_Is Mars Habitable?_

A CRITICAL EXAMINATION OF PROFESSOR PERCIVAL LOWELL'S BOOK"MARS AND ITS CANALS, " WITH AN ALTERNATIVE EXPLANATION

BY ALFRED RUSSEL WALLACE F. R. S. , ETC. 

PREFACE. 

This small volume was commenced as a review article on ProfessorPercival Lowell's book, _Mars and its Canals_, with the object ofshowing that the large amount of new and interesting facts contained inthis work did not invalidate the conclusion I had reached in 1902, andstated in my book on _Man's Place in the Universe_, that Mars was nothabitable. 

But the more complete presentation of the opposite view in the volumenow under discussion required a more detailed examination of the variousphysical problems involved, and as the subject is one of great, popular, as well as scientific interest, I determined to undertake the task. 

This was rendered the more necessary by the fact that in July lastProfessor Lowell published in the _Philosophical Magazine_ an elaboratemathematical article claiming to demonstrate that, notwithstanding itsmuch greater distance from the sun and its excessively thin atmosphere, Mars possessed a climate on the average equal to that of the south ofEngland, and in its polar and sub-polar regions even less severe thanthat of the earth. Such a contention of course required to be dealtwith, and led me to collect information bearing upon temperature in allits aspects, and so enlarging my criticism that I saw it would benecessary to issue it in book form. 

Two of my mathematical friends have pointed out the chief omission whichvitiates Professor Lowell's mathematical conclusions--that of a failureto recognise the very large conservative and _cumulative_ effect of adense atmosphere. This very point however I had already myself discussedin Chapter VI. , and by means of some remarkable researches on the heatof the moon and an investigation of the causes of its very lowtemperature, I have, I think, demonstrated the incorrectness of Mr. Lowell's results. In my last chapter, in which I briefly summarise thewhole argument, I have further strengthened the case for very severecold in Mars, by adducing the rapid lowering of temperature universallycaused by diminution of atmospheric pressure, as manifested in thewell-known phenomenon of temperate climates at moderate heights evenclose to the equator, cold climates at greater heights even on extensiveplateaux, culminating in arctic climates and perpetual snow at heightswhere the air is still far denser than it is on the surface of Mars. This argument itself is, in my opinion, conclusive; but it is enforcedby two others equally complete, neither of which is adequately met byMr. Lowell. 

The careful examination which I have been led to give to the whole ofthe phenomena which Mars presents, and especially to the discoveries ofMr. Lowell, has led me to what I hope will be considered a satisfactoryphysical explanation of them. This explanation, which occupies the wholeof my seventh chapter, is founded upon a special mode of origin forMars, derived from the Meteoritic Hypothesis, now very widely adopted byastronomers and physicists. Then, by a comparison with certainwell-known and widely spread geological phenomena, I show how the greatfeatures of Mars--the 'canals' and 'oases'--may have been caused. Thischapter will perhaps be the most interesting to the general reader, asfurnishing a quite natural explanation of features of the planet whichhave been termed 'non-natural' by Mr. Lowell. 

Incidentally, also, I have been led to an explanation of the highlyvolcanic nature of the moon's surface. This seems to me absolutely torequire some such origin as Sir George Darwin has given it, and thusfurnishes corroborative proof of the accuracy of the hypothesis that ourmoon has had an unique origin among the known satellites, in having beenthrown off from the earth itself. 

I am indebted to Professor J. H. Poynting, of the University ofBirmingham, for valuable suggestions on some of the more difficultpoints of mathematical physics here discussed, and also for the criticalnote (at the end of Chapter V. ) on Professor Lowell's estimate of thetemperature of Mars. 

BROADSTONE, DORSET, _October_ 1907. 

TABLE OF CONTENTS. 

CHAPTER I. 

EARLY OBSERVERS OF MARS, --Mars the only planet the surface of which is distinctly visible--Early observation of the snow-caps and seas--The 'canals' seen by Schiaparelli in 1877--Double canals first seen in 1881--Round spots at intersection of canals seen by Pickering in 1892--Confirmed by Lowell in 1894--Changes of colour seen in 1892 and 1894--Existence of seas doubted by Pickering and Barnard in 1894. 

CHAPTER II. 

MR. LOWELL'S DISCOVERIES AND THEORIES, --Observatory at Flagstaff, Arizona--Illustrated book on his observations of Mars--Volume on Mars and its canals, 1906--Non-natural features--The canals as irrigation works of an intelligent race--A challenge to the thinking world--The canals as described and mapped by Mr. Lowell--The double canals--Dimensions of the canals--They cross the supposed seas--Circular black spots termed oases--An interesting volume. 

CHAPTER III. 

THE CLIMATE AND PHYSIOGRAPHY OF MARS, --No permanent water on Mars--Rarely any clouds and no rain--Snow-caps the only source of water--No mountains, hills, or valleys on Mars--Two-thirds of the surface a desert--Water from the snow-caps too scanty to supply the canals--Miss Clerke's views as to the water-supply--Description of some of the chief canals--Mr. Lowell on the purpose of the canals--Remarks on the same--Mr. Lowell on relation of canals to oases and snow-caps--Critical remarks on the same. 

CHAPTER IV. 

IS ANIMAL LIFE POSSIBLE ON MARS?--Water and air essential for animal life--Atmosphere of Mars assumed to be like ours--Blue tint near melting snow the only evidence of water--Fallacy of this argument--Dr. Johnstone Stoney's proof that water-vapour cannot exist on Mars--Spectroscope gives no evidence of water. 

CHAPTER V. 

TEMPERATURE OF MARS--MR. LOWELL'S ESTIMATE, --Problem of terrestrial temperature--Ice under recent lava--Tropical oceans ice-cold at bottom--Earth's surface-heat all from the sun--Absolute zero of temperature--Complex problem of planetary temperatures--Mr. Lowell's investigation of the problem--Abstract of Mr. Lowell's paper--Critical remarks on Mr. Lowell's paper. 

CHAPTER VI. 

A NEW ESTIMATE OF THE TEMPERATURE OF MARS, --Langley's determination of lunar heat--Rapid loss of heat by radiation on the earth--Rapid loss of heat on moon during eclipse--Sir George Darwin's theory of the moon's origin--Very's researches on the moon's temperature--Application of these results to the case of Mars--Cause of great difference of temperatures of earth and moon--Special features of Mars influencing its temperature--Further criticism of Mr. Lowell's article--Very low temperature of arctic regions on Mars. 

CHAPTER VII. 

A SUGGESTION AS TO THE 'CANALS' OF MARS, --Special features of the canals--Mr. Pickering's suggested explanation--The meteoritic hypotheses of origin of planets--Probable mode of origin of Mars--Structural straight lines on the earth--Probable origin of the surface-features of Mars--Symmetry of basaltic columns--How this applies to Mars--Suggested explanation of the oases--Probable function of the great fissures--Suggested origin of blue patches adjacent to snow-caps--The double canals--Concluding remarks on the canals. 

CHAPTER VIII. 

PAGE SUMMARY AND CONCLUSION, --The canals the origin of Mr. Lowell's theory--Best explained as natural features--Evaporation difficulty not met by Mr. Lowell--How did Martians live without the canals--Radiation due to scanty atmosphere not taken account of--Three independent proofs of low temperature and uninhabitability of Mars--Conclusion. 

CHAPTER I. 

EARLY OBSERVERS OF MARS. 

Few persons except astronomers fully realise that of all the planets ofthe Solar system the only one whose solid surface has been seen withcertainty is Mars; and, very fortunately, that is also the only onewhich is sufficiently near to us for the physical features of thesurface to be determined with any accuracy, even if we could see it inthe other planets. Of Venus we probably see only the upper surface ofits cloudy atmosphere. [1] As regards Jupiter and Saturn this is stillmore certain, since their low density will only permit of acomparatively small proportion of their huge bulk being solid. Theirbelts are but the cloud-strata of their upper atmosphere, perhapsthousands of miles above their solid surfaces, and a somewhat similarcondition seems to prevail in the far more remote planets Uranus andNeptune. It has thus happened, that, although as telescopic objects ofinterest and beauty, the marvellous rings of Saturn, the belts andever-changing aspects of the satellites of Jupiter, and the moon-likephases of Venus, together with its extreme brilliancy, still remainunsurpassed, yet the greater amount of details of these features whenexamined with the powerful instruments of the nineteenth century haveneither added much to our knowledge of the planets themselves or led toany sensational theories calculated to attract the popular imagination. 

[Footnote 1: Mercury also seems to have a scanty atmosphere, but as itsmass is only one-thirtieth that of the earth it can retain only theheavier gases, and its atmosphere may be dust-laden, as is that of Mars, according to Mr. Lowell. Its dusky markings, as seen by Schiaparelli, seem to be permanent, and they are also for considerable periodsunchangeable in position, indicating that the planet keeps the same facetowards the sun as does Venus. This was confirmed by Mr. Lowell in 1896. Its distance from us and unfavourable position for observation mustprevent us from obtaining any detailed knowledge of its actual surface, though its low reflective power indicates that the surface may be reallyvisible. ]

But in the case of Mars the progress of discovery has had a verydifferent result. The most obvious peculiarity of this planet--its polarsnow-caps--were seen about 250 years ago, but they were first proved toincrease and decrease alternately, in the summer and winter of eachhemisphere, by Sir William Herschell in the latter part of theeighteenth century. This fact gave the impulse to that idea ofsimilarity in the conditions of Mars and the earth, which therecognition of many large dusky patches and streaks as water, and themore ruddy and brighter portions as land, further increased. Added tothis, a day only about half an hour longer than our own, and asuccession of seasons of the same character as ours but of nearly doublethe length owing to its much longer year, seemed to leave little wantingto render this planet a true earth on a smaller scale. It was thereforevery natural to suppose that it must be inhabited, and that we shouldsome day obtain evidence of the fact. 

_The Canals discovered by Schiaparelli. _

Hence the great interest excited when Schiaparelli, at the MilanObservatory, during the very favourable opposition of 1877 and 1879, observed that the whole of the tropical and temperate regions from 60°N. To 60° S. Lat. Were covered with a remarkable network of broadercurved and narrower straight lines of a dark colour. At each successivefavourable opposition, these strange objects called _canali_ (channels)by their discoverer, but rather misleadingly 'canals' in England andAmerica, were observed by means of all the great telescopes in theworld, and their reality and general features became well established. In Schiaparelli's first map they were represented as being much broaderand less sharply defined than he himself and other observers found bylater and equally favourable observations that they really were. 

_Discovery of the Double Canals. _

In 1881 another strange feature was discovered by Schiaparelli, whofound that about twenty canals which had previously been seen singlewere now distinctly double, that is, that they consisted of two parallellines, equally distinct and either very close together or a considerabledistance apart. This curious appearance was at first thought to be dueto some instrumental defect or optical illusion; but as it was soonconfirmed by other observers with the best instruments and in widelydifferent localities it became in time accepted as a real phenomenon ofthe planet's surface. 

_Round Spots discovered in_ 1892. 

At the favourable opposition of 1892, Mr. W. H. Pickering noticed thatbesides the 'seas' of various sizes there were numerous very small blackspots apparently quite circular and occurring at every intersection orstarting-point of the 'canals. ' Many of these had been seen bySchiaparelli as larger and ill-defined dark patches, and were termedseas or lakes; but Mr. Pickering's observatory was at Arequipa in Peru, about 8000 feet above the sea, and with such perfect atmosphericconditions as were, in his opinion, equal to a doubling of telescopicaperture. They were soon detected by other observers, especially by Mr. Lowell in 1894, who thus wrote of them:

"Scattered over the orange-ochre groundwork of the continental regionsof the planet, are any number of dark round spots. How many there may beit is not possible to state, as the better the seeing, the more of themthere seem to be. In spite, however, of their great number, there is noinstance of one unconnected with a canal. What is more, there isapparently none that does not lie at the junction of several canals. Reversely, all the junctions appear to be provided with spots. Plottedupon a globe they and their connecting canals make a most curiousnetwork over all the orange-ochre equatorial parts of the planet, a massof lines and knots, the one marking being as omnipresent as the other. "

_Changes of Colour recognised. _

During the oppositions of 1892 and 1894 it was fully recognised that aregular course of change occurred dependent upon the succession of theseasons, as had been first suggested by Schiaparelli. As the polar snowsmelt the adjacent seas appear to overflow and spread out as far as thetropics, and are often seen to assume a distinctly green colour. Theseremarkable changes and the extraordinary phenomena of perfect straightlines crossing each other over a large portion of the planet's surface, with the circular spots at their intersections, had such an appearanceof artificiality that the idea that they were really 'canals' made byintelligent beings for purposes of irrigation, was first hinted at, andthen adopted as the only intelligible explanation, by Mr. Lowell and afew other persons. This at once seized upon the public imagination andwas spread by the newspapers and magazines over the whole civilisedworld. 

_Existence of Seas doubted. _

At this time (1894) it began to be doubted whether there were any seasat all on Mars. Professor Pickering thought they were far more limitedin size than had been supposed, and even might not exist as true seas. Professor Barnard, with the Lick thirty-six inch telescope, discerned anastonishing wealth of detail on the surface of Mars, so intricate, minute, and abundant, that it baffled all attempts to delineate it; andthese peculiarities were seen upon the supposed seas as well as on theland-surfaces. In fact, under the best conditions these 'seas' lost alltrace of uniformity, their appearance being that of a mountainouscountry, broken by ridges, rifts, and canyons, seen from a greatelevation. As we shall see later on these doubts soon becamecertainties, and it is now almost universally admitted that Marspossesses no permanent bodies of water. 

CHAPTER II. 

MR. PERCIVAL LOWELL'S DISCOVERIES AND THEORIES. 

_The Observatory in Arizona. _

In 1894, after a careful search for the best atmospheric conditions, Mr. Lowell established his observatory near the town of Flagstaff inArizona, in a very dry and uniform climate, and at an elevation of 7300feet above the sea. He then possessed a fine equatorial telescope of 18inches aperture and 26 feet focal length, besides two smaller ones, allof the best quality. To these he added in 1896 a telescope with 24 inchobject glass, the last work of the celebrated firm of Alvan Clark &Sons, with which he has made his later discoveries. He thus becameperhaps more favourably situated than any astronomer in the northernhemisphere, and during the last twelve years has made a specialty of thestudy of Mars, besides doing much valuable astronomical work on otherplanets. 

_Mr, Lowell's recent Books upon Mars. _

In 1905 Mr. Lowell published an illustrated volume giving a full accountof his observations of Mars from 1894 to 1903, chiefly for the use ofastronomers; and he has now given us a popular volume summarising thewhole of his work on the planet, and published both in America andEngland by the Macmillan Company. This very interesting volume is fullyillustrated with twenty plates, four of them coloured, and more thanforty figures in the text, showing the great variety of details fromwhich the larger general maps have been constructed. 

