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+The Project Gutenberg EBook of Is Mars Habitable?, by Alfred Russel Wallace
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Is Mars Habitable?
+
+Author: Alfred Russel Wallace
+
+Release Date: January 28, 2004 [EBook #10855]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK IS MARS HABITABLE? ***
+
+
+
+
+Produced by Thaadd and the PG Distributed Proofreading Team
+
+
+
+
+_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 Professor
+Percival Lowell's book, _Mars and its Canals_, with the object of
+showing that the large amount of new and interesting facts contained in
+this work did not invalidate the conclusion I had reached in 1902, and
+stated in my book on _Man's Place in the Universe_, that Mars was not
+habitable.
+
+But the more complete presentation of the opposite view in the volume
+now under discussion required a more detailed examination of the various
+physical 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 last
+Professor Lowell published in the _Philosophical Magazine_ an elaborate
+mathematical article claiming to demonstrate that, notwithstanding its
+much greater distance from the sun and its excessively thin atmosphere,
+Mars possessed a climate on the average equal to that of the south of
+England, and in its polar and sub-polar regions even less severe than
+that of the earth. Such a contention of course required to be dealt
+with, and led me to collect information bearing upon temperature in all
+its aspects, and so enlarging my criticism that I saw it would be
+necessary to issue it in book form.
+
+Two of my mathematical friends have pointed out the chief omission which
+vitiates Professor Lowell's mathematical conclusions--that of a failure
+to recognise the very large conservative and _cumulative_ effect of a
+dense atmosphere. This very point however I had already myself discussed
+in Chapter VI., and by means of some remarkable researches on the heat
+of the moon and an investigation of the causes of its very low
+temperature, I have, I think, demonstrated the incorrectness of Mr.
+Lowell's results. In my last chapter, in which I briefly summarise the
+whole argument, I have further strengthened the case for very severe
+cold in Mars, by adducing the rapid lowering of temperature universally
+caused by diminution of atmospheric pressure, as manifested in the
+well-known phenomenon of temperate climates at moderate heights even
+close to the equator, cold climates at greater heights even on extensive
+plateaux, culminating in arctic climates and perpetual snow at heights
+where 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 enforced
+by two others equally complete, neither of which is adequately met by
+Mr. Lowell.
+
+The careful examination which I have been led to give to the whole of
+the phenomena which Mars presents, and especially to the discoveries of
+Mr. Lowell, has led me to what I hope will be considered a satisfactory
+physical explanation of them. This explanation, which occupies the whole
+of my seventh chapter, is founded upon a special mode of origin for
+Mars, derived from the Meteoritic Hypothesis, now very widely adopted by
+astronomers and physicists. Then, by a comparison with certain
+well-known and widely spread geological phenomena, I show how the great
+features of Mars--the 'canals' and 'oases'--may have been caused. This
+chapter will perhaps be the most interesting to the general reader, as
+furnishing a quite natural explanation of features of the planet which
+have been termed 'non-natural' by Mr. Lowell.
+
+Incidentally, also, I have been led to an explanation of the highly
+volcanic nature of the moon's surface. This seems to me absolutely to
+require some such origin as Sir George Darwin has given it, and thus
+furnishes corroborative proof of the accuracy of the hypothesis that our
+moon has had an unique origin among the known satellites, in having been
+thrown off from the earth itself.
+
+I am indebted to Professor J. H. Poynting, of the University of
+Birmingham, for valuable suggestions on some of the more difficult
+points of mathematical physics here discussed, and also for the critical
+note (at the end of Chapter V.) on Professor Lowell's estimate of the
+temperature 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 of
+the Solar system the only one whose solid surface has been seen with
+certainty is Mars; and, very fortunately, that is also the only one
+which is sufficiently near to us for the physical features of the
+surface to be determined with any accuracy, even if we could see it in
+the other planets. Of Venus we probably see only the upper surface of
+its cloudy atmosphere.[1] As regards Jupiter and Saturn this is still
+more certain, since their low density will only permit of a
+comparatively small proportion of their huge bulk being solid. Their
+belts are but the cloud-strata of their upper atmosphere, perhaps
+thousands of miles above their solid surfaces, and a somewhat similar
+condition seems to prevail in the far more remote planets Uranus and
+Neptune. It has thus happened, that, although as telescopic objects of
+interest and beauty, the marvellous rings of Saturn, the belts and
+ever-changing aspects of the satellites of Jupiter, and the moon-like
+phases of Venus, together with its extreme brilliancy, still remain
+unsurpassed, yet the greater amount of details of these features when
+examined with the powerful instruments of the nineteenth century have
+neither added much to our knowledge of the planets themselves or led to
+any sensational theories calculated to attract the popular imagination.
+
+[Footnote 1: Mercury also seems to have a scanty atmosphere, but as its
+mass is only one-thirtieth that of the earth it can retain only the
+heavier 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 periods
+unchangeable in position, indicating that the planet keeps the same face
+towards the sun as does Venus. This was confirmed by Mr. Lowell in 1896.
+Its distance from us and unfavourable position for observation must
+prevent us from obtaining any detailed knowledge of its actual surface,
+though its low reflective power indicates that the surface may be really
+visible.]
+
+But in the case of Mars the progress of discovery has had a very
+different result. The most obvious peculiarity of this planet--its polar
+snow-caps--were seen about 250 years ago, but they were first proved to
+increase and decrease alternately, in the summer and winter of each
+hemisphere, by Sir William Herschell in the latter part of the
+eighteenth century. This fact gave the impulse to that idea of
+similarity in the conditions of Mars and the earth, which the
+recognition of many large dusky patches and streaks as water, and the
+more ruddy and brighter portions as land, further increased. Added to
+this, a day only about half an hour longer than our own, and a
+succession of seasons of the same character as ours but of nearly double
+the length owing to its much longer year, seemed to leave little wanting
+to render this planet a true earth on a smaller scale. It was therefore
+very natural to suppose that it must be inhabited, and that we should
+some day obtain evidence of the fact.
+
+_The Canals discovered by Schiaparelli._
+
+Hence the great interest excited when Schiaparelli, at the Milan
+Observatory, during the very favourable opposition of 1877 and 1879,
+observed that the whole of the tropical and temperate regions from 60 deg.
+N. to 60 deg. S. Lat. were covered with a remarkable network of broader
+curved and narrower straight lines of a dark colour. At each successive
+favourable opposition, these strange objects called _canali_ (channels)
+by their discoverer, but rather misleadingly 'canals' in England and
+America, were observed by means of all the great telescopes in the
+world, and their reality and general features became well established.
+In Schiaparelli's first map they were represented as being much broader
+and less sharply defined than he himself and other observers found by
+later and equally favourable observations that they really were.
+
+_Discovery of the Double Canals._
+
+In 1881 another strange feature was discovered by Schiaparelli, who
+found that about twenty canals which had previously been seen single
+were now distinctly double, that is, that they consisted of two parallel
+lines, equally distinct and either very close together or a considerable
+distance apart. This curious appearance was at first thought to be due
+to some instrumental defect or optical illusion; but as it was soon
+confirmed by other observers with the best instruments and in widely
+different localities it became in time accepted as a real phenomenon of
+the planet's surface.
+
+_Round Spots discovered in_ 1892.
+
+At the favourable opposition of 1892, Mr. W. H. Pickering noticed that
+besides the 'seas' of various sizes there were numerous very small black
+spots apparently quite circular and occurring at every intersection or
+starting-point of the 'canals.' Many of these had been seen by
+Schiaparelli as larger and ill-defined dark patches, and were termed
+seas or lakes; but Mr. Pickering's observatory was at Arequipa in Peru,
+about 8000 feet above the sea, and with such perfect atmospheric
+conditions as were, in his opinion, equal to a doubling of telescopic
+aperture. 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 regions
+of the planet, are any number of dark round spots. How many there may be
+it is not possible to state, as the better the seeing, the more of them
+there seem to be. In spite, however, of their great number, there is no
+instance of one unconnected with a canal. What is more, there is
+apparently none that does not lie at the junction of several canals.
+Reversely, all the junctions appear to be provided with spots. Plotted
+upon a globe they and their connecting canals make a most curious
+network over all the orange-ochre equatorial parts of the planet, a mass
+of 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 a
+regular course of change occurred dependent upon the succession of the
+seasons, as had been first suggested by Schiaparelli. As the polar snows
+melt the adjacent seas appear to overflow and spread out as far as the
+tropics, and are often seen to assume a distinctly green colour. These
+remarkable changes and the extraordinary phenomena of perfect straight
+lines crossing each other over a large portion of the planet's surface,
+with the circular spots at their intersections, had such an appearance
+of artificiality that the idea that they were really 'canals' made by
+intelligent beings for purposes of irrigation, was first hinted at, and
+then adopted as the only intelligible explanation, by Mr. Lowell and a
+few other persons. This at once seized upon the public imagination and
+was spread by the newspapers and magazines over the whole civilised
+world.
+
+_Existence of Seas doubted._
+
+At this time (1894) it began to be doubted whether there were any seas
+at all on Mars. Professor Pickering thought they were far more limited
+in size than had been supposed, and even might not exist as true seas.
+Professor Barnard, with the Lick thirty-six inch telescope, discerned an
+astonishing wealth of detail on the surface of Mars, so intricate,
+minute, and abundant, that it baffled all attempts to delineate it; and
+these peculiarities were seen upon the supposed seas as well as on the
+land-surfaces. In fact, under the best conditions these 'seas' lost all
+trace of uniformity, their appearance being that of a mountainous
+country, broken by ridges, rifts, and canyons, seen from a great
+elevation. As we shall see later on these doubts soon became
+certainties, and it is now almost universally admitted that Mars
+possesses 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 in
+Arizona, in a very dry and uniform climate, and at an elevation of 7300
+feet above the sea. He then possessed a fine equatorial telescope of 18
+inches aperture and 26 feet focal length, besides two smaller ones, all
+of the best quality. To these he added in 1896 a telescope with 24 inch
+object glass, the last work of the celebrated firm of Alvan Clark &
+Sons, with which he has made his later discoveries. He thus became
+perhaps more favourably situated than any astronomer in the northern
+hemisphere, and during the last twelve years has made a specialty of the
+study of Mars, besides doing much valuable astronomical work on other
+planets.
+
+_Mr, Lowell's recent Books upon Mars._
+
+In 1905 Mr. Lowell published an illustrated volume giving a full account
+of his observations of Mars from 1894 to 1903, chiefly for the use of
+astronomers; and he has now given us a popular volume summarising the
+whole of his work on the planet, and published both in America and
+England by the Macmillan Company. This very interesting volume is fully
+illustrated with twenty plates, four of them coloured, and more than
+forty figures in the text, showing the great variety of details from
+which the larger general maps have been constructed.
+
+_Non-natural Features of Mars._
+
+But what renders this work especially interesting to all intelligent
+readers is, that the author has here, for the first time, fully set
+forth his views both as to the habitability of Mars and as to its being
+actually inhabited by beings comparable with ourselves in intellect. The
+larger part of the work is in fact devoted to a detailed description of
+what 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 the
+varying visibility of both, depending partly on the Martian seasons;
+while the five concluding chapters deal with the possibility of animal
+life and the evidence in favour of it. He also upholds the theory of the
+canals having been constructed for the purpose of 'husbanding' the
+scanty water-supply that exists; and throughout the whole of this
+argument he clearly shows that he considers the evidence to be
+satisfactory, and that the only intelligible explanation of the whole of
+the phenomena he so clearly sets forth is, that the inhabitants of Mars
+have carried out on their small and naturally inhospitable planet a vast
+system of irrigation-works, far greater both in its extent, in its
+utility, and its effect upon their world as a habitation for civilised
+beings, than anything we have yet done upon our earth, where our
+destructive agencies are perhaps more prominent than those of an
+improving and recuperative character.
