational faculties. It seems, however, that the ill he dreadednever came upon him, for he retained his mental powers to the close. He died on 13th July, 1762, aged seventy, and was buried atMichinghamton. 

WILLIAM HERSCHEL. 

William Herschel, one of the greatest astronomers that has everlived, was born at Hanover, on the 15th November, 1738. His father, Isaac Herschel, was a man evidently of considerable ability, whoselife was devoted to the study and practice of music, by which heearned a somewhat precarious maintenance. He had but few worldlygoods to leave to his children, but he more than compensated for thisby bequeathing to them a splendid inheritance of genius. Touches ofgenius were, indeed, liberally scattered among the members of Isaac'slarge family, and in the case of his forth child, William, and of asister several years younger, it was united with that determinedperseverance and rigid adherence to principle which enabled genius tofulfil its perfect work. 

A faithful chronicler has given us an interesting account of the wayin which Isaac Herschel educated his sons; the narrative is takenfrom the recollections of one who, at the time we are speaking of, was an unnoticed little girl five or six years old. She writes:--

"My brothers were often introduced as solo performers and assistantsin the orchestra at the Court, and I remember that I was frequentlyprevented from going to sleep by the lively criticisms on music oncoming from a concert. Often I would keep myself awake that I mightlisten to their animating remarks, for it made me so happy to seethem so happy. But generally their conversation would branch out onphilosophical subjects, when my brother William and my father oftenargued with such warmth that my mother's interference becamenecessary, when the names--Euler, Leibnitz, and Newton--soundedrather too loud for the repose of her little ones, who had to be atschool by seven in the morning. " The child whose reminiscences arehere given became afterwards the famous Caroline Herschel. Thenarrative of her life, by Mrs. John Herschel, is a most interestingbook, not only for the account it contains of the remarkable womanherself, but also because it provides the best picture we have of thegreat astronomer to whom Caroline devoted her life. 

This modest family circle was, in a measure, dispersed at theoutbreak of the Seven Years' War in 1756. The French proceeded toinvade Hanover, which, it will be remembered, belonged at this timeto the British dominions. Young William Herschel had alreadyobtained the position of a regular performer in the regimental bandof the Hanoverian Guards, and it was his fortune to obtain someexperience of actual warfare in the disastrous battle of Hastenbeck. He was not wounded, but he had to spend the night after the battle ina ditch, and his meditations on the occasion convinced him thatsoldiering was not the profession exactly adapted to his tastes. Weneed not attempt to conceal the fact that he left his regiment by thevery simple but somewhat risky process of desertion. He had, itwould seem, to adopt disguises to effect his escape. At all events, by some means he succeeded in eluding detection and reached Englandin safety. It is interesting to have learned on good authority thatmany years after this offence was committed it was solemnlyforgiven. When Herschel had become the famous astronomer, and assuch visited King George at Windsor, the King at their first meetinghanded to him his pardon for deserting from the army, written out indue form by his Majesty himself. 

It seems that the young musician must have had some difficulty inproviding for his maintenance during the first few years of his abodein England. It was not until he had reached the age of twenty-twothat he succeeded in obtaining any regular appointment. He was thenmade Instructor of Music to the Durham Militia. Shortly afterwards, his talents being more widely recognised, he was appointed asorganist at the parish church at Halifax, and his prospects in lifenow being fairly favourable, and the Seven Years' War being over, heventured to pay a visit to Hanover to see his father. We can imaginethe delight with which old Isaac Herschel welcomed his promising son, as well as his parental pride when a concert was given at which someof William's compositions were performed. If the father was sointensely gratified on this occasion, what would his feelings havebeen could he have lived to witness his son's future career? Butthis pleasure was not to be his, for he died many years beforeWilliam became an astronomer. 

In 1766, about a couple of years after his return to England fromThis visit to his old home, we find that Herschel had received afurther promotion to be organist in the Octagon Chapel, at Bath. Bath was then, as now, a highly fashionable resort, and many notablepersonages patronised the rising musician. Herschel had other pointsin his favour besides his professional skill; his appearance wasgood, his address was prepossessing, and even his nationality was adistinct advantage, inasmuch as he was a Hanoverian in the reign ofKing George the Third. On Sundays he played the organ, to the greatdelight of the congregation, and on week-days he was occupied bygiving lessons to private pupils, and in preparation for publicperformances. He thus came to be busily employed, and seems to havebeen in the enjoyment of comfortable means. 

[PLATE: 7, NEW KING STREET, BATH, WHERE HERSCHEL LIVED. ]

From his earliest youth Herschel had been endowed with thatinvaluable characteristic, an eager curiosity for knowledge. He wasnaturally desirous of perfecting himself in the theory of music, andthus he was led to study mathematics. When he had once tasted thecharms of mathematics, he saw vast regions of knowledge unfoldedbefore him, and in this way he was induced to direct his attention toastronomy. More and more this pursuit seems to have engrossed hisattention, until at last it had become an absorbing passion. Herschelwas, however, still obliged, by the exigency of procuring alivelihood, to give up the best part of his time to his profession asa musician; but his heart was eagerly fixed on another science, andevery spare moment was steadily devoted to astronomy. For manyyears, however, he continued to labour at his original calling, norwas it until he had attained middle age and become the mostcelebrated astronomer of the time, that he was enabled to concentratehis attention exclusively on his favourite pursuit. 

It was with quite a small telescope which had been lent him by afriend that Herschel commenced his career as an observer. However, he speedily discovered that to see all he wanted to see, a telescopeof far greater power would be necessary, and he determined to obtainthis more powerful instrument by actually making it with his ownhands. At first it may seem scarcely likely that one whoseoccupation had previously been the study and practice of music shouldmeet with success in so technical an operation as the construction ofa telescope. It may, however, be mentioned that the kind ofinstrument which Herschel designed to construct was formed on a verydifferent principle from the refracting telescopes with which we areordinarily familiar. His telescope was to be what is termed areflector. In this type of instrument the optical power is obtainedby the use of a mirror at the bottom of the tube, and the astronomerlooks down through the tube TOWARDS HIS MIRROR and views thereflection of the stars with its aid. Its efficiency as a telescopedepends entirely on the accuracy with which the requisite form hasbeen imparted to the mirror. The surface has to be hollowed out alittle, and this has to be done so truly that the slightest deviationfrom good workmanship in this essential particular would be fatal toefficient performance of the telescope. 

[PLATE: WILLIAM HERSCHEL. ]

The mirror that Herschel employed was composed of a mixture of twoparts of copper to one of tin; the alloy thus obtained is anintensely hard material, very difficult to cast into the propershape, and very difficult to work afterwards. It possesses, however, when polished, a lustre hardly inferior to that of silver itself. Herschel has recorded hardly any particulars as to the actual processby which he cast and figured his reflectors. We are however, toldthat in later years, after his telescopes had become famous, he madea considerable sum of money by the manufacture and sale of greatinstruments. Perhaps this may be the reason why he never found itexpedient to publish any very explicit details as to the means bywhich his remarkable successes were obtained. 

[PLATE: CAROLINE HERSCHEL. ]

Since Herschel's time many other astronomers, notably the late Earlof Rosse, have experimented in the same direction, and succeeded inmaking telescopes certainly far greater, and probably more perfect, than any which Herschel appears to have constructed. The details ofthese later methods are now well known, and have been extensivelypractised. Many amateurs have thus been able to make telescopes byfollowing the instructions so clearly laid down by Lord Rosse and theother authorities. Indeed, it would seem that any one who has alittle mechanical skill and a good deal of patience ought now toexperience no great difficulty in constructing a telescope quite aspowerful as that which first brought Herschel into fame. I should, however, mention that in these modern days the material generallyused for the mirror is of a more tractable description than themetallic substance which was employed by Herschel and by Lord Rosse. A reflecting telescope of the present day would not be fitted with amirror composed of that alloy known as speculum metal, whosecomposition I have already mentioned. It has been found moreadvantageous to employ a glass mirror carefully figured and polished, just as a metallic mirror would have been, and then to impart to thepolished glass surface a fine coating of silver laid down by achemical process. The silver-on-glass mirrors are so much lighterand so much easier to construct that the more old-fashioned metallicmirrors may be said to have fallen into almost total disuse. In onerespect however, the metallic mirror may still claim the advantagethat, with reasonable care, its surface will last bright anduntarnished for a much longer period than can the silver film on theglass. However, the operation of re-silvering a glass has now becomesuch a simple one that the advantage this indicates is not relativelyso great as might at first be supposed. 

[PLATE: STREET VIEW, HERSCHEL HOUSE, SLOUGH. ]

Some years elapsed after Herschel's attention had been first directedto astronomy, before he reaped the reward of his exertions in thepossession of a telescope which would adequately reveal some of theglories of the heavens. It was in 1774, when the astronomer wasthirty-six years old, that he obtained his first glimpse of the starswith an instrument of his own construction. Night after night, assoon as his musical labours were ended, his telescopes were broughtout, sometimes into the small back garden of his house at Bath, andsometimes into the street in front of his hall-door. It wascharacteristic of him that he was always endeavouring to improve hisapparatus. He was incessantly making fresh mirrors, or trying newlenses, or combinations of lenses to act as eye-pieces, or projectingalterations in the mounting by which the telescope was supported. Such was his enthusiasm that his house, we are told, was incessantlylittered with the usual indications of the workman's presence, greatly to the distress of his sister, who, at this time, had come totake up her abode with him and look after his housekeeping. Indeed, she complained that in his astronomical ardour he sometimes omittedto take off, before going into his workshop, the beautiful laceruffles which he wore while conducting a concert, and thatconsequently they became soiled with the pitch employed in thepolishing of his mirrors. 

This sister, who occupies such a distinct place in scientific historyis the same little girl to whom we have already referred. From herearliest days she seems to have cherished a passionate admiration forher brilliant brother William. It was the proudest delight of herchildhood as well as of her mature years to render him whateverservice she could; no man of science was ever provided with a morecapable or energetic helper than William Herschel found in thisremarkable woman. Whatever work had to be done she was willing tobear her share in it, or even to toil at it unassisted if she couldbe allowed to do so. She not only managed all his domestic affairs, but in the grinding of the lenses and in the polishing of the mirrorsshe rendered every assistance that was possible. At one stage of thevery delicate operation of fashioning a reflector, it is necessaryfor the workman to remain with his hand on the mirror for many hoursin succession. When such labours were in progress, Caroline used tosit by her brother, and enliven the time by reading stories aloud, sometimes pausing to feed him with a spoon while his hands wereengaged on the task from which he could not desist for a moment. 

When mathematical work had to be done Caroline was ready for it; shehad taught herself sufficient to enable her to perform the kind ofcalculations, not, perhaps, very difficult ones, that Herschel's workrequired; indeed, it is not too much to say that the mighty life-workwhich this man was enabled to perform could never have been accomplishedhad it not been for the self-sacrifice of this ever-loving and faithfulsister. When Herschel was at the telescope at night, Caroline sat byhim at her desk, pen in hand, ready to write down the notes of theobservations as they fell from her brother's lips. This was noinsignificant toil. The telescope was, of course, in the open air, and as Herschel not unfrequently continued his observations throughoutthe whole of a long winter's night, there were but few women who couldhave accomplished the task which Caroline so cheerfully executed. From dusk till dawn, when the sky was clear, were Herschel's observinghours, and what this sometimes implied we can realise from the factthat Caroline assures us she had sometimes to desist because the inkhad actually frozen in her pen. The night's work over, a brief restwas taken, and while William had his labours for the day to attend to, Caroline carefully transcribed the observations made during the nightbefore, reduced all the figures and prepared everything in readinessfor the observations that were to follow on the ensuing evening. 

But we have here been anticipating a little of the future which laybefore the great astronomer; we must now revert to the history of hisearly work, at Bath, in 1774, when Herschel's scrutiny of the skiesfirst commenced with an instrument of his own manufacture. For somefew years he did not attain any result of importance; no doubt hemade a few interesting observations, but the value of the work duringthose years is to be found, not in any actual discoveries which wereaccomplished, but in the practice which Herschel obtained in the useof his instruments. It was not until 1782 that the great achievementtook place by which he at once sprang into fame. 

[PLATE: GARDEN VIEW, HERSCHEL HOUSE, SLOUGH. ]

It is sometimes said that discoveries are made by accident, and, nodoubt, to a certain extent, but only, I fancy to a very small extent, this statement may be true. It is, at all events, certain that suchlucky accidents do not often fall to the lot of people unless thosepeople have done much to deserve them. This was certainly the casewith Herschel. He appears to have formed a project for making aclose examination of all the stars above a certain magnitude. Perhapshe intended to confine this research to a limited region of the sky, but, at all events, he seems to have undertaken the workenergetically and systematically. Star after star was brought to thecentre of the field of view of his telescope, and after beingcarefully examined was then displaced, while another star was broughtforward to be submitted to the same process. In the great majorityof cases such observations yield really nothing of importance; nodoubt even the smallest star in the heavens would, if we could findout all about it, reveal far more than all the astronomers that wereever on the earth have even conjectured. What we actually learnabout the great majority of stars is only information of the mostmeagre description. We see that the star is a little point of light, and we see nothing more. 

In the great review which Herschel undertook he doubtless examinedhundreds, or perhaps thousands of stars, allowing them to pass awaywithout note or comment. But on an ever-memorable night in March, 1782, it happened that he was pursuing his task among the stars inthe Constellation of Gemini. Doubtless, on that night, as on so manyother nights, one star after another was looked at only to bedismissed, as not requiring further attention. On the evening inquestion, however, one star was noticed which, to Herschel's acutevision seemed different from the stars which in so many thousands arestrewn over the sky. A star properly so called appears merely as alittle point of light, which no increase of magnifying power willever exhibit with a true disc. But there was something in thestar-like object which Herschel saw that immediately arrested hisattention and made him apply to it a higher magnifying power. Thisat once disclosed the fact that the object possessed a disc, that is, a definite, measurable size, and that it was thus totally differentfrom any one of the hundreds and thousands of stars which existelsewhere in space. Indeed, we may say at once that this littleobject was not a star at all; it was a planet. That such was itstrue nature was confirmed, after a little further observation, byperceiving that the body was shifting its place on the heavensrelatively to the stars. The organist at the Octagon Chapel at Bathhad, therefore, discovered a new planet with his home-made telescope. 

I can imagine some one will say, "Oh, there was nothing so wonderfulin that; are not planets always being discovered? Has not M. Palisa, for instance, discovered about eighty of such objects, and are therenot hundreds of them known nowadays?" This is, to a certain extent, quite true. I have not the least desire to detract from the creditof those industrious and sharp-sighted astronomers who have in moderndays brought so many of these little objects within our cognisance. Ithink, however, it must be admitted that such discoveries have atotally different importance in the history of science from thatwhich belongs to the peerless achievement of Herschel. In the firstplace, it must be observed that the minor planets now brought tolight are so minute that if a score of them were rolled to togetherinto one lump it would not be one-thousandth part of the size of thegrand planet discovered by Herschel. This is, nevertheless, not themost important point. What marks Herschel's achievement as one ofthe great epochs in the history of astronomy is the fact that thedetection of Uranus was the very first recorded occasion of thediscovery of any planet whatever. 

For uncounted ages those who watched the skies had been aware of theexistence of the five old planets--Jupiter, Mercury, Saturn, Venus, and Mars. It never seems to have occurred to any of the ancientphilosophers that there could be other similar objects as yetundetected over and above the well-known five. Great then was theastonishment of the scientific world when the Bath organist announcedhis discovery that the five planets which had been known from allantiquity must now admit the company of a sixth. And this sixthplanet was, indeed, worthy on every ground to be received into theranks of the five glorious bodies of antiquity. It was, no doubt, not so large as Saturn, it was certainly very much less than Jupiter;on the other hand, the new body was very much larger than Mercury, than Venus, or than Mars, and the earth itself seemed quite aninsignificant object in comparison with this newly added member ofthe Solar System. In one respect, too, Herschel's new planet was amuch more imposing object than any one of the older bodies; it sweptaround the sun in a majestic orbit, far outside that of Saturn, whichhad previously been regarded as the boundary of the Solar System, andits stately progress required a period of not less than eighty-oneyears. 

King George the Third, hearing of the achievements of the Hanoverianmusician, felt much interest in his discovery, and accordinglyHerschel was bidden to come to Windsor, and to bring with him thefamous telescope, in order to exhibit the new planet to the King, andto tell his Majesty all about it. The result of the interview was togive Herschel the opportunity for which he had so long wished, ofbeing able to devote himself exclusively to science for the rest ofhis life. 

[PLATE: VIEW OF THE OBSERVATORY, HERSCHEL HOUSE, SLOUGH. ]

The King took so great a fancy to the astronomer that he first, as Ihave already mentioned, duly pardoned his desertion from the army, some twenty-five years previously. As a further mark of his favourthe King proposed to confer on Herschel the title of his Majesty'sown astronomer, to assign to him a residence near Windsor, to providehim with a salary, and to furnish such funds as might be required forthe erection of great telescopes, and for the conduct of that mightyscheme of celestial observation on which Herschel was so eager toenter. Herschel's capacity for work would have been much impaired ifhe had been deprived of the aid of his admirable sister, and to her, therefore, the King also assigned a salary, and she was installed asHerschel's assistant in his new post. 

With his usually impulsive determination, Herschel immediately cuthimself free from all his musical avocations at Bath, and at onceentered on the task of making and erecting the great telescopes atWindsor. There, for more than thirty years, he and his faithfulsister prosecuted with unremitting ardour their nightly scrutiny ofthe sky. Paper after paper was sent to the Royal Society, describingthe hundreds, indeed the thousands, of objects such as double stars;nebulae and clusters, which were first revealed to human gaze duringthose midnight vigils. To the end of his life he still continued atevery possible opportunity to devote himself to that beloved pursuitin which he had such unparalleled success. No single discovery ofHerschel's later years was, however, of the same momentousdescription as that which first brought him to fame. 

[PLATE: THE 40-FOOT TELESCOPE AS IT WAS IN THE YEAR 1863, HERSCHELHOUSE, SLOUGH. ]

Herschel married when considerably advanced in life and he lived toenjoy the indescribable pleasure of finding that his only son, afterwards Sir John Herschel, was treading worthily in his footsteps, and attaining renown as an astronomical observer, second only to thatof his father. The elder Herschel died in 1822, and his illustrioussister Caroline then returned to Hanover, where she lived for manyyears to receive the respect and attention which were so justlyhers. She died at a very advanced age in 1848. 

LAPLACE. 

The author of the "Mecanique Celeste" was born at Beaumont-en-Auge, near Honfleur, in 1749, just thirteen years later than his renownedfriend Lagrange. His father was a farmer, but appears to have beenin a position to provide a good education for a son who seemedpromising. Considering the unorthodoxy in religious matters which isgenerally said to have characterized Laplace in later years, it isinteresting to note that when he was a boy the subject which firstclaimed his attention was theology. He was, however, soon introducedto the study of mathematics, in which he presently became soproficient, that while he was still no more than eighteen years old, he obtained employment as a mathematical teacher in his native town. 

Desiring wider opportunities for study and for the acquisition offame than could be obtained in the narrow associations of provinciallife, young Laplace started for Paris, being provided with letters ofintroduction to D'Alembert, who then occupied the most prominentposition as a mathematician in France, if not in the whole ofEurope. D'Alembert's fame was indeed so brilliant that Catherine theGreat wrote to ask him to undertake the education of her Son, andpromised the splendid income of a hundred thousand francs. Hepreferred, however, a quiet life of research in Paris, although therewas but a modest salary attached to his office. The philosopheraccordingly declined the alluring offer to go to Russia, even thoughCatherine wrote again to say: "I know that your refusal arises fromyour desire to cultivate your studies and your friendships in quiet. But this is of no consequence: bring all your friends with you, and Ipromise you that both you and they shall have every accommodation inmy power. " With equal firmness the illustrious mathematicianresisted the manifold attractions with which Frederick the Greatsought to induce him, to take up his residence at Berlin. In readingof these invitations we cannot but be struck at the extraordinaryrespect which was then paid to scientific distinction. It must beremembered that the discoveries of such a man as D'Alembert wereutterly incapable of being appreciated except by those who possesseda high degree of mathematical culture. We nevertheless find thepotentates of Russia and Prussia entreating and, as it happens, vainly entreating, the most distinguished mathematician in France toaccept the positions that they were proud to offer him. 

It was to D'Alembert, the profound mathematician, that young Laplace, the son of the country farmer, presented his letters ofintroduction. But those letters seem to have elicited no reply, whereupon Laplace wrote to D'Alembert submitting a discussion on somepoint in Dynamics. This letter instantly produced the desiredeffect. D'Alembert thought that such mathematical talent as theyoung man displayed was in itself the best of introductions to hisfavour. It could not be overlooked, and accordingly he invitedLaplace to come and see him. Laplace, of course, presented himself, and ere long D'Alembert obtained for the rising philosopher aprofessorship of mathematics in the Military School in Paris. Thisgave the brilliant young mathematician the opening for which hesought, and he quickly availed himself of it. 

Laplace was twenty-three years old when his first memoir on aprofound mathematical subject appeared in the Memoirs of the Academyat Turin. From this time onwards we find him publishing one memoirafter another in which he attacks, and in many cases successfullyvanquishes, profound difficulties in the application of the Newtoniantheory of gravitation to the explanation of the solar system. Likehis great contemporary Lagrange, he loftily attempted problems whichdemanded consummate analytical skill for their solution. Theattention of the scientific world thus became riveted on the splendiddiscoveries which emanated from these two men, each gifted withextraordinary genius. 

Laplace's most famous work is, of course, the "Mecanique Celeste, " inwhich he essayed a comprehensive attempt to carry out the principleswhich Newton had laid down, into much greater detail than Newton hadfound practicable. The fact was that Newton had not only toconstruct the theory of gravitation, but he had to invent themathematical tools, so to speak, by which his theory could be appliedto the explanation of the movements of the heavenly bodies. In thecourse of the century which had elapsed between the time of Newtonand the time of Laplace, mathematics had been extensively developed. In particular, that potent instrument called the infinitesimalcalculus, which Newton had invented for the investigation of nature, had become so far perfected that Laplace, when he attempted tounravel the movements of the heavenly bodies, found himself providedwith a calculus far more efficient than that which had been availableto Newton. The purely geometrical methods which Newton employed, though they are admirably adapted for demonstrating in a general waythe tendencies of forces and for explaining the more obviousphenomena by which the movements of the heavenly bodies aredisturbed, are yet quite inadequate for dealing with the more subtleeffects of the Law of Gravitation. The disturbances which one planetexercises upon the rest can only be fully ascertained by the aid oflong calculation, and for these calculations analytical methods arerequired. 

With an armament of mathematical methods which had been perfectedsince the days of Newton by the labours of two or three generationsof consummate mathematical inventors, Laplace essayed in the"Mecanique Celeste" to unravel the mysteries of the heavens. It willhardly be disputed that the book which he has produced is one of themost difficult books to understand that has ever been written. Ingreat part, of course, this difficulty arises from the very nature ofthe subject, and is so far unavoidable. No one need attempt to readthe "Mecanique Celeste" who has not been naturally endowed withconsiderable mathematical aptitude which he has cultivated by yearsof assiduous study. The critic will also note that there are gravedefects in Laplace's method of treatment. The style is oftenextremely obscure, and the author frequently leaves great gaps in hisargument, to the sad discomfiture of his reader. Nor does it mendmatters to say, as Laplace often does say, that it is "easy to see"how one step follows from another. Such inferences often presentgreat difficulties even to excellent mathematicians. Traditionindeed tells us that when Laplace had occasion to refer to his ownbook, it sometimes happened that an argument which he had dismissedwith his usual formula, "Il est facile a voir, " cost the illustriousauthor himself an hour or two of hard thinking before he couldrecover the train of reasoning which had been omitted. But there arecertain parts of this great work which have always received theenthusiastic admiration of mathematicians. Laplace has, in fact, created whole tracts of science, some of which have been subsequentlydeveloped with much advantage in the prosecution of the study ofNature. 