_Non-natural Features of Mars. _

But what renders this work especially interesting to all intelligentreaders is, that the author has here, for the first time, fully setforth his views both as to the habitability of Mars and as to its beingactually inhabited by beings comparable with ourselves in intellect. Thelarger part of the work is in fact devoted to a detailed description ofwhat he terms the 'Non-natural Features' of the planet's surface, including especially a full account of the 'Canals, ' single and double;the 'Oases, ' as he terms the dark spots at their intersections; and thevarying visibility of both, depending partly on the Martian seasons;while the five concluding chapters deal with the possibility of animallife and the evidence in favour of it. He also upholds the theory of thecanals having been constructed for the purpose of 'husbanding' thescanty water-supply that exists; and throughout the whole of thisargument he clearly shows that he considers the evidence to besatisfactory, and that the only intelligible explanation of the whole ofthe phenomena he so clearly sets forth is, that the inhabitants of Marshave carried out on their small and naturally inhospitable planet a vastsystem of irrigation-works, far greater both in its extent, in itsutility, and its effect upon their world as a habitation for civilisedbeings, than anything we have yet done upon our earth, where ourdestructive agencies are perhaps more prominent than those of animproving and recuperative character. 

_A Challenge to the Thinking World. _

This volume is therefore in the nature of a challenge, not so much toastronomers as to the educated world at large, to investigate theevidence for so portentous a conclusion. To do this requires only ageneral acquaintance with modern science, more especially with mechanicsand physics, while the main contention (with which I shall chiefly deal)that the features termed 'canals' are really works of art andnecessitate the presence of intelligent organic beings, requires onlycare and judgment in drawing conclusions from admitted facts. As I havealready paid some attention to this problem and have expressed theopinion that Mars is not habitable, [2] judging from the evidence thenavailable, and as few men of science have the leisure required for acareful examination of so speculative a subject, I propose here to pointout what the facts, as stated by Mr. Lowell himself, do _not_ rendereven probable much less prove. Incidentally, I may be able to adduceevidence of a more or less weighty character, which seems to negativethe possibility of any high form of animal life on Mars, and, _afortiori_, the development of such life as might culminate in a beingequal or superior to ourselves. As most popular works on Astronomy forthe last ten years at least, as well as many scientific periodicals andpopular magazines, have reproduced some of the maps of Mars bySchiaparelli, Lowell, and others, the general appearance of its surfacewill be familiar to most readers, who will thus be fully able toappreciate Mr. Lowell's account of his own further discoveries which Imay have to quote. One of the _best_ of these maps I am able to give asa frontispiece to this volume, and to this I shall mainly refer. 

[Footnote 2: _Man's Place in the Universe_ p. 267 (1903). ]

_The Canals as described by Mr. Lowell. _

In the clear atmosphere of Arizona, Mr. Lowell has been able on variousfavourable occasions to detect a network of straight lines, meeting orcrossing each other at various angles, and often extending to a thousandor even over two thousand miles in length. They are seen to cross boththe light and the dark regions of the planet's surface, often extendingup to or starting from the polar snow-caps. Most of these lines are sofine as only to be visible on special occasions of atmospheric clearnessand steadiness, which hardly ever occur at lowland stations, even withthe best instruments, and almost all are seen to be as perfectlystraight as if drawn with a ruler. 

_The Double Canals. _

Under exceptionally favourable conditions, many of the lines that havebeen already seen single appear double--a pair of equally fine linesexactly parallel throughout their whole length, and appearing, as Mr. Lowell says, "clear cut upon the disc, its twin lines like the rails ofa railway track. " Both Schiaparelli and Lowell were at first sosurprised at this phenomenon that they thought it must be an opticalillusion, and it was only after many observations in different years, and by the application of every conceivable test, that they both becameconvinced that they witnessed a real feature of the planet's surface. Mr. Lowell says he has now seen them hundreds of times, and that hisfirst view of one was 'the most startlingly impressive' sight he hasever witnessed. 

_Dimensions of the Canals. _

A few dimensions of these strange objects must be given in order thatreaders may appreciate their full strangeness and inexplicability. Outof more than four hundred canals seen and recorded by Mr. Lowell, fifty-one, or about one eighth, are either constantly or occasionallyseen to be double, the appearance of duplicity being more or lessperiodical. Of 'canals' generally, Mr. Lowell states that they vary inlength from a few hundred to a few thousand miles long, one of thelargest being the Phison, which he terms 'a typical double canal, ' andwhich is said to be 2250 miles long, while the distance between its twoconstituents is about 130 miles. [3] The actual width of each canal isfrom a minimum of about a mile up to several miles, in one case overtwenty. A great feature of the doubles is, that they are strictlyparallel throughout their whole course, and that in almost all casesthey are so truly straight as to form parts of a great circle of theplanet's sphere. A few however follow a gradual but very distinct curve, and such of these as are double present the same strict parallelism asthose which are straight. 

[Footnote 3: This is on the opposite side of Mars from that shown in thefrontispiece. ]

_Canals extend across the Seas. _

It was only after seventeen years of observation of the canals that itwas found that they extended also into and across the dark spots andsurfaces which by the earlier observers were termed seas, and which thenformed the only clearly distinguishable and permanent marks on theplanet's surface. At the present time, Professor Lowell states that this"curious triangulation has been traced over almost every portion of theplanet's surface, whether dark or light, whether greenish, ochre, orbrown in colour. " In some parts they are much closer together than inothers, "forming a perfect network of lines and spots, so that toidentify them all was a matter of extreme difficulty. " Two such portionsare figured at pages 247 and 256 of Mr. Lowell's volume. 

_The Oases. _

The curious circular black spots which are seen at the intersections ofmany of the canals, and which in some parts of the surface are verynumerous, are said to be more difficult of detection than even thelines, being often blurred or rendered completely invisible by slightirregularities in our own atmosphere, while the canals themselvescontinue visible. About 180 of these have now been found, and the moreprominent of them are estimated to vary from 75 to 100 miles indiameter. There are however many much smaller, down to minute and barelyvisible black points. Yet they all seem a little larger than the canalswhich enter them. Where the canals are double, the spots (or 'oases' asMr. Lowell terms them) lie between the two parallel canals. 

No one can read this book without admiration for the extremeperseverance in long continued and successful observation, the resultsof which are here recorded; and I myself accept unreservedly thesubstantial accuracy of the whole series. It must however always beremembered that the growth of knowledge of the detailed markings hasbeen very gradual, and that much of it has only been seen under veryrare and exceptional conditions. It is therefore quite possible that, ifat some future time a further considerable advance in instrumental powershould be made, or a still more favourable locality be found, the newdiscoveries might so modify present appearances as to render asatisfactory explanation of them more easy than it is at present. 

But though I wish to do the fullest justice to Mr. Lowell's technicalskill and long years of persevering work, which have brought to lightthe most complex and remarkable appearances that any of the heavenlybodies present to us, I am obliged absolutely to part company with himas regards the startling theory of artificial production which he thinksalone adequate to explain them. So much is this the case, that the veryphenomena, which to him seem to demonstrate the intervention ofintelligent beings working for the improvement of their own environment, are those which seem to me to bear the unmistakable impress of being dueto natural forces, while they are wholly unintelligible as being usefulworks of art. I refer of course to the great system of what are termed'canals, ' whether single or double. Of these I shall give my owninterpretation later on. 

CHAPTER III. 

THE CLIMATE AND PHYSIOGRAPHY OF MARS. 

Mr. Lowell admits, and indeed urges strongly, that there are nopermanent bodies of water on Mars; that the dark spaces and spots, thought by the early observers to be seas, are certainly not so now, though they may have been at an earlier period; that true clouds arerare, even if they exist, the appearances that have been taken for thembeing either dust-storms or a surface haze; that there is consequentlyno rain, and that large portions (about two-thirds) of the planet'ssurface have all the characteristics of desert regions. 

_Snow-caps the only Source of Water. _

This state of things is supposed to be ameliorated by the fact of thepolar snows, which in the winter cover the arctic and about half thetemperate regions of each hemisphere alternately. The maximum of thenorthern snow-caps is reached at a period of the Martian wintercorresponding to the end of February with us. About the end of March thecap begins to shrink in size (in the Northern Hemisphere), and this goeson so rapidly that early in the June of Mars it is reduced to itsminimum. About the same time changes of colour take place in theadjacent darker portions of the surface, which become at first bluish, and later a decided blue-green; but by far the larger portion, includingalmost all the equatorial regions of the planet, remain always of areddish-ochre tint. [4]

[Footnote 4: In 1890 at Mount Wilson, California, Mr. W. H. Pickering'sphotographs of Mars on April 9th showed the southern polar cap ofmoderate dimensions, but with a large dim adjacent area. Twenty-fourhours later a corresponding plate showed this same area brilliantlywhite; the result apparently of a great Martian snowfall. In 1882 thesame observer witnessed the steady disappearance of 1, 600, 000 squaremiles of the southern snow-cap, an area nearly one-third of thathemisphere of the planet. ]

The rapid and comparatively early disappearance of the white coveringis, very reasonably, supposed to prove that it is of small thickness, corresponding perhaps to about a foot or two of snow in north-temperateAmerica and Europe, and that by the increasing amount of sun-heat it isconverted, partly into liquid and partly into vapour. Coincident withthis disappearance and as a presumed result of the water (or otherliquid) producing inundations, the bluish-green tinge which appears onthe previously dark portion of the surface is supposed to be due to arapid growth of vegetation. 

But the evidence on this point does not seem to be clear or harmonious, for in the four coloured plates showing the planet's surface atsuccessive Martian dates from December 30th to February 21st, not onlyis a considerable extent of the south temperate zone shown to changerapidly from bluish-green to chocolate-brown and then again tobluish-green, but the portions furthest from the supposed fertilisingoverflow are permanently green, as are also considerable portions in theopposite or northern hemisphere, which one would think would then becompletely dried up. 

_No Hills upon Mars. _

The special point to which I here wish to call attention is this. Mr. Lowell's main contention is, that the surface of Mars is wonderfullysmooth and level. Not only are there no mountains, but there are nohills or valleys or plateaux. This assumption is absolutely essential tosupport the other great assumption, that the wonderful network ofperfectly straight lines over nearly the whole surface of the planet areirrigation canals. It is not alleged that irregularities or undulationsof a few hundreds or even one or two thousands of feet could possibly bedetected, while certainly all we know of planetary formation orstructure point strongly towards _some_ inequalities of surface. Mr. Lowell admits that the dark portions of the surface, when examined onthe terminator (the margin of the illuminated portion), do _look_ likehollows and _may be_ the beds of dried-up seas; yet the supposed canalsrun across these old sea-beds in perfect straight lines just as they doacross the many thousand miles of what are admitted to be deserts--whichhe describes in these forcible terms: "Pitiless as our deserts are, theyare but faint forecasts of the state of things existent on Mars at thepresent time. "

It appears, then, that Mr. Lowell has to face this dilemma--_Only if thewhole surface of Mars is an almost perfect level could the enormousnetwork of straight canals, each from hundreds to thousands of mileslong, have been possibly constructed by intelligent beings for purposesof irrigation; but, if a complete and universal level surface exists nosuch system would be necessary. _ For on a level surface--or on asurface slightly inclined from the poles towards the equator, whichwould be advantageous in either case--the melting water would of itselfspread over the ground and naturally irrigate as much of the surface asit was possible for it to reach. If the surface were not level, butconsisted of slight elevations and expressions to the extent of a fewscores or a few hundreds of feet, then there would be no possibleadvantage in cutting straight troughs through these elevations invarious directions with water flowing at the bottom of them. In neithercase, and in hardly any conceivable case, could these perfectly straightcanals, cutting across each other in every direction and at very varyingangles, be of any use, or be the work of an intelligent race, if anysuch race could possibly have been developed under the adverseconditions which exist in Mars. 

_The Scanty Water-supply. _

But further, if there were any superfluity of water derived from themelting snow beyond what was sufficient to moisten the hollows indicatedby the darker portions of the surface, which at the time the waterreaches them acquire a green tint (a superfluity under the circumstanceshighly improbable), that superfluity could be best utilised by widening, however little, the borders to which natural overflow had carried it. Any attempt to make that scanty surplus, by means of overflowing canals, travel across the equator into the opposite hemisphere, through such aterrible desert region and exposed to such a cloudless sky as Mr. Lowelldescribes, would be the work of a body of madmen rather than ofintelligent beings. It may be safely asserted that not one drop of waterwould escape evaporation or insoak at even a hundred miles from itssource. [5]

[Footnote 5: What the evaporation is likely to be in Mars may beestimated by the fact, stated by Professor J. W. Gregory in his recentvolume on 'Australia' in _Stanford's Compendium_, that in North-WestVictoria evaporation is at the rate of ten feet per annum, while inCentral Australia it is very much more. The greatly diminishedatmospheric pressure in Mars will probably more than balance the loss ofsun-heat in producing rapid evaporation. ]

_Miss Clerke on the Scanty Water-supply. _

On this point I am supported by no less an authority than the historianof modern astronomy, the late Miss Agnes Clerke. In the _EdinburghReview_ (of October 1896) there is an article entitled 'New Views aboutMars, ' exhibiting the writer's characteristic fulness of knowledge andcharm of style. Speaking of Mr. Lowell's idea of the 'canals' carryingthe surplus water across the equator, far into the opposite hemisphere, for purposes of irrigation there (which we see he again states in thepresent volume), Miss Clerke writes: "We can hardly imagine so shrewd apeople as the irrigators of Thule and Hellas[6] wasting labour, and thelife-giving fluid, after so unprofitable a fashion. There is everyreason to believe that the Martian snow-caps are quite flimsystructures. Their material might be called snow _soufflé_, since, owingto the small power of gravity on Mars, snow is almost three timeslighter there than here. Consequently, its own weight can have verylittle effect in rendering it compact. Nor, indeed, is there time formuch settling down. The calotte does not form until several months afterthe winter solstice, and it begins to melt, as a rule, shortly after thevernal equinox. (The interval between these two epochs in the southernhemisphere of Mars is 176 days. ) The snow lies on the ground, at theoutside, a couple of months. At times it melts while it is still freshfallen. Thus, at the opposition of 1881-82 the spreading of the northernsnows was delayed until seven weeks after the equinox: and they had, accordingly, no sooner reached their maximum than they began to decline. And Professor Pickering's photographs of April 9th and 10th, 1890, proved that the southern calotte may assume its definitive proportionsin a single night. 

[Footnote 6: Areas on Mars so named. ]

"No attempt has yet been made to estimate the quantity of waterderivable from the melting of one of these formations; yet theexperiment is worth trying as a help towards defining ideas. Let usgrant that the average depth of snow in them, of the delicate Martiankind, is twenty feet, equivalent at the most to one foot of water. Themaximum area covered, of 2, 400, 000 square miles, is nearly equal to thatof the United States, while the whole globe of Mars measures 55, 500, 000square miles, of which one-third, on the present hypothesis, is undercultivation, and in need of water. Nearly the whole of the dark areas, as we know, are situated in the southern hemisphere, of which theyextend over, at the very least, 17, 000, 000 square miles; that is to say, they cover an area, in round numbers, seven times that of the snow-cap. Only one-seventh of a foot of water, accordingly, could possibly bemade available for their fertilisation, supposing them to get the entireadvantage of the spring freshet. Upon a stint of less than two inches ofwater these fertile lands are expected to flourish and bear abundantcrops; and since they completely enclose the polar area they arenecessarily served first. The great emissaries for carrying off thesurplus of their aqueous riches, would then appear to be superfluousconstructions, nor is it likely that the share in those riches due tothe canals and oases, intricately dividing up the wide, dry, continentalplains, can ever be realised. 