+
+_A Challenge to the Thinking World._
+
+This volume is therefore in the nature of a challenge, not so much to
+astronomers as to the educated world at large, to investigate the
+evidence for so portentous a conclusion. To do this requires only a
+general acquaintance with modern science, more especially with mechanics
+and physics, while the main contention (with which I shall chiefly deal)
+that the features termed 'canals' are really works of art and
+necessitate the presence of intelligent organic beings, requires only
+care and judgment in drawing conclusions from admitted facts. As I have
+already paid some attention to this problem and have expressed the
+opinion that Mars is not habitable,[2] judging from the evidence then
+available, and as few men of science have the leisure required for a
+careful examination of so speculative a subject, I propose here to point
+out what the facts, as stated by Mr. Lowell himself, do _not_ render
+even probable much less prove. Incidentally, I may be able to adduce
+evidence of a more or less weighty character, which seems to negative
+the possibility of any high form of animal life on Mars, and, _a
+fortiori_, the development of such life as might culminate in a being
+equal or superior to ourselves. As most popular works on Astronomy for
+the last ten years at least, as well as many scientific periodicals and
+popular magazines, have reproduced some of the maps of Mars by
+Schiaparelli, Lowell, and others, the general appearance of its surface
+will be familiar to most readers, who will thus be fully able to
+appreciate Mr. Lowell's account of his own further discoveries which I
+may have to quote. One of the _best_ of these maps I am able to give as
+a 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 various
+favourable occasions to detect a network of straight lines, meeting or
+crossing each other at various angles, and often extending to a thousand
+or even over two thousand miles in length. They are seen to cross both
+the light and the dark regions of the planet's surface, often extending
+up to or starting from the polar snow-caps. Most of these lines are so
+fine as only to be visible on special occasions of atmospheric clearness
+and steadiness, which hardly ever occur at lowland stations, even with
+the best instruments, and almost all are seen to be as perfectly
+straight as if drawn with a ruler.
+
+_The Double Canals._
+
+Under exceptionally favourable conditions, many of the lines that have
+been already seen single appear double--a pair of equally fine lines
+exactly parallel throughout their whole length, and appearing, as Mr.
+Lowell says, "clear cut upon the disc, its twin lines like the rails of
+a railway track." Both Schiaparelli and Lowell were at first so
+surprised at this phenomenon that they thought it must be an optical
+illusion, and it was only after many observations in different years,
+and by the application of every conceivable test, that they both became
+convinced that they witnessed a real feature of the planet's surface.
+Mr. Lowell says he has now seen them hundreds of times, and that his
+first view of one was 'the most startlingly impressive' sight he has
+ever witnessed.
+
+_Dimensions of the Canals._
+
+A few dimensions of these strange objects must be given in order that
+readers may appreciate their full strangeness and inexplicability. Out
+of more than four hundred canals seen and recorded by Mr. Lowell,
+fifty-one, or about one eighth, are either constantly or occasionally
+seen to be double, the appearance of duplicity being more or less
+periodical. Of 'canals' generally, Mr. Lowell states that they vary in
+length from a few hundred to a few thousand miles long, one of the
+largest being the Phison, which he terms 'a typical double canal,' and
+which is said to be 2250 miles long, while the distance between its two
+constituents is about 130 miles.[3] The actual width of each canal is
+from a minimum of about a mile up to several miles, in one case over
+twenty. A great feature of the doubles is, that they are strictly
+parallel throughout their whole course, and that in almost all cases
+they are so truly straight as to form parts of a great circle of the
+planet's sphere. A few however follow a gradual but very distinct curve,
+and such of these as are double present the same strict parallelism as
+those which are straight.
+
+[Footnote 3: This is on the opposite side of Mars from that shown in the
+frontispiece.]
+
+_Canals extend across the Seas._
+
+It was only after seventeen years of observation of the canals that it
+was found that they extended also into and across the dark spots and
+surfaces which by the earlier observers were termed seas, and which then
+formed the only clearly distinguishable and permanent marks on the
+planet's surface. At the present time, Professor Lowell states that this
+"curious triangulation has been traced over almost every portion of the
+planet's surface, whether dark or light, whether greenish, ochre, or
+brown in colour." In some parts they are much closer together than in
+others, "forming a perfect network of lines and spots, so that to
+identify them all was a matter of extreme difficulty." Two such portions
+are figured at pages 247 and 256 of Mr. Lowell's volume.
+
+_The Oases._
+
+The curious circular black spots which are seen at the intersections of
+many of the canals, and which in some parts of the surface are very
+numerous, are said to be more difficult of detection than even the
+lines, being often blurred or rendered completely invisible by slight
+irregularities in our own atmosphere, while the canals themselves
+continue visible. About 180 of these have now been found, and the more
+prominent of them are estimated to vary from 75 to 100 miles in
+diameter. There are however many much smaller, down to minute and barely
+visible black points. Yet they all seem a little larger than the canals
+which enter them. Where the canals are double, the spots (or 'oases' as
+Mr. Lowell terms them) lie between the two parallel canals.
+
+No one can read this book without admiration for the extreme
+perseverance in long continued and successful observation, the results
+of which are here recorded; and I myself accept unreservedly the
+substantial accuracy of the whole series. It must however always be
+remembered that the growth of knowledge of the detailed markings has
+been very gradual, and that much of it has only been seen under very
+rare and exceptional conditions. It is therefore quite possible that, if
+at some future time a further considerable advance in instrumental power
+should be made, or a still more favourable locality be found, the new
+discoveries might so modify present appearances as to render a
+satisfactory explanation of them more easy than it is at present.
+
+But though I wish to do the fullest justice to Mr. Lowell's technical
+skill and long years of persevering work, which have brought to light
+the most complex and remarkable appearances that any of the heavenly
+bodies present to us, I am obliged absolutely to part company with him
+as regards the startling theory of artificial production which he thinks
+alone adequate to explain them. So much is this the case, that the very
+phenomena, which to him seem to demonstrate the intervention of
+intelligent beings working for the improvement of their own environment,
+are those which seem to me to bear the unmistakable impress of being due
+to natural forces, while they are wholly unintelligible as being useful
+works 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 own
+interpretation later on.
+
+
+
+CHAPTER III.
+
+
+THE CLIMATE AND PHYSIOGRAPHY OF MARS.
+
+Mr. Lowell admits, and indeed urges strongly, that there are no
+permanent 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 are
+rare, even if they exist, the appearances that have been taken for them
+being either dust-storms or a surface haze; that there is consequently
+no rain, and that large portions (about two-thirds) of the planet's
+surface 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 the
+polar snows, which in the winter cover the arctic and about half the
+temperate regions of each hemisphere alternately. The maximum of the
+northern snow-caps is reached at a period of the Martian winter
+corresponding to the end of February with us. About the end of March the
+cap begins to shrink in size (in the Northern Hemisphere), and this goes
+on so rapidly that early in the June of Mars it is reduced to its
+minimum. About the same time changes of colour take place in the
+adjacent darker portions of the surface, which become at first bluish,
+and later a decided blue-green; but by far the larger portion, including
+almost all the equatorial regions of the planet, remain always of a
+reddish-ochre tint.[4]
+
+[Footnote 4: In 1890 at Mount Wilson, California, Mr. W.H. Pickering's
+photographs of Mars on April 9th showed the southern polar cap of
+moderate dimensions, but with a large dim adjacent area. Twenty-four
+hours later a corresponding plate showed this same area brilliantly
+white; the result apparently of a great Martian snowfall. In 1882 the
+same observer witnessed the steady disappearance of 1,600,000 square
+miles of the southern snow-cap, an area nearly one-third of that
+hemisphere of the planet.]
+
+The rapid and comparatively early disappearance of the white covering
+is, very reasonably, supposed to prove that it is of small thickness,
+corresponding perhaps to about a foot or two of snow in north-temperate
+America and Europe, and that by the increasing amount of sun-heat it is
+converted, partly into liquid and partly into vapour. Coincident with
+this disappearance and as a presumed result of the water (or other
+liquid) producing inundations, the bluish-green tinge which appears on
+the previously dark portion of the surface is supposed to be due to a
+rapid 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 at
+successive Martian dates from December 30th to February 21st, not only
+is a considerable extent of the south temperate zone shown to change
+rapidly from bluish-green to chocolate-brown and then again to
+bluish-green, but the portions furthest from the supposed fertilising
+overflow are permanently green, as are also considerable portions in the
+opposite or northern hemisphere, which one would think would then be
+completely 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 wonderfully
+smooth and level. Not only are there no mountains, but there are no
+hills or valleys or plateaux. This assumption is absolutely essential to
+support the other great assumption, that the wonderful network of
+perfectly straight lines over nearly the whole surface of the planet are
+irrigation canals. It is not alleged that irregularities or undulations
+of a few hundreds or even one or two thousands of feet could possibly be
+detected, while certainly all we know of planetary formation or
+structure point strongly towards _some_ inequalities of surface. Mr.
+Lowell admits that the dark portions of the surface, when examined on
+the terminator (the margin of the illuminated portion), do _look_ like
+hollows and _may be_ the beds of dried-up seas; yet the supposed canals
+run across these old sea-beds in perfect straight lines just as they do
+across the many thousand miles of what are admitted to be deserts--which
+he describes in these forcible terms: "Pitiless as our deserts are, they
+are but faint forecasts of the state of things existent on Mars at the
+present time."
+
+It appears, then, that Mr. Lowell has to face this dilemma--_Only if the
+whole surface of Mars is an almost perfect level could the enormous
+network of straight canals, each from hundreds to thousands of miles
+long, have been possibly constructed by intelligent beings for purposes
+of irrigation; but, if a complete and universal level surface exists no
+such system would be necessary._ For on a level surface--or on a
+surface slightly inclined from the poles towards the equator, which
+would be advantageous in either case--the melting water would of itself
+spread over the ground and naturally irrigate as much of the surface as
+it was possible for it to reach. If the surface were not level, but
+consisted of slight elevations and expressions to the extent of a few
+scores or a few hundreds of feet, then there would be no possible
+advantage in cutting straight troughs through these elevations in
+various directions with water flowing at the bottom of them. In neither
+case, and in hardly any conceivable case, could these perfectly straight
+canals, cutting across each other in every direction and at very varying
+angles, be of any use, or be the work of an intelligent race, if any
+such race could possibly have been developed under the adverse
+conditions which exist in Mars.