Judged by a modern code the gravest defect of Laplace's great work israther of a moral than of a mathematical nature. Lagrange and headvanced together in their study of the mechanics of the heavens, atone time perhaps along parallel lines, while at other times theypursued the same problem by almost identical methods. Sometimes theimportant result was first reached by Lagrange, sometimes it wasLaplace who had the good fortune to make the discovery. It woulddoubtless be a difficult matter to draw the line which should exactlyseparate the contributions to astronomy made by one of theseillustrious mathematicians, and the contributions made by the other. But in his great work Laplace in the loftiest manner disdained toaccord more than the very barest recognition to Lagrange, or to anyof the other mathematicians, Newton alone excepted, who had advancedour knowledge of the mechanism of the heavens. It would be quiteimpossible for a student who confined his reading to the "MecaniqueCeleste" to gather from any indications that it contains whether thediscoveries about which he was reading had been really made byLaplace himself or whether they had not been made by Lagrange, or byEuler, or by Clairaut. With our present standard of morality in suchmatters, any scientific man who now brought forth a work in which hepresumed to ignore in this wholesale fashion the contributions ofothers to the subject on which he was writing, would be justlycensured and bitter controversies would undoubtedly arise. Perhapswe ought not to judge Laplace by the standard of our own time, and inany case I do not doubt that Laplace might have made a plausibledefence. It is well known that when two investigators are working atthe same subjects, and constantly publishing their results, itsometimes becomes difficult for each investigator himself todistinguish exactly between what he has accomplished and that whichmust be credited to his rival. Laplace may probably have said tohimself that he was going to devote his energies to a great work onthe interpretation of Nature, that it would take all his time and allhis faculties, and all the resources of knowledge that he couldcommand, to deal justly with the mighty problems before him. Hewould not allow himself to be distracted by any side issue. He couldnot tolerate that pages should be wasted in merely discussing to whomwe owe each formula, and to whom each deduction from such formula isdue. He would rather endeavour to produce as complete a picture ashe possibly could of the celestial mechanics, and whether it were bymeans of his mathematics alone, or whether the discoveries of othersmay have contributed in any degree to the result, is a matter soinfinitesimally insignificant in comparison with the grandeur of hissubject that he would altogether neglect it. "If Lagrange shouldthink, " Laplace might say, "that his discoveries had been undulyappropriated, the proper course would be for him to do exactly what Ihave done. Let him also write a "Mecanique Celeste, " let him employthose consummate talents which he possesses in developing his noblesubject to the utmost. Let him utilise every result that I or anyother mathematician have arrived at, but not trouble himself undulywith unimportant historical details as to who discovered this, andwho discovered that; let him produce such a work as he could write, and I shall heartily welcome it as a splendid contribution to ourscience. " Certain it is that Laplace and Lagrange continued the bestof friends, and on the death of the latter it was Laplace who wassummoned to deliver the funeral oration at the grave of his greatrival. 

The investigations of Laplace are, generally speaking, of tootechnical a character to make it possible to set forth any account ofthem in such a work as the present. He did publish, however, onetreatise, called the "Systeme du Monde, " in which, withoutintroducing mathematical symbols, he was able to give a generalaccount of the theories of the celestial movements, and of thediscoveries to which he and others had been led. In this work thegreat French astronomer sketched for the first time that remarkabledoctrine by which his name is probably most generally known to thosereaders of astronomical books who are not specially mathematicians. It is in the "Systeme du Monde" that Laplace laid down the principlesof the Nebular Theory which, in modern days, has been generallyaccepted by those philosophers who are competent to judge, assubstantially a correct expression of a great historical fact. 

[PLATE: LAPLACE. ]

The Nebular Theory gives a physical account of the origin of thesolar system, consisting of the sun in the centre, with the planetsand their attendant satellites. Laplace perceived the significanceof the fact that all the planets revolved in the same directionaround the sun; he noticed also that the movements of rotation of theplanets on their axes were performed in the same direction as that inwhich a planet revolves around the sun; he saw that the orbits of thesatellites, so far at least as he knew them, revolved around theirprimaries also in the same direction. Nor did it escape hisattention that the sun itself rotated on its axis in the same sense. His philosophical mind was led to reflect that such a remarkableunanimity in the direction of the movements in the solar systemdemanded some special explanation. It would have been in the highestdegree improbable that there should have been this unanimity unlessthere had been some physical reason to account for it. To appreciatethe argument let us first concentrate our attention on threeparticular bodies, namely the earth, the sun, and the moon. Firstthe earth revolves around the sun in a certain direction, and theearth also rotates on its axis. The direction in which the earthturns in accordance with this latter movement might have been that inwhich it revolves around the sun, or it might of course have beenopposite thereto. As a matter of fact the two agree. The moon inits monthly revolution around the earth follows also the samedirection, and our satellite rotates on its axis in the same periodas its monthly revolution, but in doing so is again observing thissame law. We have therefore in the earth and moon four movements, all taking place in the same direction, and this is also identicalwith that in which the sun rotates once every twenty-five days. Sucha coincidence would be very unlikely unless there were some physicalreason for it. Just as unlikely would it be that in tossing a coinfive heads or five tails should follow each other consecutively. Ifwe toss a coin five times the chances that it will turn up all headsor all tails is but a small one. The probability of such an event isonly one-sixteenth. 

There are, however, in the solar system many other bodies besides thethree just mentioned which are animated by this common movement. Among them are, of course, the great planets, Jupiter, Saturn, Mars, Venus, and Mercury, and the satellites which attend on theseplanets. All these planets rotate on their axes in the samedirection as they revolve around the sun, and all their satellitesrevolve also in the same way. Confining our attention merely to theearth, the sun, and the five great planets with which Laplace wasacquainted, we have no fewer than six motions of revolution and sevenmotions of rotation, for in the latter we include the rotation of thesun. We have also sixteen satellites of the planets mentioned whoserevolutions round their primaries are in the same direction. Therotation of the moon on its axis may also be reckoned, but as to therotations of the satellites of the other planets we cannot speak withany confidence, as they are too far off to be observed with thenecessary accuracy. We have thus thirty circular movements in thesolar system connected with the sun and moon and those great planetsthan which no others were known in the days of Laplace. Thesignificant fact is that all these thirty movements take place in thesame direction. That this should be the case without some physicalreason would be just as unlikely as that in tossing a coin thirtytimes it should turn up all heads or all tails every time withoutexception. 

We can express the argument numerically. Calculation proves thatsuch an event would not generally happen oftener than once out offive hundred millions of trials. To a philosopher of Laplace'spenetration, who had made a special study of the theory ofprobabilities, it seemed well-nigh inconceivable that there shouldhave been such unanimity in the celestial movements, unless there hadbeen some adequate reason to account for it. We might, indeed, addthat if we were to include all the objects which are now known tobelong to the solar system, the argument from probability might beenormously increased in strength. To Laplace the argument appearedso conclusive that he sought for some physical cause of theremarkable phenomenon which the solar system presented. Thus it wasthat the famous Nebular Hypothesis took its rise. Laplace devised ascheme for the origin of the sun and the planetary system, in whichit would be a necessary consequence that all the movements shouldtake place in the same direction as they are actually observed to do. 

Let us suppose that in the beginning there was a gigantic mass ofnebulous material, so highly heated that the iron and othersubstances which now enter into the composition of the earth andplanets were then suspended in a state of vapour. There is nothingunreasonable in such a supposition indeed, we know as a matter offact that there are thousands of such nebulae to be discerned atpresent through our telescopes. It would be extremely unlikely thatany object could exist without possessing some motion of rotation; wemay in fact assert that for rotation to be entirety absent from thegreat primeval nebula would be almost infinitely improbable. As agesrolled on, the nebula gradually dispersed away by radiation itsoriginal stores of heat, and, in accordance with well-known physicalprinciples, the materials of which it was formed would tend tocoalesce. The greater part of those materials would becomeconcentrated in a mighty mass surrounded by outlying uncondensedvapours. There would, however, also be regions throughout the extentof the nebula, in which subsidiary centres of condensation would befound. In its long course of cooling, the nebula would, therefore, tend ultimately to form a mighty central body with a number ofsmaller bodies disposed around it. As the nebula was initiallyendowed with a movement of rotation, the central mass into which ithad chiefly condensed would also revolve, and the subsidiary bodieswould be animated by movements of revolution around the centralbody. These movements would be all pursued in one common direction, and it follows, from well-known mechanical principles, that each ofthe subsidiary masses, besides participating in the generalrevolution around the central body, would also possess a rotationaround its axis, which must likewise be performed in the samedirection. Around the subsidiary bodies other objects still smallerwould be formed, just as they themselves were formed relatively tothe great central mass. 

As the ages sped by, and the heat of these bodies became graduallydissipated, the various objects would coalesce, first into moltenliquid masses, and thence, at a further stage of cooling, they wouldassume the appearance of solid masses, thus producing the planetarybodies such as we now know them. The great central mass, on accountof its preponderating dimensions, would still retain, for furtheruncounted ages, a large quantity of its primeval heat, and would thusdisplay the splendours of a glowing sun. In this way Laplace wasable to account for the remarkable phenomena presented in themovements of the bodies of the solar system. There are many otherpoints also in which the nebular theory is known to tally with thefacts of observation. In fact, each advance in science only seems tomake it more certain that the Nebular Hypothesis substantiallyrepresents the way in which our solar system has grown to its presentform. 

Not satisfied with a career which should be merely scientific, Laplace sought to connect himself with public affairs. Napoleonappreciated his genius, and desired to enlist him in the service ofthe State. Accordingly he appointed Laplace to be Minister of theInterior. The experiment was not successful, for he was not bynature a statesman. Napoleon was much disappointed at the ineptitudewhich the great mathematician showed for official life, and, indespair of Laplace's capacity as an administrator, declared that hecarried the spirit of his infinitesimal calculus into the managementof business. Indeed, Laplace's political conduct hardly admits ofmuch defence. While he accepted the honours which Napoleon showeredon him in the time of his prosperity, he seems to have forgotten allthis when Napoleon could no longer render him service. Laplace wasmade a Marquis by Louis XVIII. , a rank which he transmitted to hisson, who was born in 1789. During the latter part of his life thephilosopher lived in a retired country place at Arcueile. Here hepursued his studies, and by strict abstemiousness, preserved himselffrom many of the infirmities of old age. He died on March the 5th, 1827, in his seventy-eighth year, his last words being, "What we knowis but little, what we do not know is immense. "

BRINKLEY. 

Provost Baldwin held absolute sway in the University of Dublin forforty-one years. His memory is well preserved there. The Bursarstill dispenses the satisfactory revenues which Baldwin left to theCollege. None of us ever can forget the marble angels round thefigure of the dying Provost on which we used to gaze during the pangsof the Examination Hall. 

Baldwin died in 1785, and was succeeded by Francis Andrews, a Fellowof seventeen years' standing. As to the scholastic acquirements ofAndrews, all I can find is a statement that he was complimented bythe polite Professors of Padua on the elegance and purity with whichhe discoursed to them in Latin. Andrews was also reputed to be askilful lawyer. He was certainly a Privy Councillor and a prominentmember of the Irish House of Commons, and his social qualities wereexcellent. Perhaps it was Baldwin's example that stimulated a desirein Andrews to become a benefactor to his college. He accordinglybequeathed a sum of 3, 000 pounds and an annual income of 250 poundswherewith to build and endow an astronomical Observatory in theUniversity. The figures just stated ought to be qualified by thewords of cautious Ussher (afterwards the first Professor ofAstronomy), that "this money was to arise from an accumulation of apart of his property, to commence upon a particular contingencyhappening to his family. " The astronomical endowment was soon injeopardy by litigation. Andrews thought he had provided for hisrelations by leaving to them certain leasehold interests connectedwith the Provost's estate. The law courts, however, held that theseinterests were not at the disposal of the testator, and handed themover to Hely Hutchinson, the next Provost. The disappointedrelations then petitioned the Irish Parliament to redress thisgrievance by transferring to them the moneys designed by Andrews forthe Observatory. It would not be right, they contended, that thekindly intentions of the late Provost towards his kindred should befrustrated for the sake of maintaining what they described as "apurely ornamental institution. " The authorities of the Collegeprotested against this claim. Counsel were heard, and a Committee ofthe House made a report declaring the situation of the relations tobe a hard one. Accordingly, a compromise was made, and the disputeterminated. 

The selection of a site for the new astronomical Observatory was madeby the Board of Trinity College. The beautiful neighbourhood ofDublin offered a choice of excellent localities. On the north sideof the Liffey an Observatory could have been admirably placed, eitheron the remarkable promontory of Howth or on the elevation of whichDunsink is the summit. On the south side of Dublin there are severaleminences that would have been suitable: the breezy heaths atFoxrock combine all necessary conditions; the obelisk hill atKilliney would have given one of the most picturesque sites for anObservatory in the world; while near Delgany two or three other goodsituations could be mentioned. But the Board of those pre-railwaydays was naturally guided by the question of proximity. Dunsink wasaccordingly chosen as the most suitable site within the distance of areasonable walk from Trinity College. 

The northern boundary of the Phoenix Park approaches the little riverTolka, which winds through a succession of delightful bits of sylvanscenery, such as may be found in the wide demesne of Abbotstown andthe classic shades of Glasnevin. From the banks of the Tolka, on theopposite side of the park, the pastures ascend in a gentle slope toculminate at Dunsink, where at a distance of half a mile from thestream, of four miles from Dublin, and at a height of 300 feet abovethe sea, now stands the Observatory. From the commanding position ofDunsink a magnificent view is obtained. To the east the sea isvisible, while the southern prospect over the valley of the Liffey isbounded by a range of hills and mountains extending from Killiney toBray Head, thence to the little Sugar Loaf, the Two Rock and theThree Rock Mountains, over the flank of which the summit of the GreatSugar Loaf is just perceptible. Directly in front opens the finevalley of Glenasmole, with Kippure Mountain, while the range can befollowed to its western extremity at Lyons. The climate of Dunsinkis well suited for astronomical observation. No doubt here, aselsewhere in Ireland, clouds are abundant, but mists or haze arecomparatively unusual, and fogs are almost unknown. 

The legal formalities to be observed in assuming occupation exacted adelay of many months; accordingly, it was not until the 10thDecember, 1782, that a contract could be made with Mr. Graham Moyersfor the erection of a meridian-room and a dome for an equatorial, inconjunction with a becoming residence for the astronomer. Before thework was commenced at Dunsink, the Board thought it expedient toappoint the first Professor of Astronomy. They met for this purposeon the 22nd January, 1783, and chose the Rev. Henry Ussher, a SeniorFellow of Trinity College, Dublin. The wisdom of the appointment wasimmediately shown by the assiduity with which Ussher engaged infounding the observatory. In three years he had erected thebuildings and equipped them with instruments, several of which wereof his own invention. On the 19th of February, 1785, a special grantof 200 pounds was made by the Board to Dr. Ussher as some recompensefor his labours. It happened that the observatory was not the onlyscientific institution which came into being in Ireland at thisperiod; the newly-kindled ardour for the pursuit of knowledge led, atthe same time, to the foundation of the Royal Irish Academy. By afitting coincidence, the first memoir published in the "TransactionsOf The Royal Irish Academy, " was by the first Andrews, Professor ofAstronomy. It was read on the 13th of June, 1785, and bore thetitle, "Account of the Observatory belonging to Trinity College, " bythe Rev. H. Ussher, D. D. , M. R. I. A. , F. R. S. This communication showsthe extensive design that had been originally intended for Dunsink, only a part of which was, however, carried out. For instance, twolong corridors, running north and south from the central edifice, which are figured in the paper, never developed into bricks andmortar. We are not told why the original scheme had to becontracted; but perhaps the reason may be not unconnected with aremark of Ussher's, that the College had already advanced from itsown funds a sum considerably exceeding the original bequest. Thepicture of the building shows also the dome for the South equatorial, which was erected many years later. 

Ussher died in 1790. During his brief career at the observatory, heobserved eclipses, and is stated to have done other scientific work. The minutes of the Board declare that the infant institution hadalready obtained celebrity by his labours, and they urge the claimsof his widow to a pension, on the ground that the disease from whichhe died had been contracted by his nightly vigils. The Board alsopromised a grant of fifty guineas as a help to bring out Dr. Ussher'ssermons. They advanced twenty guineas to his widow towards thepublication of his astronomical papers. They ordered his bust to beexecuted for the observatory, and offered "The Death of Ussher" asthe subject of a prize essay; but, so far as I can find, neither thesermons nor the papers, neither the bust nor the prize essay, evercame into being. 

There was keen competition for the chair of Astronomy which the deathof Ussher vacated. The two candidates were Rev. John Brinkley, ofCaius College, Cambridge, a Senior Wrangler (born at Woodbridge, Suffolk, in 1763), and Mr. Stack, Fellow of Trinity College, Dublin, and author of a book on Optics. A majority of the Board at firstsupported Stack, while Provost Hely Hutchinson and one or two otherssupported Brinkley. In those days the Provost had a veto atelections, so that ultimately Stack was withdrawn and Brinkley waselected. This took place on the 11th December, 1790. The nationalpress of the day commented on the preference shown to the youngEnglishman, Brinkley, over his Irish rival. An animated controversyensued. The Provost himself condescended to enter the lists and tovindicate his policy by a long letter in the "Public Register" or"Freeman's Journal, " of 21st December, 1790. This letter wasanonymous, but its authorship is obvious. It gives thecorrespondence with Maskelyne and other eminent astronomers, whoseadvice and guidance had been sought by the Provost. It also contendsthat "the transactions of the Board ought not to be canvassed in thenewspapers. " For this reference, as well as for much otherinformation, I am indebted to my friend, the Rev. John Stubbs, D. D. 

[PLATE: THE OBSERVATORY, DUNSINK. From a Photograph by W. Lawrence, Upper Sackville Street, Dublin. ]

The next event in the history of the Observatory was the issue ofLetters Patent (32 Geo. III. , A. D. 1792), in which it is recited that"We grant and ordain that there shall be forever hereafter aProfessor of Astronomy, on the foundation of Dr. Andrews, to becalled and known by the name of the Royal Astronomer of Ireland. " Theletters prescribe the various duties of the astronomer and the modeof his election. They lay down regulations as to the conduct of theastronomical work, and as to the choice of an assistant. They directthat the Provost and the Senior Fellows shall make a thoroughinspection of the observatory once every year in June or July; andthis duty was first undertaken on the 5th of July, 1792. It may benoted that the date on which the celebration of the tercentenary ofthe University was held happens to coincide with the centenary of thefirst visitation of the observatory. The visitors on the firstoccasion were A. Murray, Matthew Young, George Hall, and JohnBarrett. They record that they find the buildings, books andinstruments in good condition; but the chief feature in this report, as well as in many which followed it, related to a circumstance towhich we have not yet referred. 

In the original equipment of the observatory, Ussher, with thenatural ambition of a founder, desired to place in it a telescope ofmore magnificent proportions than could be found anywhere else. TheBoard gave a spirited support to this enterprise, and negotiationswere entered into with the most eminent instrument-maker of thosedays. This was Jesse Ramsden (1735-1800), famous as the improver ofthe sextant, as the constructor of the great theodolite used byGeneral Roy in the English Survey, and as the inventor of thedividing engine for graduating astronomical instruments. Ramsden hadbuilt for Sir George Schuckburgh the largest and most perfectequatorial ever attempted. He had constructed mural quadrants forPadua and Verona, which elicited the wonder of astronomers when Dr. Maskelyne declared he could detect no error in their graduation solarge as two seconds and a half. But Ramsden maintained that evenbetter results would be obtained by superseding the entire quadrantby the circle. He obtained the means of testing this prediction whenhe completed a superb circle for Palermo of five feet diameter. Finding his anticipations were realised, he desired to apply the sameprinciples on a still grander scale. Ramsden was in this mood whenhe met with Dr. Ussher. The enthusiasm of the astronomer and theinstrument-maker communicated itself to the Board, and a tremendouscircle, to be ten feet in diameter, was forthwith projected. 

Projected, but never carried out. After Ramsden had to some extentcompleted a 10-foot circle, he found such difficulties that he trieda 9-foot, and this again he discarded for an 8-foot, which wasultimately accomplished, though not entirely by himself. Notwithstanding the contraction from the vast proportions originallydesigned, the completed instrument must still be regarded as acolossal piece of astronomical workmanship. Even at this day I donot know that any other observatory can show a circle eight feet indiameter graduated all round. 

I think it is Professor Piazzi Smith who tells us how grateful he wasto find a large telescope he had ordered finished by the opticians onthe very day they had promised it. The day was perfectly correct; itwas only the year that was wrong. A somewhat remarkable experiencein this direction is chronicled by the early reports of the visitorsto Dunsink Observatory. I cannot find the date on which the greatcircle was ordered from Ramsden, but it is fixed with sufficientprecision by an allusion in Ussher's paper to the Royal IrishAcademy, which shows that by the 13th June, 1785, the order had beengiven, but that the abandonment of the 10-foot scale had not thenbeen contemplated. It was reasonable that the board should allowRamsden ample time for the completion of a work at once so elaborateand so novel. It could not have been finished in a year, nor wouldthere have been much reason for complaint if the maker had found herequired two or even three years more. 

Seven years gone, and still no telescope, was the condition in whichthe Board found matters at their first visitation in 1792. They had, however, assurances from Ramsden that the instrument would becompleted within the year; but, alas for such promises, another sevenyears rolled on, and in 1799 the place for the great circle was stillvacant at Dunsink. Ramsden had fallen into bad health, and the Boardconsiderately directed that "inquiries should be made. " Next yearthere was still no progress, so the Board were roused to threatenRamsden with a suit at law; but the menace was never executed, forthe malady of the great optician grew worse, and he died that year. 

Affairs had now assumed a critical aspect, for the college hadadvanced much money to Ramsden during these fifteen years, and theinstrument was still unfinished. An appeal was made by the Provostto Dr. Maskelyne, the Astronomer Royal of England, for his advice andkindly offices in this emergency. Maskelyne responds--in termscalculated to allay the anxiety of the Bursar--"Mr. Ramsden has leftproperty behind him, and the College can be in no danger of losingboth their money and the instrument. " The business of Ramsden wasthen undertaken by Berge, who proceeded to finish the circle quite asdeliberately as his predecessor. After four years Berge promised theinstrument in the following August, but it did not come. Two yearslater (1806) the professor complains that he can get no answer fromBerge. In 1807, it is stated that Berge will send the telescope in amonth. He did not; but in the next year (1808), about twenty-threeyears after the great circle was ordered, it was erected at Dunsink, where it is still to be seen. 

The following circumstances have been authenticated by the signaturesof Provosts, Proctors, Bursars, and other College dignitaries:--In1793 the Board ordered two of the clocks at the observatory to besent to Mr. Crosthwaite for repairs. Seven years later, in 1800, Mr. Crosthwaite was asked if the clocks were ready. This impatience wasclearly unreasonable, for even in four more years, 1804, we find thetwo clocks were still in hand. Two years later, in 1806, the Boarddetermined to take vigorous action by asking the Bursar to call uponCrosthwaite. This evidently produced some effect, for in thefollowing year, 1807, the Professor had no doubt that the clockswould be speedily returned. After eight years more, in 1815, one ofthe clocks was still being repaired, and so it was in 1816, which isthe last record we have of these interesting timepieces. Astronomersare, however, accustomed to deal with such stupendous periods intheir calculations, that even the time taken to repair a clock seemsbut small in comparison. 

The long tenure of the chair of Astronomy by Brinkley is divided intotwo nearly equal periods by the year in which the great circle waserected. Brinkley was eighteen years waiting for his telescope, andhe had eighteen years more in which to use it. During the first ofthese periods Brinkley devoted himself to mathematical research;during the latter he became a celebrated astronomer. Brinkley'smathematical labours procured for their author some reputation as amathematician. They appear to be works of considerable mathematicalelegance, but not indicating any great power of original thought. Perhaps it has been prejudicial to Brinkley's fame in this direction, that he was immediately followed in his chair by so mighty a geniusas William Rowan Hamilton. 

After the great circle had been at last erected, Brinkley was able tobegin his astronomical work in earnest. Nor was there much time tolose. He was already forty-five years old, a year older than wasHerschel when he commenced his immortal career at Slough. Stimulatedby the consciousness of having the command of an instrument of uniqueperfection, Brinkley loftily attempted the very highest class ofastronomical research. He resolved to measure anew with his own eyeand with his own hand the constants of aberration and of nutation. Healso strove to solve that great problem of the universe, thediscovery of the distance of a fixed star. 

These were noble problems, and they were nobly attacked. But toappraise with justice this work of Brinkley, done seventy years ago, we must not apply to it the same criterion as we would think right toapply to similar work were it done now. We do not any longer useBrinkley's constant of aberration, nor do we now think thatBrinkley's determinations of the star distances were reliable. But, nevertheless, his investigations exercised a marked influence on theprogress of science; they stimulated the study of the principles onwhich exact measurements were to be conducted. 