"We have assumed, in our little calculation, that the entire contents ofa polar hood turn to water; but in actual fact a considerable proportionof them must pass directly into vapour, omitting the intermediate stage. Even with us a large quantity of snow is removed aerially; and in therare atmosphere of Mars this cause of waste must be especiallyeffective. Thus the polar reservoirs are despoiled in the act of beingopened. Further objections might be taken to Mr. Lowell's irrigationscheme, but enough has been said to show that it is hopelesslyunworkable. "

It will be seen that the writer of this article accepted the existenceof water on Mars, on the testimony of Sir W. Huggins, which, in view oflater observations, he has himself acknowledged to be valueless. Dr. Johnstone Stoney's proof of its absence, derived from the moleculartheory of gases, had not then been made public. 

_Description of some of the Canals. _

At the end of his volume Mr. Lowell gives a large chart of Mars onMercator's projection, showing the canals and other features seen duringthe opposition of 1905. This contains many canals not shown on the maphere reproduced (see frontispiece), and some of the differences betweenthe two are very puzzling. Looking at our map, which shows thenorth-polar snow below, so that the south pole is out of the view at thetop of the map, the central feature is the large spot Ascraeeus Lucus, from which ten canals diverge centrally, and four from the sides, forming wide double canals, fourteen in all. There is also a canal namedUlysses, which here passes far to the right of the spot, but in thelarge chart enters it centrally. Looking at our map we see, goingdownwards a little to the left, the canal Udon, which runs through adark area quite to the outer margin. In the dark area, however, there isshown on the chart a spot Aspledon Lucus, where five canals meet, and ifthis is taken as a terminus the Udon canal is almost exactly 2000 mileslong, and another on its right, Lapadon, is the same length, while Ich, running in a slightly curved line to a large spot (Lucus Castorius onthe chart) is still longer. The Ulysses canal, which (on the chart) runsstraight from the point of the Mare Sirenum to the Astraeeus Lucus isabout 2200 miles long. Others however are even longer, and Mr. Lowellsays: "With them 2000 miles is common; while many exceed 2500; and theEumenides-Orcus is 3540 miles from the point where it leaves LucusPhoeniceus to where it enters the Trivium Charontis. " This last canal isbarely visible on our map, its commencement being indicated by the wordEumenides. 

The Trivium Charontis is situated just beyond the right-hand margin ofour map. It is a triangular dark area, the sides about 200 miles long, and it is shown on the chart as being the centre from which radiatethirteen canals. Another centre is Aquae Calidae situated at the pointof a dark area running obliquely from 55° to 35° N. Latitude, and, asshown on a map of the opposite hemisphere to our map, has nearly twentycanals radiating from it in almost every direction. Here at all eventsthere seems to be no special connection with the polar snow-caps, andthe radiating lines seem to have no intelligent purpose whatever, butare such as might result from fractures in a glass globe produced byfiring at it with very small shots one at a time. Taking the wholeseries of them, Mr. Lowell very justly compares them to "a network whichtriangulates the surface of the planet like a geodetic survey, intopolygons of all shapes and sizes. "

At the very lowest estimate the total length of the canals observed andmapped by Mr. Lowell must be over a hundred thousand miles, while heassures us that numbers of others have been seen over the whole surface, but so faintly or on such rare occasions as to elude all attempts to fixtheir position with certainty. But these, being of the same characterand evidently forming part of the same system, must also be artificial, and thus we are led to a system of irrigation of almost unimaginablemagnitude on a planet which has no mountains, no rivers, and no rain tosupport it; whose whole water-supply is derived from polar snows, theamount of which is ludicrously inadequate to need any such world-widesystem; while the low atmospheric pressure would lead to rapidevaporation, thus greatly diminishing the small amount of moisture thatis available. Everyone must, I think, agree with Miss Clerke, that, evenadmitting the assumption that the polar snows consist of frozen water, the excessively scanty amount of water thus obtained would render anyscheme of world-wide distribution of it hopelessly unworkable. 

The very remarkable phenomena of the duplication of many of the lines, together with the darkspots--the so-called oases--at theirintersections, are doubtless all connected in some unknown way with theconstitution and past history of the planet; but, on the theory of thewhole being works of art, they certainly do _not_ help to remove any ofthe difficulties which have been shown to attend the theory that thesingle lines represent artificial canals of irrigation with a strip ofverdure on each side of them produced by their overflow. 

_Lowell on the Purpose of the Canals. _

Before leaving this subject it will be well to quote Mr. Lowell's ownwords as to the supposed perfectly level surface of Mars, and hisinterpretation of the origin and purpose of the 'canals':

"A body of planetary size, if unrotating, becomes a sphere, except forsolar tidal deformation; if rotating, it takes on a spheroidal formexactly expressive, so far as observation goes, of the so-calledcentrifugal force at work. Mars presents such a figure, being flattenedout to correspond to its axial rotation. Its surface therefore is influid equilibrium, or, in other words, a particle of liquid at any pointof its surface at the present time would stay where it was devoid ofinclination to move elsewhere. Now the water which quickens the verdureof the canals moves from the pole down to the equator as the seasonadvances. This it does then irrespective of gravity. No natural forcepropels it, and the inference is forthright and inevitable that it isartificially helped to its end. There seems to be no escape from thisdeduction. Water only flows downhill, and there is no such thing asdownhill on a surface already in fluid equilibrium. A few canals mightpresumably be so situated that their flow could, by inequality ofterrane, lie equatorward, but not all.... Now it is not in particular butby general consent that the canal-system of Mars develops from pole toequator. From the respective times at which the minima take place, itappears that the canal quickening occupies fifty-two days, as evidencedby the successive vegetal darkenings, to descend from latitude 72° northto latitude 0°, a journey of 2650 miles. This gives for the water aspeed of fifty-one miles a day, or 2. 1 miles an hour. The rate ofprogression is remarkably uniform, and this abets the deduction as toassisted transference. But the fact is more unnatural yet. The growthpays no regard to the equator, but proceeds across it as if it did notexist into the planet's other hemisphere. Here is something still moretelling than travel to this point. For even if we suppose, for the sakeof argument, that natural forces took the water down to the equator, their action must there be certainly reversed, and the equator prove adead-line, to pass which were impossible" (pp. 374-5). 

I think my readers will agree with me that this whole argument is one ofthe most curious ever put forth seriously by an eminent man of science. Because the polar compression of Mars is about what calculation shows itought to be in accordance with its rate of rotation, its surface is in astate of 'fluid equilibrium, ' and must therefore be absolutely levelthroughout. But the polar compression of the earth equally agrees withcalculation; therefore its surface is also in 'fluid equilibrium';therefore it also ought to be as perfectly level on land as it is on theocean surface! But as we know this is very far from being the case, whymust it be so in Mars? Are we to suppose Mars to have been formed insome totally different way from other planets, and that there neither isnor ever has been any reaction between its interior and exterior forces?Again, the assumption of perfect flatness is directly opposed to allobservation and all analogy with what we see on the earth and moon. Itgives no account whatever of the numerous and large dark patches, oncetermed seas, but now found to be not so, and to be full of detailedmarkings and varied depths of shadow. To suppose that these are all thesame dead-level as the light-coloured portions are assumed to be, implies that the darkness is one of material and colour only, not ofdiversified contour, which again is contrary to experience, sincedifference of material with us always leads to differences in rate ofdegradation, and hence of diversified contour, as these dark spacesactually show themselves under favourable conditions to independentobservers. 

_Lowell on the System of Canals as a whole. _

We will now see what Mr. Lowell claims to be the plain teaching of the'canals' as a whole:

"But last and all-embracing in its import is the system which the canalsform. Instead of running at hap-hazard, the canals are interconnected ina most remarkable manner. They seek centres instead of avoiding them. The centres are linked thus perfectly one with another, an arrangementwhich could not result from centres, whether of explosion or otherwise, which were themselves discrete. Furthermore, the system covers the wholesurface of the planet, dark areas and light ones alike, a world-widedistribution which exceeds the bounds of natural possibility. Any forcewhich could act longitudinally on such a scale must be limitedlatitudinally in its action, as witness the belts of Jupiter and thespots upon the sun. Rotational, climatic, or other physical cause couldnot fail of zonal expression. Yet these lines are grandly indifferent tosuch competing influences. Finally, the system, after meshing thesurface in its entirety, runs straight into the polar caps. 

"It is, then, a system whose end and aim is the tapping of the snow-capfor the water there semi-annually let loose; then to distribute it overthe planet's face" (p. 373). 

Here, again, we have curiously weak arguments adduced to support theview that these numerous straight lines imply works of art rather thanof nature, especially in the comparison made with the belts of Jupiterand the spots on the sun, both purely atmospheric phenomena, whereas thelines on Mars are on the solid surface of the planet. Why should therebe any resemblance between them? Every fact stated in the abovequotation, always keeping in mind the physical conditions of theplanet--its very tenuous atmosphere and rainless desert-surface--seemwholly in favour of a purely natural as opposed to an artificial origin;and at the close of this discussion I shall suggest one which seems tome to be at least possible, and to explain the whole series of thephenomena set forth and largely discovered by Mr. Lowell, in a simplerand more probable manner than does his tremendous assumption of theirbeing works of art. Readers who may not possess Mr. Lowell's volume willfind three of his most recent maps of the 'canals' reproduced in_Nature_ of October 11th, 1906. 

CHAPTER IV. 

IS ANIMAL LIFE POSSIBLE ON MARS?

Having now shown, that, even admitting the accuracy of all Mr. Lowell'sobservations, and provisionally accepting all his chief conclusions asto the climate, the nature of the snow-caps, the vegetation, and theanimal life of Mars, yet his interpretation of the lines on its surfaceas being veritably 'canals, ' constructed by intelligent beings for thespecial purpose of carrying water to the more arid regions, is whollyerroneous and rationally inconceivable. I now proceed to discuss hismore fundamental position as to the actual habitability of Mars by ahighly organised and intellectual race of material organic beings. 

_Water and Air essential to Life. _

Here, fortunately, the issue is rendered very simple, because Mr. Lowellfully recognises the identity of the constitution of matter and ofphysical laws throughout the solar-system, and that for any high form oforganic life certain conditions which are absolutely essential on ourearth must also exist in Mars. He admits, for example, that water isessential, that an atmosphere containing oxygen, nitrogen, aqueousvapour, and carbonic acid gas is essential, and that an abundantvegetation is essential; and these of course involve asurface-temperature through a considerable portion of the year thatrenders the existence of these--especially of water--possible andavailable for the purposes of a high and abundant animal life. 

_Blue Colour the only Evidence of Water. _

In attempting to show that these essentials actually exist on Mars he isnot very successful. He adduces evidence of an atmosphere, but of anexceedingly scanty one, since the greatest amount he can give to it is--"not more than about four inches of barometric pressure as we reckonit";[7] and he assumes, as he has a fair right to do till disproved, that it consists of oxygen and nitrogen, with carbon-dioxide andwater-vapour, in approximately the same proportions as with us. Withregard to the last item--the water-vapour--there are however manyserious difficulties. The water-vapour of our atmosphere is derived fromthe enormous area of our seas, oceans, lakes, and rivers, as well asfrom the evaporation from heated lands and tropical forests of much ofthe moisture produced by frequent and abundant rains. All these sourcesof supply are admittedly absent from Mars, which has no permanent bodiesof water, no rain, and tropical regions which are almost entirelydesert. Many writers have therefore doubted the existence of water inany form upon this planet, supposing that the snow-caps are not formedof frozen water but of carbon-dioxide, or some other heavy gas, in afrozen state; and Mr. Lowell evidently feels this to be a difficulty, since the only fact he is able to adduce in favour of the melting snowsof the polar caps producing water is, that at the time they are meltinga marginal blue band appears which accompanies them in their retreat, and this blue colour is said to prove conclusively that the liquid isnot carbonic acid but water. This point he dwells upon repeatedly, stating, of these blue borders: "This excludes the possibility of theirbeing formed by carbon-dioxide, and shows that of all the substances weknow the material composing them must be water. "

[Footnote 7: In a paper written since the book appeared the density ofair at the surface of Mars is said to be 1/12 of the earth's. ]

This is the only proof of the existence of _water_ he adduces, and it iscertainly a most extraordinary and futile one. For it is perfectly wellknown that although water, in large masses and by transmitted light, isof a blue colour, yet shallow water by reflected light is not so; and inthe case of the liquid produced by the snow-caps of Mars, which thewhole conditions of the planet show must be shallow, and also be more orless turbid, it cannot possibly be the cause of the 'deep blue' tintsaid to result from the melting of the snow. 

But there is a very weighty argument depending on the molecular theoryof gases against the polar caps of Mars being composed of frozen waterat all. The mass and elastic force of the several gases is due to thegreater or less rapidity of the vibratory motion of their moleculesunder identical conditions. The speed of these molecular motions hasbeen ascertained for all the chief gases, and it is found to be so greatas in certain cases to enable them to overcome the force of gravity andescape from a planet's surface into space. Dr. G. Johnstone Stoney hasspecially investigated this subject, and he finds that the force ofgravity on the earth is sufficient to retain all the gases composing itsatmosphere, but not sufficient to retain hydrogen; and as a consequence, although this gas is produced in small quantities by volcanoes and bydecomposing vegetation, yet no trace of it is found in our atmosphere. The moon however, having only one-eightieth the mass of the earth, cannot retain any gas, hence its airless and waterless condition. 

_Water Vapour cannot exist on Mars. _

Now, Dr. Stoney finds that in order to retain water vapour permanently aplanet must have a mass at least a quarter that of the earth. But themass of Mars is only one-ninth that of the earth; therefore, unlessthere are some special conditions that prevent its loss, this gas cannotbe present in the atmosphere. Mr. Lowell does not refer to this argumentagainst his view, neither does he claim the evidence of spectroscopy inhis favour. This was alleged more than thirty years ago to show theexistence of water-vapour in the atmosphere of Mars, but of late yearsit has been doubted, and Mr. Lowell's complete silence on the subject, while laying stress on such a very weak and inconclusive argument asthat from the tinge of colour that is observed a little distance fromthe edge of the diminishing snow-caps, shows that he himself does notthink the fact to be thus proved. If he did he would hardly adduce suchan argument for its presence as the following: "The melting of the capson the one hand and their re-forming on the other affirm the presence ofwater-vapour in the Martian atmosphere, of whatever else that airconsists" (p. 162). Yet absolutely the only proof he gives that the capsare frozen water is the almost frivolous colour-argument above referredto!

_No Spectroscopic Evidence of Water Vapour. _

As Sir William Huggins is the chief authority quoted for this fact, andis referred to as being almost conclusive in the third edition of MissClerke's _History of Astronomy_ in 1893, I have ascertained that hisopinion at the present time is that "there is no conclusive proof of thepresence of aqueous vapour in the atmosphere of Mars, and thatobservations at the Lick Observatory (in 1895), where the conditions andinstruments are of the highest order, were negative. " He also informs methat Marchand at the Pic du Midi Observatory was unable to obtain linesof aqueous vapour in the spectrum of Mars; and that in 1905, Slipher, atMr. Lowell's observatory, was unable to detect any indications ofaqueous vapour in the spectrum of Mars. 

It thus appears that spectroscopic observations are quite accordant withthe calculations founded on the molecular theory of gases as to theabsence of aqueous vapour, and therefore presumably of liquid water, from Mars. It is true that the spectroscopic argument is purelynegative, and this may be due to the extreme delicacy of theobservations required; but that dependent on the inability of the forceof gravity to retain it is positive scientific evidence against itspresence, and, till shown to be erroneous, must be held to beconclusive. 