+
+_The Scanty Water-supply._
+
+But further, if there were any superfluity of water derived from the
+melting snow beyond what was sufficient to moisten the hollows indicated
+by the darker portions of the surface, which at the time the water
+reaches them acquire a green tint (a superfluity under the circumstances
+highly 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 a
+terrible desert region and exposed to such a cloudless sky as Mr. Lowell
+describes, would be the work of a body of madmen rather than of
+intelligent beings. It may be safely asserted that not one drop of water
+would escape evaporation or insoak at even a hundred miles from its
+source. [5]
+
+[Footnote 5: What the evaporation is likely to be in Mars may be
+estimated by the fact, stated by Professor J.W. Gregory in his recent
+volume on 'Australia' in _Stanford's Compendium_, that in North-West
+Victoria evaporation is at the rate of ten feet per annum, while in
+Central Australia it is very much more. The greatly diminished
+atmospheric pressure in Mars will probably more than balance the loss of
+sun-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 historian
+of modern astronomy, the late Miss Agnes Clerke. In the _Edinburgh
+Review_ (of October 1896) there is an article entitled 'New Views about
+Mars,' exhibiting the writer's characteristic fulness of knowledge and
+charm of style. Speaking of Mr. Lowell's idea of the 'canals' carrying
+the surplus water across the equator, far into the opposite hemisphere,
+for purposes of irrigation there (which we see he again states in the
+present volume), Miss Clerke writes: "We can hardly imagine so shrewd a
+people as the irrigators of Thule and Hellas[6] wasting labour, and the
+life-giving fluid, after so unprofitable a fashion. There is every
+reason to believe that the Martian snow-caps are quite flimsy
+structures. Their material might be called snow _souffle_, since, owing
+to the small power of gravity on Mars, snow is almost three times
+lighter there than here. Consequently, its own weight can have very
+little effect in rendering it compact. Nor, indeed, is there time for
+much settling down. The calotte does not form until several months after
+the winter solstice, and it begins to melt, as a rule, shortly after the
+vernal equinox. (The interval between these two epochs in the southern
+hemisphere of Mars is 176 days.) The snow lies on the ground, at the
+outside, a couple of months. At times it melts while it is still fresh
+fallen. Thus, at the opposition of 1881-82 the spreading of the northern
+snows 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 proportions
+in a single night.
+
+[Footnote 6: Areas on Mars so named.]
+
+"No attempt has yet been made to estimate the quantity of water
+derivable from the melting of one of these formations; yet the
+experiment is worth trying as a help towards defining ideas. Let us
+grant that the average depth of snow in them, of the delicate Martian
+kind, is twenty feet, equivalent at the most to one foot of water. The
+maximum area covered, of 2,400,000 square miles, is nearly equal to that
+of the United States, while the whole globe of Mars measures 55,500,000
+square miles, of which one-third, on the present hypothesis, is under
+cultivation, and in need of water. Nearly the whole of the dark areas,
+as we know, are situated in the southern hemisphere, of which they
+extend 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 be
+made available for their fertilisation, supposing them to get the entire
+advantage of the spring freshet. Upon a stint of less than two inches of
+water these fertile lands are expected to flourish and bear abundant
+crops; and since they completely enclose the polar area they are
+necessarily served first. The great emissaries for carrying off the
+surplus of their aqueous riches, would then appear to be superfluous
+constructions, nor is it likely that the share in those riches due to
+the canals and oases, intricately dividing up the wide, dry, continental
+plains, can ever be realised.
+
+"We have assumed, in our little calculation, that the entire contents of
+a polar hood turn to water; but in actual fact a considerable proportion
+of them must pass directly into vapour, omitting the intermediate stage.
+Even with us a large quantity of snow is removed aerially; and in the
+rare atmosphere of Mars this cause of waste must be especially
+effective. Thus the polar reservoirs are despoiled in the act of being
+opened. Further objections might be taken to Mr. Lowell's irrigation
+scheme, but enough has been said to show that it is hopelessly
+unworkable."
+
+It will be seen that the writer of this article accepted the existence
+of water on Mars, on the testimony of Sir W. Huggins, which, in view of
+later observations, he has himself acknowledged to be valueless. Dr.
+Johnstone Stoney's proof of its absence, derived from the molecular
+theory 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 on
+Mercator's projection, showing the canals and other features seen during
+the opposition of 1905. This contains many canals not shown on the map
+here reproduced (see frontispiece), and some of the differences between
+the two are very puzzling. Looking at our map, which shows the
+north-polar snow below, so that the south pole is out of the view at the
+top 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 named
+Ulysses, which here passes far to the right of the spot, but in the
+large chart enters it centrally. Looking at our map we see, going
+downwards a little to the left, the canal Udon, which runs through a
+dark area quite to the outer margin. In the dark area, however, there is
+shown on the chart a spot Aspledon Lucus, where five canals meet, and if
+this is taken as a terminus the Udon canal is almost exactly 2000 miles
+long, and another on its right, Lapadon, is the same length, while Ich,
+running in a slightly curved line to a large spot (Lucus Castorius on
+the chart) is still longer. The Ulysses canal, which (on the chart) runs
+straight from the point of the Mare Sirenum to the Astraeeus Lucus is
+about 2200 miles long. Others however are even longer, and Mr. Lowell
+says: "With them 2000 miles is common; while many exceed 2500; and the
+Eumenides-Orcus is 3540 miles from the point where it leaves Lucus
+Phoeniceus to where it enters the Trivium Charontis." This last canal is
+barely visible on our map, its commencement being indicated by the word
+Eumenides.
+
+The Trivium Charontis is situated just beyond the right-hand margin of
+our 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 radiate
+thirteen canals. Another centre is Aquae Calidae situated at the point
+of a dark area running obliquely from 55 deg. to 35 deg. N. latitude, and, as
+shown on a map of the opposite hemisphere to our map, has nearly twenty
+canals radiating from it in almost every direction. Here at all events
+there seems to be no special connection with the polar snow-caps, and
+the radiating lines seem to have no intelligent purpose whatever, but
+are such as might result from fractures in a glass globe produced by
+firing at it with very small shots one at a time. Taking the whole
+series of them, Mr. Lowell very justly compares them to "a network which
+triangulates the surface of the planet like a geodetic survey, into
+polygons of all shapes and sizes."
+
+At the very lowest estimate the total length of the canals observed and
+mapped by Mr. Lowell must be over a hundred thousand miles, while he
+assures 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 fix
+their position with certainty. But these, being of the same character
+and evidently forming part of the same system, must also be artificial,
+and thus we are led to a system of irrigation of almost unimaginable
+magnitude on a planet which has no mountains, no rivers, and no rain to
+support it; whose whole water-supply is derived from polar snows, the
+amount of which is ludicrously inadequate to need any such world-wide
+system; while the low atmospheric pressure would lead to rapid
+evaporation, thus greatly diminishing the small amount of moisture that
+is available. Everyone must, I think, agree with Miss Clerke, that, even
+admitting the assumption that the polar snows consist of frozen water,
+the excessively scanty amount of water thus obtained would render any
+scheme 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 their
+intersections, are doubtless all connected in some unknown way with the
+constitution and past history of the planet; but, on the theory of the
+whole being works of art, they certainly do _not_ help to remove any of
+the difficulties which have been shown to attend the theory that the
+single lines represent artificial canals of irrigation with a strip of
+verdure 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 own
+words as to the supposed perfectly level surface of Mars, and his
+interpretation of the origin and purpose of the 'canals':
+
+"A body of planetary size, if unrotating, becomes a sphere, except for
+solar tidal deformation; if rotating, it takes on a spheroidal form
+exactly expressive, so far as observation goes, of the so-called
+centrifugal force at work. Mars presents such a figure, being flattened
+out to correspond to its axial rotation. Its surface therefore is in
+fluid equilibrium, or, in other words, a particle of liquid at any point
+of its surface at the present time would stay where it was devoid of
+inclination to move elsewhere. Now the water which quickens the verdure
+of the canals moves from the pole down to the equator as the season
+advances. This it does then irrespective of gravity. No natural force
+propels it, and the inference is forthright and inevitable that it is
+artificially helped to its end. There seems to be no escape from this
+deduction. Water only flows downhill, and there is no such thing as
+downhill on a surface already in fluid equilibrium. A few canals might
+presumably be so situated that their flow could, by inequality of
+terrane, lie equatorward, but not all....Now it is not in particular but
+by general consent that the canal-system of Mars develops from pole to
+equator. From the respective times at which the minima take place, it
+appears that the canal quickening occupies fifty-two days, as evidenced
+by the successive vegetal darkenings, to descend from latitude 72 deg. north
+to latitude 0 deg., a journey of 2650 miles. This gives for the water a
+speed of fifty-one miles a day, or 2.1 miles an hour. The rate of
+progression is remarkably uniform, and this abets the deduction as to
+assisted transference. But the fact is more unnatural yet. The growth
+pays no regard to the equator, but proceeds across it as if it did not
+exist into the planet's other hemisphere. Here is something still more
+telling than travel to this point. For even if we suppose, for the sake
+of argument, that natural forces took the water down to the equator,
+their action must there be certainly reversed, and the equator prove a
+dead-line, to pass which were impossible" (pp. 374-5).
+
+I think my readers will agree with me that this whole argument is one of
+the most curious ever put forth seriously by an eminent man of science.
+Because the polar compression of Mars is about what calculation shows it
+ought to be in accordance with its rate of rotation, its surface is in a
+state of 'fluid equilibrium,' and must therefore be absolutely level
+throughout. But the polar compression of the earth equally agrees with
+calculation; therefore its surface is also in 'fluid equilibrium';
+therefore it also ought to be as perfectly level on land as it is on the
+ocean surface! But as we know this is very far from being the case, why
+must it be so in Mars? Are we to suppose Mars to have been formed in
+some totally different way from other planets, and that there neither is
+nor ever has been any reaction between its interior and exterior forces?
+Again, the assumption of perfect flatness is directly opposed to all
+observation and all analogy with what we see on the earth and moon. It
+gives no account whatever of the numerous and large dark patches, once
+termed seas, but now found to be not so, and to be full of detailed
+markings and varied depths of shadow. To suppose that these are all the
+same dead-level as the light-coloured portions are assumed to be,
+implies that the darkness is one of material and colour only, not of
+diversified contour, which again is contrary to experience, since
+difference of material with us always leads to differences in rate of
+degradation, and hence of diversified contour, as these dark spaces
+actually show themselves under favourable conditions to independent
+observers.
+
+_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 canals
+form. Instead of running at hap-hazard, the canals are interconnected in
+a most remarkable manner. They seek centres instead of avoiding them.
+The centres are linked thus perfectly one with another, an arrangement
+which could not result from centres, whether of explosion or otherwise,
+which were themselves discrete. Furthermore, the system covers the whole
+surface of the planet, dark areas and light ones alike, a world-wide
+distribution which exceeds the bounds of natural possibility. Any force
+which could act longitudinally on such a scale must be limited
+latitudinally in its action, as witness the belts of Jupiter and the
+spots upon the sun. Rotational, climatic, or other physical cause could
+not fail of zonal expression. Yet these lines are grandly indifferent to
+such competing influences. Finally, the system, after meshing the
+surface in its entirety, runs straight into the polar caps.
+
+"It is, then, a system whose end and aim is the tapping of the snow-cap
+for the water there semi-annually let loose; then to distribute it over
+the planet's face" (p. 373).