Brinkley had another profession in addition to that of anastronomer. He was a divine. When a man endeavours to pursue twodistinct occupations concurrently, it will be equally easy to explainwhy his career should be successful, or why it should be thereverse. If he succeeds, he will, of course, exemplify the wisdom ofhaving two strings to his bow. Should he fail, it is, of course, because he has attempted to sit on two stools at once. In Brinkley'scase, his two professions must be likened to the two strings ratherthan to the two stools. It is true that his practical experience ofhis clerical life was very slender. He had made no attempt tocombine the routine of a parish with his labours in the observatory. Nor do we associate a special eminence in any department of religiouswork with his name. If, however, we are to measure Brinkley's meritsas a divine by the ecclesiastical preferment which he received, hisservices to theology must have rivalled his services to astronomy. Having been raised step by step in the Church, he was at lastappointed to the See of Cloyne, in 1826, as the successor of BishopBerkeley. 

Now, though it was permissible for the Archdeacon to be also theAndrews Professor, yet when the Archdeacon became a Bishop, it wasunderstood that he should transfer his residence from the observatoryto the palace. The chair of Astronomy accordingly became vacant. Brinkley's subsequent career seems to have been devoted entirely toecclesiastical matters, and for the last ten years of his life he didnot contribute a paper to any scientific society. Arago, after acharacteristic lament that Brinkley should have forsaken the pursuitof science for the temporal and spiritual attractions of a bishopric, pays a tribute to the conscientiousness of the quondam astronomer, who would not even allow a telescope to be brought into the palacelest his mind should be distracted from his sacred duties. 

The good bishop died on the 13th September, 1835. He was buried inthe chapel of Trinity College, and a fine monument to his memory is afamiliar object at the foot of the noble old staircase of the library. The best memorial of Brinkley is his admirable book on the "Elementsof Plane Astronomy. " It passed through many editions in his lifetime, and even at the present day the same work, revised first by Dr. Luby, and more recently by the Rev. Dr. Stubbs and Dr. Brunnow, has a largeand well-merited circulation. 

JOHN HERSCHEL. 

This illustrious son of an illustrious father was born at Slough, near Windsor, on the 7th March, 1792. He was the only child of SirWilliam Herschel, who had married somewhat late in life, as we havealready mentioned. 

[PLATE: ASTRONOMETER MADE BY SIR J. HERSCHEL to compare the lightof certain stars by the intervention of the moon. ]

The surroundings among which the young astronomer was reared affordedhim an excellent training for that career on which he was to enter, and in which he was destined to attain a fame only less brilliantthan that of his father. The circumstances of his youth permittedhim to enjoy one great advantage which was denied to the elderHerschel. He was able, from his childhood, to devote himself almostexclusively to intellectual pursuits. William Herschel, in the earlypart of his career, had only been able to snatch occasional hours forstudy from his busy life as a professional musician. But the son, having been born with a taste for the student's life, was fortunateenough to have been endowed with the leisure and the means to enjoyit from the commencement. His early years have been so welldescribed by the late Professor Pritchard in the "Report of theCouncil of the Royal Astronomical Society for 1872, " that I ventureto make an extract here:--

"A few traits of John Herschel's boyhood, mentioned by himself in hismaturer life, have been treasured up by those who were dear to him, and the record of some of them may satisfy a curiosity as pardonableas inevitable, which craves to learn through what early steps greatmen or great nations become illustrious. His home was singular, andsingularly calculated to nurture into greatness any child born asJohn Herschel was with natural gifts, capable of wide development. Atthe head of the house there was the aged, observant, reticentphilosopher, and rarely far away his devoted sister, CarolineHerschel, whose labours and whose fame are still cognisable as abeneficent satellite to the brighter light of her illustriousbrother. It was in the companionship of these remarkable persons, and under the shadow of his father's wonderful telescope, that JohnHerschel passed his boyish years. He saw them, in silent butceaseless industry, busied about things which had no apparent concernwith the world outside the walls of that well-known house, but which, at a later period of his life, he, with an unrivalled eloquence, taught his countrymen to appreciate as foremost among those livinginfluences which but satisfy and elevate the noblest instincts of ournature. What sort of intercourse passed between the father and theboy may be gathered from an incident or two which he narrated ashaving impressed themselves permanently on the memory of his youth. He once asked his father what he thought was the oldest of allthings. The father replied, after the Socratic method, by puttinganother question: 'And what do you yourself suppose is the oldest ofall things?' The boy was not successful in his answers, thereon theold astronomer took up a small stone from the garden walk: 'There, mychild, there is the oldest of all the things that I certainly know. 'On another occasion his father is said to have asked the boy, 'Whatsort of things, do you think, are most alike?' The delicate, blue-eyed boy, after a short pause, replied, 'The leaves of the sametree are most like each other. ' 'Gather, then, a handful of leaves ofthat tree, ' rejoined the philosopher, 'and choose two that arealike. ' The boy failed; but he hid the lesson in his heart, and histhoughts were revealed after many days. These incidents may betrifles; nor should we record them here had not John Herschelhimself, though singularly reticent about his personal emotions, recorded them as having made a strong impression on his mind. Beyondall doubt we can trace therein, first, that grasp and grouping ofmany things in one, implied in the stone as the oldest of things;and, secondly, that fine and subtle discrimination of each thing outof many like things as forming the main features which characterizedthe habit of our venerated friend's philosophy. "

John Herschel entered St. John's College, Cambridge, when he wasseventeen years of age. His university career abundantly fulfilledhis father's eager desire, that his only son should develop acapacity for the pursuit of science. After obtaining many lesserdistinctions, he finally came out as Senior Wrangler in 1813. Itwas, indeed, a notable year in the mathematical annals of theUniversity. Second on that list, in which Herschel's name was first, appeared that of the illustrious Peacock, afterwards Dean of Ely, whoremained throughout life one of Herschel's most intimate friends. 

Almost immediately after taking his degree, Herschel gave evidence ofpossessing a special aptitude for original scientific investigation. He sent to the Royal Society a mathematical paper which was publishedin the PHILOSOPHICAL TRANSACTIONS. Doubtless the splendour thatattached to the name he bore assisted him in procuring earlyrecognition of his own great powers. Certain it is that he was madea Fellow of the Royal Society at the unprecedentedly early age oftwenty-one. Even after this remarkable encouragement to adopt ascientific career as the business of his life, it does not seem thatJohn Herschel at first contemplated devoting himself exclusively toscience. He commenced to prepare for the profession of the Law byentering as a student at the Middle Temple, and reading with apractising barrister. 

But a lawyer John Herschel was not destined to become. Circumstancesbrought him into association with some leading scientific men. Hepresently discovered that his inclinations tended more and more inthe direction of purely scientific pursuits. Thus it came to passthat the original intention as to the calling which he should followwas gradually abandoned. Fortunately for science Herschel found itspursuit so attractive that he was led, as his father had been beforehim, to give up his whole life to the advancement of knowledge. Norwas it unnatural that a Senior Wrangler, who had once tasted thedelights of mathematical research, should have been tempted to devotemuch time to this fascinating pursuit. By the time John Herschel wastwenty-nine he had published so much mathematical work, and hisresearches were considered to possess so much merit, that the RoyalSociety awarded him the Copley Medal, which was the highestdistinction it was capable of conferring. 

At the death of his father in 1822, John Herschel, with his tastesalready formed for a scientific career, found himself in thepossession of ample means. To him also passed all his father's greattelescopes and apparatus. These material aids, together with adutiful sense of filial obligation, decided him to make practicalastronomy the main work of his life. He decided to continue to itscompletion that great survey of the heavens which had already beeninaugurated, and, indeed, to a large extent accomplished, by hisfather. 

The first systematic piece of practical astronomical work which JohnHerschel undertook was connected with the measurement of what areknown as "Double Stars. " It should be observed, that there are inthe heavens a number of instances in which two stars are seen in veryclose association. In the case of those objects to which theexpression "Double Stars" is generally applied, the two luminouspoints are so close together that even though they might each bequite bright enough to be visible to the unaided eye, yet theirproximity is such that they cannot be distinguished as two separateobjects without optical aid. The two stars seem fused together intoone. In the telescope, however, the bodies may be discernedseparately, though they are frequently so close together that ittaxes the utmost power of the instrument to indicate the divisionbetween them. 

The appearance presented by a double star might arise from thecircumstance that the two stars, though really separated from eachother by prodigious distances, happened to lie nearly in the sameline of vision, as seen from our point of view. No doubt, many ofthe so-called double stars could be accounted for on thissupposition. Indeed, in the early days when but few double starswere known, and when telescopes were not powerful enough to exhibitthe numerous close doubles which have since been brought to light, there seems to have been a tendency to regard all double stars asmerely such perspective effects. It was not at first suggested thatthere could be any physical connection between the components of eachpair. The appearance presented was regarded as merely due to thecircumstance that the line joining the two bodies happened to passnear the earth. 

[PLATE: SIR JOHN HERSCHEL. ]

In the early part of his career, Sir William Herschel seems to haveentertained the view then generally held by other astronomers withregard to the nature of these stellar pairs. The great observerthought that the double stars could therefore be made to afford ameans of solving that problem in which so many of the observers ofthe skies had been engaged, namely, the determination of thedistances of the stars from the earth. Herschel saw that thedisplacement of the earth in its annual movement round the sun wouldproduce an apparent shift in the place of the nearer of the two starsrelatively to the other, supposed to be much more remote. If thisshift could be measured, then the distance of the nearer of the starscould be estimated with some degree of precision. 

As has not unfrequently happened in the history of science, an effectwas perceived of a very different nature from that which had beenanticipated. If the relative places of the two stars had beenapparently deranged merely in consequence of the motion of the earth, then the phenomenon would be an annual one. After the lapse of ayear the two stars would have regained their original relativepositions. This was the effect for which William Herschel waslooking. In certain of the so called double stars, he, no doubt, didfind a movement. He detected the remarkable fact that both theapparent distance and the relative positions of the two bodies werechanging. But what was his surprise to observe that thesealterations were not of an annually periodic character. It becameevident then that in some cases one of the component stars wasactually revolving around the other, in an orbit which required manyyears for its completion. Here was indeed a remarkable discovery. Itwas clearly impossible to suppose that movements of this kind couldbe mere apparent displacements, arising from the annual shift in ourpoint of view, in consequence of the revolution of the earth. Herschel's discovery established the interesting fact that, incertain of these double stars, or binary stars, as these particularobjects are more expressively designated, there is an actual orbitalrevolution of a character similar to that which the earth performsaround the sun. Thus it was demonstrated that in these particulardouble stars the nearness of the two components was not merelyapparent. The objects must actually lie close together at a distancewhich is small in comparison with the distance at which either ofthem is separated from the earth. The fact that the heavens containpairs of twin suns in mutual revolution was thus brought to light. 

In consequence of this beautiful discovery, the attention ofastronomers was directed to the subject of double stars with a degreeof interest which these objects had never before excited. It wastherefore not unnatural that John Herschel should have been attractedto this branch of astronomical work. Admiration for his father'sdiscovery alone might have suggested that the son should strive todevelop this territory newly opened up to research. But it alsohappened that the mathematical talents of the younger Herschelinclined his inquiries in the same direction. He saw clearly that, when sufficient observations of any particular binary star had beenaccumulated, it would then be within the power of the mathematicianto elicit from those observations the shape and the position in spaceof the path which each of the revolving stars described around theother. Indeed, in some cases he would be able to perform theastonishing feat of determining from his calculations the weight ofthese distant suns, and thus be enabled to compare them with the massof our own sun. 

[PLATE: NEBULA IN SOUTHERN HEMISPHERE, drawn by Sir John Herschel. ]

But this work must follow the observations, it could not precedethem. The first step was therefore to observe and to measure withthe utmost care the positions and distances of those particulardouble stars which appear to offer the greatest promise in thisparticular research. In 1821, Herschel and a friend of his, Mr. James South, agreed to work together with this object. South was amedical man with an ardent devotion to science, and possessed ofconsiderable wealth. He procured the best astronomical instrumentsthat money could obtain, and became a most enthusiastic astronomerand a practical observer of tremendous energy. 

South and John Herschel worked together for two years in theobservation and measurement of the double stars discovered by SirWilliam Herschel. In the course of this time their assiduity wasrewarded by the accumulation of so great a mass of carefulmeasurements that when published, they formed quite a volume in the"Philosophical Transactions. " The value and accuracy of the work, when estimated by standards which form proper criteria for thatperiod, is universally recognised. It greatly promoted the progressof sidereal astronomy, and the authors were in consequence awardedmedals from the Royal Society, and the Royal Astronomical Society, as well as similar testimonials from various foreign institutions. 

This work must, however, be regarded as merely introductory to themain labours of John Herschel's life. His father devoted the greaterpart of his years as an observer to what he called his "sweeps" ofthe heavens. The great reflecting telescope, twenty feet long, wasmoved slowly up and down through an arc of about two degrees towardsand from the pole, while the celestial panorama passed slowly in thecourse of the diurnal motion before the keenly watching eye of theastronomer. Whenever a double star traversed the field Herscheldescribed it to his sister Caroline, who, as we have alreadymentioned, was his invariable assistant in his midnight watches. Whena nebula appeared, then he estimated its size and its brightness, henoticed whether it had a nucleus, or whether it had stars disposed inany significant manner with regard to it. He also dictated any othercircumstance which he deemed worthy of record. These observationswere duly committed to writing by the same faithful and indefatigablescribe, whose business it also was to take a memorandum of the exactposition of the object as indicated by a dial placed in front of herdesk, and connected with the telescope. 

John Herschel undertook the important task of re-observing thevarious double stars and nebulae which had been discovered duringthese memorable vigils. The son, however, lacked one inestimableadvantage which had been possessed by the father. John Herschel hadno assistant to discharge all those duties which Caroline had soefficiently accomplished. He had, therefore, to modify the system ofsweeping previously adopted in order to enable all the work both ofobserving and of recording to be done by himself. This, in manyways, was a great drawback to the work of the younger astronomer. Thedivision of labour between the observer and the scribe enables agreatly increased quantity of work to be got through. It is alsodistinctly disadvantageous to an observer to have to use his eye atthe telescope directly after he has been employing it for reading thegraduations on a circle, by the light of a lamp, or for enteringmemoranda in a note book. Nebulae, especially, are often soexcessively faint that they can only be properly observed by an eyewhich is in that highly sensitive condition which is obtained by longcontinuance in darkness. The frequent withdrawal of the eye from thedark field of the telescope, and the application of it to reading byartificial light, is very prejudicial to its use for the moredelicate purpose. John Herschel, no doubt, availed himself of everyprecaution to mitigate the ill effects of this inconvenience as muchas possible, but it must have told upon his labours as compared withthose of his father. 

But nevertheless John Herschel did great work during his "sweeps. " Hewas specially particular to note all the double stars which presentedthemselves to his observation. Of course some little discretion mustbe allowed in deciding as to what degree of proximity in adjacentstars does actually bring them within the category of "doublestars. " Sir John set down all such objects as seemed to him likelyto be of interest, and the results of his discoveries in this branchof astronomy amount to some thousands. Six or seven great memoirs inthe TRANSACTIONS of the Royal Astronomical Society have been devotedto giving an account of his labours in this department of astronomy. 

[PLATE: THE CLUSTER IN THE CENTAUR, drawn by Sir John Herschel. ]

One of the achievements by which Sir John Herschel is best known ishis invention of a method by which the orbits of binary stars couldbe determined. It will be observed that when one star revolvesaround another in consequence of the law of gravitation, the orbitdescribed must be an ellipse. This ellipse, however, generallyspeaking, appears to us more or less foreshortened, for it is easilyseen that only under highly exceptional circumstances would the planein which the stars move happen to be directly square to the line ofview. It therefore follows that what we observe is not exactly thetrack of one star around the other; it is rather the projection ofthat track as seen on the surface of the sky. Now it is remarkablethat this apparent path is still an ellipse. Herschel contrived avery ingenious and simple method by which he could discover from theobservations the size and position of the ellipse in which therevolution actually takes place. He showed how, from the study ofthe apparent orbit of the star, and from certain measurements whichcould easily be effected upon it, the determination of the trueellipse in which the movement is performed could be arrived at. Inother words, Herschel solved in a beautiful manner the problem offinding the true orbits of double stars. The importance of this workmay be inferred from the fact that it has served as the basis onwhich scores of other investigators have studied the fascinatingsubject of the movement of binary stars. 

The labours, both in the discovery and measurement of the doublestars, and in the discussion of the observations with the object offinding the orbits of such stars as are in actual revolution, received due recognition in yet another gold medal awarded by theRoyal Society. An address was delivered on the occasion by the Dukeof Sussex (30th November, 1833), in the course of which, afterstating that the medal had been conferred on Sir John Herschel, heremarks:--

"It has been said that distance of place confers the same privilegeas distance of time, and I should gladly avail myself of theprivilege which is thus afforded me by Sir John Herschel's separationfrom his country and friends, to express my admiration of hischaracter in stronger terms than I should otherwise venture to use;for the language of panegyric, however sincerely it may flow from theheart, might be mistaken for that of flattery, if it could not thusclaim somewhat of an historical character; but his great attainmentsin almost every department of human knowledge, his fine powers as aphilosophical writer, his great services and his distinguisheddevotion to science, the high principles which have regulated hisconduct in every relation of life, and, above all, his engagingmodesty, which is the crown of all his other virtues, presenting sucha model of an accomplished philosopher as can rarely be found beyondthe regions of fiction, demand abler pens than mine to describe themin adequate terms, however much inclined I might feel to undertakethe task. "

The first few lines of the eulogium just quoted allude to Herschel'sabsence from England. This was not merely an episode of interest inthe career of Herschel, it was the occasion of one of the greatestscientific expeditions in the whole history of astronomy. 

Herschel had, as we have seen, undertaken a revision of his father's"sweeps" for new objects, in those skies which are visible from ourlatitudes in the northern hemisphere. He had well-nigh completedthis task. Zone by zone the whole of the heavens which could beobserved from Windsor had passed under his review. He had addedhundreds to the list of nebulae discovered by his father. He hadannounced thousands of double stars. At last, however, the greatsurvey was accomplished. The contents of the northern hemisphere, sofar at least as they could be disclosed by his telescope of twentyfeet focal length, had been revealed. 

[PLATE: SIR JOHN HERSCHEL'S OBSERVATORY AT FELDHAUSEN, Cape of Good Hope. ]

But Herschel felt that this mighty task had to be supplemented byanother of almost equal proportions, before it could be said that thetwenty-foot telescope had done its work. It was only the northernhalf of the celestial sphere which had been fully explored. Thesouthern half was almost virgin territory, for no other astronomerwas possessed of a telescope of such power as those which theHerschels had used. It is true, of course, that as a certain marginof the southern hemisphere was visible from these latitudes, it hadbeen more or less scrutinized by observers in northern skies. Andthe glimpses which had thus been obtained of the celestial objects inthe southern sky, were such as to make an eager astronomer long for acloser acquaintance with the celestial wonders of the south. Themost glorious object in the sidereal heavens, the Great Nebula inOrion, lies indeed in that southern hemisphere to which the youngerHerschel's attention now became directed. It fortunately happens, however, for votaries of astronomy all the world over, that Naturehas kindly placed her most astounding object, the great Nebula inOrion, in such a favoured position, near the equator, that from aconsiderable range of latitudes, both north and south, the wonders ofthe Nebula can be explored. There are grounds for thinking that thesouthern heavens contain noteworthy objects which, on the whole, arenearer to the solar system than are the noteworthy objects in thenorthern skies. The nearest star whose distance is known, AlphaCentauri, lies in the southern hemisphere, and so also does the mostsplendid cluster of stars. 

Influenced by the desire to examine these objects, Sir John Herscheldetermined to take his great telescope to a station in the southernhemisphere, and thus complete his survey of the sidereal heavens. Thelatitude of the Cape of Good Hope is such that a suitable site couldbe there found for his purpose. The purity of the skies in SouthAfrica promised to provide for the astronomer those clear nightswhich his delicate task of surveying the nebulae would require. 

On November 13, 1833, Sir John Herschel, who had by this timereceived the honour of knighthood from William IV. , sailed fromPortsmouth for the Cape of Good Hope, taking with him his giganticinstruments. After a voyage of two months, which was considered tobe a fair passage in those days, he landed in Table Bay, and havingduly reconnoitred various localities, he decided to place hisobservatory at a place called Feldhausen, about six miles from CapeTown, near the base of the Table Mountain. A commodious residencewas there available, and in it he settled with his family. Atemporary building was erected to contain the equatorial, but thegreat twenty-foot telescope was accommodated with no more shelterthan is provided by the open canopy of heaven. 

As in his earlier researches at home, the attention of the greatastronomer at the Cape of Good Hope was chiefly directed to themeasurement of the relative positions and distances apart of thedouble stars, and to the close examination of the nebulae. In thedelineation of the form of these latter objects Herschel found ampleemployment for his skilful pencil. Many of the drawings he has madeof the celestial wonders in the southern sky are admirable examplesof celestial portraiture. 

The number of the nebulae and of those kindred objects, the starclusters, which Herschel studied in the southern heavens, during fouryears of delightful labour, amount in all to one thousand sevenhundred and seven. His notes on their appearance, and thedeterminations of their positions, as well as his measurements ofdouble stars, and much other valuable astronomical research, werepublished in a splendid volume, brought out at the cost of the Dukeof Northumberland. This is, indeed, a monumental work, full ofinteresting and instructive reading for any one who has a taste forastronomy. 

Herschel had the good fortune to be at the Cape on the occasion ofthe periodical return of Halley's great comet in 1833. To the studyof this body he gave assiduous attention, and the records of hisobservations form one of the most interesting chapters in thatremarkable volume to which we have just referred. 

[PLATE: COLUMN AT FELDHAUSEN, CAPE TOWN, to commemorate Sir JohnHerschel's survey of the Southern Heavens. ]

Early in 1838 Sir John Herschel returned to England. He had mademany friends at the Cape, who deeply sympathised with his self-imposed labours while he was resident among them. They desired topreserve the recollection of this visit, which would always, theyconsidered, be a source of gratification in the colony. Accordingly, a number of scientific friends in that part of the world raised amonument with a suitable inscription, on the spot which had beenoccupied by the great twenty-foot reflector at Feldhausen. 

His return to England after five years of absence was naturally anoccasion for much rejoicing among the lovers of astronomy. He wasentertained at a memorable banquet, and the Queen, at her coronation, made him a baronet. His famous aunt Caroline, at that time agedeighty, was still in the enjoyment of her faculties, and was able toestimate at its true value the further lustre which was added to thename she bore. But there is reason to believe that her satisfactionwas not quite unmixed with other feelings. With whatever favour shemight regard her nephew, he was still not the brother to whom herlife had been devoted. So jealous was this vigorous old lady of thefame of the great brother William, that she could hardly hear withpatience of the achievements of any other astronomer, and thisfailing existed in some degree even when that other astronomerhappened to be her illustrious nephew. 

With Sir John Herschel's survey of the Southern Hemisphere it may besaid that his career as an observing astronomer came to a close. Hedid not again engage in any systematic telescopic research. But itmust not be inferred from this statement that he desisted from activeastronomical work. It has been well observed that Sir John Herschelwas perhaps the only astronomer who has studied with success, andadvanced by original research, every department of the great sciencewith which his name is associated. It was to some other branches ofastronomy besides those concerned with looking through telescopes, that the rest of the astronomer's life was to be devoted. 

To the general student Sir John Herschel is best known by the volumewhich he published under the title of "Outlines of Astronomy. " Thisis, indeed, a masterly work, in which the characteristic difficultiesof the subject are resolutely faced and expounded with as muchsimplicity as their nature will admit. As a literary effort thiswork is admirable, both on account of its picturesque language andthe ennobling conceptions of the universe which it unfolds. Thestudent who desires to become acquainted with those reconditedepartments of astronomy, in which the effects of the disturbingaction of one planet upon the motions of another planet areconsidered, will turn to the chapters in Herschel's famous work onthe subject. There he will find this complex matter elucidated, without resort to difficult mathematics. Edition after edition ofthis valuable work has appeared, and though the advances of modernastronomy have left it somewhat out of date in certain departments, yet the expositions it contains of the fundamental parts of thescience still remain unrivalled. 