This absence of water is of itself conclusive against the existence ofanimal life, unless we enter the regions of pure conjecture as to thepossibility of some other liquid being able to take its place, and thatliquid being as omnipresent there as water is here. Mr. Lowell howevernever takes this ground, but bases his whole theory on the fundamentalidentity of the substance of the bodies of living organisms whereverthey may exist in the solar system. In the next two chapters I shalldiscuss an equally essential condition, that of temperature, whichaffords a still more conclusive and even crushing argument against thesuitability of Mars for the existence of organic life. 

CHAPTER V. 

THE TEMPERATURE OF MARS--MR. LOWELL'S ESTIMATE. 

We have now to consider a still more important and fundamental question, and one which Mr. Lowell does not grapple with in this volume, theactual temperatures on Mars due to its distance from the sun and theatmospheric conditions on which he himself lays so much stress. If I amnot greatly mistaken we shall arrive at conclusions on this subjectwhich are absolutely fatal to the conception of any high form of organiclife being possible on this planet. 

_The Problem of Terrestrial Temperatures. _

In order that the problem may be understood and its importanceappreciated, it is necessary to explain the now generally acceptedprinciples as to the causes which determine the temperatures on ourearth, and, presumably, on all other planets whose conditions are notwholly unlike ours. The fact of the internal heat of the earth whichbecomes very perceptible even at the moderate depths reached in minesand deep borings, and in the deepest mines becomes a positiveinconvenience, leads many people to suppose that the surface-temperatures of the earth are partly due to this cause. But it is nowgenerally admitted that this is not the case, the reason being that allrocks and soils, in their natural compacted state, are exceedingly badconductors of heat. 

A striking illustration of this is the fact, that a stream of lava oftencontinues to be red hot at a few feet depth for years after the surfaceis consolidated, and is hardly any warmer than that of the surroundingland. A still more remarkable case is that of a glacier on thesouth-east side of the highest cone of Etna underneath a lava streamwith an intervening bed of volcanic sand only ten feet thick. This wasvisited by Sir Charles Lyell in 1828, and a second time thirty yearslater, when he made a very careful examination of the strata, and wasquite satisfied that the sand and the lava stream together had actuallypreserved this mass of ice, which neither the heat of the lava above itat its first outflow, nor the continued heat rising from the greatvolcano below it, had been able to melt or perceptibly to diminish inthirty years. Another fact that points in the same direction is theexistence over the whole floor of the deepest oceans of ice-cold water, which, originating in the polar seas, owing to its greater density sinksand creeps slowly along the ocean bottom to the depths of the Atlanticand Pacific, and is not perceptibly warmed by the internal heat of theearth. 

Now the solid crust of the earth is estimated as at least about twentymiles in average thickness; and, keeping in mind the other facts justreferred to, we can understand that the heat from the interior passesthrough it with such extreme slowness as not to be detected at thesurface, the varying temperatures of which are due entirely to solarheat. A large portion of this heat is stored up in the surface soil, andespecially in the surface layer of the oceans and seas, thus partlyequalising the temperatures of day and night, of winter and summer, soas greatly to ameliorate the rapid changes of temperature that wouldotherwise occur. Our dense atmosphere is also of immense advantage to usas an equaliser of temperature, charged as it almost always is with alarge quantity of water-vapour. This latter gas, when not condensed intocloud, allows the solar heat to pass freely to the earth; but it has thesingular and highly beneficial property of absorbing and retaining thedark or lower-grade heat given off by the earth which would otherwiseradiate into space much more rapidly. We must therefore always rememberthat, very nearly if not quite, the _whole_ of _the warmth we experienceon the earth is derived from the sun. _[8]

[Footnote 8: Professor J. H. Poynting, in his lecture to the BritishAssociation at Cambridge in 1904, says: "The surface of the earthreceives, we know, an amount of heat from the inside almostinfinitesimal compared with that which it receives from the sun, and onthe sun, therefore, we depend for our temperature. "]

In order to understand the immense significance of this conclusion wemust know what is meant by the _whole_ heat or warmth; as unless we knowthis we cannot define what half or any other proportion of sun-heatreally means. Now I feel pretty sure that nine out of ten of the averageeducated public would answer the following question incorrectly: Themean temperature of the southern half of England is about 48° F. Supposing the earth received only half the sun-heat it now receives, what would then be the probable mean temperature of the South ofEngland? The majority would, I think, answer at once--About 24° F. Nearly as many would perhaps say--48° F. Is 16° above the freezingpoint; therefore half the heat received would bring us down to 8° abovethe freezing point, or 40° F. Very few, I think, would realise that ourshare of half the amount of sun-heat received by the earth wouldprobably result in reducing our mean temperature to about 100° F. Belowthe freezing point, and perhaps even lower. This is about the verylowest temperature yet experienced on the earth's surface. To understandhow such results are obtained a few words must be said about theabsolute zero of temperature. 

_The Zero of Temperature. _

Heat is now believed to be entirely due to ether-vibration, whichproduces a correspondingly rapid vibration of the molecules of matter, causing it to expand and producing all the phenomena we term 'heat. ' Wecan conceive this vibration to increase indefinitely, and thus therewould appear to be no necessary limit to the amount of heat possible, but we cannot conceive it to decrease indefinitely at the same uniformrate, as it must soon inevitably come to nothing. Now it has been foundby experiment that gases under uniform pressure expand 1/273 of theirvolume for each degree Centigrade of increased temperature, so that inpassing from 0° C. To 273° C. They are doubled in volume. They alsodecrease in volume at the same rate for each degree below 0° C. (thefreezing point of water). Hence if this goes on to-273° C. A gas willhave no volume, or it will undergo some change of nature. Hence this iscalled the zero of temperature, or the temperature to which any matterfalls which receives no heat from any other matter. It is also sometimescalled the temperature of space, or of the ether in a state of rest, ifthat is possible. All the gases have now been proved to become, firstliquid and then (most of them) solid, at temperatures considerably abovethis zero. 

The only way to compare the proportional temperatures of bodies, whetheron the earth or in space, is therefore by means of a scale beginning atthis natural zero, instead of those scales founded on the artificialzero of the freezing point of water, or, as in Fahrenheit's, 32° belowit. Only by using the natural zero and measuring continuously from itcan we estimate temperatures in relative proportion to the amount ofheat received. This is termed the absolute zero, and so that we startreckoning from that point it does not matter whether the scale adoptedis the Centigrade or that of Fahrenheit. 

_The Complex Problem of Planetary Temperatures. _

Now if, as is the case with Mars, a planet receives only half the amountof solar heat that we receive, owing to its greater distance from thesun, and if the mean temperature of our earth is 60° F. , this is equalto 551° F. On the absolute scale. It would therefore appear very simpleto halve this amount and obtain 275. 5° F. As the mean temperature ofthat planet. But this result is erroneous, because the actual amount ofsun heat intercepted by a planet is only one condition out of many thatdetermine its resulting temperature. Radiation, that is loss of heat, isgoing on concurrently with gain, and the rate of loss varies with thetemperature according to a law recently discovered, the loss being muchgreater at high temperatures in proportion to the 4th power of theabsolute temperature. Then, again, the whole heat intercepted by aplanet does not reach its surface unless it has no atmosphere. When ithas one, much is reflected or absorbed according to complex lawsdependent on the density and composition of the atmosphere. Then, again, the heat that reaches the actual surface is partly reflected and partlyabsorbed, according to the nature of that surface--land or water, desertor forest or snow-clad--that part which is absorbed being the chiefagent in raising the temperature of the surface and of the air incontact with it. Very important too is the loss of heat by radiationfrom these various heated surfaces at different rates; while theatmosphere itself sends back to the surface an ever varying portion ofboth this radiant and reflected heat according to distinct laws. Furtherdifficulties arise from the fact that much of the sun's heat consists ofdark or invisible rays, and it cannot therefore be measured by thequantity of light only. 

From this rough statement it will be seen that the problem is anexceedingly complex one, not to be decided off-hand, or by any simplemethod. It has in fact been usually considered as (strictly speaking)insoluble, and only to be estimated by a more or less roughapproximation, or by the method of general analogy from certain knownfacts. It will be seen, from what has been said in previous chapters, that Mr. Lowell, in his book, has used the latter method, and, by takingthe presence of water and water-vapour in Mars as proved by thebehaviour of the snow-caps and the bluish colour that results from theirmelting, has deduced a temperature above the freezing point of water, asprevalent in the equatorial regions permanently, and in the temperateand arctic zones during a portion of each year. 

_Mr. Lowell's Mathematical Investigation of the Problem. _

But as this result has been held to be both improbable in itself andfounded on no valid evidence, he has now, in the _London, Edinburgh, andDublin Philosophical Magazine_ of July 1907, published an elaboratepaper of 15 pages, entitled _A General Method for Evaluating theSurface-Temperatures of the Planets; with special reference to theTemperature of Mars_, by Professor Percival Lowell; and in this paper, by what purports to be strict mathematical reasoning based on the mostrecent discoveries as to the laws of heat, as well as on measurements orestimates of the various elements and constants used in thecalculations, he arrives at a conclusion strikingly accordant with thatput forward in the recently published volume. Having myself neithermathematical nor physical knowledge sufficient to enable me to criticisethis elaborate paper, except on a few points, I will here limit myselfto giving a short account of it, so as to explain its method ofprocedure; after which I may add a few notes on what seem to me doubtfulpoints; while I also hope to be able to give the opinions of some morecompetent critics than myself. 

_Mr. Lowell's Mode of Estimating the Surface-temperature of Mars. _

The author first states, that Professor Young, in his _GeneralAstronomy_ (1898), makes the mean temperature of Mars 223. 6° absolute, by using Newton's law of heat being radiated in proportion totemperature, and 363° abs. (=-96° F. ) by Dulong and Petit's law; butadds, that a closer determination has been made by Professor Moulton, using Stefan's law, that radiation is as the _/4th_ power of thetemperature, whence results a mean temperature of-31° F. These estimatesassume identity of atmospheric conditions of Mars and the Earth. 

But as none of these estimates take account of the many complex factorswhich interfere with such direct and simple calculations, Mr. Lowellthen proceeds to enunciate them, and work out mathematically the effectsthey produce:

(1) The whole radiant energy of the sun on striking a planet becomesdivided as follows: Part is reflected back into space, part absorbed bythe atmosphere, part transmitted to the surface of the planet. Thissurface again reflects a portion and only the balance left goes to warmthe planet. 

(2) To solve this complex problem we are helped by the _albedoes_ orintrinsic brilliancy of the planets, which depend on the proportion ofthe visible rays which are reflected and which determines thecomparative brightness of their respective surfaces. We also have tofind the ratio of the invisible to the visible rays and the heatingpower of each. 

(3) He then refers to the actinometer and pyroheliometer, instrumentsfor measuring the actual heat derived from the sun, and also to theBolometer, an instrument invented by Professor Langley for measuring theinvisible heat rays, which he has proved to extend to more than threetimes the length of the whole heat-spectrum as previously known, andhas also shown that the invisible rays contribute 68 per cent, of thesun's total energy. [9]

[Footnote 9: For a short account of this remarkable instrument, see my_Wonderful Century_, new ed. , pp. 143-145. ]

(4) Then follows an elaborate estimate of the loss of heat during thepassage of the sun's rays through our atmosphere from experiments madeat different altitudes and from the estimated reflective power of thevarious parts of the earth's surface--rocks and soil, ocean, forest andsnow--the final result being that three-fourths of the whole sun-heatis reflected back into space, forming our _albedo_, while onlyone-fourth is absorbed by the soil and goes to warm the air anddetermine our mean temperature. 

(5) We now have another elaborate estimate of the comparative amounts ofheat actually received by Mars and the Earth, dependent on their verydifferent amounts of atmosphere, and this estimate depends almost whollyon the comparative _albedoes_, that of Mars, as observed by astronomersbeing 0. 27, while ours has been estimated in a totally different way asbeing 0. 75, whence he concludes that nearly three-fourths of thesun-heat that Mars receives reaches the surface and determines itstemperature, while we get only one-fourth of our total amount. Then byapplying Stefan's law, that the radiation is as the 4th power of thesurface temperature, he reaches the final result that the actual heatingpower at the surface of Mars is considerably _more_ than on the Earth, and would produce a mean temperature of 72° F. , if it were not for thegreater conservative or blanketing power of our denser and morewater-laden atmosphere. The difference produced by this latter fact heminimises by dwelling on the probability of a greater proportion ofcarbonic-acid gas and water-vapour in the Martian atmosphere, and thusbrings down the mean temperature of Mars to 48° F. , which is almostexactly the same as that of the southern half of England. He has also, as the result of observations, reduced the probable density of theatmosphere of Mars to 2-1/2 inches of mercury, or only one-twelfth ofthat of the Earth. 

_Critical Remarks on Mr. Lowell's Paper. _

The last part of this paper, indicated under pars. 4 and 5, is the mostelaborate, occupying eight pages, and it contains much that seemsuncertain, if not erroneous. In particular, it seems very unlikely thatunder a clear sky over the whole earth we should only receive at thesea-level 0. 23 of the solar rays which the earth intercepts (p. 167). These data largely depend on observations made in California and otherparts of the southern United States, where the lower atmosphere isexceptionally dust-laden. Till we have similar observations made in thetropical forest-regions, which cover so large an area, and where theatmosphere is purified by frequent rains, and also on the prairies andthe great oceans, we cannot trust these very local observations forgeneral conclusions affecting the whole earth. Later, in the samearticle (p. 170), Mr. Lowell says: "Clouds transmit approximately 20 percent. Of the heat reaching them: a clear sky at sea-level 60 per cent. As the sky is half the time cloudy the mean transmission is 35 percent. " These statements seem incompatible with that quoted above. 

The figure he uses in his calculations for the actual albedo of theearth, 0. 75, is also not only improbable, but almost self-contradictory, because the albedo of cloud is 0. 72, and that of the great cloud-coveredplanet, Jupiter, is given by Lowell as 0. 75, while Zollner made it only0. 62. Again, Lowell gives Venus an albedo of 0. 92, while Zollner made itonly 0. 50 and Mr. Gore 0. 65. This shows the extreme uncertainty of theseestimates, while the fact that both Venus and Jupiter are whollycloud-covered, while we are only half-covered, renders it almostcertain that our albedo is far less than Mr. Lowell makes it. It isevident that mathematical calculations founded upon such uncertain datacannot yield trustworthy results. But this is by no means the only casein which the data employed in this paper are of uncertain value. Everywhere we meet with figures of somewhat doubtful accuracy. Here wehave somebody's 'estimate' quoted, there another person's 'observation, 'and these are adopted without further remark and used in the variouscalculations leading to the result above quoted. It requires a practisedmathematician, and one fully acquainted with the extensive literature ofthis subject, to examine these various data, and track them through themaze of formulae and figures so as to determine to what extent theyaffect the final result. 