+
+Here, again, we have curiously weak arguments adduced to support the
+view that these numerous straight lines imply works of art rather than
+of nature, especially in the comparison made with the belts of Jupiter
+and the spots on the sun, both purely atmospheric phenomena, whereas the
+lines on Mars are on the solid surface of the planet. Why should there
+be any resemblance between them? Every fact stated in the above
+quotation, always keeping in mind the physical conditions of the
+planet--its very tenuous atmosphere and rainless desert-surface--seem
+wholly 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 to
+me to be at least possible, and to explain the whole series of the
+phenomena set forth and largely discovered by Mr. Lowell, in a simpler
+and more probable manner than does his tremendous assumption of their
+being works of art. Readers who may not possess Mr. Lowell's volume will
+find 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's
+observations, and provisionally accepting all his chief conclusions as
+to the climate, the nature of the snow-caps, the vegetation, and the
+animal life of Mars, yet his interpretation of the lines on its surface
+as being veritably 'canals,' constructed by intelligent beings for the
+special purpose of carrying water to the more arid regions, is wholly
+erroneous and rationally inconceivable. I now proceed to discuss his
+more fundamental position as to the actual habitability of Mars by a
+highly organised and intellectual race of material organic beings.
+
+_Water and Air essential to Life._
+
+Here, fortunately, the issue is rendered very simple, because Mr. Lowell
+fully recognises the identity of the constitution of matter and of
+physical laws throughout the solar-system, and that for any high form of
+organic life certain conditions which are absolutely essential on our
+earth must also exist in Mars. He admits, for example, that water is
+essential, that an atmosphere containing oxygen, nitrogen, aqueous
+vapour, and carbonic acid gas is essential, and that an abundant
+vegetation is essential; and these of course involve a
+surface-temperature through a considerable portion of the year that
+renders the existence of these--especially of water--possible and
+available 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 is
+not very successful. He adduces evidence of an atmosphere, but of an
+exceedingly scanty one, since the greatest amount he can give to it is--
+"not more than about four inches of barometric pressure as we reckon
+it";[7] and he assumes, as he has a fair right to do till disproved,
+that it consists of oxygen and nitrogen, with carbon-dioxide and
+water-vapour, in approximately the same proportions as with us. With
+regard to the last item--the water-vapour--there are however many
+serious difficulties. The water-vapour of our atmosphere is derived from
+the enormous area of our seas, oceans, lakes, and rivers, as well as
+from the evaporation from heated lands and tropical forests of much of
+the moisture produced by frequent and abundant rains. All these sources
+of supply are admittedly absent from Mars, which has no permanent bodies
+of water, no rain, and tropical regions which are almost entirely
+desert. Many writers have therefore doubted the existence of water in
+any form upon this planet, supposing that the snow-caps are not formed
+of frozen water but of carbon-dioxide, or some other heavy gas, in a
+frozen 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 snows
+of the polar caps producing water is, that at the time they are melting
+a marginal blue band appears which accompanies them in their retreat,
+and this blue colour is said to prove conclusively that the liquid is
+not carbonic acid but water. This point he dwells upon repeatedly,
+stating, of these blue borders: "This excludes the possibility of their
+being formed by carbon-dioxide, and shows that of all the substances we
+know the material composing them must be water."
+
+[Footnote 7: In a paper written since the book appeared the density of
+air 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 is
+certainly a most extraordinary and futile one. For it is perfectly well
+known that although water, in large masses and by transmitted light, is
+of a blue colour, yet shallow water by reflected light is not so; and in
+the case of the liquid produced by the snow-caps of Mars, which the
+whole conditions of the planet show must be shallow, and also be more or
+less turbid, it cannot possibly be the cause of the 'deep blue' tint
+said to result from the melting of the snow.
+
+But there is a very weighty argument depending on the molecular theory
+of gases against the polar caps of Mars being composed of frozen water
+at all. The mass and elastic force of the several gases is due to the
+greater or less rapidity of the vibratory motion of their molecules
+under identical conditions. The speed of these molecular motions has
+been ascertained for all the chief gases, and it is found to be so great
+as in certain cases to enable them to overcome the force of gravity and
+escape from a planet's surface into space. Dr. G. Johnstone Stoney has
+specially investigated this subject, and he finds that the force of
+gravity on the earth is sufficient to retain all the gases composing its
+atmosphere, but not sufficient to retain hydrogen; and as a consequence,
+although this gas is produced in small quantities by volcanoes and by
+decomposing 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 a
+planet must have a mass at least a quarter that of the earth. But the
+mass of Mars is only one-ninth that of the earth; therefore, unless
+there are some special conditions that prevent its loss, this gas cannot
+be present in the atmosphere. Mr. Lowell does not refer to this argument
+against his view, neither does he claim the evidence of spectroscopy in
+his favour. This was alleged more than thirty years ago to show the
+existence of water-vapour in the atmosphere of Mars, but of late years
+it has been doubted, and Mr. Lowell's complete silence on the subject,
+while laying stress on such a very weak and inconclusive argument as
+that from the tinge of colour that is observed a little distance from
+the edge of the diminishing snow-caps, shows that he himself does not
+think the fact to be thus proved. If he did he would hardly adduce such
+an argument for its presence as the following: "The melting of the caps
+on the one hand and their re-forming on the other affirm the presence of
+water-vapour in the Martian atmosphere, of whatever else that air
+consists" (p. 162). Yet absolutely the only proof he gives that the caps
+are frozen water is the almost frivolous colour-argument above referred
+to!
+
+_No Spectroscopic Evidence of Water Vapour._
+
+As Sir William Huggins is the chief authority quoted for this fact, and
+is referred to as being almost conclusive in the third edition of Miss
+Clerke's _History of Astronomy_ in 1893, I have ascertained that his
+opinion at the present time is that "there is no conclusive proof of the
+presence of aqueous vapour in the atmosphere of Mars, and that
+observations at the Lick Observatory (in 1895), where the conditions and
+instruments are of the highest order, were negative." He also informs me
+that Marchand at the Pic du Midi Observatory was unable to obtain lines
+of aqueous vapour in the spectrum of Mars; and that in 1905, Slipher, at
+Mr. Lowell's observatory, was unable to detect any indications of
+aqueous vapour in the spectrum of Mars.
+
+It thus appears that spectroscopic observations are quite accordant with
+the calculations founded on the molecular theory of gases as to the
+absence of aqueous vapour, and therefore presumably of liquid water,
+from Mars. It is true that the spectroscopic argument is purely
+negative, and this may be due to the extreme delicacy of the
+observations required; but that dependent on the inability of the force
+of gravity to retain it is positive scientific evidence against its
+presence, and, till shown to be erroneous, must be held to be
+conclusive.
+
+This absence of water is of itself conclusive against the existence of
+animal life, unless we enter the regions of pure conjecture as to the
+possibility of some other liquid being able to take its place, and that
+liquid being as omnipresent there as water is here. Mr. Lowell however
+never takes this ground, but bases his whole theory on the fundamental
+identity of the substance of the bodies of living organisms wherever
+they may exist in the solar system. In the next two chapters I shall
+discuss an equally essential condition, that of temperature, which
+affords a still more conclusive and even crushing argument against the
+suitability 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, the
+actual temperatures on Mars due to its distance from the sun and the
+atmospheric conditions on which he himself lays so much stress. If I am
+not greatly mistaken we shall arrive at conclusions on this subject
+which are absolutely fatal to the conception of any high form of organic
+life being possible on this planet.
+
+_The Problem of Terrestrial Temperatures._
+
+In order that the problem may be understood and its importance
+appreciated, it is necessary to explain the now generally accepted
+principles as to the causes which determine the temperatures on our
+earth, and, presumably, on all other planets whose conditions are not
+wholly unlike ours. The fact of the internal heat of the earth which
+becomes very perceptible even at the moderate depths reached in mines
+and deep borings, and in the deepest mines becomes a positive
+inconvenience, leads many people to suppose that the surface-
+temperatures of the earth are partly due to this cause. But it is now
+generally admitted that this is not the case, the reason being that all
+rocks and soils, in their natural compacted state, are exceedingly bad
+conductors of heat.
+
+A striking illustration of this is the fact, that a stream of lava often
+continues to be red hot at a few feet depth for years after the surface
+is consolidated, and is hardly any warmer than that of the surrounding
+land. A still more remarkable case is that of a glacier on the
+south-east side of the highest cone of Etna underneath a lava stream
+with an intervening bed of volcanic sand only ten feet thick. This was
+visited by Sir Charles Lyell in 1828, and a second time thirty years
+later, when he made a very careful examination of the strata, and was
+quite satisfied that the sand and the lava stream together had actually
+preserved this mass of ice, which neither the heat of the lava above it
+at its first outflow, nor the continued heat rising from the great
+volcano below it, had been able to melt or perceptibly to diminish in
+thirty years. Another fact that points in the same direction is the
+existence over the whole floor of the deepest oceans of ice-cold water,
+which, originating in the polar seas, owing to its greater density sinks
+and creeps slowly along the ocean bottom to the depths of the Atlantic
+and Pacific, and is not perceptibly warmed by the internal heat of the
+earth.
+
+Now the solid crust of the earth is estimated as at least about twenty
+miles in average thickness; and, keeping in mind the other facts just
+referred to, we can understand that the heat from the interior passes
+through it with such extreme slowness as not to be detected at the
+surface, the varying temperatures of which are due entirely to solar
+heat. A large portion of this heat is stored up in the surface soil, and
+especially in the surface layer of the oceans and seas, thus partly
+equalising the temperatures of day and night, of winter and summer, so
+as greatly to ameliorate the rapid changes of temperature that would
+otherwise occur. Our dense atmosphere is also of immense advantage to us
+as an equaliser of temperature, charged as it almost always is with a
+large quantity of water-vapour. This latter gas, when not condensed into
+cloud, allows the solar heat to pass freely to the earth; but it has the
+singular and highly beneficial property of absorbing and retaining the
+dark or lower-grade heat given off by the earth which would otherwise
+radiate into space much more rapidly. We must therefore always remember
+that, very nearly if not quite, the _whole_ of _the warmth we experience
+on the earth is derived from the sun._[8]
+
+[Footnote 8: Professor J.H. Poynting, in his lecture to the British
+Association at Cambridge in 1904, says: "The surface of the earth
+receives, we know, an amount of heat from the inside almost
+infinitesimal compared with that which it receives from the sun, and on
+the sun, therefore, we depend for our temperature."]
+
+In order to understand the immense significance of this conclusion we
+must know what is meant by the _whole_ heat or warmth; as unless we know
+this we cannot define what half or any other proportion of sun-heat
+really means. Now I feel pretty sure that nine out of ten of the average
+educated public would answer the following question incorrectly: The
+mean temperature of the southern half of England is about 48 deg. F.
+Supposing the earth received only half the sun-heat it now receives,
+what would then be the probable mean temperature of the South of
+England? The majority would, I think, answer at once--About 24 deg. F.
+Nearly as many would perhaps say--48 deg. F. is 16 deg. above the freezing
+point; therefore half the heat received would bring us down to 8 deg. above
+the freezing point, or 40 deg. F. Very few, I think, would realise that our
+share of half the amount of sun-heat received by the earth would
+probably result in reducing our mean temperature to about 100 deg. F. below
+the freezing point, and perhaps even lower. This is about the very
+lowest temperature yet experienced on the earth's surface. To understand
+how such results are obtained a few words must be said about the
+absolute zero of temperature.