Another great work which Sir John undertook after his return from theCape, was a natural climax to those labours on which his father andhe had been occupied for so many years. We have already explainedhow the work of both these observers had been mainly devoted to thestudy of the nebulae and the star clusters. The results of theirdiscoveries had been announced to the world in numerous isolatedmemoirs. The disjointed nature of these publications made their usevery inconvenient. But still it was necessary for those who desiredto study the marvellous objects discovered by the Herschels, to havefrequent recourse to the original works. To incorporate all theseveral observations of nebular into one great systematic catalogue, seemed, therefore, to be an indispensable condition of progress inthis branch of knowledge. No one could have been so fitted for thistask as Sir John Herschel. He, therefore, attacked and carriedthrough the great undertaking. Thus at last a grand catalogue ofnebulae and clusters was produced. Never before was there somajestic an inventory. If we remember that each of the nebulae is anobject so vast, that the whole of the solar system would form aninconsiderable speck by comparison, what are we to think of acollection in which these objects are enumerated in thousands? Inthis great catalogue we find arranged in systematic order all thenebulae and all the clusters which had been revealed by the diligenceof the Herschels, father and son, in the Northern Hemisphere, and ofthe son alone in the Southern Hemisphere. Nor should we omit tomention that the labours of other astronomers were likewiseincorporated. It was unavoidable that the descriptions given to eachof the objects should be very slight. Abbreviations are used, whichindicate that a nebula is bright, or very bright, or extremelybright, or faint, or very faint, or extremely faint. Such phraseshave certainly but a relative and technical meaning in such acatalogue. The nebulae entered as extremely bright by theexperienced astronomer are only so described by way of contrast tothe great majority of these delicate telescopic objects. Most of thenebulae, indeed, are so difficult to see, that they admit of but veryslight description. It should be observed that Herschel's catalogueaugmented the number of known nebulous objects to more than ten timesthat collected into any catalogue which had ever been compiled beforethe days of William Herschel's observing began. But the study ofthese objects still advances, and the great telescopes now in usecould probably show at least twice as many of these objects as arecontained in the list of Herschel, of which a new and enlargededition has since been brought out by Dr. Dreyer. 

One of the best illustrations of Sir John Herschel's literary powersis to be found in the address which he delivered at the RoyalAstronomical Society, on the occasion of presenting a medal to Mr. Francis Baily, in recognition of his catalogue of stars. The passageI shall here cite places in its proper aspect the true merit of thelaborious duty involved in such a task as that which Mr. Baily hadcarried through with such success:--

"If we ask to what end magnificent establishments are maintained bystates and sovereigns, furnished with masterpieces of art, and placedunder the direction of men of first-rate talent and high-mindedenthusiasm, sought out for those qualities among the foremost in theranks of science, if we demand QUI BONO? for what good a Bradley hastoiled, or a Maskelyne or a Piazzi has worn out his venerable age inwatching, the answer is--not to settle mere speculative points in thedoctrine of the universe; not to cater for the pride of man byrefined inquiries into the remoter mysteries of nature; not to tracethe path of our system through space, or its history through past andfuture eternities. These, indeed, are noble ends and which I am farfrom any thought of depreciating; the mind swells in theircontemplation, and attains in their pursuit an expansion and ahardihood which fit it for the boldest enterprise. But the directpractical utility of such labours is fully worthy of theirspeculative grandeur. The stars are the landmarks of the universe;and, amidst the endless and complicated fluctuations of our system, seem placed by its Creator as guides and records, not merely toelevate our minds by the contemplation of what is vast, but to teachus to direct our actions by reference to what is immutable in Hisworks. It is, indeed, hardly possible to over-appreciate their valuein this point of view. Every well-determined star, from the momentits place is registered, becomes to the astronomer, the geographer, the navigator, the surveyor, a point of departure which can neverdeceive or fail him, the same for ever and in all places, of adelicacy so extreme as to be a test for every instrument yet inventedby man, yet equally adapted for the most ordinary purposes; asavailable for regulating a town clock as for conducting a navy to theIndies; as effective for mapping down the intricacies of a pettybarony as for adjusting the boundaries of Transatlantic empires. Whenonce its place has been thoroughly ascertained and carefullyrecorded, the brazen circle with which that useful work was done maymoulder, the marble pillar may totter on its base, and the astronomerhimself survive only in the gratitude of posterity; but the recordremains, and transfuses all its own exactness into everydetermination which takes it for a groundwork, giving to inferiorinstruments--nay, even to temporary contrivances, and to theobservations of a few weeks or days--all the precision attainedoriginally at the cost of so much time, labour, and expense. "

Sir John Herschel wrote many other works besides those we havementioned. His "Treatise on Meteorology" is, indeed, a standard workon this subject, and numerous articles from the same pen onmiscellaneous subjects, which have been collected and reprinted, seemed as a relaxation from his severe scientific studies. Likecertain other great mathematicians Herschel was also a poet, and hepublished a translation of the Iliad into blank verse. 

In his later years Sir John Herschel lived a retired life. For abrief period he had, indeed, been induced to accept the office ofMaster of the Mint. It was, however, evident that the routine ofsuch an occupation was not in accordance with his tastes, and hegladly resigned it, to return to the seclusion of his study in hisbeautiful home at Collingwood, in Kent. 

His health having gradually failed, he died on the 11th May, 1871, inthe seventy-ninth year of his age. 

THE EARL OF ROSSE. 

The subject of our present sketch occupies quite a distinct positionin scientific history. Unlike many others who have risen by theirscientific discoveries from obscurity to fame, the great Earl ofRosse was himself born in the purple. His father, who, under thetitle of Sir Lawrence Parsons, had occupied a distinguished positionin the Irish Parliament, succeeded on the death of his father to theEarldom which had been recently created. The subject of our presentmemoir was, therefore, the third of the Earls of Rosse, and he wasborn in York on June 17, 1800. Prior to his father's death in 1841, he was known as Lord Oxmantown. 

The University education of the illustrious astronomer was begun inDublin and completed at Oxford. We do not hear in his case of anyvery remarkable University career. Lord Rosse was, however, adiligent student, and obtained a first-class in mathematics. Healways took a great deal of interest in social questions, and was aprofound student of political economy. He had a seat in the House ofCommons, as member for King's County, from 1821 to 1834, hisancestral estate being situated in this part of Ireland. 

[PLATE: THE EARL OF ROSSE. ]

Lord Rosse was endowed by nature with a special taste for mechanicalpursuits. Not only had he the qualifications of a scientificengineer, but he had the manual dexterity which qualified himpersonally to carry out many practical arts. Lord Rosse was, infact, a skilful mechanic, an experienced founder, and an ingeniousoptician. His acquaintances were largely among those who wereinterested in mechanical pursuits, and it was his delight to visitthe works or engineering establishments where refined processes inthe arts were being carried on. It has often been stated--and as Ihave been told by members of his family, truly stated--that on oneoccasion, after he had been shown over some large works in the northof England, the proprietor bluntly said that he was greatly in wantof a foreman, and would indeed be pleased if his visitor, who hadevinced such extraordinary capacity for mechanical operations, wouldaccept the post. Lord Rosse produced his card, and gently explainedthat he was not exactly the right man, but he appreciated thecompliment, and this led to a pleasant dinner, and was the basis of along friendship. 

I remember on one occasion hearing Lord Rosse explain how it was thathe came to devote his attention to astronomy. It appears that whenhe found himself in the possession of leisure and of means, hedeliberately cast around to think how that means and that leisurecould be most usefully employed. Nor was it surprising that heshould search for a direction which would offer special scope for hismechanical tastes. He came to the conclusion that the building ofgreat telescopes was an art which had received no substantial advancesince the great days of William Herschel. He saw that to constructmighty instruments for studying the heavens required at once thecommand of time and the command of wealth, while he also felt thatthis was a subject the inherent difficulties of which would tax tothe uttermost whatever mechanical skill he might possess. Thus itwas he decided that the construction of great telescopes shouldbecome the business of his life. 

[PLATE: BIRR CASTLE. 

PLATE: THE MALL, PARSONSTOWN. ]

In the centre of Ireland, seventy miles from Dublin, on the borderbetween King's County and Tipperary, is a little town whereof we mustbe cautious before writing the name. The inhabitants of that townfrequently insist that its name is Birr, * while the officialdesignation is Parsonstown, and to this day for every six people whoapply one name to the town, there will be half a dozen who use theother. But whichever it may be, Birr or Parsonstown--and I shallgenerally call it by the latter name--it is a favourable specimen ofan Irish county town. The widest street is called the OxmantownMall. It is bordered by the dwelling-houses of the chief residents, and adorned with rows of stately trees. At one end of thisdistinctly good feature in the town is the Parish Church, while atthe opposite end are the gates leading into Birr Castle, theancestral home of the house of Parsons. Passing through the gatesthe visitor enters a spacious demesne, possessing much beauty of woodand water, one of the most pleasing features being the junction ofthe two rivers, which unite at a spot ornamented by beautifultimber. At various points illustrations of the engineering skill ofthe great Earl will be observed. The beauty of the park has beengreatly enhanced by the construction of an ample lake, designed withthe consummate art by which art is concealed. Even in mid-summer itis enlivened by troops of wild ducks preening themselves in thatconfidence which they enjoy in those happy localities where the soundof a gun is seldom heard. The water is led into the lake by a tubewhich passes under one of the two rivers just mentioned, while theoverflow from the lake turns a water-wheel, which works a pair ofelevators ingeniously constructed for draining the low-lying parts ofthe estate. 

 * Considering the fame acquired by Parsonstown from Lord Rosse's mirrors, it may be interesting to note the following extract from "The Natural History of Ireland, " by Dr. Gerard Boate, Thomas Molyneux M. D. , F. R. S. , and others, which shows that 150 years ago Parsonstown was famous for its glass:--

 "We shall conclude this chapter with the glass, there having been several glasshouses set up by the English in Ireland, none in Dublin or other cities, but all of them in the country; amongst which the principal was that of Birre, a market town, otherwise called Parsonstown, after one Sir Lawrence Parsons, who, having purchased that lordship, built a goodly house upon it; his son William Parsons having succeeded him in the possession of it; which town is situate in Queen's County, about fifty miles (Irish) to the southwest of Dublin, upon the borders of the two provinces of Leinster and Munster; from this place Dublin was furnished with all sorts of window and drinking glasses, and such other as commonly are in use. One part of the materials, viz. , the sand, they had out of England; the other, to wit the ashes, they made in the place of ash-tree, and used no other. The chiefest difficulty was to get the clay for the pots to melt the materials in; this they had out of the north. "--Chap. XXI. , Sect. VIII. "Of the Glass made in Ireland. "

Birr Castle itself is a noble mansion with reminiscences from thetime of Cromwell. It is surrounded by a moat and a drawbridge ofmodern construction, and from its windows beautiful views can be hadover the varied features of the park. But while the visitors toParsonstown will look with great interest on this residence of anIrish landlord, whose delight it was to dwell in his own country, andamong his own people, yet the feature which they have specially cometo observe is not to be found in the castle itself. On an extensivelawn, sweeping down from the moat towards the lake, stand two noblemasonry walls. They are turreted and clad with ivy, and considerablyloftier than any ordinary house. As the visitor approaches, he willsee between those walls what may at first sight appear to him to bethe funnel of a steamer lying down horizontally. On closer approachhe will find that it is an immense wooden tube, sixty feet long, andupwards of six feet in diameter. It is in fact large enough to admitof a tall man entering into it and walking erect right through fromone end to the other. This is indeed the most gigantic instrumentwhich has ever been constructed for the purpose of exploring theheavens. Closely adjoining the walls between which the great tubeswings, is a little building called "The Observatory. " In this thesmaller instruments are contained, and there are kept the books whichare necessary for reference. The observatory also offers shelter tothe observers, and provides the bright fire and the cup of warm tea, which are so acceptable in the occasional intervals of a night'sobservation passed on the top of the walls with no canopy but thewinter sky. 

Almost the first point which would strike the visitor to Lord Rosse'stelescope is that the instrument at which he is looking is not onlyenormously greater than anything of the kind that he has ever seenbefore, but also that it is something of a totally different nature. In an ordinary telescope he is accustomed to find a tube with lensesof glass at either end, while the large telescopes that we see in ourobservatories are also in general constructed on the same principle. At one end there is the object-glass, and at the other end theeye-piece, and of course it is obvious that with an instrument ofthis construction it is to the lower end of the tube that the eye ofthe observer must be placed when the telescope is pointed to theskies. But in Lord Rosse's telescope you would look in vain forthese glasses, and it is not at the lower end of the instrument thatyou are to take your station when you are going to make yourobservations. The astronomer at Parsonstown has rather to availhimself of the ingenious system of staircases and galleries, by whichhe is enabled to obtain access to the mouth of the great tube. Thecolossal telescope which swings between the great walls, likeHerschel's great telescope already mentioned, is a reflector, theoriginal invention of which is due of course to Newton. The opticalwork which is accomplished by the lenses in the ordinary telescope iseffected in the type of instrument constructed by Lord Rosse by areflecting mirror which is placed at the lower end of the vast tube. The mirror in this instrument is made of a metal consisting of twoparts of copper to one of tin. As we have already seen, this mixtureforms an alloy of a very peculiar nature. The copper and the tinboth surrender their distinctive qualities, and unite to form amaterial of a very different physical character. The copper is toughand brown, the tin is no doubt silvery in hue, but soft and almostfibrous in texture. When the two metals are mixed together in theproportions I have stated, the alloy obtained is intensely hard andquite brittle being in both these respects utterly unlike either ofthe two ingredients of which it is composed. It does, however, resemble the tin in its whiteness, but it acquires a lustre farbrighter than tin; in fact, this alloy hardly falls short of silveritself in its brilliance when polished. 

[PLATE: LORD ROSSE'S TELESCOPE. From a photograph by W. Lawrence, Upper Sackville Street, Dublin. ]

The first duty that Lord Rosse had to undertake was the constructionof this tremendous mirror, six feet across, and about four or fiveinches thick. The dimensions were far in excess of those which hadbeen contemplated in any previous attempt of the same kind. Herschelhad no doubt fashioned one mirror of four feet in diameter, and manyothers of smaller dimensions, but the processes which he employed hadnever been fully published, and it was obvious that, with a largeincrease in dimensions, great additional difficulties had to beencountered. Difficulties began at the very commencement of theprocess, and were experienced in one form or another at everysubsequent stage. In the first place, the mere casting of a greatdisc of this mixture of tin and copper, weighing something like threeor four tons, involved very troublesome problems. No doubt a castingof this size, if the material had been, for example, iron, would haveoffered no difficulties beyond those with which every practicalfounder is well acquainted, and which he has to encounter daily inthe course of his ordinary work. But speculum metal is a material ofa very intractable description. There is, of course, no practicaldifficulty in melting the copper, nor in adding the proper proportionof tin when the copper has been melted. There may be no greatdifficulty in arranging an organization by which several crucibles, filled with the molten material, shall be poured simultaneously so asto obtain the requisite mass of metal, but from this point thedifficulties begin. For speculum metal when cold is excessivelybrittle, and were the casting permitted to cool like an ordinarycopper or iron casting, the mirror would inevitably fly into pieces. Lord Rosse, therefore, found it necessary to anneal the casting withextreme care by allowing it to cool very slowly. This wasaccomplished by drawing the disc of metal as soon as it had enteredinto the solid state, though still glowing red, into an annealingoven. There the temperature was allowed to subside so gradually, that six weeks elapsed before the mirror had reached the temperatureof the external air. The necessity for extreme precaution in theoperation of annealing will be manifest if we reflect on one of theaccidents which happened. On a certain occasion, after the coolingof a great casting had been completed, it was found, on withdrawingthe speculum, that it was cracked into two pieces. This mishap waseventually traced to the fact that one of the walls of the oven hadonly a single brick in its thickness, and that therefore the heat hadescaped more easily through that side than through the other sideswhich were built of double thickness. The speculum had, consequently, not cooled uniformly, and hence the fracture hadresulted. Undeterred, however, by this failure, as well as by not afew other difficulties, into a description of which we cannot nowenter, Lord Rosse steadily adhered to his self-imposed task, and atlast succeeded in casting two perfect discs on which to commence thetedious processes of grinding and polishing. The magnitude of theoperations involved may perhaps be appreciated if I mention that thevalue of the mere copper and tin entering into the composition ofeach of the mirrors was about 500 pounds. 

In no part of his undertaking was Lord Rosse's mechanical ingenuitymore taxed than in the devising of the mechanism for carrying out thedelicate operations of grinding and polishing the mirrors, whosecasting we have just mentioned. In the ordinary operations of thetelescope-maker, such processes had hitherto been generally effectedby hand, but, of course, such methods became impossible when dealingwith mirrors which were as large as a good-sized dinner table, andwhose weight was measured by tons. The rough grinding was effectedby means of a tool of cast iron about the same size as the mirror, which was moved by suitable machinery both backwards and forwards, and round and round, plenty of sand and water being supplied betweenthe mirror and the tool to produce the necessary attrition. As theprocess proceeded and as the surface became smooth, emery was usedinstead of sand; and when this stage was complete, the grinding toolwas removed and the polishing tool was substituted. The essentialpart of this was a surface of pitch, which, having been temporarilysoftened by heat, was then placed on the mirror, and accepted fromthe mirror the proper form. Rouge was then introduced as thepolishing powder, and the operation was continued about nine hours, by which time the great mirror had acquired the appearance of highlypolished silver. When completed, the disc of speculum metal wasabout six feet across and four inches thick. The depression in thecentre was about half an inch. Mounted on a little truck, the greatspeculum was then conveyed to the instrument, to be placed in itsreceptacle at the bottom of the tube, the length of which was sixtyfeet, this being the focal distance of the mirror. Another smallreflector was inserted in the great tube sideways, so as to directthe gaze of the observer down upon the great reflector. Thus wascompleted the most colossal instrument for the exploration of theheavens which the art of man has ever constructed. 

[PLATE: ROMAN CATHOLIC CHURCH AT PARSONSTOWN. ]

It was once my privilege to be one of those to whom the illustriousbuilder of the great telescope entrusted its use. For two seasons in1865 and 1866 I had the honour of being Lord Rosse's astronomer. During that time I passed many a fine night in the observer'sgallery, examining different objects in the heavens with the aid ofthis remarkable instrument. At the time I was there, the objectsprincipally studied were the nebulae, those faint stains of lightwhich lie on the background of the sky. Lord Rosse's telescope wasspecially suited for the scrutiny of these objects, inasmuch as theirdelicacy required all the light-grasping power which could beprovided. 

One of the greatest discoveries made by Lord Rosse, when his hugeinstrument was first turned towards the heavens, consisted in thedetection of the spiral character of some of the nebulous forms. When the extraordinary structure of these objects was firstannounced, the discovery was received with some degree ofincredulity. Other astronomers looked at the same objects, and whenthey failed to discern--and they frequently did fail to discern--thespiral structure which Lord Rosse had indicated, they drew theconclusion that this spiral structure did not exist. They thought itmust be due possibly to some instrumental defect or to theimagination of the observer. It was, however, hardly possible forany one who was both willing and competent to examine into theevidence, to doubt the reality of Lord Rosse's discoveries. Ithappens, however, that they have been recently placed beyond alldoubt by testimony which it is impossible to gainsay. A witnessnever influenced by imagination has now come forward, and theinfallible photographic plate has justified Lord Rosse. Among theremarkable discoveries which Dr. Isaac Roberts has recently made inthe application of his photographic apparatus to the heavens, thereis none more striking than that which declares, not only that thenebulae which Lord Rosse described as spirals, actually do possessthe character so indicated, but that there are many others of thesame description. He has even brought to light the astonishinglyinteresting fact that there are invisible objects of this class whichhave never been seen by human eye, but whose spiral character isvisible to the peculiar delicacy of the photographic telescope. 

In his earlier years, Lord Rosse himself used to be a diligentobserver of the heavenly bodies with the great telescope which wascompleted in the year 1845. But I think that those who knew LordRosse well, will agree that it was more the mechanical processesincidental to the making of the telescope which engaged his interestthan the actual observations with the telescope when it wascompleted. Indeed one who was well acquainted with him believed LordRosse's special interest in the great telescope ceased when the lastnail had been driven into it. But the telescope was never allowed tolie idle, for Lord Rosse always had associated with him some ardentyoung astronomer, whose delight it was to employ to the uttermost theadvantages of his position in exploring the wonders of the sky. Amongthose who were in this capacity in the early days of the greattelescope, I may mention my esteemed friend Dr. Johnston Stoney. 

Such was the renown of Lord Rosse himself, brought about by hisconsummate mechanical genius and his astronomical discoveries, andsuch the interest which gathered around the marvellous workshops atBirr castle, wherein his monumental exhibitions of optical skill wereconstructed, that visitors thronged to see him from all parts of theworld. His home at Parsonstown became one of the most remarkablescientific centres in Great Britain; thither assembled from time totime all the leading men of science in the country, as well as manyillustrious foreigners. For many years Lord Rosse filled with markeddistinction the exalted position of President of the Royal Society, and his advice and experience in practical mechanical matters werealways at the disposal of those who sought his assistance. Personallyand socially Lord Rosse endeared himself to all with whom he came incontact. I remember one of the attendants telling me that on oneoccasion he had the misfortune to let fall and break one of the smallmirrors on which Lord Rosse had himself expended many hours of hardpersonal labour. The only remark of his lordship was that "accidentswill happen. "

The latter years of his life Lord Rosse passed in comparativeseclusion; he occasionally went to London for a brief sojourn duringthe season, and he occasionally went for a cruise in his yacht; butthe greater part of the year he spent at Birr Castle, devotinghimself largely to the study of political and social questions, andrarely going outside the walls of his demesne, except to church onSunday mornings. He died on October 31, 1867. 

He was succeeded by his eldest son, the present Earl of Rosse, whohas inherited his father's scientific abilities, and done muchnotable work with the great telescope. 

AIRY. 

In our sketch of the life of Flamsteed, we have referred to thecircumstances under which the famous Observatory that crownsGreenwich Hill was founded. We have also had occasion to mentionthat among the illustrious successors of Flamsteed both Halley andBradley are to be included. But a remarkable development ofGreenwich Observatory from the modest establishment of early daystook place under the direction of the distinguished astronomer whosename is at the head of this chapter. By his labours this temple ofscience was organised to such a degree of perfection that it hasserved in many respects as a model for other astronomicalestablishments in various parts of the world. An excellent accountof Airy's career has been given by Professor H. H. Turner, in theobituary notice published by the Royal Astronomical Society. To thisI am indebted for many of the particulars here to be set downconcerning the life of the illustrious Astronomer Royal. 

The family from which Airy took his origin came from Kentmere, inWestmoreland. His father, William Airy, belonged to a Lincolnshirebranch of the same stock. His mother's maiden name was Ann Biddell, and her family resided at Playford, near Ipswich. William Airy heldsome small government post which necessitated an occasional change ofresidence to different parts of the country, and thus it was that hisson, George Biddell, came to be born at Alnwick, on 27th July, 1801. The boy's education, so far as his school life was concerned waspartly conducted at Hereford and partly at Colchester. He does not, however, seem to have derived much benefit from the hours which hepassed in the schoolroom. But it was delightful to him to spend hisholidays on the farm at Playford, where his uncle, Arthur Biddell, showed him much kindness. The scenes of his early youth remaineddear to Airy throughout his life, and in subsequent years he himselfowned a house at Playford, to which it was his special delight toresort for relaxation during the course of his arduous career. Inspite of the defects of his school training he seems to havemanifested such remarkable abilities that his uncle decided to enterhim in Cambridge University. He accordingly joined Trinity Collegeas a sizar in 1819, and after a brilliant career in mathematical andphysical science he graduated as Senior Wrangler in 1823. It may benoted as an exceptional circumstance that, notwithstanding thedemands on his time in studying for his tripos, he was able, afterhis second term of residence, to support himself entirely by takingprivate pupils. In the year after he had taken his degree he waselected to a Fellowship at Trinity College. 

Having thus gained an independent position, Airy immediately enteredupon that career of scientific work which he prosecuted withoutintermission almost to the very close of his life. One of his mostinteresting researches in these early days is on the subject ofAstigmatism, which defect he had discovered in his own eyes. Hisinvestigations led him to suggest a means of correcting this defectby using a pair of spectacles with lenses so shaped as to counteractthe derangement which the astigmatic eye impressed upon the rays oflight. His researches on this subject were of a very completecharacter, and the principles he laid down are to the present daypractically employed by oculists in the treatment of thismalformation. 

On the 7th of December, 1826, Airy was elected to the LucasianProfessorship of Mathematics in the University of Cambridge, thechair which Newton's occupancy had rendered so illustrious. Histenure of this office only lasted for two years, when he exchanged itfor the Plumian Professorship. The attraction which led him todesire this change is doubtless to be found in the circumstance thatthe Plumian Professorship of Astronomy carried with it at that timethe appointment of director of the new astronomical observatory, theorigin of which must now be described. 