There is however one curious oversight which I must refer to, as it is apoint to which I have given much attention. Not only does Mr. Lowellassume, as in his book, that the 'snows' of Mars consist of frozenwater, and that therefore there _is_ water on its surface andwater-vapour in its atmosphere, not only does he ignore altogether Dr. Johnstone Stoney's calculations with regard to it, which I have alreadyreferred to, but he uses terms that imply that water-vapour is one ofthe heavier components of our atmosphere. The passage is at p. 168 ofthe _Philosophical Magazine. _ After stating that, owing to the verysmall barometric pressure in Mars, water would boil at 110° F. , he adds:"The sublimation at lower temperatures would be correspondinglyincreased. Consequently the amount of water-vapour in the Martian airmust on that score be relatively greater than our own. " Then followsthis remarkable passage: "Carbon-dioxide, because of its greaterspecific gravity, would also be in relatively greater amount so far asthis cause is considered. For the planet would part, _caeteris paribus_, with its lighter gases the quickest. Whence as regards both water-vapourand carbon-dioxide we have reason to think them in relatively greaterquantity than in our own air at corresponding barometric pressure. " Icannot understand this passage except as implying that 'water-vapour andcarbon-dioxide' are among the heavier and not among the lighter gases ofthe atmosphere--those which the planet 'parts with quickest. ' But thisis just what water-vapour _is_, being a little less than two-thirds theweight of air (0. 6225), and one of those which the planet _would_ partwith the quickest, and which, according to Dr. Johnstone Stoney, itloses altogether. * * * * *

Note on Professor Lowell's article in the _Philosophical Magazine_; byJ. H. Poynting, F. R. S. , Professor of Physics in the University ofBirmingham. 

"I think Professor Lowell's results are erroneous through his neglect ofthe heat stored in the air by its absorption of radiation both from thesun and from the surface. The air thus heated radiates to the surfaceand keeps up the temperature. I have sent to the _PhilosophicalMagazine_ a paper in which I think it is shown that when the radiationby the atmosphere is taken into account the results are entirelychanged. The temperature of Mars, with Professor Lowell's data, stillcomes out far below the freezing-point--still further below than theincreased distance alone would make it. Indeed, the lower temperature onelevated regions of the earth's surface would lead us to expect this. Ithink it is impossible to raise the temperature of Mars to anything likethe value obtained by Professor Lowell, unless we assume some quality inhis atmosphere entirely different from any found in our own atmosphere. "J. H. POYNTING. October 19, 1907. 

CHAPTER VI. 

A NEW ESTIMATE OF THE TEMPERATURE OF MARS. 

When we are presented with a complex problem depending on a great numberof imperfectly ascertained data, we may often check the results thusobtained by the comparison of cases in which some of the more importantof these data are identical, while others are at a maximum or a minimum. In the present case we can do this by a consideration of the Moon ascompared with the Earth and with Mars. 

_Langley's Determination of the Moon's Temperature. _

In the moon we see the conditions that prevail in Mars both exaggeratedand simplified. Mars has a very scanty atmosphere, the moon none at all, or if there is one it is so excessively scanty that the most refinedobservations have not detected it. All the complications arising fromthe possible nature of the atmosphere, and its complex effects uponreflection, absorption, and radiation are thus eliminated. The meandistance of the moon from the sun being identical with that of theearth, the total amount of heat intercepted must also be identical; onlyin this case the whole of it reaches the surface instead of one-fourthonly, according to Mr. Lowell's estimate for the earth. 

Now, by the most refined observations with his Bolometer, Mr. Langleywas able to determine the temperature of the moon's surface exposed toundimmed sunshine for fourteen days together; and he found that, even inthat portion of it on which the sun was shining almost vertically, thetemperature rarely rose above the freezing point of water. Howeverextraordinary this result may seem, it is really a striking confirmationof the accuracy of the general laws determining temperature which I haveendeavoured to explain in the preceding chapter. For the same surfacewhich has had fourteen days of sunshine has also had a precedingfourteen days of darkness, during which the heat which it hadaccumulated in its surface layers would have been lost by free radiationinto stellar space. It thus acquires during its day a maximumtemperature of only 491° F. Absolute, while its minimum, after 14 days'continuous radiation, must be very low, and is, with much reason, supposed to approach the absolute zero. 

_Rapid Loss of Heat by Radiation on the Earth. _

In order better to comprehend what this minimum may be under extremeconditions, it will be useful to take note of the effects it actuallyproduces on the earth in places where the conditions are nearest tothose existing on the moon or on Mars, though never quite equalling, oreven approaching very near them. It is in our great desert regions, andespecially on high plateaux, that extreme aridity prevails, and it is insuch districts that the differences between day and night temperaturesreach their maximum. It is stated by geographers that in parts of theGreat Sahara the surface temperature is sometimes 150° F. , while duringthe night it falls nearly or quite to the freezing point--a differenceof 118 degrees in little more than 12 hours. [10] In the high desertplains of Central Asia the extremes are said to be even greater. [11]Again, in his _Universal Geography_, Reclus states that in the ArmenianHighlands the thermometer oscillates between 13° F. And 112°F. We maytherefore, without any fear of exaggeration, take it as proved that afall of 100° F. In twelve or fifteen hours not infrequently occurs wherethere is a very dry and clear atmosphere permitting continuousinsolation by day and rapid radiation by night. 

[Footnote 10: Keith Johnston's 'Africa' in _Stanford's Compendium. _]

[Footnote 11: _Chambers's Encyclopaedia_, Art. 'Deserts. ']

Now, as it is admitted that our dense atmosphere, however dry and clear, absorbs and reflects some considerable portion of the solar heat, weshall certainly underestimate the radiation from the moon's surfaceduring its long night if we take as the basis of our calculation alowering of temperature amounting to 100° F. During twelve hours, as notunfrequently occurs with us. Using these data--with Stefan's law ofdecrease of radiation as the 4th power of the temperature--amathematical friend finds that the temperature of the moon's surfacewould be reduced during the lunar night to nearly 200° F. Absolute(equal to-258° F. ). 

_More Rapid Loss of Heat by the Moon. _

Although such a calculation as the above may afford us a goodapproximation to the rate of loss of heat by Mars with its very scantyatmosphere, we have now good evidence that in the case of the moon theloss is much more rapid. Two independent workers have investigated thissubject with very accordant results--Dr. Boeddicker, with Lord Rosse's3-foot reflector and a Thermopile to measure the heat, and Mr. FrankVery, with a glass reflector of 12 inches diameter and the Bolometerinvented by Mr. Langley. The very striking and unexpected fact in whichthese observers agree is the sudden disappearance of much of thestored-up heat during the comparatively short duration of a totaleclipse of the moon--less than two hours of complete darkness, and abouttwice that period of partial obscuration. 

Dr. Boeddicker was unable to detect any appreciable heat at the periodof greatest obscuration; but, owing to the extreme sensitiveness of theBolometer, Mr. Very ascertained that those parts of the surface whichhad been longest in the shadow still emitted heat "to the amount of oneper cent. Of the heat to be expected from the full moon. " This howeveris the amount of radiation measured by the Bolometer, and to get thetemperature of the radiating surface we must apply Stefan's law of the4th power. Hence the temperature of the moon's dark surface will be the[fourth root of (1 over 100)] = 1 over 3. 2 [A] of the highest temperature (which we may take at the freezing-point, 491° F. Abs. ), or 154° F. Abs. , just below the liquefaction point of air. This is about 50° lower than theamount found by calculation from our most rapid radiation; and as thisamount is produced in a few hours, it is not too much to expect that, when continued for more than two weeks (the lunar night), it might reacha temperature sufficient to liquefy hydrogen (60° F. Abs. ), or perhapseven below it. 

[Note A: LaTex markup $\root 4 \of {1 \over 100} = {1 \over 3. 2}$ ]

_Theory of the Moon's Origin. _

This extremely rapid loss of heat by radiation, at first sight soimprobable as to be almost incredible, may perhaps be to some extentexplained by the physical constitution of the moon's surface, which, from a theoretical point of view, does not appear to have received theattention it deserves. It is clear that our satellite has been longsubjected to volcanic eruptions over its whole visible face, and thesehave evidently been of an explosive nature, so as to build up the verylofty cones and craters, as well as thousands of smaller ones, which, owing to the absence of any degrading or denuding agencies, haveremained piled up as they were first formed. 

This highly volcanic structure can, I think, be well explained by anorigin such as that attributed to it by Sir George Darwin, and which hasbeen so well described by Sir Robert Ball in his small volume, _Time andTide. _ These astronomers adduce strong evidence that the earth oncerotated so rapidly that the equatorial protuberance was almost at thepoint of separation from the planet as a ring. Before this occurred, however, the tension was so great that one large portion of theprotuberance where it was weakest broke away, and began to move aroundthe earth at some considerable distance from it. As about 1/50 of thebulk of the earth thus escaped, it must have consisted of a considerableportion of the solid crust and a much larger quantity of the liquid orsemi-liquid interior, together with a proportionate amount of the gaseswhich we know formed, and still form, an important part of the earth'ssubstance. 

As the surface layers of the earth must have been the lightest, theywould necessarily, when broken up by this gigantic convulsion, have cometogether to form the exterior of the new satellite, and be soon adjustedby the forces of gravity and tidal disturbance into a more or lessirregular spheroidal form, all whose interstices and cavities would befilled up and connected together by the liquid or semi-liquid massforced up between them. Thence-forward, as the moon increased itsdistance and reduced its time of rotation, in the way explained by SirRobert Ball, there would necessarily commence a process of escape of theimprisoned gases at every fissure and at all points and lines ofweakness, giving rise to numerous volcanic outlets, which, beingsubjected only to the small force of lunar gravity (only one-sixth thatof the earth), would, in the course of ages, pile up those giganticcones and ridges which form its great characteristic. 

But this small gravitative power of the moon would prevent its retainingon its surface any of the gases forming our atmosphere, which would allescape from it and probably be recaptured by the earth. By no process ofexternal aggregation of solid matter to such a relatively small amountas that forming the moon, even if the aggregation was so violent as toproduce heat enough to cause liquefaction, could any suchlong-continued volcanic action arise by gradual cooling, in the absenceof internal gases. There might be fissures, and even some outflows ofmolten rock; but without imprisoned gases, and especially without waterand water-vapour producing explosive outbursts, could any such amount ofscoriae and ashes be produced as were necessary for the building up ofthe vast volcanic cones, craters, and craterlets we see upon the moon'ssurface. 

I am not aware that either Sir Robert Ball or Sir George Darwin haveadduced this highly volcanic condition of the moon's surface as aphenomenon which can _only_ be explained by our satellite having beenthrown off a very much larger body, whose gravitative force wassufficient to acquire and retain the enormous quantity of gases and ofwater which we possess, and which are _absolutely essential_ for that_special form of cone-building volcanic action_ which the moon exhibitsin so pre-eminent a degree. Yet it seems to me clear, that some suchhypothetical origin for our satellite would have had to be assumed ifSir George Darwin had not deduced it by means of purely mathematicalargument based upon astronomical facts. 

Returning now to the problem of the moon's temperature, I think thephenomena this presents may be in part due to the mode of formation heredescribed. For, its entire surface being the result of long-continuedgaseous explosions, all the volcanic products--scoriae, pumice, andashes--would necessarily be highly porous throughout; and, never havingbeen compacted by water-action, as on the earth, and there having beenno winds to carry the finer dust so as to fill up their pores andfissures, the whole of the surface material to a very considerable depthmust be loose and porous to a high degree. This condition has beenfurther increased owing to the small power of gravity and the extremeirregularity of the surface, consisting very largely of lofty cones andridges very loosely piled up to enormous heights. 

Now this condition of the substance of the moon's surface is such aswould produce a high specific heat, so that it would absorb a largeamount of heat in proportion to the rise of temperature produced, theheat being conducted downwards to a considerable depth. Owing, however, to the total absence of atmosphere radiation would very rapidly cool thesurface, but afterwards more slowly, both on account of the action ofStefan's law and because the heat stored up in the deeper portions couldbe carried to the surface by conduction only, and with extreme slowness. 

_Very's Researches on the Moon's Heat. _

The results of the eclipse observations are supported by the detailedexamination of the surface-temperature of the moon by Mr. Very in his_Prize Essay on the Distribution of the Moon's Heat_ (published by theUtrecht Society of Arts and Sciences in 1891). He shows, by a diagram ofthe 'Phase-curve, ' that at the commencement of the Lunar day the surfacejust within the illuminated limb has acquired about 1/7 of its maximumtemperature, or about 70° F. Abs. As the surface exposed to theBolometer at each observation is about 1/30 of the moon's surface, andin order to ensure accuracy the instrument has to be directed to a spotlying wholly within the edge of the moon, it is evident that the surfacemeasured has already been for several hours exposed to oblique sunshine. The curve of temperature then rises gradually and afterwards morerapidly, till it attains its maximum (of about +30 to 40° F. ) a fewhours _before_ noon. This, Mr. Very thinks, is due to the fact that thehalf of the moon's face first illuminated for us has, on the average, adarker surface than that of the afternoon, or second quarter, duringwhich the curve descends not quite so rapidly, the temperature nearsunset being only a little higher than that near sunrise. This rapidfall while exposed to oblique sunshine is quite in harmony with therapid loss of heat during the few hours of darkness during an eclipse, both showing the prepotency of radiation over insolation on the moon. 

Two other diagrams show the distribution of heat at the time offull-moon, one half of the curve showing the temperatures along theequator from the edge of the disc to the centre, the other along ameridian from this centre to the pole. This diagram (here reproduced)exhibits the quick rise of temperature of the oblique rim of the moonand the nearly uniform heat of the central half of its surface; thediminution of heat towards the pole, however, is slower for the firsthalf and more rapid for the latter portion. 

It is an interesting fact that the temperature near the margin of thefull-moon increases towards the centre more rapidly than it does whenthe same parts are observed during the early phases of the firstquarter. Mr. Very explains this difference as being due to the fact thatthe full-moon to its very edges is fully illuminated, all the shadows ofthe ridges and mountains being thrown vertically or obliquely _behindthem. _ We thus measure the heat reflected from the _whole_ visiblesurface. But at new moon, and somewhat beyond the first quarter, thedeep shadows thrown by the smallest cones and ridges, as well as by theloftiest mountains, cover a considerable portion of the visible surface, thus largely reducing the quantity of light and heat reflected orradiated in our direction. It is only at the full, therefore, that themaximum temperature of the whole lunar surface can be measured. It mustbe considered a proof of the delicacy of the heat-measuring instrumentsthat this difference in the curves of temperature of the different partsof the moon's surface and under different conditions is so clearlyshown. 

_The Application of the Preceding Results to the Case of Mars. _

This somewhat lengthy account of the actual state of the moon's surfaceand temperature is of very great importance in our present enquiry, because it shows us the extraordinary difference in mean and extremetemperatures of two bodies situated at the same distance from the sun, and therefore receiving exactly the same amount of solar heat per unitof surface. We have learned also what are the main causes of this almostincredible difference, namely: (1) a remarkably rugged surface withporous and probably cavernous rock-texture, leading to extremely rapidradiation of heat in the one; as compared with a comparatively even andwell-compacted surface largely clad with vegetation, leading tocomparatively slow and gradual loss by radiation in the other: and (2), these results being greatly intensified by the total absence of aprotecting atmosphere in the former, while a dense and cloudy atmospherewith an ever-present supply of water-vapour, accumulates and equalisesthe heat received by the latter. 

The only other essential difference in the two bodies which may possiblyaid in the production of this marvellous result, is the fact of our dayand night having a mean length of 12 hours, while those of the moon areabout 14-1/2 of our days. But the altogether unexpected fact, in whichtwo independent enquirers agree, that during the few hours' duration ofa total eclipse of the moon so large a proportion of the heat is lost byradiation renders it almost certain that the resulting low temperaturewould be not very much less if the moon had a day and night the samelength as our own. 