+
+_The Zero of Temperature._
+
+Heat is now believed to be entirely due to ether-vibration, which
+produces a correspondingly rapid vibration of the molecules of matter,
+causing it to expand and producing all the phenomena we term 'heat.' We
+can conceive this vibration to increase indefinitely, and thus there
+would appear to be no necessary limit to the amount of heat possible,
+but we cannot conceive it to decrease indefinitely at the same uniform
+rate, as it must soon inevitably come to nothing. Now it has been found
+by experiment that gases under uniform pressure expand 1/273 of their
+volume for each degree Centigrade of increased temperature, so that in
+passing from 0 deg. C. to 273 deg. C. they are doubled in volume. They also
+decrease in volume at the same rate for each degree below 0 deg. C. (the
+freezing point of water). Hence if this goes on to-273 deg. C. a gas will
+have no volume, or it will undergo some change of nature. Hence this is
+called the zero of temperature, or the temperature to which any matter
+falls which receives no heat from any other matter. It is also sometimes
+called the temperature of space, or of the ether in a state of rest, if
+that is possible. All the gases have now been proved to become, first
+liquid and then (most of them) solid, at temperatures considerably above
+this zero.
+
+The only way to compare the proportional temperatures of bodies, whether
+on the earth or in space, is therefore by means of a scale beginning at
+this natural zero, instead of those scales founded on the artificial
+zero of the freezing point of water, or, as in Fahrenheit's, 32 deg. below
+it. Only by using the natural zero and measuring continuously from it
+can we estimate temperatures in relative proportion to the amount of
+heat received. This is termed the absolute zero, and so that we start
+reckoning from that point it does not matter whether the scale adopted
+is 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 amount
+of solar heat that we receive, owing to its greater distance from the
+sun, and if the mean temperature of our earth is 60 deg. F., this is equal
+to 551 deg. F. on the absolute scale. It would therefore appear very simple
+to halve this amount and obtain 275.5 deg. F. as the mean temperature of
+that planet. But this result is erroneous, because the actual amount of
+sun heat intercepted by a planet is only one condition out of many that
+determine its resulting temperature. Radiation, that is loss of heat, is
+going on concurrently with gain, and the rate of loss varies with the
+temperature according to a law recently discovered, the loss being much
+greater at high temperatures in proportion to the 4th power of the
+absolute temperature. Then, again, the whole heat intercepted by a
+planet does not reach its surface unless it has no atmosphere. When it
+has one, much is reflected or absorbed according to complex laws
+dependent on the density and composition of the atmosphere. Then, again,
+the heat that reaches the actual surface is partly reflected and partly
+absorbed, according to the nature of that surface--land or water, desert
+or forest or snow-clad--that part which is absorbed being the chief
+agent in raising the temperature of the surface and of the air in
+contact with it. Very important too is the loss of heat by radiation
+from these various heated surfaces at different rates; while the
+atmosphere itself sends back to the surface an ever varying portion of
+both this radiant and reflected heat according to distinct laws. Further
+difficulties arise from the fact that much of the sun's heat consists of
+dark or invisible rays, and it cannot therefore be measured by the
+quantity of light only.
+
+From this rough statement it will be seen that the problem is an
+exceedingly complex one, not to be decided off-hand, or by any simple
+method. It has in fact been usually considered as (strictly speaking)
+insoluble, and only to be estimated by a more or less rough
+approximation, or by the method of general analogy from certain known
+facts. 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 taking
+the presence of water and water-vapour in Mars as proved by the
+behaviour of the snow-caps and the bluish colour that results from their
+melting, has deduced a temperature above the freezing point of water, as
+prevalent in the equatorial regions permanently, and in the temperate
+and 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 and
+founded on no valid evidence, he has now, in the _London, Edinburgh, and
+Dublin Philosophical Magazine_ of July 1907, published an elaborate
+paper of 15 pages, entitled _A General Method for Evaluating the
+Surface-Temperatures of the Planets; with special reference to the
+Temperature of Mars_, by Professor Percival Lowell; and in this paper,
+by what purports to be strict mathematical reasoning based on the most
+recent discoveries as to the laws of heat, as well as on measurements or
+estimates of the various elements and constants used in the
+calculations, he arrives at a conclusion strikingly accordant with that
+put forward in the recently published volume. Having myself neither
+mathematical nor physical knowledge sufficient to enable me to criticise
+this elaborate paper, except on a few points, I will here limit myself
+to giving a short account of it, so as to explain its method of
+procedure; after which I may add a few notes on what seem to me doubtful
+points; while I also hope to be able to give the opinions of some more
+competent critics than myself.
+
+_Mr. Lowell's Mode of Estimating the Surface-temperature of Mars._
+
+The author first states, that Professor Young, in his _General
+Astronomy_ (1898), makes the mean temperature of Mars 223.6 deg. absolute,
+by using Newton's law of heat being radiated in proportion to
+temperature, and 363 deg. abs. (=-96 deg. F.) by Dulong and Petit's law; but
+adds, that a closer determination has been made by Professor Moulton,
+using Stefan's law, that radiation is as the _/4th_ power of the
+temperature, whence results a mean temperature of-31 deg. F. These estimates
+assume identity of atmospheric conditions of Mars and the Earth.
+
+But as none of these estimates take account of the many complex factors
+which interfere with such direct and simple calculations, Mr. Lowell
+then proceeds to enunciate them, and work out mathematically the effects
+they produce:
+
+(1) The whole radiant energy of the sun on striking a planet becomes
+divided as follows: Part is reflected back into space, part absorbed by
+the atmosphere, part transmitted to the surface of the planet. This
+surface again reflects a portion and only the balance left goes to warm
+the planet.
+
+(2) To solve this complex problem we are helped by the _albedoes_ or
+intrinsic brilliancy of the planets, which depend on the proportion of
+the visible rays which are reflected and which determines the
+comparative brightness of their respective surfaces. We also have to
+find the ratio of the invisible to the visible rays and the heating
+power of each.
+
+(3) He then refers to the actinometer and pyroheliometer, instruments
+for measuring the actual heat derived from the sun, and also to the
+Bolometer, an instrument invented by Professor Langley for measuring the
+invisible heat rays, which he has proved to extend to more than three
+times the length of the whole heat-spectrum as previously known, and
+has also shown that the invisible rays contribute 68 per cent, of the
+sun'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 the
+passage of the sun's rays through our atmosphere from experiments made
+at different altitudes and from the estimated reflective power of the
+various parts of the earth's surface--rocks and soil, ocean, forest and
+snow--the final result being that three-fourths of the whole sun-heat
+is reflected back into space, forming our _albedo_, while only
+one-fourth is absorbed by the soil and goes to warm the air and
+determine our mean temperature.
+
+(5) We now have another elaborate estimate of the comparative amounts of
+heat actually received by Mars and the Earth, dependent on their very
+different amounts of atmosphere, and this estimate depends almost wholly
+on the comparative _albedoes_, that of Mars, as observed by astronomers
+being 0.27, while ours has been estimated in a totally different way as
+being 0.75, whence he concludes that nearly three-fourths of the
+sun-heat that Mars receives reaches the surface and determines its
+temperature, while we get only one-fourth of our total amount. Then by
+applying Stefan's law, that the radiation is as the 4th power of the
+surface temperature, he reaches the final result that the actual heating
+power at the surface of Mars is considerably _more_ than on the Earth,
+and would produce a mean temperature of 72 deg. F., if it were not for the
+greater conservative or blanketing power of our denser and more
+water-laden atmosphere. The difference produced by this latter fact he
+minimises by dwelling on the probability of a greater proportion of
+carbonic-acid gas and water-vapour in the Martian atmosphere, and thus
+brings down the mean temperature of Mars to 48 deg. F., which is almost
+exactly the same as that of the southern half of England. He has also,
+as the result of observations, reduced the probable density of the
+atmosphere of Mars to 2-1/2 inches of mercury, or only one-twelfth of
+that of the Earth.
+
+_Critical Remarks on Mr. Lowell's Paper._
+
+The last part of this paper, indicated under pars. 4 and 5, is the most
+elaborate, occupying eight pages, and it contains much that seems
+uncertain, if not erroneous. In particular, it seems very unlikely that
+under a clear sky over the whole earth we should only receive at the
+sea-level 0.23 of the solar rays which the earth intercepts (p. 167).
+These data largely depend on observations made in California and other
+parts of the southern United States, where the lower atmosphere is
+exceptionally dust-laden. Till we have similar observations made in the
+tropical forest-regions, which cover so large an area, and where the
+atmosphere is purified by frequent rains, and also on the prairies and
+the great oceans, we cannot trust these very local observations for
+general conclusions affecting the whole earth. Later, in the same
+article (p. 170), Mr. Lowell says: "Clouds transmit approximately 20 per
+cent. 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 per
+cent." These statements seem incompatible with that quoted above.
+
+The figure he uses in his calculations for the actual albedo of the
+earth, 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-covered
+planet, Jupiter, is given by Lowell as 0.75, while Zollner made it only
+0.62. Again, Lowell gives Venus an albedo of 0.92, while Zollner made it
+only 0.50 and Mr. Gore 0.65. This shows the extreme uncertainty of these
+estimates, while the fact that both Venus and Jupiter are wholly
+cloud-covered, while we are only half-covered, renders it almost
+certain that our albedo is far less than Mr. Lowell makes it. It is
+evident that mathematical calculations founded upon such uncertain data
+cannot yield trustworthy results. But this is by no means the only case
+in which the data employed in this paper are of uncertain value.
+Everywhere we meet with figures of somewhat doubtful accuracy. Here we
+have somebody's 'estimate' quoted, there another person's 'observation,'
+and these are adopted without further remark and used in the various
+calculations leading to the result above quoted. It requires a practised
+mathematician, and one fully acquainted with the extensive literature of
+this subject, to examine these various data, and track them through the
+maze of formulae and figures so as to determine to what extent they
+affect the final result.
+
+There is however one curious oversight which I must refer to, as it is a
+point to which I have given much attention. Not only does Mr. Lowell
+assume, as in his book, that the 'snows' of Mars consist of frozen
+water, and that therefore there _is_ water on its surface and
+water-vapour in its atmosphere, not only does he ignore altogether Dr.
+Johnstone Stoney's calculations with regard to it, which I have already
+referred to, but he uses terms that imply that water-vapour is one of
+the heavier components of our atmosphere. The passage is at p. 168 of
+the _Philosophical Magazine._ After stating that, owing to the very
+small barometric pressure in Mars, water would boil at 110 deg. F., he adds:
+"The sublimation at lower temperatures would be correspondingly
+increased. Consequently the amount of water-vapour in the Martian air
+must on that score be relatively greater than our own." Then follows
+this remarkable passage: "Carbon-dioxide, because of its greater
+specific gravity, would also be in relatively greater amount so far as
+this cause is considered. For the planet would part, _caeteris paribus_,
+with its lighter gases the quickest. Whence as regards both water-vapour
+and carbon-dioxide we have reason to think them in relatively greater
+quantity than in our own air at corresponding barometric pressure." I
+cannot understand this passage except as implying that 'water-vapour and
+carbon-dioxide' are among the heavier and not among the lighter gases of
+the atmosphere--those which the planet 'parts with quickest.' But this
+is just what water-vapour _is_, being a little less than two-thirds the
+weight of air (0.6225), and one of those which the planet _would_ part
+with the quickest, and which, according to Dr. Johnstone Stoney, it
+loses altogether.
+       *       *       *       *       *
+
+Note on Professor Lowell's article in the _Philosophical Magazine_; by
+J.H. Poynting, F.R.S., Professor of Physics in the University of
+Birmingham.