Those most interested in the scientific side of University lifedecided in 1820 that it would be proper to found an astronomicalobservatory at Cambridge. Donations were accordingly sought for thispurpose, and upwards of 6, 000 pounds were contributed by members ofthe University and the public. To this sum 5, 000 pounds were addedby a grant from the University chest, and in 1824 further sumsamounting altogether to 7, 115 pounds were given by the University forthe same object. The regulations as to the administration of the newobservatory placed it under the management of the Plumian Professor, who was to be provided with two assistants. Their duties were toconsist in making meridian observations of the sun, moon, and thestars, and the observations made each year were to be printed andpublished. The observatory was also to be used in the educationalwork of the University, for it was arranged that smaller instrumentswere to be provided by which students could be instructed in thepractical art of making astronomical observations. 

The building of the Cambridge Astronomical Observatory was completedin 1824, but in 1828, when Airy entered on the discharge of hisduties as Director, the establishment was still far from completion, in so far as its organisation was concerned. Airy commenced his workso energetically that in the next year after his appointment he wasable to publish the first volume of "Cambridge AstronomicalObservations, " notwithstanding that every part of the work, from themaking of observations to the revising of the proof-sheets, had to bedone by himself. 

It may here be remarked that these early volumes of the publicationsof the Cambridge Observatory contained the first exposition of thosesystematic methods of astronomical work which Airy afterwardsdeveloped to such a great extent at Greenwich, and which have beensubsequently adopted in many other places. No more profitableinstruction for the astronomical beginner can be found than thatwhich can be had by the study of these volumes, in which the PlumianProfessor has laid down with admirable clearness the true principleson which meridian work should be conducted. 

[PLATE: SIR GEORGE AIRY. From a Photograph by Mr. E. P. Adams, Greenwich. ]

Airy gradually added to the instruments with which the observatorywas originally equipped. A mural circle was mounted in 1832, and inthe same year a small equatorial was erected by Jones. This was madeuse of by Airy in a well-known series of observations of Jupiter'sfourth satellite for the determination of the mass of the greatplanet. His memoir on this subject fully ex pounds the method offinding the weight of a planet from observations of the movements ofa satellite by which the planet is attended. This is, indeed, avaluable investigation which no student of astronomy can afford toneglect. The ardour with which Airy devoted himself to astronomicalstudies may be gathered from a remarkable report on the progress ofastronomy during the present century, which he communicated to theBritish Association at its second meeting in 1832. In the earlyyears of his life at Cambridge his most famous achievement wasconnected with a research in theoretical astronomy for whichconsummate mathematical power was required. We can only give a briefaccount of the Subject, for to enter into any full detail with regardto it would be quite out of the question. 

Venus is a planet of about the same size and the same weight as theearth, revolving in an orbit which lies within that described by ourglobe. Venus, consequently, takes less time than the earth toaccomplish one revolution round the sun, and it happens that therelative movements of Venus and the earth are so proportioned that inthe time in which our earth accomplishes eight of her revolutions theother planet will have accomplished almost exactly thirteen. It, therefore, follows that if the earth and Venus are in line with thesun at one date, then in eight years later both planets will again befound at the same points in their orbits. In those eight years theearth has gone round eight times, and has, therefore, regained itsoriginal position, while in the same period Venus has accomplishedthirteen complete revolutions, and, therefore, this planet also hasreached the same spot where it was at first. Venus and the earth, ofcourse, attract each other, and in consequence of these mutualattractions the earth is swayed from the elliptic track which itwould otherwise pursue. In like manner Venus is also forced by theattraction of the earth to revolve in a track which deviates fromthat which it would otherwise follow. Owing to the fact that the sunis of such preponderating magnitude (being, in fact, upwards of300, 000 times as heavy as either Venus or the earth), thedisturbances induced in the motion of either planet, in consequenceof the attraction of the other, are relatively insignificant to themain controlling agency by which each of the movements is governed. It is, however, possible under certain circumstances that thedisturbing effects produced upon one planet by the other can becomeso multiplied as to produce peculiar effects which attain measurabledimensions. Suppose that the periodic times in which the earth andVenus revolved had no simple relation to each other, then the pointsof their tracks in which the two planets came into line with the sunwould be found at different parts of the orbits, and consequently thedisturbances would to a great extent neutralise each other, andproduce but little appreciable effect. As, however, Venus and theearth come back every eight years to nearly the same positions at thesame points of their track, an accumulative effect is produced. Forthe disturbance of one planet upon the other will, of course, begreatest when those two planets are nearest, that is, when they liein line with the sun and on the same side of it. Every eight years acertain part of the orbit of the earth is, therefore, disturbed bythe attraction of Venus with peculiar vigour. The consequence isthat, owing to the numerical relation between the movements of theplanets to which I have referred, disturbing effects becomeappreciable which would otherwise be too small to permit ofrecognition. Airy proposed to himself to compute the effects whichVenus would have on the movement of the earth in consequence of thecircumstance that eight revolutions of the one planet required almostthe same time as thirteen revolutions of the other. This is amathematical inquiry of the most arduous description, but the PlumianProfessor succeeded in working it out, and he had, accordingly, thegratification of announcing to the Royal Society that he had detectedthe influence which Venus was thus able to assert on the movement ofour earth around the sun. This remarkable investigation gained forits author the gold medal of the Royal Astronomical Society in theyear 1832. 

In consequence Of his numerous discoveries, Airy's scientific famehad become so well recognised that the Government awarded him aspecial pension, and in 1835, when Pond, who was then AstronomerRoyal, resigned, Airy was offered the post at Greenwich. There wasin truth, no scientific inducement to the Plumian Professor to leavethe comparatively easy post he held at Cambridge, in which he hadample leisure to devote himself to those researches which speciallyinterested him, and accept that of the much more arduous observatoryat Greenwich. There were not even pecuniary inducements to make thechange; however, he felt it to be his duty to accede to the requestwhich the Government had made that he would take up the positionwhich Pond had vacated, and accordingly Airy went to Greenwich asAstronomer Royal on October 1st, 1835. 

He immediately began with his usual energy to organise the systematicconduct of the business of the National Observatory. To realise oneof the main characteristics of Airy's great work at Greenwich, it isnecessary to explain a point that might not perhaps be understoodwithout a little explanation by those who have no practicalexperience in an observatory. In the work of an establishment suchas Greenwich, an observation almost always consists of a measurementof some kind. The observer may, for instance, be making ameasurement of the time at which a star passes across a spider linestretched through the field of view; on another occasion his objectmay be the measurement of an angle which is read off by examiningthrough a microscope the lines of division on a graduated circle whenthe telescope is so pointed that the star is placed on a certain markin the field of view. In either case the immediate result of theastronomical observation is a purely numerical one, but it rarelyhappens, indeed we may say it never happens, that the immediatenumerical result which the observation gives expresses directly thequantity which we are really seeking for. No doubt the observationhas been so designed that the quantity we want to find can beobtained from the figures which the measurement gives, but the objectsought is not those figures, for there are always a multitude ofother influences by which those figures are affected. For example, if an observation were to be perfect, then the telescope with whichthe observation is made should be perfectly placed in the exactposition which it ought to occupy; this is, however, never the case, for no mechanic can ever construct or adjust a telescope so perfectlyas the wants of the astronomer demand. The clock also by which wedetermine the time of the observation should be correct, but this israrely if ever the case. We have to correct our observations forsuch errors, that is to say, we have to determine the errors in thepositions of our telescopes and the errors in the going of ourclocks, and then we have to determine what the observations wouldhave been had our telescopes been absolutely perfect, and had ourclocks been absolutely correct. There are also many other matterswhich have to be attended to in order to reduce our observations soas to obtain from the figures as yielded to the observer at thetelescope the actual quantities which it is his object to determine. 

The work of effecting these reductions is generally a very intricateand laborious matter, so that it has not unfrequently happened thatwhile observations have accumulated in an observatory, yet thetedious duty of reducing these observations has been allowed to fallinto arrear. When Airy entered on his duties at Greenwich he foundthere an enormous mass of observations which, though implicitlycontaining materials of the greatest value to astronomers, were, intheir unreduced form, entirely unavailable for any useful purpose. He, therefore, devoted himself to coping with the reduction of theobservations of his predecessors. He framed systematic methods bywhich the reductions were to be effected, and he so arranged the workthat little more than careful attention to numerical accuracy wouldbe required for the conduct of the operations. Encouraged by theAdmiralty, for it is under this department that Greenwich Observatoryis placed, the Astronomer Royal employed a large force of computersto deal with the work. BY his energy and admirable organisation hemanaged to reduce an extremely valuable series of planetaryobservations, and to publish the results, which have been of thegreatest importance to astronomical investigation. 

The Astronomer Royal was a capable, practical engineer as well as anoptician, and he presently occupied himself by designing astronomicalinstruments of improved pattern, which should replace the antiquatedinstruments he found in the observatory. In the course of years theentire equipment underwent a total transformation. He ordered agreat meridian circle, every part of which may be said to have beenformed from his own designs. He also designed the mounting for afine equatorial telescope worked by a driving clock, which he hadhimself invented. Gradually the establishment at Greenwich waxedgreat under his incessant care. It was the custom for theobservatory to be inspected every year by a board of visitors, whosechairman was the President of the Royal Society. At each annualvisitation, held on the first Saturday in June, the visitors receiveda report from the Astronomer Royal, in which he set forth thebusiness which had been accomplished during the past year. It was onthese occasions that applications were made to the Admiralty, eitherfor new instruments or for developing the work of the observatory insome other way. After the more official business of the inspectionwas over, the observatory was thrown open to visitors, and hundredsof people enjoyed on that day the privilege of seeing the nationalobservatory. These annual gatherings are happily still continued, and the first Saturday in June is known to be the occasion of one ofthe most interesting reunions of scientific men which takes place inthe course of the year. 

Airy's scientific work was, however, by no means confined to theobservatory. He interested himself largely in expeditions for theobservation of eclipses and in projects for the measurement of arcson the earth. He devoted much attention to the collection of magneticobservations from various parts of the world. Especially will it beremembered that the circumstances of the transits of Venus, whichoccurred in 1874 and in 1882, were investigated by him, and under hisguidance expeditions were sent forth to observe the transits fromthose localities in remote parts of the earth where observations mostsuitable for the determination of the sun's distance from the earthcould be obtained. The Astronomer Royal also studied tidalphenomena, and he rendered great service to the country in therestoration of the standards of length and weight which had beendestroyed in the great fire at the House of Parliament in October, 1834. In the most practical scientific matters his advice was oftensought, and was as cheerfully rendered. Now we find him engaged inan investigation of the irregularities of the compass in iron ships, with a view to remedying its defects; now we find him reporting onthe best gauge for railways. Among the most generally usefuldevelopments of the observatory must be mentioned the telegraphicmethod for the distribution of exact time. By arrangement with thePost Office, the astronomers at Greenwich despatch each morning asignal from the observatory to London at ten o'clock precisely. Byspecial apparatus, this signal is thence distributed automaticallyover the country, so as to enable the time to be known everywhereaccurately to a single second. It was part of the same system that atime ball should be dropped daily at one o'clock at Deal, as well asat other places, for the purpose of enabling ship's chronometers tobe regulated. 

Airy's writings were most voluminous, and no fewer than forty-eightmemoirs by him are mentioned in the "Catalogue of ScientificMemoirs, " published by the Royal Society up to the year 1873, andthis only included ten years out of an entire life of mostextraordinary activity. Many other subjects besides those of apurely scientific character from time to time engaged his attention. He wrote, for instance, a very interesting treatise on the Romaninvasion of Britain, especially with a view of determining the portfrom which Caesar set forth from Gaul, and the point at which helanded on the British coast. Airy was doubtless led to thisinvestigation by his study of the tidal phenomena in the Straits ofDover. Perhaps the Astronomer Royal is best known to the generalreading public by his excellent lectures on astronomy, delivered atthe Ipswich Museum in 1848. This book has passed through manyeditions, and it gives a most admirable account of the manner inwhich the fundamental problems in astronomy have to be attacked. 

As years rolled by almost every honour and distinction that could beconferred upon a scientific man was awarded to Sir George Airy. Hewas, indeed, the recipient of other honours not often awarded forscientific distinction. Among these we may mention that in 1875 hereceived the freedom of the City of London, "as a recognition of hisindefatigable labours in astronomy, and of his eminent services inthe advancement of practical science, whereby he has so materiallybenefited the cause of commerce and civilisation. "

Until his eightieth year Airy continued to discharge his labours atGreenwich with unflagging energy. At last, on August 15th, 1881, heresigned the office which he had held so long with such distinctionto himself and such benefit to his country. He had married in 1830the daughter of the Rev. Richard Smith, of Edensor. Lady Airy diedin 1875, and three sons and three daughters survived him. Onedaughter is the wife of Dr. Routh, of Cambridge, and his otherdaughters were the constant companions of their father during thedeclining years of his life. Up to the age of ninety he enjoyedperfect physical health, but an accidental fall which then occurredwas attended with serious results. He died on Saturday, January 2nd, 1892, and was buried in the churchyard at Playford. 

HAMILTON. 

William Rowan Hamilton was born at midnight between the 3rd and 4thof August, 1805, at Dublin, in the house which was then 29, butsubsequently 36, Dominick Street. His father, Archibald Hamilton, was a solicitor, and William was the fourth of a family of nine. Withreference to his descent, it may be sufficient to notice that hisancestors appear to have been chiefly of gentle Irish families, butthat his maternal grandmother was of Scottish birth. When he wasabout a year old, his father and mother decided to hand over theeducation of the child to his uncle, James Hamilton, a clergyman ofTrim, in County Meath. James Hamilton's sister, Sydney, resided withhim, and it was in their home that the days of William's childhoodwere passed. 

In Mr. Graves' "Life of Sir William Rowan Hamilton" a series ofletters will be found, in which Aunt Sydney details the progress ofthe boy to his mother in Dublin. Probably there is no record of aninfant prodigy more extraordinary than that which these letterscontain. At three years old his aunt assured the mother that Williamis "a hopeful blade, " but at that time it was his physical vigour towhich she apparently referred; for the proofs of his capacity, whichshe adduces, related to his prowess in making boys older than himselffly before him. In the second letter, a month later, we hear thatWilliam is brought in to read the Bible for the purpose of putting toshame other boys double his age who could not read nearly so well. Uncle James appears to have taken much pains with William'sschooling, but his aunt said that "how he picks up everything isastonishing, for he never stops playing and jumping about. " When hewas four years and three months old, we hear that he went out to dineat the vicar's, and amused the company by reading for them equallywell whether the book was turned upside down or held in any otherfashion. His aunt assures the mother that "Willie is a most sensiblelittle creature, but at the same time has a great deal of roguery. "At four years and five months old he came up to pay his mother avisit in town, and she writes to her sister a description of theboy;--

"His reciting is astonishing, and his clear and accurate knowledge ofgeography is beyond belief; he even draws the countries with a pencilon paper, and will cut them out, though not perfectly accurate, yetso well that a anybody knowing the countries could not mistake them;but, you will think this nothing when I tell you that he reads Latin, Greek, and Hebrew. "

Aunt Sydney recorded that the moment Willie got back to Trim he wasdesirous of at once resuming his former pursuits. He would not eathis breakfast till his uncle had heard him his Hebrew, and hecomments on the importance of proper pronunciation. At five he wastaken to see a friend, to whom he repeated long passages fromDryden. A gentleman present, who was not unnaturally sceptical aboutWillie's attainments, desired to test him in Greek, and took down acopy of Homer which happened to have the contracted type, and to hisamazement Willie went on with the greatest ease. At six years andnine months he was translating Homer and Virgil; a year later hisuncle tells us that William finds so little difficulty in learningFrench and Italian, that he wishes to read Homer in French. He isenraptured with the Iliad, and carries it about with him, repeatingfrom it whatever particularly pleases him. At eight years and onemonth the boy was one of a party who visited the Scalp in the Dublinmountains, and he was so delighted with the scenery that he forthwithdelivered an oration in Latin. At nine years and six months he isnot satisfied until he learns Sanscrit; three months later his thirstfor the Oriental languages is unabated, and at ten years and fourmonths he is studying Arabic and Persian. When nearly twelve heprepared a manuscript ready for publication. It was a "SyriacGrammar, " in Syriac letters and characters compiled from that ofBuxtorf, by William Hamilton, Esq. , of Dublin and Trim. When he wasfourteen, the Persian ambassador, Mirza Abul Hassan Khan, paid avisit to Dublin, and, as a practical exercise in his Orientallanguages, the young scholar addressed to his Excellency a letter inPersian; a translation of which production is given by Mr. Graves. When William was fourteen he had the misfortune to lose his father;and he had lost his mother two years previously. The boy and histhree sisters were kindly provided for by different members of thefamily on both sides. 

It was when William was about fifteen that his attention began to beturned towards scientific subjects. These were at first regardedrather as a relaxation from the linguistic studies with which he hadbeen so largely occupied. On November 22nd, 1820, he notes in hisjournal that he had begun Newton's "Principia": he commenced also thestudy of astronomy by observing eclipses, occultations, and similarphenomena. When he was sixteen we learn that he had read conicsections, and that he was engaged in the study of pendulums. Afteran attack of illness, he was moved for change to Dublin, and in May, 1822, we find him reading the differential calculus and Laplace's"Mecanique Celeste. " He criticises an important part of Laplace'swork relative to the demonstration of the parallelogram of forces. Inthis same year appeared the first gushes of those poems whichafterwards flowed in torrents. 

His somewhat discursive studies had, however, now to give place to amore definite course of reading in preparation for entrance to theUniversity of Dublin. The tutor under whom he entered, CharlesBoyton, was himself a distinguished man, but he frankly told theyoung William that he could be of little use to him as a tutor, forhis pupil was quite as fit to be his tutor. Eliza Hamilton, by whomthis is recorded, adds, "But there is one thing which Boyton wouldpromise to be to him, and that was a FRIEND; and that one proof hewould give of this should be that, if ever he saw William beginningto be UPSET by the sensation he would excite, and the notice he wouldattract, he would tell him of it. " At the beginning of his collegecareer he distanced all his competitors in every intellectualpursuit. At his first term examination in the University he wasfirst in Classics and first in Mathematics, while he received theChancellor's prize for a poem on the Ionian Islands, and another forhis poem on Eustace de St. Pierre. 

There is abundant testimony that Hamilton had "a heart for friendshipformed. " Among the warmest of the friends whom he made in theseearly days was the gifted Maria Edgeworth, who writes to her sisterabout "young Mr. Hamilton, an admirable Crichton of eighteen, a realprodigy of talents, who Dr. Brinkley says may be a second Newton, quiet, gentle, and simple. " His sister Eliza, to whom he wasaffectionately attached, writes to him in 1824:--

"I had been drawing pictures of you in my mind in your study atCumberland Street with 'Xenophon, ' &c. , on the table, and you, withyour most awfully sublime face of thought, now sitting down, and nowwalking about, at times rubbing your hands with an air ofsatisfaction, and at times bursting forth into some very heroicalstrain of poetry in an unknown language, and in your own internalsolemn ventriloquist-like voice, when you address yourself to thesilence and solitude of your own room, and indeed, at times, evenwhen your mysterious poetical addresses are not quite unheard. "

This letter is quoted because it refers to a circumstance which allwho ever met with Hamilton, even in his latest years, will remember. He was endowed with two distinct voices, one a high treble, the othera deep bass, and he alternately employed these voices not only inordinary conversation, but when he was delivering an address on theprofundities of Quaternions to the Royal Irish Academy, or onsimilar occasions. His friends had long grown so familiar with thispeculiarity that they were sometimes rather surprised to find howludicrous it appeared to strangers. 

Hamilton was fortunate in finding, while still at a very early age, acareer open before him which was worthy of his talents. He had notceased to be an undergraduate before he was called to fill anillustrious chair in his university. The circumstances are brieflyas follows. 

We have already mentioned that, in 1826, Brinkley was appointedBishop of Cloyne, and the professorship of astronomy thereupon becamevacant. Such was Hamilton's conspicuous eminence that, notwithstanding he was still an undergraduate, and had only justcompleted his twenty-first year, he was immediately thought of as asuitable successor to the chair. Indeed, so remarkable were histalents in almost every direction that had the vacancy been in theprofessorship of classics or of mathematics, of English literature orof metaphysics, of modern or of Oriental languages, it seemsdifficult to suppose that he would not have occurred to every one asa possible successor. The chief ground, however, on which thefriends of Hamilton urged his appointment was the earnest of originalpower which he had already shown in a research on the theory ofSystems of Rays. This profound work created a new branch of optics, and led a few years later to a superb discovery, by which the fame ofits author became world-wide. 

At first Hamilton thought it would be presumption for him to applyfor so exalted a position; he accordingly retired to the country, andresumed his studies for his degree. Other eminent candidates cameforward, among them some from Cambridge, and a few of the Fellowsfrom Trinity College, Dublin, also sent in their claims. It was notuntil Hamilton received an urgent letter from his tutor Boyton, inwhich he was assured of the favourable disposition of the Boardtowards his candidature, that he consented to come forward, and onJune 16th, 1827, he was unanimously chosen to succeed the Bishop ofCloyne as Professor of Astronomy in the University. The appointmentmet with almost universal approval. It should, however, be notedthat Brinkley, whom Hamilton succeeded, did not concur in the generalsentiment. No one could have formed a higher opinion than he haddone of Hamilton's transcendent powers; indeed, it was on that veryground that he seemed to view the appointment with disapprobation. He considered that it would have been wiser for Hamilton to haveobtained a Fellowship, in which capacity he would have been able toexercise a greater freedom in his choice of intellectual pursuits. The bishop seems to have thought, and not without reason, thatHamilton's genius would rather recoil from much of the routine workof an astronomical establishment. Now that Hamilton's whole life isbefore us, it is easy to see that the bishop was entirely wrong. Itis quite true that Hamilton never became a skilled astronomicalobserver; but the seclusion of the observatory was eminentlyfavourable to those gigantic labours to which his life was devoted, and which have shed so much lustre, not only on Hamilton himself, butalso on his University and his country. 

In his early years at Dunsink, Hamilton did make some attempts at apractical use of the telescopes, but he possessed no natural aptitudefor such work, while exposure which it involved seems to have actedinjuriously on his health. He, therefore, gradually allowed hisattention to be devoted to those mathematical researches in which hehad already given such promise of distinction. Although it was inpure mathematics that he ultimately won his greatest fame, yet healways maintained and maintained with justice, that he had ampleclaims to the title of an astronomer. In his later years he setforth this position himself in a rather striking manner. De Morganhad written commending to Hamilton's notice Grant's "History ofPhysical Astronomy. " After becoming acquainted with the book, Hamilton writes to his friend as follows:--

"The book is very valuable, and very creditable to its composer. Butyour humble servant may be pardoned if he finds himself somewhatamused at the title, `History of Physical Astronomy from the EarliestAges to the Middle of the Nineteenth Century, ' when he fails toobserve any notice of the discoveries of Sir W. R. Hamilton in thetheory of the 'Dynamics of the Heavens. '"

The intimacy between the two correspondents will account for the toneof this letter; and, indeed, Hamilton supplies in the lines whichfollow ample grounds for his complaint. He tells how Jacobi spoke ofhim in Manchester in 1842 as "le Lagrange de votre pays, " and howDonkin had said that, "The Analytical Theory of Dynamics as it existsat present is due mainly to the labours of La Grange Poisson, Sir W. R. Hamilton, and Jacobi, whose researches on this subjectpresent a series of discoveries hardly paralleled for their eleganceand importance in any other branch of mathematics. " In the sameletter Hamilton also alludes to the success which had attended theapplications of his methods in other hands than his own to theelucidation of the difficult subject of Planetary Perturbations. Even had his contributions to science amounted to no more than thesediscoveries, his tenure of the chair would have been an illustriousone. It happens, however, that in the gigantic mass of hisintellectual work these researches, though intrinsically of suchimportance, assume what might almost be described as a relativeinsignificance. 

The most famous achievement of Hamilton's earlier years at theobservatory was the discovery of conical refraction. This was one ofthose rare events in the history of science, in which a sagaciouscalculation has predicted a result of an almost startling character, subsequently confirmed by observation. At once this conferred on theyoung professor a world-wide renown. Indeed, though he was stillonly twenty-seven, he had already lived through an amount ofintellectual activity which would have been remarkable for a man ofthreescore and ten. 