The great lesson we learn by this extreme contrast of conditionssupplied to us by nature, as if to enable us to solve some of herproblems, is, the overwhelming importance, first, of a dense andwell-compacted surface, due to water-action and strong gravitativeforce; secondly, of a more or less general coat of vegetation; and, thirdly, of a dense vapour-laden atmosphere. These three favourableconditions result in a mean temperature of about +60° F. With a rangeseldom exceeding 40° above or below it, while over more than half theland-surface of the earth the temperature rarely falls below thefreezing point. On the other hand, we have a globe of the same materialsand at the same distance from the sun, with a maximum temperature offreezing water, and a minimum not very far from the absolute zero, themonthly mean being probably much below the freezing point ofcarbonic-acid gas--a difference entirely due to the absence of thesethree favourable conditions. 

_The Special Features of Mars as influencing Temperature. _

Coming now to the special feature of Mars and its probable temperature, we find that most writers have arrived at a very different conclusionfrom that of Mr. Lowell, who himself quotes Mr. Moulton as an authoritywho 'recently, by the application of Stefan's law, ' has found the meantemperature of this planet to be-35° F. Again, Professor J. H. Poynting, in his lecture on 'Radiation in the Solar System, ' delivered before theBritish Association at Cambridge in 1904, gave an estimate of the meantemperature of the planets, arrived at from measurements of the sun'semissive power and the application of Stefan's law to the distances ofthe several planets, and he thus finds the earth to have a meantemperature of 17° C. (=62-1/2° F. ) and Mars one of-38° C. (=-36-1/2°F. ), a wonderfully close approximation to the mean temperature of theearth as determined by direct measurement, and therefore, presumably, anequally near approximation to that of Mars as dependent on distance fromthe sun, and '_on the supposition that it is earth-like in all itsconditions. _'

But we know that it is far from being earth-like in the very conditionswhich we have found to be those which determine the extremely differenttemperatures of the earth, and moon; and, as regards each of these, weshall find that, so far as it differs from the earth, it approximates tothe less favourable conditions that prevail in the moon. The first ofthese conditions which we have found to be essential in regulating theabsorption and radiation of heat, and thus raising the mean temperatureof a planet, is a compact surface well covered with vegetation, twoconditions arising from, and absolutely dependent on, an ample amount ofwater. But Mr. Lowell himself assures us, as a fact of which he has nodoubt, that there are no permanent bodies of water, great or small, uponMars; that rain, and consequently rivers, are totally wanting; that itssky is almost constantly clear, and that what appear to be clouds arenot formed of water-vapour but of dust. He dwells, emphatically, on theterrible desert conditions of the greater part of the surface of theplanet. 

That being the case now, we have no right to assume that it has everbeen otherwise; and, taking full account of the fact, neither denied nordisputed by Mr. Lowell, that the force of gravity on Mars is notsufficient to retain water-vapour in its atmosphere, we must concludethat the surface of that planet, like that of the moon, has been mouldedby some form of volcanic action modified probably by wind, but not bywater. Adding to this, that the force of gravity on Mars is nearer thatof the moon than to that of the earth, and we may r reasonably concludethat its surface is formed of volcanic matter in a light and porouscondition, and therefore highly favourable for the rapid loss of surfaceheat by radiation. The surface-conditions of Mars are therefore, presumably, much more like those of the moon than like those of theearth. 

The next condition favourable to the storing up of heat--a covering ofvegetation--is almost certainly absent from Mars except, possibly, overlimited areas and for short periods. In this feature also the surface ofMars approximates much nearer to lunar than to earth-conditions. Thethird condition--a dense, vapour-laden atmosphere--is also wanting inMars. For although it possesses an atmosphere it is estimated by Mr. Lowell (in his latest article) to have a pressure equivalent to only2-1/2 inches of mercury with us, giving it a density of only one-twelfthpart that of ours; while aqueous vapour, the chief accumulator of heat, cannot permanently exist in it, and, notwithstanding repeatedspectroscopic observations for the purpose of detecting it, has neverbeen proved to exist. 

I submit that I have now shown from the statements--and largely as theresult of the long-continued observations--of Mr. Lowell himself, that, so far as the physical conditions of Mars are known to differ from thoseof the earth, the differences are all _unfavourable_ to the conservationand _favourable_ to the dissipation of the scanty heat it receives fromthe sun--that they point unmistakeably towards the temperatureconditions of the moon rather than to those of the earth, and that thecumulative effect of these adverse conditions, acting upon aheat-supply, reduced by solar distance to less than one-half of ours, _must_ result in a mean temperature (as well as in the extremes) nearerto that of our satellite than to that of our own earth. 

_Further Criticism of Mr. Lowell's Article. _

We are now in a position to test some further conclusions of Mr. Lowell's _Phil. Mag. _ article by comparison with actual phenomena. Wehave seen, in the outline I have given of this article, that heendeavours to show how the small amount of solar heat received by Marsis counterbalanced, largely by the greater transparency to light andheat of its thin and cloudless atmosphere, and partially also by agreater conservative or 'blanketing' power of its atmosphere due to thepresence in it of a large proportion of carbonic acid gas and aqueousvapour. The first of these statements may be admitted as a fact which heis entitled to dwell upon, but the second--the presence of largequantities of carbon-dioxide and aqueous vapour is a pure hypothesisunsupported by any item of scientific evidence, while in the case ofaqueous vapour it is directly opposed to admitted results founded uponthe molecular theory of gaseous elasticity. But, although Mr. Lowellrefers to the conservative or 'blanketing' effect of the earth'satmosphere, he does not consider or allow for its very great cumulativeeffect, as is strikingly shown by the comparison with the actualtemperature conditions of the moon. This cumulative effect is due to the_continuous_ reflection and radiation of heat from the clouds as well asfrom the vapour-laden strata of air in our lower atmosphere, whichlatter, though very transparent to the luminous and accompanying heatrays of the sun, are opaque to the dark heat-rays whether radiated orreflected from the earth's surface. We are therefore in a positionstrictly comparable with that of the interior of some huge glass house, which not only becomes intensely heated by the direct rays of the sun, but also to a less degree by reflected rays from the sky and thoseradiated from the clouds, so that even on a cloudy or misty day itstemperature rises many degrees above that of the outer air. Such abuilding, if of large size, of suitable form, and well protected atnight by blinds or other covering, might be so arranged as to accumulateheat in its soil and walls so as to maintain a tolerably uniformtemperature though exposed to a considerable range of external heat andcold. It is to such a power of accumulation of heat in our soil andlower atmosphere that we must impute the overwhelming contrast betweenour climate and that of the moon. With us, the solar heat thatpenetrates our vapour-laden and cloudy atmosphere is shut in by thatsame atmosphere, accumulates there for weeks and months together, andcan only slowly escape. It is this great cumulative power which Mr. Lowell has not taken account of, while he certainly has not estimatedthe enormous loss of heat by free radiation, which entirely neutralisesthe effects of increase of sun-heat, however great, when thesecumulative agencies are not present. [12]

[Footnote 12: The effects of this 'cumulative' power of a denseatmosphere are further discussed and illustrated in the last chapter ofthis book, where I show that the universal fact of steadily diminishingtemperatures at high altitudes is due solely to the diminution of thiscumulative power of our atmosphere, and that from this cause alone thetemperature of Mars must be that which would be found on a lofty plateauabout 18, 000 feet higher than the average of the peaks of the Andes!]

_Temperature on Polar Regions of Mars. _

There is also a further consideration which I think Mr. Lowell hasaltogether omitted to discuss. Whatever may be the _mean_ temperatureof Mars, we must take account of the long nights in its polar andhigh-temperate latitudes, lasting nearly twice as long as ours, with theresulting lowering of temperature by radiation into a constantly clearsky. Even in Siberia, in Lat. 67-1/2°N. A cold of-88°F. Has beenattained; while over a large portion of N. Asia and America above 60°Lat. The _mean_ January temperature is from-30°F. To-60°F. , and thewhole subsoil is permanently frozen from a depth of 6 or 7 feet toseveral hundreds. But the winter temperatures, _over the same latitudes_in Mars, must be very much lower; and it must require a proportionallylarger amount of its feeble sun-heat to raise the surface even to thefreezing-point, and an additional very large amount to melt anyconsiderable depth of snow. But this identical area, from a little below60° to the pole, is that occupied by the snow-caps of Mars, and over thewhole of it the winter temperature must be far lower than theearth-minimum of-88°F. Then, as the Martian summer comes on, there isless than half the sun-heat available to raise this low temperatureafter a winter nearly double the length of ours. And when the summerdoes come with its scanty sun-heat, that heat is not accumulated as itis by our dense and moisture-laden atmosphere, the marvellous effects ofwhich we have already shown. Yet with all these adverse conditions, eachassisting the other to produce a climate approximating to that which theearth would have if it had no atmosphere (but retaining our superiorityover Mars in receiving double the amount of sun-heat), we are asked toaccept a mean temperature for the more distant planet almost exactly thesame as that of mild and equable southern England, and a disappearanceof the vast snowfields of its polar regions as rapid and complete aswhat occurs with us! If the moon, even at its equator, has not itstemperature raised above the freezing-point of water, how can the more_distant_ Mars, with its _oblique_ noon-day sun falling upon thesnow-caps, receive heat enough, first to raise their temperature to 32°F. , and then to melt with marked rapidity the vast frozen plains of itspolar regions?

Mr. Lowell is however so regardless of the ordinary teachings ofmeteorological science that he actually accounts for the supposed mildclimate of the polar regions of Mars by the absence of water on itssurface and in its atmosphere. He concludes his fifth chapter with thefollowing words: "Could our earth but get rid of its oceans, we toomight have temperate regions stretching to the poles. " Here he runscounter to two of the best-established laws of terrestrial climatology--the wonderful equalising effects of warm ocean-currents which are thechief agents in diminishing polar cold; the equally striking effects ofwarm moist winds derived from these oceans, and the great storehouse ofheat we possess in our vapour-laden atmosphere, its vapour beingprimarily derived from these same oceans! But, in Mr. Lowell's opinion, all our meteorologists are quite mistaken. Our oceans are our greatdrawbacks. Only get rid of them and we should enjoy the exquisiteclimate of Mars--with its absence of clouds and fog, of rain or rivers, and its delightful expanses of perennial deserts, varied towards thepoles by a scanty snow-fall in winter, the melting of which might, withgreat care, supply us with the necessary moisture to grow wheat andcabbages for about one-tenth, or more likely one-hundredth, of ourpresent population. I hope I may be excused for not treating such anargument seriously. The various considerations now advanced, especiallythose which show the enormous cumulative and conservative effect of ourdense and water-laden atmosphere, and the disastrous effect--judging bythe actual condition of the moon--which the loss of it would have uponour temperature, seem to me quite sufficient to demonstrate importanterrors in the data or fallacies in the complex mathematical argument bywhich Mr. Lowell has attempted to uphold his views as to the temperatureand consequent climatic conditions of Mars. In concluding this portionof my discussion of the problem of Mars, I wish to call attention to thefact that my argument, founded upon a comparison of the physicalconditions of the earth and moon with those of Mars, is dependent upon asmall number of generally admitted scientific facts; while theconclusions drawn from those facts are simple and direct, requiring nomathematical knowledge to follow them, or to appreciate their weight andcogency. I claim for them, therefore, that they are in no degreespeculative, but in their data and methods exclusively scientific. Inthe next chapter I will put forward a suggestion as to how the verycurious markings upon the surface of Mars may possibly be interpreted, so as to be in harmony with the planet's actual physical condition andits not improbable origin and past history. 

CHAPTER VII. 

A SUGGESTION AS TO THE 'CANALS' OF MARS. 

The special characteristics of the numerous lines which intersect thewhole of the equatorial and temperate regions of Mars are, theirstraightness combined with their enormous length. It is this which hasled Mr. Lowell to term them 'non-natural features. ' Schiaparelli, in hisearlier drawings, showed them curved and of comparatively great width. Later, he found them to be straight fine lines when seen under the bestconditions, just as Mr. Lowell has always seen them in the pureatmosphere of his observatory. Both of these observers were at firstdoubtful of their reality, but persistent observation continued at manysuccessive oppositions compelled acceptance of them as actual featuresof the planet's disc. So many other observers have now seen them thatthe objection of unreality seems no longer valid. 

Mr. Lowell urges, however, that their perfect straightness, theirextreme tenuity, their uniformity throughout their whole length, thedual character of many of them, their relation to the 'oases' and theform and position of these round black spots, are all proofs ofartificiality and are suggestive of design. And considering that some ofthem are actually as long as from Boston to San Francisco, andrelatively to their globe as long as from London to Bombay, hisobjection that "no natural phenomena within our knowledge show suchregularity on such a scale" seems, at first, a mighty one. 

It is certainly true that we can point to nothing exactly like themeither on the earth or on the moon, and these are the only two planetarybodies we are in a position to compare with Mars. Yet even these do, Ithink, afford us some hints towards an interpretation of the mysteriouslines. But as our knowledge of the internal structure and past historyeven of our earth is still imperfect, that of the moon only conjectural, and that of Mars a perfect blank, it is not perhaps surprising that thesurface-features of the latter do not correspond with those of either ofthe others. 

_Mr. Pickering's Suggestion. _

The best clue to a natural interpretation of the strange features of thesurface of Mars is that suggested by the American astronomer Mr. W. H. Pickering in _Popular Astronomy_ (1904). Briefly it is, that both the'canals' of Mars and the rifts as well as the luminous streaks on themoon are cracks in the volcanic crust, caused by internal stresses dueto the action of the heated interior. These cracks he considers to besymmetrically arranged with regard to small 'craterlets' (Mr. Lowell's'oases') because they have originated from them, just as the whitestreaks on the moon radiate from the larger craters as centres. Hefurther supposes that water and carbon-dioxide issue from the interiorinto these fissures, and, in conjunction with sunlight, promote thegrowth of vegetation. Owing to the very rare atmosphere, the vapours, hethinks, would not ascend but would roll down the outsides of thecraterlets and along the borders of the canals, thus irrigating theimmediate vicinity and serving to promote the growth of some form ofvegetation which renders the canals and oases visible. [13]

[Footnote 13: _Nature_, vol. 70, p. 536. ]

This opinion is especially important because, next to Mr. Lowell, Mr. Pickering is perhaps the astronomer who has given most attention to Marsduring the last fifteen years. He was for some time at Flagstaff withMr. Lowell, and it was he who discovered the oases or craterlets, andwho originated the idea that we did not see the 'canals' themselves butonly the vegetable growth on their borders. He also observed Mars in theSouthern Hemisphere at Arequipa; and he has since made an elaboratestudy of the moon by means of a specially constructed telescope of 135feet focal length, which produced a direct image on photographic platesnearly 16 inches in diameter. [14]

[Footnote 14: _Nature_, vol. 70, May 5, p. Xi, supplement. ]

It is clear therefore that Mr. Lowell's views as to the artificialnature of the 'canals' of Mars are not accepted by an astronomer ofequal knowledge and still wider experience. Yet Professor Pickering'salternative view is more a suggestion than an explanation, because thereis no attempt to account for the enormous length and perfectstraightness of the lines on Mars, so different from anything that isfound either on our earth or on the moon. There must evidently be somegreat peculiarity of structure or of conditions on Mars to account forthese features, and I shall now attempt to point out what thispeculiarity is and how it may have arisen. 