+
+"I think Professor Lowell's results are erroneous through his neglect of
+the heat stored in the air by its absorption of radiation both from the
+sun and from the surface. The air thus heated radiates to the surface
+and keeps up the temperature. I have sent to the _Philosophical
+Magazine_ a paper in which I think it is shown that when the radiation
+by the atmosphere is taken into account the results are entirely
+changed. The temperature of Mars, with Professor Lowell's data, still
+comes out far below the freezing-point--still further below than the
+increased distance alone would make it. Indeed, the lower temperature on
+elevated regions of the earth's surface would lead us to expect this. I
+think it is impossible to raise the temperature of Mars to anything like
+the value obtained by Professor Lowell, unless we assume some quality in
+his 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 number
+of imperfectly ascertained data, we may often check the results thus
+obtained by the comparison of cases in which some of the more important
+of 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 as
+compared 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 exaggerated
+and 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 refined
+observations have not detected it. All the complications arising from
+the possible nature of the atmosphere, and its complex effects upon
+reflection, absorption, and radiation are thus eliminated. The mean
+distance of the moon from the sun being identical with that of the
+earth, the total amount of heat intercepted must also be identical; only
+in this case the whole of it reaches the surface instead of one-fourth
+only, according to Mr. Lowell's estimate for the earth.
+
+Now, by the most refined observations with his Bolometer, Mr. Langley
+was able to determine the temperature of the moon's surface exposed to
+undimmed sunshine for fourteen days together; and he found that, even in
+that portion of it on which the sun was shining almost vertically, the
+temperature rarely rose above the freezing point of water. However
+extraordinary this result may seem, it is really a striking confirmation
+of the accuracy of the general laws determining temperature which I have
+endeavoured to explain in the preceding chapter. For the same surface
+which has had fourteen days of sunshine has also had a preceding
+fourteen days of darkness, during which the heat which it had
+accumulated in its surface layers would have been lost by free radiation
+into stellar space. It thus acquires during its day a maximum
+temperature of only 491 deg. 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 extreme
+conditions, it will be useful to take note of the effects it actually
+produces on the earth in places where the conditions are nearest to
+those existing on the moon or on Mars, though never quite equalling, or
+even approaching very near them. It is in our great desert regions, and
+especially on high plateaux, that extreme aridity prevails, and it is in
+such districts that the differences between day and night temperatures
+reach their maximum. It is stated by geographers that in parts of the
+Great Sahara the surface temperature is sometimes 150 deg. F., while during
+the night it falls nearly or quite to the freezing point--a difference
+of 118 degrees in little more than 12 hours.[10] In the high desert
+plains of Central Asia the extremes are said to be even greater.[11]
+Again, in his _Universal Geography_, Reclus states that in the Armenian
+Highlands the thermometer oscillates between 13 deg. F. and 112 deg.F. We may
+therefore, without any fear of exaggeration, take it as proved that a
+fall of 100 deg. F. in twelve or fifteen hours not infrequently occurs where
+there is a very dry and clear atmosphere permitting continuous
+insolation 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, we
+shall certainly underestimate the radiation from the moon's surface
+during its long night if we take as the basis of our calculation a
+lowering of temperature amounting to 100 deg. F. during twelve hours, as not
+unfrequently occurs with us. Using these data--with Stefan's law of
+decrease of radiation as the 4th power of the temperature--a
+mathematical friend finds that the temperature of the moon's surface
+would be reduced during the lunar night to nearly 200 deg. F. absolute
+(equal to-258 deg. F.).
+
+_More Rapid Loss of Heat by the Moon._
+
+Although such a calculation as the above may afford us a good
+approximation to the rate of loss of heat by Mars with its very scanty
+atmosphere, we have now good evidence that in the case of the moon the
+loss is much more rapid. Two independent workers have investigated this
+subject with very accordant results--Dr. Boeddicker, with Lord Rosse's
+3-foot reflector and a Thermopile to measure the heat, and Mr. Frank
+Very, with a glass reflector of 12 inches diameter and the Bolometer
+invented by Mr. Langley. The very striking and unexpected fact in which
+these observers agree is the sudden disappearance of much of the
+stored-up heat during the comparatively short duration of a total
+eclipse of the moon--less than two hours of complete darkness, and about
+twice that period of partial obscuration.
+
+Dr. Boeddicker was unable to detect any appreciable heat at the period
+of greatest obscuration; but, owing to the extreme sensitiveness of the
+Bolometer, Mr. Very ascertained that those parts of the surface which
+had been longest in the shadow still emitted heat "to the amount of one
+per cent. of the heat to be expected from the full moon." This however
+is the amount of radiation measured by the Bolometer, and to get the
+temperature of the radiating surface we must apply Stefan's law of the
+4th 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 deg. F. abs.), or 154 deg. F. abs.,
+ just below the liquefaction point of air. This is about 50 deg. lower than the
+amount found by calculation from our most rapid radiation; and as this
+amount 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 reach
+a temperature sufficient to liquefy hydrogen (60 deg. F. abs.), or perhaps
+even 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 so
+improbable as to be almost incredible, may perhaps be to some extent
+explained by the physical constitution of the moon's surface, which,
+from a theoretical point of view, does not appear to have received the
+attention it deserves. It is clear that our satellite has been long
+subjected to volcanic eruptions over its whole visible face, and these
+have evidently been of an explosive nature, so as to build up the very
+lofty cones and craters, as well as thousands of smaller ones, which,
+owing to the absence of any degrading or denuding agencies, have
+remained piled up as they were first formed.
+
+This highly volcanic structure can, I think, be well explained by an
+origin such as that attributed to it by Sir George Darwin, and which has
+been so well described by Sir Robert Ball in his small volume, _Time and
+Tide._ These astronomers adduce strong evidence that the earth once
+rotated so rapidly that the equatorial protuberance was almost at the
+point of separation from the planet as a ring. Before this occurred,
+however, the tension was so great that one large portion of the
+protuberance where it was weakest broke away, and began to move around
+the earth at some considerable distance from it. As about 1/50 of the
+bulk of the earth thus escaped, it must have consisted of a considerable
+portion of the solid crust and a much larger quantity of the liquid or
+semi-liquid interior, together with a proportionate amount of the gases
+which we know formed, and still form, an important part of the earth's
+substance.
+
+As the surface layers of the earth must have been the lightest, they
+would necessarily, when broken up by this gigantic convulsion, have come
+together to form the exterior of the new satellite, and be soon adjusted
+by the forces of gravity and tidal disturbance into a more or less
+irregular spheroidal form, all whose interstices and cavities would be
+filled up and connected together by the liquid or semi-liquid mass
+forced up between them. Thence-forward, as the moon increased its
+distance and reduced its time of rotation, in the way explained by Sir
+Robert Ball, there would necessarily commence a process of escape of the
+imprisoned gases at every fissure and at all points and lines of
+weakness, giving rise to numerous volcanic outlets, which, being
+subjected only to the small force of lunar gravity (only one-sixth that
+of the earth), would, in the course of ages, pile up those gigantic
+cones and ridges which form its great characteristic.
+
+But this small gravitative power of the moon would prevent its retaining
+on its surface any of the gases forming our atmosphere, which would all
+escape from it and probably be recaptured by the earth. By no process of
+external aggregation of solid matter to such a relatively small amount
+as that forming the moon, even if the aggregation was so violent as to
+produce heat enough to cause liquefaction, could any such
+long-continued volcanic action arise by gradual cooling, in the absence
+of internal gases. There might be fissures, and even some outflows of
+molten rock; but without imprisoned gases, and especially without water
+and water-vapour producing explosive outbursts, could any such amount of
+scoriae and ashes be produced as were necessary for the building up of
+the vast volcanic cones, craters, and craterlets we see upon the moon's
+surface.
+
+I am not aware that either Sir Robert Ball or Sir George Darwin have
+adduced this highly volcanic condition of the moon's surface as a
+phenomenon which can _only_ be explained by our satellite having been
+thrown off a very much larger body, whose gravitative force was
+sufficient to acquire and retain the enormous quantity of gases and of
+water which we possess, and which are _absolutely essential_ for that
+_special form of cone-building volcanic action_ which the moon exhibits
+in so pre-eminent a degree. Yet it seems to me clear, that some such
+hypothetical origin for our satellite would have had to be assumed if
+Sir George Darwin had not deduced it by means of purely mathematical
+argument based upon astronomical facts.
+
+Returning now to the problem of the moon's temperature, I think the
+phenomena this presents may be in part due to the mode of formation here
+described. For, its entire surface being the result of long-continued
+gaseous explosions, all the volcanic products--scoriae, pumice, and
+ashes--would necessarily be highly porous throughout; and, never having
+been compacted by water-action, as on the earth, and there having been
+no winds to carry the finer dust so as to fill up their pores and
+fissures, the whole of the surface material to a very considerable depth
+must be loose and porous to a high degree. This condition has been
+further increased owing to the small power of gravity and the extreme
+irregularity of the surface, consisting very largely of lofty cones and
+ridges very loosely piled up to enormous heights.
+
+Now this condition of the substance of the moon's surface is such as
+would produce a high specific heat, so that it would absorb a large
+amount of heat in proportion to the rise of temperature produced, the
+heat being conducted downwards to a considerable depth. Owing, however,
+to the total absence of atmosphere radiation would very rapidly cool the
+surface, but afterwards more slowly, both on account of the action of
+Stefan's law and because the heat stored up in the deeper portions could
+be 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 detailed
+examination of the surface-temperature of the moon by Mr. Very in his
+_Prize Essay on the Distribution of the Moon's Heat_ (published by the
+Utrecht Society of Arts and Sciences in 1891). He shows, by a diagram of
+the 'Phase-curve,' that at the commencement of the Lunar day the surface
+just within the illuminated limb has acquired about 1/7 of its maximum
+temperature, or about 70 deg. F. abs. As the surface exposed to the
+Bolometer at each observation is about 1/30 of the moon's surface, and
+in order to ensure accuracy the instrument has to be directed to a spot
+lying wholly within the edge of the moon, it is evident that the surface
+measured has already been for several hours exposed to oblique sunshine.
+The curve of temperature then rises gradually and afterwards more
+rapidly, till it attains its maximum (of about +30 to 40 deg. F.) a few
+hours _before_ noon. This, Mr. Very thinks, is due to the fact that the
+half of the moon's face first illuminated for us has, on the average, a
+darker surface than that of the afternoon, or second quarter, during
+which the curve descends not quite so rapidly, the temperature near
+sunset being only a little higher than that near sunrise. This rapid
+fall while exposed to oblique sunshine is quite in harmony with the
+rapid 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 of
+full-moon, one half of the curve showing the temperatures along the
+equator from the edge of the disc to the centre, the other along a
+meridian from this centre to the pole. This diagram (here reproduced)
+exhibits the quick rise of temperature of the oblique rim of the moon
+and the nearly uniform heat of the central half of its surface; the
+diminution of heat towards the pole, however, is slower for the first
+half and more rapid for the latter portion.