Simultaneously with his growth in fame came the growth of his severalfriendships. There were, in the first place, his scientificfriendships with Herschel, Robinson, and many others with whom he hadcopious correspondence. In the excellent biography to which I havereferred, Hamilton's correspondence with Coleridge may be read, ascan also the letters to his lady correspondents, among them beingMaria Edgeworth, Lady Dunraven, and Lady Campbell. Many of thesesheets relate to literary matters, but they are largely intermingledWith genial pleasantry, and serve at all events to show the affectionand esteem with which he was regarded by all who had the privilege ofknowing him. There are also the letters to the sisters whom headored, letters brimming over with such exalted sentiment, that mostordinary sisters would be tempted to receive them with a smile in theexcessively improbable event of their still more ordinary brothersattempting to pen such effusions. There are also indications ofletters to and from other young ladies who from time to time were theobjects of Hamilton's tender admiration. We use the pluraladvisedly, for, as Mr. Graves has set forth, Hamilton's love affairspursued a rather troubled course. The attention which he lavished onone or two fair ones was not reciprocated, and even the intensecharms of mathematical discovery could not assuage the pangs whichthe disappointed lover experienced. At last he reached the haven ofmatrimony in 1833, when he was married to Miss Bayly. Of his marriedlife Hamilton said, many years later to De Morgan, that it was ashappy as he expected, and happier than he deserved. He had two sons, William and Archibald, and one daughter, Helen, who became the wifeof Archdeacon O'Regan. 

[PLATE: SIR W. ROWAN HAMILTON. ]

The most remarkable of Hamilton's friendships in his early years wasunquestionably that with Wordsworth. It commenced with Hamilton'svisit to Keswick; and on the first evening, when the poet met theyoung mathematician, an incident occurred which showed the mutualinterest that was aroused. Hamilton thus describes it in a letter tohis sister Eliza:--

"He (Wordsworth) walked back with our party as far as their lodge, and then, on our bidding Mrs. Harrison good-night, I offered to walkback with him while my party proceeded to the hotel. This offer heaccepted, and our conversation had become so interesting that when wehad arrived at his home, a distance of about a mile, he proposed towalk back with me on my way to Ambleside, a proposal which you may besure I did not reject; so far from it that when he came to turn oncemore towards his home I also turned once more along with him. It wasvery late when I reached the hotel after all this walking. "

Hamilton also submitted to Wordsworth an original poem, entitled"It Haunts me Yet. " The reply of Wordsworth is worth repeating:--

"With a safe conscience I can assure you that, in my judgment, yourverses are animated with the poetic spirit, as they are evidently theproduct of strong feeling. The sixth and seventh stanzas affected memuch, even to the dimming of my eyes and faltering of my voice whileI was reading them aloud. Having said this, I have said enough. Nowfor the per contra. You will not, I am sure, be hurt when I tell youthat the workmanship (what else could be expected from so young awriter?) is not what it ought to be. . . 

"My household desire to be remembered to you in no formal way. Seldom have I parted--never, I was going to say--with one whom afterso short an acquaintance I lost sight of with more regret. I trustwe shall meet again. "

The further affectionate intercourse between Hamilton and Wordsworthis fully set forth, and to Hamilton's latest years a recollection ofhis "Rydal hours" was carefully treasured and frequently referredto. Wordsworth visited Hamilton at the observatory, where abeautiful shady path in the garden is to the present day spoken of as"Wordsworth's Walk. "

It was the practice of Hamilton to produce a sonnet on almost everyoccasion which admitted of poetical treatment, and it was his delightto communicate his verses to his friends all round. When Whewell wasproducing his "Bridgewater Treatises, " he writes to Hamilton in1833:--

"Your sonnet which you showed me expressed much better than I couldexpress it the feeling with which I tried to write this book, and Ionce intended to ask your permission to prefix the sonnet to my book, but my friends persuaded me that I ought to tell my story in my ownprose, however much better your verse might be. "

The first epoch-marking contribution to Theoretical Dynamics afterthe time of Newton was undoubtedly made by Lagrange, in his discoveryof the general equations of Motion. The next great step in the samedirection was that taken by Hamilton in his discovery of a still morecomprehensive method. Of this contribution Hamilton writes toWhewell, March 31st, 1834:--

"As to my late paper, a day or two ago sent off to London, it ismerely mathematical and deductive. I ventured, indeed, to call itthe 'Mecanique Analytique' of Lagrange, 'a scientific poem'; andspoke of Dynamics, or the Science of Force, as treating of 'Poweracting by Law in Space and Time. ' In other respects it is asunpoetical and unmetaphysical as my gravest friends could desire. "

It may well be doubted whether there is a more beautiful chapter inthe whole of mathematical philosophy than that which containsHamilton's dynamical theory. It is disfigured by no tediouscomplexity of symbols; it condescends not to any particular problems;it is an all embracing theory, which gives an intellectual grasp ofthe most appropriate method for discovering the result of theapplication of force to matter. It is the very generality of thisdoctrine which has somewhat impeded the applications of which it issusceptible. The exigencies of examinations are partly responsiblefor the fact that the method has not become more familiar to studentsof the higher mathematics. An eminent professor has complained thatHamilton's essay on dynamics was of such an extremely abstractcharacter, that he found himself unable to extract from it problemssuitable for his examination papers. 

The following extract is from a letter of Professor Sylvester toHamilton, dated 20th of September, 1841. It will show how his workswere appreciated by so consummate a mathematician as the writer:--

"Believe me, sir, it is not the least of my regrets in quitting thisempire to feel that I forego the casual occasion of meeting thosemasters of my art, yourself chief amongst the number, whoseacquaintance, whose conversation, or even notice, have in themselvesthe power to inspire, and almost to impart fresh vigour to theunderstanding, and the courage and faith without which the efforts ofinvention are in vain. The golden moments I enjoyed under yourhospitable roof at Dunsink, or moments such as they were, mayprobably never again fall to my lot. 

"At a vast distance, and in an humble eminence, I still promise myselfthe calm satisfaction of observing your blazing course in theelevated regions of discovery. Such national honour as you are ableto confer on your country is, perhaps, the only species of thatluxury for the rich (I mean what is termed one's glory) which is notbought at the expense of the comforts of the million. "

The study of metaphysics was always a favourite recreation whenHamilton sought for a change from the pursuit of mathematics. In theyear 1834 we find him a diligent student of Kant; and, to show theviews of the author of Quaternions and of Algebra as the Science ofPure Time on the "Critique of the Pure Reason, " we quote thefollowing letter, dated 18th of July, 1834, from Hamilton to ViscountAdare:--

"I have read a large part of the 'Critique of the Pure Reason, ' andfind it wonderfully clear, and generally quite convincing. Notwithstanding some previous preparation from Berkeley, and from myown thoughts, I seem to have learned much from Kant's own statementof his views of 'Space and Time. ' Yet, on the whole, a large part ofmy pleasure consists in recognising through Kant's works, opinions, or rather views, which have been long familiar to myself, althoughfar more clearly and systematically expressed and combined by him. . . . Kant is, I think, much more indebted than he owns, or, perhapsknows, to Berkeley, whom he calls by a sneer, `GUTEM Berkeley'. . . As it were, `good soul, well meaning man, ' who was able for all thatto shake to its centre the world of human thought, and to effect arevolution among the early consequences of which was the growth ofKant himself. "

At several meetings of the British Association Hamilton was a veryconspicuous figure. Especially was this the case in 1835, when theAssociation met in Dublin, and when Hamilton, though then but thirtyyears old, had attained such celebrity that even among a verybrilliant gathering his name was perhaps the most renowned. Abanquet was given at Trinity College in honour of the meeting. Thedistinguished visitors assembled in the Library of the University. The Earl of Mulgrave, then Lord Lieutenant of Ireland, made this theopportunity of conferring on Hamilton the honour of knighthood, gracefully adding, as he did so: "I but set the royal, and thereforethe national mark, on a distinction already acquired by your geniusand labours. "

The banquet followed, writes Mr. Graves. "It was no little additionto the honour Hamilton had already received that, when ProfessorWhewell returned thanks for the toast of the University of Cambridge, he thought it appropriate to add the words, 'There was one pointwhich strongly pressed upon him at that moment: it was now onehundred and thirty years since a great man in another Trinity Collegeknelt down before his sovereign, and rose up Sir Isaac Newton. ' Thecompliment was welcomed by immense applause. "

A more substantial recognition of the labours of Hamilton took placesubsequently. He thus describes it in a letter to Mr. Graves of 14thof November, 1843:--

"The Queen has been pleased--and you will not doubt that it wasentirely unsolicited, and even unexpected, on my part--'to expressher entire approbation of the grant of a pension of two hundredpounds per annum from the Civil List' to me for scientific services. The letters from Sir Robert Peel and from the Lord Lieutenant ofIreland in which this grant has been communicated or referred to havebeen really more gratifying to my feelings than the addition to myincome, however useful, and almost necessary, that may have been. "

The circumstances we have mentioned might lead to the suppositionthat Hamilton was then at the zenith of his fame but this was notso. It might more truly be said, that his achievements up to thispoint were rather the preliminary exercises which fitted him for thegigantic task of his life. The name of Hamilton is now chieflyassociated with his memorable invention of the calculus ofQuaternions. It was to the creation of this branch of mathematicsthat the maturer powers of his life were devoted; in fact he gives ushimself an illustration of how completely habituated he became to thenew modes of thought which Quaternions originated. In one of hislater years he happened to take up a copy of his famous paper onDynamics, a paper which at the time created such a sensation amongmathematicians, and which is at this moment regarded as one of theclassics of dynamical literature. He read, he tells us, his paperwith considerable interest, and expressed his feelings ofgratification that he found himself still able to follow itsreasoning without undue effort. But it seemed to him all the time asa work belonging to an age of analysis now entirely superseded. 

In order to realise the magnitude of the revolution which Hamiltonhas wrought in the application of symbols to mathematicalinvestigation, it is necessary to think of what Hamilton did besidethe mighty advance made by Descartes. To describe the character ofthe quaternion calculus would be unsuited to the pages of this work, but we may quote an interesting letter, written by Hamilton from hisdeathbed, twenty-two years later, to his son Archibald, in which hehas recorded the circumstances of the discovery:--

"Indeed, I happen to be able to put the finger of memory upon the yearand month--October, 1843--when having recently returned from visitsto Cork and Parsonstown, connected with a meeting of the BritishAssociation, the desire to discover the laws of multiplicationreferred to, regained with me a certain strength and earnestnesswhich had for years been dormant, but was then on the point of beinggratified, and was occasionally talked of with you. Every morning inthe early part of the above-cited month, on my coming down tobreakfast, your (then) little brother William Edwin, and yourself, used to ask me, 'Well papa, can you multiply triplets?' Whereto Iwas always obliged to reply, with a sad shake of the head: 'No, Ican only ADD and subtract them, '

"But on the 16th day of the same month--which happened to be Monday, and a Council day of the Royal Irish Academy--I was walking in toattend and preside, and your mother was walking with me along theRoyal Canal, to which she had perhaps driven; and although she talkedwith me now and then, yet an UNDERCURRENT of thought was going on inmy mind which gave at last a RESULT, whereof it is not too much tosay that I felt AT ONCE the importance. An ELECTRIC circuit seemedto CLOSE; and a spark flashed forth the herald (as I FORESAWIMMEDIATELY) of many long years to come of definitely directedthought and work by MYSELF, if spared, and, at all events, on thepart of OTHERS if I should even be allowed to live long enoughdistinctly to communicate the discovery. Nor could I resist theimpulse--unphilosophical as it may have been--to cut with a knife ona stone of Brougham Bridge as we passed it, the fundamental formulawhich contains the SOLUTION of the PROBLEM, but, of course, theinscription has long since mouldered away. A more durable noticeremains, however, on the Council Books of the Academy for that day(October 16, 1843), which records the fact that I then asked for andobtained leave to read a Paper on 'Quaternions, ' at the First GeneralMeeting of the Session; which reading took place accordingly, onMonday, the 13th of November following. "

Writing to Professor Tait, Hamilton gives further particulars of thesame event. And again in a letter to the Rev. J. W. Stubbs:--

"To-morrow will be the fifteenth birthday of the Quaternions. Theystarted into life full-grown on the 16th October, 1843, as I waswalking with Lady Hamilton to Dublin, and came up to BroughamBridge--which my boys have since called Quaternion Bridge. I pulledout a pocketbook which still exists, and made entry, on which at thevery moment I felt that it might be worth my while to expend thelabour of at least ten or fifteen years to come. But then it is fairto say that this was because I felt a problem to have been at thatmoment solved, an intellectual want relieved which had haunted me forat least fifteen years before. 

"But did the thought of establishing such a system, in whichgeometrically opposite facts--namely, two lines (or areas) which areopposite IN SPACE give ALWAYS a positive product--ever come intoanybody's head till I was led to it in October, 1843, by trying toextend my old theory of algebraic couples, and of algebra as thescience of pure time? As to my regarding geometrical addition oflines as equivalent to composition of motions (and as performed bythe same rules), that is indeed essential in my theory but notpeculiar to it; on the contrary, I am only one of many who have beenled to this view of addition. "

Pilgrims in future ages will doubtless visit the spot commemorated bythe invention of Quaternions. Perhaps as they look at that by nomeans graceful structure Quaternion Bridge, they will regret that thehand of some Old Mortality had not been occasionally employed incutting the memorable inscription afresh. It is now irrecoverablylost. 

It was ten years after the discovery that the great volume appearedunder the title of "Lectures on Quaternions, " Dublin, 1853. Thereception of this work by the scientific world was such as might havebeen expected from the extraordinary reputation of its author, andthe novelty and importance of the new calculus. His valued friend, Sir John Herschel, writes to him in that style of which he was amaster:--

"Now, most heartily let me congratulate you on getting out yourbook--on having found utterance, ore rotundo, for all that labouringand seething mass of thought which has been from time to time sendingout sparks, and gleams, and smokes, and shaking the soil about you;but now breaks into a good honest eruption, with a lava stream and ashower of fertilizing ashes. 

"Metaphor and simile apart, there is work for a twelve-month to anyman to read such a book, and for half a lifetime to digest it, and Iam glad to see it brought to a conclusion. "

We may also record Hamilton's own opinion expressed to HumphreyLloyd:--

"In general, although in one sense I hope that I am actually growingmodest about the quaternions, from my seeing so many peeps and vistasinto future expansions of their principles, I still must assert thatthis discovery appears to me to be as important for the middle of thenineteenth century as the discovery of fluxions was for the close ofthe seventeenth. "

Bartholomew Lloyd died in 1837. He had been the Provost of TrinityCollege, and the President of the Royal Irish Academy. Threecandidates were put forward by their respective friends for thevacant Presidency. One was Humphrey Lloyd, the son of the lateProvost, and the two others were Hamilton and Archbishop Whately. Lloyd from the first urged strongly the claims of Hamilton, anddeprecated the putting forward of his own name. Hamilton in likemanner desired to withdraw in favour of Lloyd. The wish was stronglyfelt by many of the Fellows of the College that Lloyd should beelected, in consequence of his having a more intimate associationwith collegiate life than Hamilton; while his scientific eminence wasworld-wide. The election ultimately gave Hamilton a considerablemajority over Lloyd, behind whom the Archbishop followed at aconsiderable distance. All concluded happily, for both Lloyd and theArchbishop expressed, and no doubt felt, the pre-eminent claims ofHamilton, and both of them cordially accepted the office of aVice-President, to which, according to the constitution of theAcademy, it is the privilege of the incoming President to nominate. 

In another chapter I have mentioned as a memorable episode inastronomical history, that Sir J. Herschel went for a prolongedsojourn to the Cape of Good Hope, for the purpose of submitting thesouthern skies to the same scrutiny with the great telescope that hisfather had given to the northern skies. The occasion of Herschel'sreturn after the brilliant success of his enterprise, was celebratedby a banquet. On June 15th, 1838, Hamilton was assigned the highhonour of proposing the health of Herschel. This banquet isotherwise memorable in Hamilton's career as being one of the twooccasions in which he was in the company of his intimate friend DeMorgan. 

In the year 1838 a scheme was adopted by the Royal Irish Academy forthe award of medals to the authors of papers which appeared topossess exceptionally high merit. At the institution of the medaltwo papers were named in competition for the prize. One wasHamilton's "Memoir on Algebra, as the Science of Pure Time. " Theother was Macullagh's paper on the "Laws of Crystalline Reflectionand Refraction. " Hamilton expresses his gratification that, mainlyin consequence of his own exertions, he succeeded in having the medalawarded to Macullagh rather than to himself. Indeed, it would almostappear as if Hamilton had procured a letter from Sir J. Herschel, which indicated the importance of Macullagh's memoir in such a way asto decide the issue. It then became Hamilton's duty to award themedal from the chair, and to deliver an address in which he expressedhis own sense of the excellence of Macullagh's scientific work. Itis the more necessary to allude to these points, because in the wholeof his scientific career it would seem that Macullagh was the onlyman with whom Hamilton had ever even an approach to a dispute aboutpriority. The incident referred to took place in connection with thediscovery of conical refraction, the fame of which Macullagh made apreposterous attempt to wrest from Hamilton. This is evidentlyalluded to in Hamilton's letter to the Marquis of Northampton, datedJune 28th, 1838, in which we read:--

"And though some former circumstances prevented me from applying tothe person thus distinguished the sacred name of FRIEND, I had thepleasure of doing justice. .. To his high intellectual merits. .. Ibelieve he was not only gratified but touched, and may, perhaps, regard me in future with feelings more like those which I long toentertain towards him. "

Hamilton was in the habit, from time to time, of commencing thekeeping of a journal, but it does not appear to have beensystematically conducted. Whatever difficulties the biographer mayhave experienced from its imperfections and irregularities, seem tobe amply compensated for by the practice which Hamilton had ofpreserving copies of his letters, and even of comparativelyinsignificant memoranda. In fact, the minuteness with whichapparently trivial matters were often noted down appears almostwhimsical. He frequently made a memorandum of the name of the personwho carried a letter to the post, and of the hour in which it wasdespatched. On the other hand, the letters which he received werealso carefully preserved in a mighty mass of manuscripts, with whichhis study was encumbered, and with which many other parts of thehouse were not unfrequently invaded. If a letter was laid aside fora few hours, it would become lost to view amid the seething mass ofpapers, though occasionally, to use his own expression, it might beseen "eddying" to the surface in some later disturbance. 

The great volume of "Lectures on Quaternions" had been issued, andthe author had received the honours which the completion of such atask would rightfully bring him. The publication of an immortal workdoes not, however, necessarily provide the means for paying theprinter's bill. The printing of so robust a volume was necessarilycostly; and even if all the copies could be sold, which at the timedid not seem very likely, they would hardly have met the inevitableexpenses. The provision of the necessary funds was, therefore, amatter for consideration. The Board of Trinity College had alreadycontributed 200 pounds to the printing, but yet another hundred wasrequired. Even the discoverer of Quaternions found this a source ofmuch anxiety. However, the board, urged by the representation ofHumphrey Lloyd, now one of its members, and, as we have already seen, one of Hamilton's staunchest friends, relieved him of all liability. We may here note that, notwithstanding the pension which Hamiltonenjoyed in addition to the salary of his chair, he seems always tohave been in some what straitened circumstances, or, to use his ownwords in one of his letters to De Morgan, "Though not an embarrassedman, I am anything rather than a rich one. " It appears that, notwithstanding the world-wide fame of Hamilton's discoveries, theonly profit in a pecuniary sense that he ever obtained from any ofhis works was by the sale of what he called his Icosian Game. Someenterprising publisher, on the urgent representations of one ofHamilton's friends in London, bought the copyright of the IcosianGame for 25 pounds. Even this little speculation proved unfortunatefor the purchaser, as the public could not be induced to take thenecessary interest in the matter. 

After the completion of his great book, Hamilton appeared for awhileto permit himself a greater indulgence than usual in literaryrelaxations. He had copious correspondence with his intimate friend, Aubrey de Vere, and there were multitudes of letters from thosetroops of friends whom it was Hamilton's privilege to possess. Hehad been greatly affected by the death of his beloved sister Eliza, apoetess of much taste and feeling. She left to him her many papersto preserve or to destroy, but he said it was only after theexpiration of four years of mourning that he took courage to open herpet box of letters. 

The religious side of Hamilton's character is frequently illustratedin these letters; especially is this brought out in thecorrespondence with De Vere, who had seceded to the Church of Rome. Hamilton writes, August 4, 1855:--

"If, then, it be painfully evident to both, that under suchcircumstances there CANNOT (whatever we may both DESIRE) be NOW inthe nature of things, or of minds, the same degree of INTIMACYbetween us as of old; since we could no longer TALK with the samedegree of unreserve on every subject which happened to presentitself, but MUST, from the simplest instincts of courtesy, be each onhis guard not to say what might be offensive, or, at least, painfulto the other; yet WE were ONCE so intimate, an retain still, and, asI trust, shall always retain, so much of regard and esteem andappreciation for each other, made tender by so many associations ofmy early youth and your boyhood, which can never be forgotten byeither of us, that (as times go) TWO OR THREE VERY RESPECTABLEFRIENDSHIPS might easily be carved out from the fragments of ourformer and ever-to-be-remembered INTIMACY. It would be noexaggeration to quote the words: 'Heu! quanto minus est cum reliquisversari, quam tui meminisse!'"

In 1858 a correspondence on the subject of Quaternions commencedbetween Professor Tait and Sir William Hamilton. It was particularlygratifying to the discoverer that so competent a mathematician asProfessor Tait should have made himself acquainted with the newcalculus. It is, of course, well known that Professor Taitsubsequently brought out a most valuable elementary treatise onQuaternions, to which those who are anxious to become acquainted withthe subject will often turn in preference to the tremendous work ofHamilton. 

In the year 1861 gratifying information came to hand of the progresswhich the study of Quaternions was making abroad. Especially did thesubject attract the attention of that accomplished mathematician, Moebius, who had already in his "Barycentrische Calculus" been led toconceptions which bore more affinity to Quaternions than could befound in the writings of any other mathematician. Such notices ofhis work were always pleasing to Hamilton, and they served, perhaps, as incentives to that still closer and more engrossing labour bywhich he became more and more absorbed. During the last few years ofhis life he was observed to be even more of a recluse than he hadhitherto been. His powers of long and continuous study seemed togrow with advancing years, and his intervals of relaxation, such asthey were, became more brief and more infrequent. 

It was not unusual for him to work for twelve hours at a stretch. The dawn would frequently surprise him as he looked up to snuff hiscandles after a night of fascinating labour at original research. Regularity in habits was impossible to a student who had prolongedfits of what he called his mathematical trances. Hours for rest andhours for meals could only be snatched in the occasional the lucidintervals between one attack of Quaternions and the next. Whenhungry, he would go to see whether any thing could be found on thesideboard; when thirsty, he would visit the locker, and the oneblemish in the man's personal character is that these latter visitswere sometimes paid too often. 

As an example of one of Hamilton's rare diversions from the all-absorbing pursuit of Quaternions, we find that he was seized withcuriosity to calculate back to the date of the Hegira, which he foundon the 15th July, 622. He speaks of the satisfaction with which heascertained subsequently that Herschel had assigned precisely thesame date. Metaphysics remained also, as it had ever been, afavourite subject of Hamilton's readings and meditations and ofcorrespondence with his friends. He wrote a very long letter to Dr. Ingleby on the subject of his "Introduction to Metaphysics. " In itHamilton alludes, as he has done also in other places, to apeculiarity of his own vision. It was habitual to him, by somedefect in the correlation of his eyes, to see always a distinct imagewith each; in fact, he speaks of the remarkable effect which the useof a good stereoscope had on his sensations of vision. It was then, for the first time, that he realised how the two images which he hadalways seen hitherto would, under normal circumstances, be blendedinto one. He cites this fact as bearing on the phenomena ofbinocular vision, and he draws from it the inference that thenecessity of binocular vision for the correct appreciation ofdistance is unfounded. "I am quite sure, " he says, "that I SEEDISTANCE with EACH EYE SEPARATELY. "

The commencement of 1865, the last year of his life saw Hamilton asdiligent as ever, and corresponding with Salmon and Cayley. On April26th he writes to a friend to say, that his health has not been goodfor years past, and that so much work has injured his constitution;and he adds, that it is not conducive to good spirits to find that heis accumulating another heavy bill with the printer for thepublication of the "Elements. " This was, indeed, up to the day ofhis death, a cause for serious anxiety. It may, however, bementioned that the whole cost, which amounted to nearly 500 pounds, was, like that of the previous volume, ultimately borne by theCollege. Contrary to anticipation, the enterprise, even in apecuniary sense, cannot have been a very unprofitable one. The wholeedition has long been out of print, and as much as 5 pounds has sincebeen paid for a single copy. 

It was on the 9th of May, 1865, that Hamilton was in Dublin for thelast time. A few days later he had a violent attack of gout, and onthe 4th of June he became alarmingly ill, and on the next day had anattack of epileptic convulsions. However, he slightly rallied, sothat before the end of the month he was again at work at the"Elements. " A gratifying incident brightened some of the last daysof his life. The National Academy of Science in America had thenbeen just formed. A list of foreign Associates had to be chosen fromthe whole world, and a discussion took place as to what name shouldbe placed first on the list. Hamilton was informed by privatecommunication that this great distinction was awarded to him by amajority of two-thirds. 