_The Meteoritic Hypothesis. _

During the last quarter of a century a considerable change has come overthe opinions of astronomers as regards the probable origin of the SolarSystem. The large amount of knowledge of the stellar universe, andespecially of nebulae, of comets and of meteor-streams which we nowpossess, together with many other phenomena, such as the constitution ofSaturn's rings, the great number and extent of the minor planets, andgenerally of the vast amount of matter in the form of meteor-rings andmeteoric dust in and around our system, have all pointed to a differentorigin for the planets and their satellites than that formulated byLaplace as the Nebular hypothesis. 

It is now seen more clearly than at any earlier period, that most of theplanets possess special characteristics which distinguish them from oneanother, and that such an origin as Laplace suggested--the slow coolingand contraction of one vast sun-mist or nebula, besides presentinginherent difficulties--many think them impossibilities--in itself doesnot afford an adequate explanation of these peculiarities. Hence hasarisen what is termed the Meteoritic theory, which has been ablyadvocated for many years by Sir Norman Lockyer, and with someunimportant modifications is now becoming widely accepted. Briefly, thistheory is, that the planets have been formed by the slow aggregation ofsolid particles around centres of greatest condensation; but as many ofmy readers may be altogether unacquainted with it, I will here give avery clear statement of what it is, from Professor J. W. Gregory'spresidential address to the Geological Section of the BritishAssociation of the present year. He began by saying that these modernviews were of far more practical use to men of science than that ofLaplace, and that they give us a history of the world consistent withthe actual records of geology. He then continues:

"According to Sir Norman Lockyer's Meteoritic Hypothesis, nebulae, comets, and many so-called stars consist of swarms of meteorites which, though normally cold and dark, are heated by repeated collisions, and sobecome luminous. They may even be volatilised into glowing meteoricvapour; but in time this heat is dissipated, and the force of gravitycondenses a meteoritic swarm into a single globe. 'Some of the swarmsare, ' says Lockyer, 'truly members of the solar system, ' and some ofthese travel round the sun in nearly circular orbits, like planets. Theymay be regarded as infinitesimal planets, and so Chamberlain calls them'planetismals. '

"The planetismal theory is a development of the meteoritic theory, andpresents it in an especially attractive guise. It regards meteorites asvery sparsely distributed through space, and gravity as powerless tocollect them into dense groups. So it assigns the parentage of the solarsystem to a spiral nebula composed of planetismals, and the planets asformed from knots in the nebula, where many planetismals had beenconcentrated near the intersections of their orbits. These groups ofmeteorites, already as dense as a swarm of bees, were then packed closerby the influence of gravity, and the contracting mass was heated by thepressure, even above the normal melting-point of the material, which waskept rigid by the weight of the overlying layers. "

Now, adopting this theory as the last word of science upon the subjectof the origin of planets, we see that it affords immense scope fordiversity in results depending on the total _amount_ of matter availablewithin the range of attraction of an incipient planetary mass, and the_rates_ at which this matter becomes available. By a special combinationof these two quantities (which have almost certainly been different foreach planet) I think we may be able to throw some light upon thestructure and physical features of Mars. 

_The Probable Mode of Origin of Mars. _

This planet, lying between two of much greater mass, has evidently hadless material from which to be formed by aggregation; and if weassume--as in the absence of evidence to the contrary we have a right todo--that its beginnings were not much later (or earlier) than those ofthe earth, then its smaller size shows that it has in all probabilityaggregated very much more slowly. But the internal heat acquired by aplanet while forming in this manner will depend upon the rate at whichit aggregates and the velocity with which the planetismals' fall intoit, and this velocity will increase with its mass and consequent forceof gravity. In the early stages of a planet's growth it will probablyremain cold, the small amount of heat produced by each impact being lostby radiation before the next one occurs; and with a small and slowlyaggregating planet this condition will prevail till it approaches itsfull size. Then only will its gravitative force be sufficient to causeincoming matter to fall upon it with so powerful an impact as to produceintense heat. Further, the compressive force of a small planet will be aless effective heat-producing agency than in the case of a larger one. 

The earth we know has acquired a large amount of internal heat, probablysufficient to liquefy its whole interior; but Mars has only one-ninthpart the mass of the earth, and it is quite possible, and even probable, that its comparatively small attractive force would never have liquefiedor even permanently heated the more central portions of its mass. Thisbeing admitted, I suggest the following course of events as quitepossible, and not even improbable, in the case of this planet. Duringthe whole of its early growth, and till it acquired nearly its presentdiameter, its rate of aggregation was so slow that the planetismalsfalling upon it, though they might have been heated and even partiallyliquefied by the impact, were never in such quantity as to produce anyconsiderable heating effect on the whole mass, and each local rise oftemperature was soon lost by radiation. The planet thus grew as a solidand cold mass, compacted together by the impact of the incoming matteras well as by its slowly increasing gravitative force. But when it hadattained to within perhaps 100, perhaps 50 miles, or less, of itspresent diameter, a great change occurred in the opportunity for furthergrowth. Some large and dense swarm of meteorites, perhaps containing anumber of bodies of the size of the asteroids, came within the range ofthe sun's attraction and were drawn by it into an orbit which crossedthat of Mars at such a small angle that the planet was able at eachrevolution to capture a considerable number of them. The result mightthen be that, as in the case of the earth, the continuous inpour of thefresh matter first heated, and later on liquefied the greater part of itas well perhaps as a thin layer of the planet's original surface; sothat when in due course the whole of the meteor-swarm had been captured, Mars had acquired its present mass, but would consist of an intenselyheated, and either liquid or plastic thin outer shell resting upon acold and solid interior. 

The size and position of the two recently discovered satellites of Mars, which are believed to be not more than ten miles in diameter, the moreremote revolving around its primary very little slower than the planetrotates, while the nearer one, which is considerably less distant fromthe planet's surface than its own antipodes and revolves around it morethan three times during the Martian day, may perhaps be looked upon asthe remnants of the great meteor-swarm which completed the Martiandevelopment, and which are perhaps themselves destined at some distantperiod to fall into the planet. Should future astronomers witness thephenomenon the effect produced upon its surface would be full ofinstruction. 

As the result of such an origin as that suggested, Mars would possess astructure which, in the essential feature of heat-distribution, would bethe very opposite of that which is believed to characterise the earth, yet it might have been produced by a very slight modification of thesame process. This peculiar heat-distribution, together with a muchsmaller mass and gravitative force, would lead to a very differentdevelopment of the surface and an altogether diverse geological historyfrom ours, which has throughout been profoundly influenced by its heatedinterior, its vast supply of water, and the continuous physical andchemical reactions between the interior and the crust. 

These reactions have, in our case, been of substantially the samenature, and very nearly of the same degree of intensity throughout thewhole vast eons of geological time, and they have resulted in awonderfully complex succession of rock-formations--volcanic, plutonic, and sedimentary--more or less intermingled throughout the whole series, here remaining horizontal as when first deposited, there upheaved ordepressed, fractured or crushed, inclined or contorted; denuded by rainand rivers with the assistance of heat and cold, of frost and ice, in anunceasing series of changes, so that however varied the surface may be, with hill and dale, plains and uplands, mountain ranges and deepintervening valleys, these are as nothing to the diversities of interiorstructure, as exhibited in the sides of every alpine valley orprecipitous escarpment, and made known to us by the work of the minerand the well-borer in every part of the world. 

_Structural Straight Lines on the Earth. _

The great characteristic of the earth, both on its surface and in itsinterior, is thus seen to be extreme diversity both of form andstructure, and this is further intensified by the varied texture, constitution, hardness, and density of the various rocks and debris ofwhich it is composed. It is therefore not surprising that, with such acomplex outer crust, we should nowhere find examples of thosegeometrical forms and almost world-wide straight lines that give such aremarkable, and as Mr. Lowell maintains, 'non-natural' character to thesurface of Mars, but which, as it seems to me, of themselves afford_prima facie_ evidence of a corresponding simplicity and uniformity inits internal structure. 

Yet we are not ourselves by any means devoid of 'straight lines'structurally produced, in spite of every obstacle of diversity of formand texture, of softness and hardness, of lamination or crystallisation, which are adverse to such developments. Examples of these are thenumerous 'faults' which occur in the harder rocks, and which oftenextend for great distances in almost perfect straight lines. In our owncountry we have the Tyneside and Craven faults in the North of England, which are 30 miles long and often 20 yards wide; but even more strikingis the great Cleveland Dyke--a wall of volcanic rock dipping slightlytowards the south, but sometimes being almost vertical, and stretchingacross the country, over hill and dale, in an almost perfect straightline from a point on the coast ten miles north of Scarborough, in awest-by-north direction, passing about two miles south of Stockton andterminating about six miles north-by-east of Barnard Castle, a distanceof very nearly 60 miles. The great fault between the Highlands andLowlands of Scotland extends across the country from Stonehaven to nearHelensburgh, a distance of 120 miles; and there are very many more ofless importance. 

Much more extensive are some of the great continental dislocations, often forming valleys of considerable width and length. The Upper Rhineflows in one of these great valleys of subsidence for about 180 miles, from Mulhausen to Frankfort, in a generally straight line, thoughmodified by denudation. Vaster still is the valley of the Jordan throughthe Sea of Galilee to the Dead Sea, continued by the Wady Arabah to theGulf of Akaba, believed to form one vast geological depression orfracture extending in a straight line for 400 miles. 

Thousands of such faults, dykes, or depressions exist in every part ofthe world, all believed to be due to the gradual shrinking of the heatedinterior to which the solid crust has to accommodate itself, and theyare especially interesting and instructive for our present purpose asshowing the tendency of such fractures of solid rock-material to extendto great lengths in straight lines, notwithstanding the extremeirregularity both in the surface contour as well as in the internalstructures of the varied deposits and formations through which theypass. 

_Probable Origin of the Surface-features of Mars. _

Returning now to Mars, let us consider the probable course of eventsfrom the point at which we left it. The heat produced by impact andcondensation would be likely to release gases which had been incombination with some of the solid matter, or perhaps been itself in asolid state due to intense cold, and these, escaping outwards to thesurface, would produce on a small scale a certain amount of upheaval andvolcanic disturbance; and as an outer crust rapidly formed, a number ofvents might remain as craters or craterlets in a moderate state ofactivity. Owing to the comparatively small force of gravity, the outercrust would become scoriaceous and more or less permeated by the gases, which would continue to escape through it, and this would facilitate thecooling of the whole of the heated outer crust, and allow it to becomerather densely compacted. When the greater portion of the gases had thusescaped to the outer surface and assisted to form a scanty atmosphere, such as now exists, there would be no more internal disturbance and thecooling of the heated outer coating would steadily progress, resultingat last in a slightly heated, and later in a cold layer of moderatethickness and great general uniformity. Owing to the absence of rain andrivers, denudation such as we experience would be unknown, though thesuperficial scoriaceous crust might be partially broken up by expansionand contraction, and suffer a certain amount of atmospheric erosion. 

The final result of this mode of aggregation would be, that the planetwould consist of an outer layer of moderate thickness as compared withthe central mass, which outer layer would have cooled from a highlyheated state to a temperature considerably below the freezing-point, andthis would have been all the time _contracting upon a previously cold, and therefore non-contracting nucleus. _ The result would be that veryearly in the process great superficial tensions would be produced, whichcould only be relieved by cracks or fissures, which would initiate atpoints of weakness--probably at the craterlets already referred to--fromwhich they would radiate in several directions. Each crack thus formednear the surface would, as cooling progressed, develop in length anddepth; and owing to the general uniformity of the material, and possiblysome amount of crystalline structure due to slow and continuous coolingdown to a very low temperature, the cracks would tend to run on instraight lines and to extend vertically downwards, which twocircumstances would necessarily result in their forming portions of'great circles' on the planet's surface--the two great facts which Mr. Lowell appeals to as being especially 'non-natural. '

_Symmetry of Basaltic Columns. _

We have however one quite natural fact on our earth which serves toillustrate one of these two features, the direction of the downwardfissure. This is, the comparatively common phenomenon of basalticcolumns and 'Giant's Causeways. ' The wonderful regularity of these, andespecially the not unfrequent upright pillars in serried ranks, as inthe palisades of the Hudson river, must have always impressed observerswith their appearance of artificiality. Yet they are undoubtedly theresult of the very slow cooling and contraction of melted rocks undercompression by strata _below and above them_, so that, when oncesolidified, the mass was held in position and the tension produced bycontraction could only be relieved by numerous very small cracks atshort distances from each other in every direction, resulting in five, six, or seven-sided polygons, with sides only a few inches long. Thiscontraction began of course at the coolest surface, generally the upperone; and observation of these columns in various positions hasestablished the rule that their direction lengthways _is always at rightangles to the cooling surface_, and thus, whenever this surface washorizontal, the columns became almost exactly vertical. 

_How this applies to Mars. _

One of the features of the surface of Mars that Mr. Lowell describeswith much confidence is, that it is wonderfully uniform and level, whichof course it would be if it had once been in a liquid or plastic state, and not much disturbed since by volcanic or other internal movements. The result would be that cracks formed by contraction of the hardenedouter crust would be vertical; and, in a generally uniform material at avery uniform temperature, these cracks would continue almostindefinitely in straight lines. The hardened and contracting surfacebeing free to move laterally on account of there being a more heated andplastic layer below it, the cracks once initiated above wouldcontinually widen at the surface as they penetrated deeper and deeperinto the slightly heated substratum. Now, as basalt begins to soften atabout 1400° F. And the surface of Mars has cooled to at least thefreezing-point--perhaps very much below it--the contraction would be sogreat that if the fissures produced were 500 miles apart they might bethree miles wide at the surface, and, if only 100 miles apart, thenabout two-thirds of a mile wide. [15] But as the production of thefissures might have occupied perhaps millions of years, a considerableamount of atmospheric denudation would result, however slowly it acted. Expansion and contraction would wear away the edges and sides of thefissures, fill up many of them with the debris, and widen them at thesurfaces to perhaps double their original size. [16]

[Footnote 15: The coefficient of contraction of basalt is 0. 000006 for1° F. , which would lead to the results given here. ]

[Footnote 16: Mr. W. H. Pickering observed clouds on Mars 15 miles high;these are the 'projections' seen on the terminator when the planet ispartially illuminated. They were at first thought to be mountains; butduring the opposition of 1894, more than 400 of them were seen atFlagstaff during nine months' observation. Usually they are of rareoccurrence. They are seen to change in form and position from day today, and Mr. Lowell is strongly of opinion that they are dust-storms, not what we term clouds. They were mostly about 13 miles high, indicating considerable aerial disturbance on the planet, and thereforecapable of producing proportional surface denudation. ]

_Suggested Explanation of the 'Oases. '_

The numerous round dots seen upon the 'canals, ' and especially at pointsfrom which several canals radiate and where they intersect--termed'oases' by Mr. Lowell and 'craterlets' by Mr. Pickering may be explainedin two ways. Those from which several canals radiate may be true cratersfrom which the gases imprisoned in the heated surface layers havegradually escaped. They would be situated at points of weakness in thecrust, and become centres from which cracks would start duringcontraction. Those dots which occur at the crossing of two straightcanals or cracks may have originated from the fact that at suchintersections there would be four sharply-projecting angles, which, being exposed to the influence of alternate heat and cold (during dayand night) on the two opposite surfaces, would inevitably in time becomefractured and crumbled away, resulting in the formation of a roughlycircular chasm which would become partly filled up by the debris. Thoseformed by cracks radiating from craterlets would also be subject to thesame process of rounding off to an even greater extent; and thus wouldbe produced the 'oases' of various sizes up to 50 miles or more indiameter recorded by Mr. Lowell and other observers. 