+
+It is an interesting fact that the temperature near the margin of the
+full-moon increases towards the centre more rapidly than it does when
+the same parts are observed during the early phases of the first
+quarter. Mr. Very explains this difference as being due to the fact that
+the full-moon to its very edges is fully illuminated, all the shadows of
+the ridges and mountains being thrown vertically or obliquely _behind
+them._ We thus measure the heat reflected from the _whole_ visible
+surface. But at new moon, and somewhat beyond the first quarter, the
+deep shadows thrown by the smallest cones and ridges, as well as by the
+loftiest mountains, cover a considerable portion of the visible surface,
+thus largely reducing the quantity of light and heat reflected or
+radiated in our direction. It is only at the full, therefore, that the
+maximum temperature of the whole lunar surface can be measured. It must
+be considered a proof of the delicacy of the heat-measuring instruments
+that this difference in the curves of temperature of the different parts
+of the moon's surface and under different conditions is so clearly
+shown.
+
+_The Application of the Preceding Results to the Case of Mars._
+
+This somewhat lengthy account of the actual state of the moon's surface
+and temperature is of very great importance in our present enquiry,
+because it shows us the extraordinary difference in mean and extreme
+temperatures of two bodies situated at the same distance from the sun,
+and therefore receiving exactly the same amount of solar heat per unit
+of surface. We have learned also what are the main causes of this almost
+incredible difference, namely: (1) a remarkably rugged surface with
+porous and probably cavernous rock-texture, leading to extremely rapid
+radiation of heat in the one; as compared with a comparatively even and
+well-compacted surface largely clad with vegetation, leading to
+comparatively slow and gradual loss by radiation in the other: and (2),
+these results being greatly intensified by the total absence of a
+protecting atmosphere in the former, while a dense and cloudy atmosphere
+with an ever-present supply of water-vapour, accumulates and equalises
+the heat received by the latter.
+
+The only other essential difference in the two bodies which may possibly
+aid in the production of this marvellous result, is the fact of our day
+and night having a mean length of 12 hours, while those of the moon are
+about 14-1/2 of our days. But the altogether unexpected fact, in which
+two independent enquirers agree, that during the few hours' duration of
+a total eclipse of the moon so large a proportion of the heat is lost by
+radiation renders it almost certain that the resulting low temperature
+would be not very much less if the moon had a day and night the same
+length as our own.
+
+The great lesson we learn by this extreme contrast of conditions
+supplied to us by nature, as if to enable us to solve some of her
+problems, is, the overwhelming importance, first, of a dense and
+well-compacted surface, due to water-action and strong gravitative
+force; secondly, of a more or less general coat of vegetation; and,
+thirdly, of a dense vapour-laden atmosphere. These three favourable
+conditions result in a mean temperature of about +60 deg. F. with a range
+seldom exceeding 40 deg. above or below it, while over more than half the
+land-surface of the earth the temperature rarely falls below the
+freezing point. On the other hand, we have a globe of the same materials
+and at the same distance from the sun, with a maximum temperature of
+freezing water, and a minimum not very far from the absolute zero, the
+monthly mean being probably much below the freezing point of
+carbonic-acid gas--a difference entirely due to the absence of these
+three 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 conclusion
+from that of Mr. Lowell, who himself quotes Mr. Moulton as an authority
+who 'recently, by the application of Stefan's law,' has found the mean
+temperature of this planet to be-35 deg. F. Again, Professor J.H. Poynting,
+in his lecture on 'Radiation in the Solar System,' delivered before the
+British Association at Cambridge in 1904, gave an estimate of the mean
+temperature of the planets, arrived at from measurements of the sun's
+emissive power and the application of Stefan's law to the distances of
+the several planets, and he thus finds the earth to have a mean
+temperature of 17 deg. C. (=62-1/2 deg. F.) and Mars one of-38 deg. C. (=-36-1/2 deg.
+F.), a wonderfully close approximation to the mean temperature of the
+earth as determined by direct measurement, and therefore, presumably, an
+equally near approximation to that of Mars as dependent on distance from
+the sun, and '_on the supposition that it is earth-like in all its
+conditions._'
+
+But we know that it is far from being earth-like in the very conditions
+which we have found to be those which determine the extremely different
+temperatures of the earth, and moon; and, as regards each of these, we
+shall find that, so far as it differs from the earth, it approximates to
+the less favourable conditions that prevail in the moon. The first of
+these conditions which we have found to be essential in regulating the
+absorption and radiation of heat, and thus raising the mean temperature
+of a planet, is a compact surface well covered with vegetation, two
+conditions arising from, and absolutely dependent on, an ample amount of
+water. But Mr. Lowell himself assures us, as a fact of which he has no
+doubt, that there are no permanent bodies of water, great or small, upon
+Mars; that rain, and consequently rivers, are totally wanting; that its
+sky is almost constantly clear, and that what appear to be clouds are
+not formed of water-vapour but of dust. He dwells, emphatically, on the
+terrible desert conditions of the greater part of the surface of the
+planet.
+
+That being the case now, we have no right to assume that it has ever
+been otherwise; and, taking full account of the fact, neither denied nor
+disputed by Mr. Lowell, that the force of gravity on Mars is not
+sufficient to retain water-vapour in its atmosphere, we must conclude
+that the surface of that planet, like that of the moon, has been moulded
+by some form of volcanic action modified probably by wind, but not by
+water. Adding to this, that the force of gravity on Mars is nearer that
+of the moon than to that of the earth, and we may r reasonably conclude
+that its surface is formed of volcanic matter in a light and porous
+condition, and therefore highly favourable for the rapid loss of surface
+heat by radiation. The surface-conditions of Mars are therefore,
+presumably, much more like those of the moon than like those of the
+earth.
+
+The next condition favourable to the storing up of heat--a covering of
+vegetation--is almost certainly absent from Mars except, possibly, over
+limited areas and for short periods. In this feature also the surface of
+Mars approximates much nearer to lunar than to earth-conditions. The
+third condition--a dense, vapour-laden atmosphere--is also wanting in
+Mars. For although it possesses an atmosphere it is estimated by Mr.
+Lowell (in his latest article) to have a pressure equivalent to only
+2-1/2 inches of mercury with us, giving it a density of only one-twelfth
+part that of ours; while aqueous vapour, the chief accumulator of heat,
+cannot permanently exist in it, and, notwithstanding repeated
+spectroscopic observations for the purpose of detecting it, has never
+been proved to exist.
+
+I submit that I have now shown from the statements--and largely as the
+result of the long-continued observations--of Mr. Lowell himself, that,
+so far as the physical conditions of Mars are known to differ from those
+of the earth, the differences are all _unfavourable_ to the conservation
+and _favourable_ to the dissipation of the scanty heat it receives from
+the sun--that they point unmistakeably towards the temperature
+conditions of the moon rather than to those of the earth, and that the
+cumulative effect of these adverse conditions, acting upon a
+heat-supply, reduced by solar distance to less than one-half of ours,
+_must_ result in a mean temperature (as well as in the extremes) nearer
+to 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. We
+have seen, in the outline I have given of this article, that he
+endeavours to show how the small amount of solar heat received by Mars
+is counterbalanced, largely by the greater transparency to light and
+heat of its thin and cloudless atmosphere, and partially also by a
+greater conservative or 'blanketing' power of its atmosphere due to the
+presence in it of a large proportion of carbonic acid gas and aqueous
+vapour. The first of these statements may be admitted as a fact which he
+is entitled to dwell upon, but the second--the presence of large
+quantities of carbon-dioxide and aqueous vapour is a pure hypothesis
+unsupported by any item of scientific evidence, while in the case of
+aqueous vapour it is directly opposed to admitted results founded upon
+the molecular theory of gaseous elasticity. But, although Mr. Lowell
+refers to the conservative or 'blanketing' effect of the earth's
+atmosphere, he does not consider or allow for its very great cumulative
+effect, as is strikingly shown by the comparison with the actual
+temperature conditions of the moon. This cumulative effect is due to the
+_continuous_ reflection and radiation of heat from the clouds as well as
+from the vapour-laden strata of air in our lower atmosphere, which
+latter, though very transparent to the luminous and accompanying heat
+rays of the sun, are opaque to the dark heat-rays whether radiated or
+reflected from the earth's surface. We are therefore in a position
+strictly 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 those
+radiated from the clouds, so that even on a cloudy or misty day its
+temperature rises many degrees above that of the outer air. Such a
+building, if of large size, of suitable form, and well protected at
+night by blinds or other covering, might be so arranged as to accumulate
+heat in its soil and walls so as to maintain a tolerably uniform
+temperature though exposed to a considerable range of external heat and
+cold. It is to such a power of accumulation of heat in our soil and
+lower atmosphere that we must impute the overwhelming contrast between
+our climate and that of the moon. With us, the solar heat that
+penetrates our vapour-laden and cloudy atmosphere is shut in by that
+same atmosphere, accumulates there for weeks and months together, and
+can only slowly escape. It is this great cumulative power which Mr.
+Lowell has not taken account of, while he certainly has not estimated
+the enormous loss of heat by free radiation, which entirely neutralises
+the effects of increase of sun-heat, however great, when these
+cumulative agencies are not present.[12]
+
+[Footnote 12: The effects of this 'cumulative' power of a dense
+atmosphere are further discussed and illustrated in the last chapter of
+this book, where I show that the universal fact of steadily diminishing
+temperatures at high altitudes is due solely to the diminution of this
+cumulative power of our atmosphere, and that from this cause alone the
+temperature of Mars must be that which would be found on a lofty plateau
+about 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 has
+altogether omitted to discuss. Whatever may be the _mean_ temperature
+of Mars, we must take account of the long nights in its polar and
+high-temperate latitudes, lasting nearly twice as long as ours, with the
+resulting lowering of temperature by radiation into a constantly clear
+sky. Even in Siberia, in Lat. 67-1/2 deg.N. a cold of-88 deg.F. has been
+attained; while over a large portion of N. Asia and America above 60 deg.
+Lat. the _mean_ January temperature is from-30 deg.F. to-60 deg.F., and the
+whole subsoil is permanently frozen from a depth of 6 or 7 feet to
+several hundreds. But the winter temperatures, _over the same latitudes_
+in Mars, must be very much lower; and it must require a proportionally
+larger amount of its feeble sun-heat to raise the surface even to the
+freezing-point, and an additional very large amount to melt any
+considerable depth of snow. But this identical area, from a little below
+60 deg. to the pole, is that occupied by the snow-caps of Mars, and over the
+whole of it the winter temperature must be far lower than the
+earth-minimum of-88 deg.F. Then, as the Martian summer comes on, there is
+less than half the sun-heat available to raise this low temperature
+after a winter nearly double the length of ours. And when the summer
+does come with its scanty sun-heat, that heat is not accumulated as it
+is by our dense and moisture-laden atmosphere, the marvellous effects of
+which we have already shown. Yet with all these adverse conditions, each
+assisting the other to produce a climate approximating to that which the
+earth would have if it had no atmosphere (but retaining our superiority
+over Mars in receiving double the amount of sun-heat), we are asked to
+accept a mean temperature for the more distant planet almost exactly the
+same as that of mild and equable southern England, and a disappearance
+of the vast snowfields of its polar regions as rapid and complete as
+what occurs with us! If the moon, even at its equator, has not its
+temperature raised above the freezing-point of water, how can the more
+_distant_ Mars, with its _oblique_ noon-day sun falling upon the
+snow-caps, receive heat enough, first to raise their temperature to 32 deg.
+F., and then to melt with marked rapidity the vast frozen plains of its
+polar regions?