In August he was still at work on the table of contents of the"Elements, " and one of his very latest efforts was his letter to Mr. Gould, in America, communicating his acknowledgements of the honourwhich had been just conferred upon him by the National Academy. Onthe 2nd of September Mr. Graves went to the observatory, in responseto a summons, and the great mathematician at once admitted to hisfriend that he felt the end was approaching. He mentioned that hehad found in the 145th Psalm a wonderfully suitable expression of histhoughts and feelings, and he wished to testify his faith andthankfulness as a Christian by partaking of the Lord's Supper. Hedied at half-past two on the afternoon of the 2nd of September, 1865, aged sixty years and one month. He was buried in Mount JeromeCemetery on the 7th of September. 

Many were the letters and other more public manifestations of thefeelings awakened by Hamilton's death. Sir John Herschel wrote tothe widow:--

"Permit me only to add that among the many scientific friends whomtime has deprived me of, there has been none whom I more deeplylament, not only for his splendid talents, but for the excellence ofhis disposition and the perfect simplicity of his manners--so great, and yet devoid of pretensions. "

De Morgan, his old mathematical crony, as Hamilton affectionatelystyled him, also wrote to Lady Hamilton:--

"I have called him one of my dearest friends, and most truly; for Iknow not how much longer than twenty-five years we have been inintimate correspondence, of most friendly agreement or disagreement, of most cordial interest in each other. And yet we did not know eachother's faces. I met him about 1830 at Babbage's breakfast table, and there for the only time in our lives we conversed. I saw him, along way off, at the dinner given to Herschel (about 1838) on hisreturn from the Cape and there we were not near enough, nor on thatcrowded day could we get near enough, to exchange a word. And thisis all I ever saw, and, so it has pleased God, all I shall see inthis world of a man whose friendly communications were among mygreatest social enjoyments, and greatest intellectual treats. "

There is a very interesting memoir of Hamilton written by De Morgan, in the "Gentleman's Magazine" for 1866, in which he produces anexcellent sketch of his friend, illustrated by personal reminiscencesand anecdotes. He alludes, among other things, to the picturesqueconfusion of the papers in his study. There was some sort of orderin the mass, discernible however, by Hamilton alone, and any invasionof the domestics, with a view to tidying up, would throw themathematician as we are informed, into "a good honest thunderingpassion. "

Hardly any two men, who were both powerful mathematicians, could havebeen more dissimilar in every other respect than were Hamilton and DeMorgan. The highly poetical temperament of Hamilton was remarkablycontrasted with the practical realism of De Morgan. Hamilton sendssonnets to his friend, who replies by giving the poet advice aboutmaking his will. The metaphysical subtleties, with which Hamiltonoften filled his sheets, did not seem to have the same attraction forDe Morgan that he found in battles about the quantification of thePredicate. De Morgan was exquisitely witty, and though his jokeswere always appreciated by his correspondent, yet Hamilton seldomventured on anything of the same kind in reply; indeed his rareattempts at humour only produced results of the most ponderousdescription. But never were two scientific correspondents moreperfectly in sympathy with each other. Hamilton's work onQuaternions, his labours in Dynamics, his literary tastes, hismetaphysics, and his poetry, were all heartily welcomed by hisfriend, whose letters in reply invariably evince the kindliestinterest in all Hamilton's concerns. In a similar way De Morgan'sletters to Hamilton always met with a heartfelt response. 

Alike for the memory of Hamilton, for the credit of his University, and for the benefit of science, let us hope that a collected editionof his works will ere long appear--a collection which shall showthose early achievements in splendid optical theory, thoseachievements of his more mature powers which made him the Lagrange ofhis country, and finally those creations of the Quaternion Calculusby which new capabilities have been bestowed on the human intellect. 

LE VERRIER. 

The name of Le Verrier is one that goes down to fame on account ofvery different discoveries from those which have given renown toseveral of the other astronomers whom we have mentioned. We aresometimes apt to identify the idea of an astronomer with that of aman who looks through a telescope at the stars; but the wordastronomer has really much wider significance. No man who ever livedhas been more entitled to be designated an astronomer than LeVerrier, and yet it is certain that he never made a telescopicdiscovery of any kind. Indeed, so far as his scientific achievementshave been concerned, he might never have looked through a telescopeat all. 

For the full interpretation of the movements of the heavenly bodies, mathematical knowledge of the most advanced character is demanded. The mathematician at the outset calls upon the astronomer who usesthe instruments in the observatory, to ascertain for him at varioustimes the exact positions occupied by the sun, the moon, and theplanets. These observations, obtained with the greatest care, andpurified as far as possible from the errors by which they may beaffected form, as it were, the raw material on which themathematician exercises his skill. It is for him to elicit from theobserved places the true laws which govern the movements of theheavenly bodies. Here is indeed a task in which the highest powersof the human intellect may be worthily employed. 

Among those who have laboured with the greatest success in theinterpretation of the observations made with instruments ofprecision, Le Verrier holds a highly honoured place. To him it hasbeen given to provide a superb illustration of the success with whichthe mind of man can penetrate the deep things of Nature. 

The illustrious Frenchman, Urban Jean Joseph Le Verrier, was born onthe 11th March, 1811, at St. Lo, in the department of Manche. Hereceived his education in that famous school for education in thehigher branches of science, the Ecole Polytechnique, and acquiredthere considerable fame as a mathematician. On leaving the school LeVerrier at first purposed to devote himself to the public service, inthe department of civil engineering; and it is worthy of note thathis earliest scientific work was not in those mathematical researchesin which he was ultimately to become so famous. His duties in theengineering department involved practical chemical research in thelaboratory. In this he seems to have become very expert, andprobably fame as a chemist would have been thus attained, had notdestiny led him into another direction. As it was, he did engage insome original chemical research. His first contributions to sciencewere the fruits of his laboratory work; one of his papers was on thecombination of phosphorus and hydrogen, and another on thecombination of phosphorus and oxygen. 

His mathematical labours at the Ecole Polytechnique had, however, revealed to Le Verrier that he was endowed with the powers requisitefor dealing with the subtlest instruments of mathematical analysis. When he was twenty-eight years old, his first great astronomicalinvestigation was brought forth. It will be necessary to enter intosome explanation as to the nature of this, inasmuch as it was thecommencement of the life-work which he was to pursue. 

If but a single planet revolved around the sun, then the orbit ofthat planet would be an ellipse, and the shape and size, as well asthe position of the ellipse, would never alter. One revolution afteranother would be traced out, exactly in the same manner, incompliance with the force continuously exerted by the sun. Suppose, however, that a second planet be introduced into the system. The sunwill exert its attraction on this second planet also, and it willlikewise describe an orbit round the central globe. We can, however, no longer assert that the orbit in which either of the planets movesremains exactly an ellipse. We may, indeed, assume that the mass ofthe sun is enormously greater than that of either of the planets. Inthis case the attraction of the sun is a force of such preponderatingmagnitude, that the actual path of each planet remains nearly thesame as if the other planet were absent. But it is impossible forthe orbit of each planet not to be affected in some degree by theattraction of the other planet. The general law of nature assertsthat every body in space attracts every other body. So long as thereis only a single planet, it is the single attraction between the sunand that planet which is the sole controlling principle of themovement, and in consequence of it the ellipse is described. Butwhen a second planet is introduced, each of the two bodies is notonly subject to the attraction of the sun, but each one of theplanets attracts the other. It is true that this mutual attractionis but small, but, nevertheless, it produces some effect. It"disturbs, " as the astronomer says, the elliptic orbit which wouldotherwise have been pursued. Hence it follows that in the actualplanetary system where there are several planets disturbing eachother, it is not true to say that the orbits are absolutely elliptic. 

At the same time in any single revolution a planet may for mostpractical purposes be said to be actually moving in an ellipse. As, however, time goes on, the ellipse gradually varies. It alters itsshape, it alters its plane, and it alters its position in thatplane. If, therefore, we want to study the movements of the planets, when great intervals of time are concerned, it is necessary to havethe means of learning the nature of the movement of the orbit inconsequence of the disturbances it has experienced. 

We may illustrate the matter by supposing the planet to be runninglike a railway engine on a track which has been laid in a longelliptic path. We may suppose that while the planet is coursingalong, the shape of the track is gradually altering. But thisalteration may be so slow, that it does not appreciably affect themovement of the engine in a single revolution. We can also supposethat the plane in which the rails have been laid has a slowoscillation in level, and that the whole orbit is with more or lessuniformity moved slowly about in the plane. 

In short periods of time the changes in the shapes and positions ofthe planetary orbits, in consequence of their mutual attractions, areof no great consequence. When, however, we bring thousands of yearsinto consideration, then the displacements of the planetary orbitsattain considerable dimensions, and have, in fact, produced aprofound effect on the system. 

It is of the utmost interest to investigate the extent to which oneplanet can affect another in virtue of their mutual attractions. Suchinvestigations demand the exercise of the highest mathematicalgifts. But not alone is intellectual ability necessary for successin such inquiries. It must be united with a patient capacity forcalculations of an arduous type, protracted, as they frequently haveto be, through many years of labour. Le Verrier soon found in theseprofound inquiries adequate scope for the exercise of his peculiargifts. His first important astronomical publication contained aninvestigation of the changes which the orbits of several of theplanets, including the earth, have undergone in times past, and whichthey will undergo in times to come. 

As an illustration of these researches, we may take the case of theplanet in which we are, of course, especially interested, namely, theearth, and we can investigate the changes which, in the lapse oftime, the earth's orbit has undergone, in consequence of thedisturbance to which it has been subjected by the other planets. Ina century, or even in a thousand years, there is but littlerecognisable difference in the shape of the track pursued by theearth. Vast periods of time are required for the development of thelarge consequences of planetary perturbation. Le Verrier has, however, given us the particulars of what the earth's journey throughspace has been at intervals of 20, 000 years back from the presentdate. His furthest calculation throws our glance back to the stateof the earth's track 100, 000 years ago, while, with a bound forward, he shows us what the earth's orbit is to be in the future, atsuccessive intervals of 20, 000 years, till a date is reached which is100, 000 years in advance Of A. D. 1800. 

The talent which these researches displayed brought Le Verrier intonotice. At that time the Paris Observatory was presided over byArago, a SAVANT who occupies a distinguished position in Frenchscientific annals. Arago at once perceived that Le Verrier was justthe man who possessed the qualifications suitable for undertaking aproblem of great importance and difficulty that had begun to forceitself on the attention of astronomers. What this great problem was, and how astonishing was the solution it received, must now beconsidered. 

Ever since Herschel brought himself into fame by his superb discoveryof the great planet Uranus, the movements of this new addition to thesolar system were scrutinized with care and attention. The positionof Uranus was thus accurately determined from time to time. Atlength, when sufficient observations of this remote planet had beenbrought together, the route which the newly-discovered body pursuedthrough the heavens was ascertained by those calculations with whichastronomers are familiar. It happens, however, that Uranus possessesa superficial resemblance to a star. Indeed the resemblance is sooften deceptive that long ere its detection as a planet by Herschel, it had been observed time after time by skilful astronomers, wholittle thought that the star-like point at which they looked wasanything but a star. From these early observations it was possibleto determine the track of Uranus, and it was found that the greatplanet takes a period of no less than eighty-four years to accomplisha circuit. Calculations were made of the shape of the orbit in whichit revolved before its discovery by Herschel, and these were comparedwith the orbit which observations showed the same body to pursue inthose later years when its planetary character was known. It couldnot, of course, be expected that the orbit should remain unaltered;the fact that the great planets Jupiter and Saturn revolve in thevicinity of Uranus must necessarily imply that the orbit of thelatter undergoes considerable changes. When, however, due allowancehas been made for whatever influence the attraction of Jupiter andSaturn, and we may add of the earth and all the other Planets, couldpossibly produce, the movements of Uranus were still inexplicable. Itwas perfectly obvious that there must be some other influence at workbesides that which could be attributed to the planets already known. 

Astronomers could only recognise one solution of such a difficulty. It was impossible to doubt that there must be some other planet inaddition to the bodies at that time known, and that the perturbationsof Uranus hitherto unaccounted for, were due to the disturbancescaused by the action of this unknown planet. Arago urged Le Verrierto undertake the great problem of searching for this body, whosetheoretical existence seemed demonstrated. But the conditions of thesearch were such that it must needs be conducted on principles whollydifferent from any search which had ever before been undertaken for acelestial object. For this was not a case in which mere survey witha telescope might be expected to lead to the discovery. 

Certain facts might be immediately presumed with reference to theunknown object. There could be no doubt that the unknown disturberof Uranus must be a large body with a mass far exceeding that of theearth. It was certain, however, that it must be so distant that itcould only appear from our point of view as a very small object. Uranus itself lay beyond the range, or almost beyond the range, ofunassisted vision. It could be shown that the planet by which thedisturbance was produced revolved in an orbit which must lie outsidethat of Uranus. It seemed thus certain that the planet could not bea body visible to the unaided eye. Indeed, had it been at allconspicuous its planetary character would doubtless have beendetected ages ago. The unknown body must therefore be a planet whichwould have to be sought for by telescopic aid. 

There is, of course, a profound physical difference between a planetand a star, for the star is a luminous sun, and the planet is merelya dark body, rendered visible by the sunlight which falls upon it. Notwithstanding that a star is a sun thousands of times larger thanthe planet and millions of times more remote, yet it is a singularfact that telescopic planets possess an illusory resemblance to thestars among which their course happens to lie. So far as actualappearance goes, there is indeed only one criterion by which a planetof this kind can be discriminated from a star. If the planet belarge enough the telescope will show that it possesses a disc, andhas a visible and measurable circular outline. This feature a stardoes not exhibit. The stars are indeed so remote that no matter howlarge they may be intrinsically, they only exhibit radiant points oflight, which the utmost powers of the telescope fail to magnify intoobjects with an appreciable diameter. The older and well-knownplanets, such as Jupiter and Mars, possess discs, which, though notvisible to the unaided eye, were clearly enough discernible with theslightest telescopic power. But a very remote planet like Uranus, though it possessed a disc large enough to be quickly appreciated bythe consummate observing skill of Herschel, was nevertheless sostellar in its appearance, that it had been observed no fewer thanseventeen times by experienced astronomers prior to Herschel. Ineach case the planetary nature of the object had been overlooked, andit had been taken for granted that it was a star. It presented nodifference which was sufficient to arrest attention. 

As the unknown body by which Uranus was disturbed was certainly muchmore remote than Uranus, it seemed to be certain that though it mightshow a disc perceptible to very close inspection, yet that the discmust be so minute as not to be detected except with extreme care. Inother words, it seemed probable that the body which was to be soughtfor could not readily be discriminated from a small star, to whichclass of object it bore a superficial resemblance, though, as amatter of fact, there was the profoundest difference between the twobodies. 

There are on the heavens many hundreds of thousands of stars, and theproblem of identifying the planet, if indeed it should lie amongthese stars, seemed a very complex matter. Of course it is theabundant presence of the stars which causes the difficulty. If thestars could have been got rid of, a sweep over the heavens would atonce disclose all the planets which are bright enough to be visiblewith the telescopic power employed. It is the fortuitous resemblanceof the planet to the stars which enables it to escape detection. Todiscriminate the planet among stars everywhere in the sky would bealmost impossible. If, however, some method could be devised forlocalizing that precise region in which the planet's existence mightbe presumed, then the search could be undertaken with some prospectof success. 

To a certain extent the problem of localizing the region on the skyin which the planet might be expected admitted of an immediatelimitation. It is known that all the planets, or perhaps I oughtrather to say, all the great planets, confine their movements to acertain zone around the heavens. This zone extends some way oneither side of that line called the ecliptic in which the earthpursues its journey around the sun. It was therefore to be inferredthat the new planet need not be sought for outside this zone. It isobvious that this consideration at once reduces the area to bescrutinized to a small fraction of the entire heavens. But evenwithin the zone thus defined there are many thousands of stars. Itwould seem a hopeless task to detect the new planet unless somefurther limitation to its position could be assigned. 

It was accordingly suggested to Le Verrier that he should endeavourto discover in what particular part of the strip of the celestialsphere which we have indicated the search for the unknown planetshould be instituted. The materials available to the mathematicianfor the solution of this problem were to be derived solely from thediscrepancies between the calculated places in which Uranus should befound, taking into account the known causes of disturbance, and theactual places in which observation had shown the planet to exist. Here was indeed an unprecedented problem, and one of extraordinarydifficulty. Le Verrier, however, faced it, and, to the astonishmentof the world, succeeded in carrying it through to a brilliantsolution. We cannot here attempt to enter into any account of themathematical investigations that were necessary. All that we can dois to give a general indication of the method which had to beadopted. 

Let us suppose that a planet is revolving outside Uranus, at adistance which is suggested by the several distances at which theother planets are dispersed around the sun. Let us assume that thisouter planet has started on its course, in a prescribed path, andthat it has a certain mass. It will, of course, disturb the motionof Uranus, and in consequence of that disturbance Uranus will followa path the nature of which can be determined by calculation. Itwill, however, generally be found that the path so ascertained doesnot tally with the actual path which observations have indicated forUranus. This demonstrates that the assumed circumstances of theunknown planet must be in some respects erroneous, and the astronomercommences afresh with an amended orbit. At last after many trials, Le Verrier ascertained that, by assuming a certain size, shape, andposition for the unknown Planet's orbit, and a certain value for themass of the hypothetical body, it would be possible to account forthe observed disturbances of Uranus. Gradually it became clear tothe perception of this consummate mathematician, not only that thedifficulties in the movements of Uranus could be thus explained, butthat no other explanation need be sought for. It accordinglyappeared that a planet possessing the mass which he had assigned, andmoving in the orbit which his calculations had indicated, must indeedexist, though no eye had ever beheld any such body. Here was, indeed, an astonishing result. The mathematician sitting at hisdesk, by studying the observations which had been supplied to him ofone planet, is able to discover the existence of another planet, andeven to assign the very position which it must occupy, ere ever thetelescope is invoked for its discovery. 

Thus it was that the calculations of Le Verrier narrowed greatly thearea to be scrutinised in the telescopic search which was presentlyto be instituted. It was already known, as we have just pointed out, that the planet must lie somewhere on the ecliptic. The Frenchmathematician had now further indicated the spot on the ecliptic atwhich, according to his calculations, the planet must actually befound. And now for an episode in this history which will becelebrated so long as science shall endure. It is nothing less thanthe telescopic confirmation of the existence of this new planet, which had previously been indicated only by mathematicalcalculation. Le Verrier had not himself the instruments necessaryfor studying the heavens, nor did he possess the skill of thepractical astronomer. He, therefore, wrote to Dr. Galle, of theObservatory at Berlin, requesting him to undertake a telescopicsearch for the new planet in the vicinity which the mathematicalcalculation had indicated for the whereabouts of the planet at thatparticular time. Le Verrier added that he thought the planet oughtto admit of being recognised by the possession of a disc sufficientlydefinite to mark the distinction between it and the surroundingstars. 

It was the 23rd September, 1846, when the request from Le Verrierreached the Berlin Observatory, and the night was clear, so that thememorable search was made on the same evening. The investigation wasfacilitated by the circumstance that a diligent observer had recentlycompiled elaborate star maps for certain tracts of the heavens lyingin a sufficiently wide zone on both sides of the equator. These mapswere as yet only partially complete, but it happened that Hora. XXI. , which included the very spot which Le Verrier's results referred to, had been just issued. Dr. Galle had thus before his, eyes a chart ofall the stars which were visible in that part of the heavens at thetime when the map was made. The advantage of such an assistance tothe search could hardly be over-estimated. It at once gave theastronomer another method of recognising the planet besides thatafforded by its possible possession of a disc. For as the planet wasa moving body, it would not have been in the same place relatively tothe stars at the time when the map was constructed, as it occupiedsome years later when the search was being made. If the body shouldbe situated in the spot which Le Verrier's calculations indicated inthe autumn of 1846, then it might be regarded as certain that itwould not be found in that same place on a map drawn some yearspreviously. 

The search to be undertaken consisted in a comparison made point bypoint between the bodies shown on the map, and those stars in the skywhich Dr. Galle's telescope revealed. In the course of thiscomparison it presently appeared that a star-like object of theeighth magnitude, which was quite a conspicuous body in thetelescope, was not represented in the map. This at once attractedthe earnest attention of the astronomer, and raised his hopes thathere was indeed the planet. Nor were these hopes destined to bedisappointed. It could not be supposed that a star of the eighthmagnitude would have been overlooked in the preparation of a chartwhereon stars of many lower degrees of brightness were set down. Oneother supposition was of course conceivable. It might have been thatthis suspicious object belonged to the class of variables, for thereare many such stars whose brightness fluctuates, and if it hadhappened that the map was constructed at a time when the star inquestion had but feeble brilliance, it might have escaped notice. Itis also well known that sometimes new stars suddenly develop, so thatthe possibility that what Dr. Galle saw should have been a variablestar or should have been a totally new star had to be providedagainst. 

Fortunately a test was immediately available to decide whether thenew object was indeed the long sought for planet, or whether it was astar of one of the two classes to which I have just referred. A starremains fixed, but a planet is in motion. No doubt when a planetlies at the distance at which this new planet was believed to besituated, its apparent motion would be so slow that it would not beeasy to detect any change in the course of a single night'sobservation. Dr. Galle, however, addressed himself with much skillto the examination of the place of the new body. Even in the courseof the night he thought he detected slight movements, and he awaitedwith much anxiety the renewal of his observations on the subsequentevenings. His suspicions as to the movement of the body were thenamply confirmed, and the planetary nature of the new object was thusunmistakably detected. 

Great indeed was the admiration of the scientific world at thissuperb triumph. Here was a mighty planet whose very existence wasrevealed by the indications afforded by refined mathematicalcalculation. At once the name of Le Verrier, already known to thoseconversant with the more profound branches of astronomy, becameeverywhere celebrated. It soon, however, appeared, that the famebelonging to this great achievement had to be shared between LeVerrier and another astronomer, J. C. Adams, of Cambridge. In ourchapter on this great English mathematician we shall describe themanner in which he was independently led to the same discovery. 

Directly the planetary nature of the newly-discovered body had beenestablished, the great observatories naturally included thisadditional member of the solar system in their working lists, so thatday after day its place was carefully determined. When sufficienttime had elapsed the shape and position of the orbit of the bodybecame known. Of course, it need hardly be said that observationsapplied to the planet itself must necessarily provide a far moreaccurate method of determining the path which it follows, than wouldbe possible to Le Verrier, when all he had to base his calculationsupon was the influence of the planet reflected, so to speak, fromUranus. It may be noted that the true elements of the planet, whenrevealed by direct observation, showed that there was a considerablediscrepancy between the track of the planet which Le Verrier hadannounced, and that which the planet was actually found to pursue. 

The name of the newly-discovered body had next to be considered. Asthe older members of the system were already known by the same namesas great heathen divinities, it was obvious that some similar sourceshould be invoked for a suggestion as to a name for the most recentplanet. The fact that this body was so remote in the depths ofspace, not unnaturally suggested the name "Neptune. " Such isaccordingly the accepted designation of that mighty globe whichrevolves in the track that at present seems to trace out thefrontiers of our system. 

Le Verrier attained so much fame by this discovery, that when, in1854, Arago's place had to be filled at the head of the great ParisObservatory, it was universally felt that the discoverer of Neptunewas the suitable man to assume the office which corresponds in Franceto that of the Astronomer Royal in England. It was true that thework of the astronomical mathematician had hitherto been of anabstract character. His discoveries had been made at his desk andnot in the observatory, and he had no practical acquaintance with theuse of astronomical instruments. However, he threw himself into thetechnical duties of the observatory with vigour and determination. Heendeavoured to inspire the officers of the establishment withenthusiasm for that systematic work which is so necessary for theaccomplishment of useful astronomical research. It must, however, beadmitted that Le Verrier was not gifted with those natural qualitieswhich would make him adapted for the successful administration ofsuch an establishment. Unfortunately disputes arose between theDirector and his staff. At last the difficulties of the situationbecame so great that the only possible solution was to supersede LeVerrier, and he was accordingly obliged to retire. He was succeededin his high office by another eminent mathematician, M. Delaunay, only less distinguished than Le Verrier himself. 