_Probable Function of the Great Fissures. _

Mr. Pickering, as we have seen, supposes that these fissures give outthe gases which, overflowing on each side, favour the growth of thesupposed vegetation which renders the course of the canals visible, andthis no doubt may have been the case during the remote periods whenthese cracks gave access to the heated portions of the surface layer. But it seems more probable that Mars has now cooled down to the almostuniform mean temperature it derives from solar heat, and that thefissures--now for the most part broad shallow valleys--serve merely aschannels along which the liquids and heavy gases derived from themelting of the polar snows naturally flow, and, owing to their nearlylevel surfaces, overflow to a certain distance on each side of them. 

_Suggested Origin of the Blue Patches. _

These heavy gases, mainly perhaps, as has been often suggested, carbon-dioxide, would, when in large quantity and of considerable depth, reflect a good deal of light, and, being almost inevitably dust-laden, might produce that blue tinge adjacent to the melting snow-caps whichMr. Lowell has erroneously assumed to be itself a proof of the presenceof liquid water. Just as the blue of our sky is undoubtedly due toreflection from the ultra-minute dust particles in our higheratmosphere, similar particles brought down by the 'snow' from the higherMartian atmosphere might produce the blue tinge in the great volumes ofheavy gas produced by its evaporation or liquefaction. 

It may be noted that Mr. Lowell objects to the carbon-dioxide theory ofthe formation of the snow-caps, that this gas at low pressures does notliquefy, but passes at once from the solid to the gaseous state, andthat only water remains liquid sufficiently long to produce the bluecolour' which plays so large a part in his argument for the mild climateessential for an inhabited planet. But this argument, as I have alreadyshown, is valueless. For only very deep water can possibly show a bluecolour by reflected light, while a dust-laden atmosphere--especiallywith a layer of very dense gas at the bottom of it, as would be the casewith the newly evaporated carbon-dioxide from the diminishing snow-cap--would provide the very conditions likely to produce this blue tinge ofcolour. 

It may be considered a support to this view that carbonic-acid gasbecomes liquid at--140° F. And solid at--162° F. , temperatures farhigher than we should expect to prevail in the polar and north temperateregions of Mars during a considerable part of the year, but such asmight be reached there during the summer solstice when the `snows' sorapidly disappear, to be re-formed a few months later. 

_The Double Canals. _

The curious phenomena of the 'double canals' are undoubtedly the mostdifficult to explain satisfactorily on any theory that has yet beensuggested. They vary in distance apart from about 100 to 400 miles. Inmany cases they appear perfectly parallel, and Mr. Lowell gives us theimpression that they are almost always so. But his maps show, in somecases, decided differences of width at the two extremities, indicatingconsiderable want of parallelism. A few of the curved canals are alsodouble. 

There is one drawing in Mr. Lowell's book (p. 219) of the mouths, orstarting points, of the Euphrates and Phison, two widely separateddouble canals diverging at an angle of about 40° from the same twooases, so that the two inner canals cross each other. Now this suggeststwo wide bands of weakness in the planet's crust radiating probably fromwithin the dark tract called the 'Mare Icarium, ' and that somewidespread volcanic outburst initiated diverging cracks on either sideof these bands. Something of this kind may have been the cause of mostof the double canals, or they may have been started from two or morecraterlets not far apart, the direction being at first decided by somelocal peculiarity of structure; and where begun continuing in straightlines owing to homogeneity or uniform density of material. This is veryvague, but the phenomena are so remarkable, and so very imperfectlyknown at present, that nothing but suggestion can be attempted. 

_Concluding Remarks on the 'Canals. '_

In this somewhat detailed exposition of a possible, and, I hope, aprobable explanation of the surface-features of Mars, I haveendeavoured to be guided by known facts or accepted theories bothastronomical and geological. I think I may claim to have shown thatthere are some analogous features of terrestrial rock-structure toserve as guides towards a natural and intelligible explanation of thestrange geometric markings discovered during the last thirty years, andwhich have raised this planet from comparative obscurity into a positionof the very first rank both in astronomical and popular interest. 

This wide-spread interest is very largely due to Mr. Lowell's devotionto its study, both in seeking out so admirable a position as regardsaltitude and climate, and in establishing there a first-classobservatory; and also in bringing his discoveries before the public inconnection with a theory so startling as to compel attention. I ventureto think that his merit as one of our first astronomical observers willin no way be diminished by the rejection of his theory, and thesubstitution of one more in accordance with the actually observed facts. 

APPENDIX. 

_A Suggested Experiment to Illustrate the 'Canals' of Mars. _

If my explanation of the 'canals' should be substantially correct--thatis, if they were produced by the contraction of a heated outward crustupon a cold, and therefore non-contracting interior, the result of sucha condition might be shown experimentally. 

Several baked clay balls might be formed to serve as cores, say of 8 to10 inches in diameter. These being fixed within moulds of say half aninch to an inch greater diameter, the outer layer would be formed bypouring in some suitable heated liquid material, and releasing it fromthe mould as soon as consolidation occurs, so that it may cool rapidlyfrom the _outside. _ Some kinds of impure glass, or the brittle metalsbismuth or antimony or alloys of these might be used, in order to seewhat form the resulting fractures would take. It would be well to haveseveral duplicates of each ball, and, as soon as tension throughcontraction manifests itself, to try the effect of firing very smallcharges of small shot to ascertain whether such impacts would startradiating fractures. When taken from the moulds, the balls should besuspended in a slight current of air, and kept rotating, to reproducethe planetary condition as nearly as possible. 

The exact size and material of the cores, the thickness of the heatedouter crust, the material best suited to show fracture by contraction, and the details of their treatment, might be modified in various ways assuggested by the results first obtained. Such a series of experimentswould probably throw further light on the physical conditions which haveproduced the gigantic system of fissures or channels we see upon thesurface of Mars, though it would not, of course, prove that suchconditions actually existed there. In such a speculative matter we canonly be guided by probabilities, based upon whatever evidence isavailable. 

CHAPTER VIII. 

SUMMARY AND CONCLUSION. 

This little volume has necessarily touched upon a great variety ofsubjects, in order to deal in a tolerably complete manner with the veryextraordinary theories by which Mr. Lowell attempts to explain theunique features of the surface of the planet, which, by long-continuedstudy, he has almost made his own. It may therefore be well to sum upthe main points of the arguments against his view, introducing a fewother facts and considerations which greatly strengthen my argument. 

The one great feature of Mars which led Mr. Lowell to adopt the view ofits being inhabited by a race of highly intelligent beings, and, withever-increasing discovery to uphold this theory to the present time, isundoubtedly that of the so-called 'canals'--their straightness, theirenormous length, their great abundance, and their extension over theplanet's whole surface from one polar snow-cap to the other. The veryimmensity of this system, and its constant growth and extension duringfifteen years of persistent observation, have so completely takenpossession of his mind, that, after a very hasty glance at analogousfacts and possibilities, he has declared them to be 'non-natural'--therefore to be works of art--therefore to necessitate thepresence of highly intelligent beings who have designed and constructedthem. This idea has coloured or governed all his writings on thesubject. The innumerable difficulties which it raises have been eitherignored, or brushed aside on the flimsiest evidence. As examples, henever even discusses the totally inadequate water-supply for suchworldwide irrigation, or the extreme irrationality of constructing sovast a canal-system the waste from which, by evaporation, when exposedto such desert conditions as he himself describes, would use up tentimes the probable supply. 

Again, he urges the 'purpose' displayed in these 'canals. ' Their being_all_ so straight, _all_ describing great circles of the 'sphere, ' allbeing so evidently arranged (as he thinks) either to carry water to some'oasis' 2000 miles away, or to reach some arid region far over theequator in the opposite hemisphere! But he never considers thedifficulties this implies. Everywhere these canals run for thousands ofmiles across waterless deserts, forming a system and indicating apurpose, the wonderful perfection of which he is never tired of dwellingupon (but which I myself can nowhere perceive). 

Yet he never even attempts to explain how the Martians could have lived_before_ this great system was planned and executed, or why they did not_first_ utilise and render fertile the belt of land adjacent to thelimits of the polar snows--why the method of irrigation did not, as withall human arts, begin gradually, at home, with terraces and channels toirrigate the land close to the source of the water. How, with such adesert as he describes three-fourths of Mars to be, did the inhabitantsever get to _know_ anything of the equatorial regions and its needs, soas to start right away to supply those needs? All this, to my mind, isquite opposed to the idea of their being works of art, and altogether infavour of their being natural features of a globe as peculiar in originand internal structure as it is in its surface-features. The explanationI have given, though of course hypothetical, is founded on knowncosmical and terrestrial facts, and is, I suggest, far more scientificas well as more satisfactory than Mr. Lowell's wholly unsupportedspeculation. This view I have explained in some detail in the precedingchapter. 

Mr. Lowell never even refers to the important question of loss byevaporation in these enormous open canals, or considers the undoubtedfact that the only intelligent and practical way to convey a limitedquantity of water such great distances would be by a system ofwater-tight and air-tight tubes laid _under the ground. _ The mereattempt to use open canals for such a purpose shows complete ignoranceand stupidity in these alleged very superior beings; while it is certainthat, long before half of them were completed their failure to be of anyuse would have led any rational beings to cease constructing them. 

He also fails to consider the difficulty, that, if these canals arenecessary for existence in Mars, how did the inhabitants ever reach asufficiently large population with surplus food and leisure enablingthem to rise from the low condition of savages to one of civilisation, and ultimately to scientific knowledge? Here again is a dilemma which ishard to overcome. Only a _dense_ population with _ample_ means ofsubsistence could possibly have constructed such gigantic works; but, given these two conditions, no adequate motive existed for theconception and execution of them--even if they were likely to be of anyuse, which I have shown they could not be. 

_Further Considerations on the Climate of Mars. _

Recurring now to the question of climate, which is all-important, Mr. Lowell never even discusses the essential point--the temperature thatmust _necessarily_ result from an atmospheric envelope one-twelfth (orat most one-seventh) the density of our own; in either casecorresponding to an altitude far greater than that of our highestmountains. [17] Surely this phenomenon, everywhere manifested on theearth even under the equator, of a regular decrease of temperature withaltitude, the only cause of which is a less dense atmosphere, shouldhave been fairly grappled with, and some attempt made to show why itshould not apply to Mars, except the weak remark that on a level surfaceit will not have the same effect as on exposed mountain heights. But it_does_ have the same effect, or very nearly so, on our lofty plateauxoften hundreds of miles in extent, in proportion to their altitude. Quito, at 9350 ft. Above the sea, has a mean temperature of about 57°F. , giving a lowering of 23° from that of Manaos at the mouth of the RioNegro. This is about a degree for each 400 feet, while the general fallfor isolated mountains is about one degree in 340 feet according toHumboldt, who notes the above difference between the rate of cooling foraltitude of the plains--or more usually sheltered valleys in which thetowns are situated--and the exposed mountain sides. It will be seen thatthis lower rate would bring the temperature of Mars at the equator downto 20° F. Below the freezing point of water from this cause alone. 

[Footnote 17: A four inches barometer is equivalent to a height of40, 000 feet above sea-level with us. ]

But all enquirers have admitted, that if conditions as to atmospherewere the same as on the earth, its greater distance from the sun wouldreduce the temperature to-31° F. , equal to 63° below the freezingpoint. It is therefore certain that the combined effect of both causesmust bring the temperature of Mars down to at least 70° or 80°below thefreezing point. 

The cause of this absolute dependence of terrestrial temperatures upondensity of the air-envelope is seldom discussed in text-books either ofgeography or of physics, and there seems to be still some uncertaintyabout it. Some impute it wholly to the thinner air being unable toabsorb and retain so much heat as that which is more dense; but if thiswere the case the soil at great altitudes not having so much of its heattaken up by the air should be warmer than below, since it undoubtedly_receives_ more heat owing to the greater transparency of the air aboveit; but it certainly does not become warmer. The more correct view seemsto be that the loss of heat by radiation is increased so much throughthe rarity of the air above it as to _more_ than counterbalance theincreased insolation, so that though the surface of the earth at a givenaltitude may receive 10 per cent. More direct sun-heat it loses bydirect radiation, combined with diminished air and cloud-radiation, perhaps 20 or 25 per cent. More, whence there is a resultant coolingeffect of 10 or 15 per cent. This acts by day as well as by night, sothat the greater heat received at high altitudes does not warm the soilso much as a less amount of heat with a denser atmosphere. 

This effect is further intensified by the fact that a less dense cannotabsorb and transmit so much heat as a more dense atmosphere. Here thenwe have an absolute law of nature to be observed operating everywhere onthe earth, and the mode of action of which is fairly well understood. This law is, that reduced atmospheric pressure increases radiation, orloss of heat, _more rapidly_ than it increases insolation or gain ofheat, so that the result is _always_ a considerable _lowering_ oftemperature. What this lowering is can be seen in the universal fact, that even within the tropics perpetual snow covers the higher mountainsummits, while on the high plains of the Andes, at 15, 000 or 16, 000 feetaltitude, where there is very little or no snow, travellers are oftenfrozen to death when delayed by storms; yet at this elevation theatmosphere has much more than double the density of that of Mars!

The error in Mr. Lowell's argument is, that he claims for the scantyatmosphere of Mars that it allows more sun-heat to reach the surface;but he omits to take account of the enormously increased loss of heat bydirect radiation, as well as by the diminution of air-radiation, whichtogether necessarily produce a great reduction of temperature. 

It is this great principle of the prepotency of radiation overabsorption with a diminishing atmosphere that explains the excessivelylow temperature of the moon's surface, a fact which also serves toindicate a very low temperature for Mars, as I have shown in Chapter VI. These two independent arguments--from alpine temperatures and from thoseof the moon--support and enforce each other, and afford a conclusiveproof (as against anything advanced by Mr. Lowell) that the temperatureof Mars must be far too low to support animal life. 

A third independent argument leading to the same result is Dr. JohnstoneStoney's proof that aqueous vapour cannot exist on Mars; and this factMr. Lowell does not attempt to controvert. 

To put the whole case in the fewest possible words:

All physicists are agreed that, owing to the distance of Mars from thesun, it would have a mean temperature of about-35° F. (= 456° F. Abs. )even if it had an atmosphere as dense as ours. 

(2) But the very low temperatures on the earth under the equator, at aheight where the barometer stands at about three times as high as onMars, proves, that from scantiness of atmosphere alone Mars cannotpossibly have a temperature as high as the freezing point of water; andthis proof is supported by Langley's determination of the low _maximum_temperature of the full moon. 

The combination of these two results must bring down the temperature ofMars to a degree wholly incompatible with the existence of animal life. 

(3) The quite independent proof that water-vapour cannot exist on Mars, and that therefore, the first essential of organic life--water--isnon-existent. 

The conclusion from these three independent proofs, which enforce eachother in the multiple ratio of their respective weights, is thereforeirresistible--that animal life, especially in its higher forms, cannotexist on the planet. 

Mars, therefore, is not only uninhabited by intelligent beings such asMr. Lowell postulates, but is absolutely UNINHABITABLE.