Relieved of his official duties, Le Verrier returned to themathematics he loved. In his non-official capacity he continued towork with the greatest ardour at his researches on the movements ofthe planets. After the death of M. Delaunay, who was accidentallydrowned in 1873, Le Verrier was restored to the directorship of theobservatory, and he continued to hold the office until his death. 

The nature of the researches to which the life of Le Verrier wassubsequently devoted are not such as admit of description in ageneral sketch like this, where the language, and still less thesymbols, of mathematics could not be suitably introduced. It may, however, be said in general that he was particularly engaged with thestudy of the effects produced on the movements of the planets bytheir mutual attractions. The importance of this work to astronomyconsists, to a considerable extent, in the fact that by suchcalculations we are enabled to prepare tables by which the places ofthe different heavenly bodies can be predicted for our almanacs. Tothis task Le Verrier devoted himself, and the amount of work he hasaccomplished would perhaps have been deemed impossible had it notbeen actually done. 

The superb success which had attended Le Verrier's efforts to explainthe cause of the perturbations of Uranus, naturally led thiswonderful computer to look for a similar explanation of certain otherirregularities in planetary movements. To a large extent hesucceeded in showing how the movements of each of the great planetscould be satisfactorily accounted for by the influence of theattractions of the other bodies of the same class. One circumstancein connection with these investigations is sufficiently noteworthy torequire a few words here. Just as at the opening of his career, LeVerrier had discovered that Uranus, the outermost planet of the thenknown system, exhibited the influence of an unknown external body, sonow it appeared to him that Mercury, the innermost body of oursystem, was also subjected to some disturbances, which could not besatisfactorily accounted for as consequences of any known agents ofattraction. The ellipse in which Mercury revolved was animated by aslow movement, which caused it to revolve in its plane. It appearedto Le Verrier that this displacement was incapable of explanation bythe action of any of the known bodies of our system. He was, therefore, induced to try whether he could not determine from thedisturbances of Mercury the existence of some other planet, atpresent unknown, which revolved inside the orbit of the knownplanet. Theory seemed to indicate that the observed alteration inthe track of the planet could be thus accounted for. He naturallydesired to obtain telescopic confirmation which might verify theexistence of such a body in the same way as Dr. Galle verified theexistence of Neptune. If there were, indeed, an intramercurialplanet, then it must occasionally cross between the earth and thesun, and might now and then be expected to be witnessed in the actualact of transit. So confident did Le Verrier feel in the existence ofsuch a body that an observation of a dark object in transit, byLescarbault on 26th March, 1859, was believed by the mathematician tobe the object which his theory indicated. Le Verrier also thought itlikely that another transit of the same object would be seen inMarch, 1877. Nothing of the kind was, however, witnessed, notwithstanding that an assiduous watch was kept, and the explanationof the change in Mercury's orbit must, therefore, be regarded asstill to be sought for. 

Le Verrier naturally received every honour that could be bestowedupon a man of science. The latter part of his life was passed duringthe most troubled period of modern French history. He was asupporter of the Imperial Dynasty, and during the Commune heexperienced much anxiety; indeed, at one time grave fears wereentertained for his personal safety. 

Early in 1877 his health, which had been gradually failing for someyears, began to give way. He appeared to rally somewhat in thesummer, but in September he sank rapidly, and died on Sunday, the23rd of that month. 

His remains were borne to the cemetery on Mont Parnasse in a publicfuneral. Among his pallbearers were leading men of science, fromother countries as well as France, and the memorial discoursespronounced at the grave expressed their admiration of his talents andof the greatness of the services he had rendered to science. 

ADAMS. 

The illustrious mathematician who, among Englishmen, at all events, was second only to Newton by his discoveries in theoreticalastronomy, was born on June the 5th, 1819, at the farmhouse ofLidcot, seven miles from Launceston, in Cornwall. His earlyeducation was imparted under the guidance of the Rev. John CouchGrylls, a first cousin of his mother. He appears to have received aneducation of the ordinary school type in classics and mathematics, but his leisure hours were largely devoted to studying whatastronomical books he could find in the library of the Mechanics'Institute at Devonport. He was twenty years old when he entered St. John's College, Cambridge. His career in the University was one ofalmost unparalleled distinction, and it is recorded that hisanswering at the Wranglership examination, where he came out at thehead of the list in 1843, was so high that he received more thandouble the marks awarded to the Second Wrangler. 

Among the papers found after his death was the following memorandum, dated July the 3rd, 1841: "Formed a design at the beginning of thisweek of investigating, as soon as possible after taking my degree, the irregularities in the motion of Uranus, Which are as yetunaccounted for, in order to find whether they may be attributed tothe action of an undiscovered planet beyond it; and, if possible, thence to determine the elements of its orbit approximately, whichwould lead probably to its discovery. "

After he had taken his degree, and had thus obtained a littlerelaxation from the lines within which his studies had previouslybeen necessarily confined, Adams devoted himself to the study of theperturbations of Uranus, in accordance with the resolve which we havejust seen that he formed while he was still an undergraduate. As afirst attempt he made the supposition that there might be a planetexterior to Uranus, at a distance which was double that of Uranusfrom the sun. Having completed his calculation as to the effectwhich such a hypothetical planet might exercise upon the movement ofUranus, he came to the conclusion that it would be quite possible toaccount completely for the unexplained difficulties by the action ofan exterior planet, if only that planet were of adequate size and hadits orbit properly placed. It was necessary, however, to follow upthe problem more precisely, and accordingly an application was madethrough Professor Challis, the Director of the Cambridge Observatory, to the Astronomer Royal, with the object of obtaining from theobservations made at Greenwich Observatory more accurate values forthe disturbances suffered by Uranus. Basing his work on the moreprecise materials thus available, Adams undertook his calculationsanew, and at last, with his completed results, he called at GreenwichObservatory on October the 21st, 1845. He there left for theAstronomer Royal a paper which contained the results at which he hadarrived for the mass and the mean distance of the hypothetical planetas well as the other elements necessary for calculating its exactposition. 

[PLATE: JOHN COUCH ADAMS. ]

As we have seen in the preceding chapter, Le Verrier had been alsoinvestigating the same problem. The place which Le Verrier assignedto the hypothetical disturbing planet for the beginning of the year1847, was within a degree of that to which Adams's computationspointed, and which he had communicated to the Astronomer Royal sevenmonths before Le Verrier's work appeared. On July the 29th, 1846, Professor Challis commenced to search for the unknown object with theNorthumberland telescope belonging to the Cambridge Observatory. Heconfined his attention to a limited region in the heavens, extendingaround that point to which Mr. Adams' calculations pointed. Therelative places of all the stars, or rather star-like objects withinthis area, were to be carefully measured. When the same observationswere repeated a week or two later, then the distances of the severalpairs of stars from each other would be found unaltered, but anyplanet which happened to lie among the objects measured woulddisclose its existence by the alterations in distance due to itsmotion in the interval. This method of search, though no doubt itmust ultimately have proved successful, was necessarily a verytedious one, but to Professor Challis, unfortunately, no other methodwas available. Thus it happened that, though Challis commenced hissearch at Cambridge two months earlier than Galle at Berlin, yet, aswe have already explained, the possession of accurate star-maps byDr. Galle enabled him to discover the planet on the very first nightthat he looked for it. 

The rival claims of Adams and Le Verrier to the discovery of Neptune, or rather, we should say, the claims put forward by their respectivechampions, for neither of the illustrious investigators themselvescondescended to enter into the personal aspect of the question, neednot be further discussed here. The main points of the controversyhave been long since settled, and we cannot do better than quote thewords of Sir John Herschel when he addressed the Royal AstronomicalSociety in 1848:--

"As genius and destiny have joined the names of Le Verrier and Adams, I shall by no means put them asunder; nor will they ever bepronounced apart so long as language shall celebrate the triumphs Ofscience in her sublimest walks. On the great discovery of Neptune, which may be said to have surpassed, by intelligible and legitimatemeans, the wildest pretensions of clairvoyance, it Would now be quitesuperfluous for me to dilate. That glorious event and the stepswhich led to it, and the various lights in which it has been placed, are already familiar to every one having the least tincture ofscience. I will only add that as there is not, nor henceforth evercan be, the slightest rivalry on the subject between these twoillustrious men--as they have met as brothers, and as such will, Itrust, ever regard each other--we have made, we could make, nodistinction between then, on this occasion. May they both long adornand augment our science, and add to their own fame already so highand pure, by fresh achievements. "

Adams was elected a Fellow of St. John's College, Cambridge, in 1843;but as he did not take holy orders, his Fellowship, in accordancewith the rules then existing came to an end in 1852. In thefollowing year he was, however, elected to a Fellowship at PembrokeCollege, which he retained until the end of his life. In 1858 he wasappointed Professor of Mathematics in the University of St. Andrews, but his residence in the north was only a brief one, for in the sameyear he was recalled to Cambridge as Lowndean Professor of Astronomyand Geometry, in succession to Peacock. In 1861 Challis retired fromthe Directorship of the Cambridge Observatory, and Adams wasappointed to succeed him. 

The discovery of Neptune was a brilliant inauguration of theastronomical career of Adams. He worked at, and wrote upon, thetheory of the motions of Biela's comet; he made important correctionsto the theory of Saturn; he investigated the mass of Uranus, asubject in which he was naturally interested from its importance inthe theory of Neptune; he also improved the methods of computing theorbits of double stars. But all these must be regarded as his minorlabours, for next to the discovery of Neptune the fame of Adamsmainly rests on his researches upon certain movements of the moon, and upon the November meteors. 

The periodic time of the moon is the interval required for onecircuit of its orbit. This interval is known with accuracy at thepresent day, and by means of the ancient eclipses the period of themoon's revolution two thousand years ago can be also ascertained. Ithad been discovered by Halley that the period which the moon requiresto accomplish each of its revolutions around the earth has beensteadily, though no doubt slowly, diminishing. The change thusproduced is not appreciable when only small intervals of time areconsidered, but it becomes appreciable when we have to deal withintervals of thousands of years. The actual effect which is producedby the lunar acceleration, for so this phenomenon is called, may bethus estimated. If we suppose that the moon had, throughout theages, revolved around the earth in precisely the same periodic timewhich it has at present, and if from this assumption we calculateback to find where the moon must have been about two thousand yearsago, we obtain a position which the ancient eclipses show to bedifferent from that in which the moon was actually situated. Theinterval between the position in which the moon would have been foundtwo thousand years ago if there had been no acceleration, and theposition in which the moon was actually placed, amounts to about adegree, that is to say, to an arc on the heavens which is twice themoon's apparent diameter. 

If no other bodies save the earth and the moon were present in theuniverse, it seems certain that the motion of the moon would neverhave exhibited this acceleration. In such a simple case as thatwhich I have supposed the orbit of the moon would have remained forever absolutely unchanged. It is, however, well known that thepresence of the sun exerts a disturbing influence upon the movementsof the moon. In each revolution our satellite is continually drawnaside by the action of the sun from the place which it wouldotherwise have occupied. These irregularities are known as theperturbations of the lunar orbit, they have long been studied, andthe majority of them have been satisfactorily accounted for. Itseems, however, to those who first investigated the question that thephenomenon of the lunar acceleration could not be explained as aconsequence of solar perturbation, and, as no other agent competentto produce such effects was recognised by astronomers, the lunaracceleration presented an unsolved enigma. 

At the end of the last century the illustrious French mathematicianLaplace undertook a new investigation of the famous problem, and wasrewarded with a success which for a long time appeared to be quitecomplete. Let us suppose that the moon lies directly between theearth and the sun, then both earth and moon are pulled towards thesun by the solar attraction; as, however, the moon is the nearer ofthe two bodies to the attracting centre it is pulled the moreenergetically, and consequently there is an increase in the distancebetween the earth and the moon. Similarly when the moon happens tolie on the other side of the earth, so that the earth is interposeddirectly between the moon and the sun, the solar attraction exertedupon the earth is more powerful than the same influence upon themoon. Consequently in this case, also, the distance of the moon fromthe earth is increased by the solar disturbance. These instanceswill illustrate the general truth, that, as one of the consequencesof the disturbing influence exerted by the sun upon the earth-moonsystem, there is an increase in the dimensions of the average orbitwhich the moon describes around the earth. As the time required bythe moon to accomplish a journey round the earth depends upon itsdistance from the earth, it follows that among the influences of thesun upon the moon there must be an enlargement of the periodic time, from what it would have been had there been no solar disturbingaction. 

This was known long before the time of Laplace, but it did notdirectly convey any explanation of the lunar acceleration. It nodoubt amounted to the assertion that the moon's periodic time wasslightly augmented by the disturbance, but it did not give anygrounds for suspecting that there was a continuous change inprogress. It was, however, apparent that the periodic time wasconnected with the solar disturbance, so that, if there were anyalteration in the amount of the sun's disturbing effect, there mustbe a corresponding alteration in the moon's periodic time. Laplace, therefore, perceived that, if he could discover any continuous changein the ability of the sun for disturbing the moon, he would then haveaccounted for a continuous change in the moon's periodic time, andthat thus an explanation of the long-vexed question of the lunaracceleration might be forthcoming. 

The capability of the sun for disturbing the earth-moon system isobviously connected with the distance of the earth from the sun. Ifthe earth moved in an orbit which underwent no change whatever, thenthe efficiency of the sun as a disturbing agent would not undergo anychange of the kind which was sought for. But if there were anyalteration in the shape or size of the earth's orbit, then that mightinvolve such changes in the distance between the earth and the sun aswould possibly afford the desired agent for producing the observedlunar effect. It is known that the earth revolves in an orbit which, though nearly circular, is strictly an ellipse. If the earth werethe only planet revolving around the sun then that ellipse wouldremain unaltered from age to age. The earth is, however, only one ofa large number of planets which circulate around the great luminary, and are guided and controlled by his supreme attracting power. Theseplanets mutually attract each other, and in consequence of theirmutual attractions the orbits of the planets are disturbed from thesimple elliptic form which they would otherwise possess. Themovement of the earth, for instance, is not, strictly speaking, performed in an elliptical orbit. We may, however, regard it asrevolving in an ellipse provided we admit that the ellipse is itselfin slow motion. 

It is a remarkable characteristic of the disturbing effects of theplanets that the ellipse in which the earth is at any moment movingalways retains the same length; that is to say, its longest diameteris invariable. In all other respects the ellipse is continuallychanging. It alters its position, it changes its plane, and, mostimportant of all, it changes its eccentricity. Thus, from age to agethe shape of the track which the earth describes may at one time begrowing more nearly a circle, or at another time may be departingmore widely from a circle. These alterations are very small inamount, and they take place with extreme slowness, but they are inincessant progress, and their amount admits of being accuratelycalculated. At the present time, and for thousands of years past, aswell as for thousands of years to come, the eccentricity of theearth's orbit is diminishing, and consequently the orbit described bythe earth each year is becoming more nearly circular. We must, however, remember that under all circumstances the length of thelongest axis of the ellipse is unaltered, and consequently the sizeof the track which the earth describes around the sun is graduallyincreasing. In other words, it may be said that during the presentages the average distance between the earth and the sun is waxinggreater in consequence of the perturbations which the earthexperiences from the attraction of the other planets. We have, however, already seen that the efficiency of the solar attraction fordisturbing the moon's movement depends on the distance between theearth and the sun. As therefore the average distance between theearth and the sun is increasing, at all events during the thousandsof years over which our observations extend, it follows that theability of the sun for disturbing the moon must be graduallydiminishing. 

[PLATE: CAMBRIDGE OBSERVATORY. ]

It has been pointed out that, in consequence of the solardisturbance, the orbit of the moon must be some what enlarged. As itnow appears that the solar disturbance is on the whole declining, itfollows that the orbit of the moon, which has to be adjustedrelatively to the average value of the solar disturbance, must alsobe gradually declining. In other words, the moon must be approachingnearer to the earth in consequence of the alterations in theeccentricity of the earth's orbit produced by the attraction of theother planets. It is true that the change in the moon's positionthus arising is an extremely small one, and the consequent effect inaccelerating the moon's motion is but very slight. It is in factalmost imperceptible, except when great periods of time areinvolved. Laplace undertook a calculation on this subject. He knewwhat the efficiency of the planets in altering the dimensions of theearth's orbit amounted to; from this he was able to determine thechanges that would be propagated into the motion of the moon. Thushe ascertained, or at all events thought he had ascertained, that theacceleration of the moon's motion, as it had been inferred from theobservations of the ancient eclipses which have been handed down tous, could be completely accounted for as a consequence of planetaryperturbation. This was regarded as a great scientific triumph. Ourbelief in the universality of the law of gravitation would, in fact, have been seriously challenged unless some explanation of the lunaracceleration had been forthcoming. For about fifty years no onequestioned the truth of Laplace's investigation. When amathematician of his eminence had rendered an explanation of theremarkable facts of observation which seemed so complete, it is notsurprising that there should have been but little temptation to doubtit. On undertaking a new calculation of the same question, ProfessorAdams found that Laplace had not pursued this approximationsufficiently far, and that consequently there was a considerableerror in the result of his analysis. Adams, it must be observed, didnot impugn the value of the lunar acceleration which Halley haddeduced from the observations, but what he did show was, that thecalculation by which Laplace thought he had provided an explanationof this acceleration was erroneous. Adams, in fact, proved that theplanetary influence which Laplace had detected only possessed abouthalf the efficiency which the great French mathematician hadattributed to it. There were not wanting illustrious mathematicianswho came forward to defend the calculations of Laplace. Theycomputed the question anew and arrived at results practicallycoincident with those he had given. On the other hand certaindistinguished mathematicians at home and abroad verified the resultsof Adams. The issue was merely a mathematical one. It had only onecorrect solution. Gradually it appeared that those who opposed Adamspresented a number of different solutions, all of them discordantwith his, and, usually, discordant with each other. Adams showeddistinctly where each of these investigators had fallen into error, and at last it became universally admitted that the CambridgeProfessor had corrected Laplace in a very fundamental point ofastronomical theory. 

Though it was desirable to have learned the truth, yet the breachbetween observation and calculation which Laplace was believed tohave closed thus became reopened. Laplace's investigation, had itbeen correct, would have exactly explained the observed facts. Itwas, however, now shown that his solution was not correct, and thatthe lunar acceleration, when strictly calculated as a consequence ofsolar perturbations, only produced about half the effect which waswanted to explain the ancient eclipses completely. It now seemscertain that there is no means of accounting for the lunaracceleration as a direct consequence of the laws of gravitation, ifwe suppose, as we have been in the habit of supposing, that themembers of the solar system concerned may be regarded as rigidparticles. It has, however, been suggested that another explanationof a very interesting kind may be forthcoming, and this we mustendeavour to set forth. 

It will be remembered that we have to explain why the period ofrevolution of the moon is now shorter than it used to be. If weimagine the length of the period to be expressed in terms of days andfractions of a day, that is to say, in terms of the rotations of theearth around its axis, then the difficulty encountered is, that themoon now requires for each of its revolutions around the earth rathera smaller number of rotations of the earth around its axis than usedformerly to be the case. Of course this may be explained by the factthat the moon is now moving more swiftly than of yore, but it isobvious that an explanation of quite a different kind might beconceivable. The moon may be moving just at the same pace as ever, but the length of the day may be increasing. If the length of theday is increasing, then, of course, a smaller number of days will berequired for the moon to perform each revolution even though themoon's period was itself really unchanged. It would, therefore, seemas if the phenomenon known as the lunar acceleration is the result ofthe two causes. The first of these is that discovered by Laplace, though its value was overestimated by him, in which the perturbationsof the earth by the planets indirectly affect the motion of themoon. The remaining part of the acceleration of our satellite isapparent rather than real, it is not that the moon is moving morequickly, but that our time-piece, the earth, is revolving moreslowly, and is thus actually losing time. It is interesting to notethat we can detect a physical explanation for the apparent checkingof the earth's motion which is thus manifested. The tides which ebband flow on the earth exert a brake-like action on the revolvingglobe, and there can be no doubt that they are gradually reducing itsspeed, and thus lengthening the day. It has accordingly beensuggested that it is this action of the tides which produces thesupplementary effect necessary to complete the physical explanationof the lunar acceleration, though it would perhaps be a littlepremature to assert that this has been fully demonstrated. 

The third of Professor Adams' most notable achievements was connectedwith the great shower of November meteors which astonished the worldin 1866. This splendid display concentrated the attention ofastronomers on the theory of the movements of the little objects bywhich the display was produced. For the definite discovery of thetrack in which these bodies revolve, we are indebted to the laboursof Professor Adams, who, by a brilliant piece of mathematical work, completed the edifice whose foundations had been laid by ProfessorNewton, of Yale, and other astronomers. 

Meteors revolve around the sun in a vast swarm, every individualmember of which pursues an orbit in accordance with the well-knownlaws of Kepler. In order to understand the movements of theseobjects, to account satisfactorily for their periodic recurrence, andto predict the times of their appearance, it became necessary tolearn the size and the shape of the track which the swarm followed, as well as the position which it occupied. Certain features of thetrack could no doubt be readily assigned. The fact that the showerrecurs on one particular day of the year, viz. , November 13th, defines one point through which the orbit must pass. The position onthe heavens of the radiant point from which the meteors appear todiverge, gives another element in the track. The sun must of coursebe situated at the focus, so that only one further piece ofinformation, namely, the periodic time, will be necessary to completeour knowledge of the movements of the system. Professor H. Newton, of Yale, had shown that the choice of possible orbits for themeteoric swarm is limited to five. There is, first, the greatellipse in which we now know the meteors revolve once every thirtythree and one quarter years. There is next an orbit of a nearlycircular kind in which the periodic time would be a little more thana year. There is a similar track in which the periodic time would bea few days short of a year, while two other smaller orbits would alsobe conceivable. Professor Newton had pointed out a test by which itwould be possible to select the true orbit, which we know must be oneor other of these five. The mathematical difficulties which attendedthe application of this test were no doubt great, but they did notbaffle Professor Adams. 

There is a continuous advance in the date of this meteoric shower. The meteors now cross our track at the point occupied by the earth onNovember 13th, but this point is gradually altering. The onlyinfluence known to us which could account for the continuous changein the plane of the meteor's orbit arises from the attraction of thevarious planets. The problem to be solved may therefore be attackedin this manner. A specified amount of change in the plane of theorbit of the meteors is known to arise, and the changes which oughtto result from the attraction of the planets can be computed for eachof the five possible orbits, in one of which it is certain that themeteors must revolve. Professor Adams undertook the work. Itsdifficulty principally arises from the high eccentricity of thelargest of the orbits, which renders the more ordinary methods ofcalculation inapplicable. After some months of arduous labour thework was completed, and in April, 1867, Adams announced his solutionof the problem. He showed that if the meteors revolved in thelargest of the five orbits, with the periodic time of thirty threeand one quarter years, the perturbations of Jupiter would account fora change to the extent of twenty minutes of arc in the point in whichthe orbit crosses the earth's track. The attraction of Saturn wouldaugment this by seven minutes, and Uranus would add one minute more, while the influence of the Earth and of the other planets would beinappreciable. The accumulated effect is thus twenty-eight minutes, which is practically coincident with the observed value as determinedby Professor Newton from an examination of all the showers of whichthere is any historical record. Having thus showed that the greatorbit was a possible path for the meteors, Adams next proved that noone of the other four orbits would be disturbed in the same manner. Indeed, it appeared that not half the observed amount of change couldarise in any orbit except in that one with the long period. Thus wasbrought to completion the interesting research which demonstrated thetrue relation of the meteor swarm to the solar system. 

Besides those memorable scientific labours with which his attentionwas so largely engaged, Professor Adams found time for much otherstudy. He occasionally allowed himself to undertake as a relaxationsome pieces of numerical calculation, so tremendously long that wecan only look on them with astonishment. He has calculated certainimportant mathematical constants accurately to more than two hundredplaces of decimals. He was a diligent reader of works on history, geology, and botany, and his arduous labours were often beguiled bynovels, of which, like many other great men, he was very fond. Hehad also the taste of a collector, and he brought together abouteight hundred volumes of early printed works, many of considerablerarity and value. As to his personal character, I may quote thewords of Dr. Glaisher when he says, "Strangers who first met him wereinvariably struck by his simple and unaffected manner. He was adelightful companion, always cheerful and genial, showing in societybut few traces of his really shy and retiring disposition. Hisnature was sympathetic and generous, and in few men have the moraland intellectual qualities been more perfectly balanced. "

In 1863 he married the daughter of Haliday Bruce, Esq. , of Dublin andup to the close of his life he lived at the Cambridge Observatory, pursuing his mathematical work and enjoying the society of hisfriends. 

He died, after a long illness, on 21st January, 1892, and wasinterred in St. Giles's Cemetery, on the Huntingdon Road, Cambridge.