+
+Mr. Lowell is however so regardless of the ordinary teachings of
+meteorological science that he actually accounts for the supposed mild
+climate of the polar regions of Mars by the absence of water on its
+surface and in its atmosphere. He concludes his fifth chapter with the
+following words: "Could our earth but get rid of its oceans, we too
+might have temperate regions stretching to the poles." Here he runs
+counter to two of the best-established laws of terrestrial climatology--
+the wonderful equalising effects of warm ocean-currents which are the
+chief agents in diminishing polar cold; the equally striking effects of
+warm moist winds derived from these oceans, and the great storehouse of
+heat we possess in our vapour-laden atmosphere, its vapour being
+primarily derived from these same oceans! But, in Mr. Lowell's opinion,
+all our meteorologists are quite mistaken. Our oceans are our great
+drawbacks. Only get rid of them and we should enjoy the exquisite
+climate of Mars--with its absence of clouds and fog, of rain or rivers,
+and its delightful expanses of perennial deserts, varied towards the
+poles by a scanty snow-fall in winter, the melting of which might, with
+great care, supply us with the necessary moisture to grow wheat and
+cabbages for about one-tenth, or more likely one-hundredth, of our
+present population. I hope I may be excused for not treating such an
+argument seriously. The various considerations now advanced, especially
+those which show the enormous cumulative and conservative effect of our
+dense and water-laden atmosphere, and the disastrous effect--judging by
+the actual condition of the moon--which the loss of it would have upon
+our temperature, seem to me quite sufficient to demonstrate important
+errors in the data or fallacies in the complex mathematical argument by
+which Mr. Lowell has attempted to uphold his views as to the temperature
+and consequent climatic conditions of Mars. In concluding this portion
+of my discussion of the problem of Mars, I wish to call attention to the
+fact that my argument, founded upon a comparison of the physical
+conditions of the earth and moon with those of Mars, is dependent upon a
+small number of generally admitted scientific facts; while the
+conclusions drawn from those facts are simple and direct, requiring no
+mathematical knowledge to follow them, or to appreciate their weight and
+cogency. I claim for them, therefore, that they are in no degree
+speculative, but in their data and methods exclusively scientific. In
+the next chapter I will put forward a suggestion as to how the very
+curious markings upon the surface of Mars may possibly be interpreted,
+so as to be in harmony with the planet's actual physical condition and
+its 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 the
+whole of the equatorial and temperate regions of Mars are, their
+straightness combined with their enormous length. It is this which has
+led Mr. Lowell to term them 'non-natural features.' Schiaparelli, in his
+earlier drawings, showed them curved and of comparatively great width.
+Later, he found them to be straight fine lines when seen under the best
+conditions, just as Mr. Lowell has always seen them in the pure
+atmosphere of his observatory. Both of these observers were at first
+doubtful of their reality, but persistent observation continued at many
+successive oppositions compelled acceptance of them as actual features
+of the planet's disc. So many other observers have now seen them that
+the objection of unreality seems no longer valid.
+
+Mr. Lowell urges, however, that their perfect straightness, their
+extreme tenuity, their uniformity throughout their whole length, the
+dual character of many of them, their relation to the 'oases' and the
+form and position of these round black spots, are all proofs of
+artificiality and are suggestive of design. And considering that some of
+them are actually as long as from Boston to San Francisco, and
+relatively to their globe as long as from London to Bombay, his
+objection that "no natural phenomena within our knowledge show such
+regularity on such a scale" seems, at first, a mighty one.
+
+It is certainly true that we can point to nothing exactly like them
+either on the earth or on the moon, and these are the only two planetary
+bodies we are in a position to compare with Mars. Yet even these do, I
+think, afford us some hints towards an interpretation of the mysterious
+lines. But as our knowledge of the internal structure and past history
+even 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 the
+surface-features of the latter do not correspond with those of either of
+the others.
+
+_Mr. Pickering's Suggestion._
+
+The best clue to a natural interpretation of the strange features of the
+surface 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 the
+moon are cracks in the volcanic crust, caused by internal stresses due
+to the action of the heated interior. These cracks he considers to be
+symmetrically arranged with regard to small 'craterlets' (Mr. Lowell's
+'oases') because they have originated from them, just as the white
+streaks on the moon radiate from the larger craters as centres. He
+further supposes that water and carbon-dioxide issue from the interior
+into these fissures, and, in conjunction with sunlight, promote the
+growth of vegetation. Owing to the very rare atmosphere, the vapours, he
+thinks, would not ascend but would roll down the outsides of the
+craterlets and along the borders of the canals, thus irrigating the
+immediate vicinity and serving to promote the growth of some form of
+vegetation 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 Mars
+during the last fifteen years. He was for some time at Flagstaff with
+Mr. Lowell, and it was he who discovered the oases or craterlets, and
+who originated the idea that we did not see the 'canals' themselves but
+only the vegetable growth on their borders. He also observed Mars in the
+Southern Hemisphere at Arequipa; and he has since made an elaborate
+study of the moon by means of a specially constructed telescope of 135
+feet focal length, which produced a direct image on photographic plates
+nearly 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 artificial
+nature of the 'canals' of Mars are not accepted by an astronomer of
+equal knowledge and still wider experience. Yet Professor Pickering's
+alternative view is more a suggestion than an explanation, because there
+is no attempt to account for the enormous length and perfect
+straightness of the lines on Mars, so different from anything that is
+found either on our earth or on the moon. There must evidently be some
+great peculiarity of structure or of conditions on Mars to account for
+these features, and I shall now attempt to point out what this
+peculiarity is and how it may have arisen.
+
+_The Meteoritic Hypothesis._
+
+During the last quarter of a century a considerable change has come over
+the opinions of astronomers as regards the probable origin of the Solar
+System. The large amount of knowledge of the stellar universe, and
+especially of nebulae, of comets and of meteor-streams which we now
+possess, together with many other phenomena, such as the constitution of
+Saturn's rings, the great number and extent of the minor planets, and
+generally of the vast amount of matter in the form of meteor-rings and
+meteoric dust in and around our system, have all pointed to a different
+origin for the planets and their satellites than that formulated by
+Laplace as the Nebular hypothesis.
+
+It is now seen more clearly than at any earlier period, that most of the
+planets possess special characteristics which distinguish them from one
+another, and that such an origin as Laplace suggested--the slow cooling
+and contraction of one vast sun-mist or nebula, besides presenting
+inherent difficulties--many think them impossibilities--in itself does
+not afford an adequate explanation of these peculiarities. Hence has
+arisen what is termed the Meteoritic theory, which has been ably
+advocated for many years by Sir Norman Lockyer, and with some
+unimportant modifications is now becoming widely accepted. Briefly, this
+theory is, that the planets have been formed by the slow aggregation of
+solid particles around centres of greatest condensation; but as many of
+my readers may be altogether unacquainted with it, I will here give a
+very clear statement of what it is, from Professor J.W. Gregory's
+presidential address to the Geological Section of the British
+Association of the present year. He began by saying that these modern
+views were of far more practical use to men of science than that of
+Laplace, and that they give us a history of the world consistent with
+the 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 so
+become luminous. They may even be volatilised into glowing meteoric
+vapour; but in time this heat is dissipated, and the force of gravity
+condenses a meteoritic swarm into a single globe. 'Some of the swarms
+are,' says Lockyer, 'truly members of the solar system,' and some of
+these travel round the sun in nearly circular orbits, like planets. They
+may be regarded as infinitesimal planets, and so Chamberlain calls them
+'planetismals.'
+
+"The planetismal theory is a development of the meteoritic theory, and
+presents it in an especially attractive guise. It regards meteorites as
+very sparsely distributed through space, and gravity as powerless to
+collect them into dense groups. So it assigns the parentage of the solar
+system to a spiral nebula composed of planetismals, and the planets as
+formed from knots in the nebula, where many planetismals had been
+concentrated near the intersections of their orbits. These groups of
+meteorites, already as dense as a swarm of bees, were then packed closer
+by the influence of gravity, and the contracting mass was heated by the
+pressure, even above the normal melting-point of the material, which was
+kept rigid by the weight of the overlying layers."
+
+Now, adopting this theory as the last word of science upon the subject
+of the origin of planets, we see that it affords immense scope for
+diversity in results depending on the total _amount_ of matter available
+within the range of attraction of an incipient planetary mass, and the
+_rates_ at which this matter becomes available. By a special combination
+of these two quantities (which have almost certainly been different for
+each planet) I think we may be able to throw some light upon the
+structure and physical features of Mars.
+
+_The Probable Mode of Origin of Mars._
+
+This planet, lying between two of much greater mass, has evidently had
+less material from which to be formed by aggregation; and if we
+assume--as in the absence of evidence to the contrary we have a right to
+do--that its beginnings were not much later (or earlier) than those of
+the earth, then its smaller size shows that it has in all probability
+aggregated very much more slowly. But the internal heat acquired by a
+planet while forming in this manner will depend upon the rate at which
+it aggregates and the velocity with which the planetismals' fall into
+it, and this velocity will increase with its mass and consequent force
+of gravity. In the early stages of a planet's growth it will probably
+remain cold, the small amount of heat produced by each impact being lost
+by radiation before the next one occurs; and with a small and slowly
+aggregating planet this condition will prevail till it approaches its
+full size. Then only will its gravitative force be sufficient to cause
+incoming matter to fall upon it with so powerful an impact as to produce
+intense heat. Further, the compressive force of a small planet will be a
+less effective heat-producing agency than in the case of a larger one.
+
+The earth we know has acquired a large amount of internal heat, probably
+sufficient to liquefy its whole interior; but Mars has only one-ninth
+part the mass of the earth, and it is quite possible, and even probable,
+that its comparatively small attractive force would never have liquefied
+or even permanently heated the more central portions of its mass. This
+being admitted, I suggest the following course of events as quite
+possible, and not even improbable, in the case of this planet. During
+the whole of its early growth, and till it acquired nearly its present
+diameter, its rate of aggregation was so slow that the planetismals
+falling upon it, though they might have been heated and even partially
+liquefied by the impact, were never in such quantity as to produce any
+considerable heating effect on the whole mass, and each local rise of
+temperature was soon lost by radiation. The planet thus grew as a solid
+and cold mass, compacted together by the impact of the incoming matter
+as well as by its slowly increasing gravitative force. But when it had
+attained to within perhaps 100, perhaps 50 miles, or less, of its
+present diameter, a great change occurred in the opportunity for further
+growth. Some large and dense swarm of meteorites, perhaps containing a
+number of bodies of the size of the asteroids, came within the range of
+the sun's attraction and were drawn by it into an orbit which crossed
+that of Mars at such a small angle that the planet was able at each
+revolution to capture a considerable number of them. The result might
+then be that, as in the case of the earth, the continuous inpour of the
+fresh matter first heated, and later on liquefied the greater part of it
+as well perhaps as a thin layer of the planet's original surface; so
+that when in due course the whole of the meteor-swarm had been captured,
+Mars had acquired its present mass, but would consist of an intensely
+heated, and either liquid or plastic thin outer shell resting upon a
+cold 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 more
+remote revolving around its primary very little slower than the planet
+rotates, while the nearer one, which is considerably less distant from
+the planet's surface than its own antipodes and revolves around it more
+than three times during the Martian day, may perhaps be looked upon as
+the remnants of the great meteor-swarm which completed the Martian
+development, and which are perhaps themselves destined at some distant
+period to fall into the planet. Should future astronomers witness the
+phenomenon the effect produced upon its surface would be full of
+instruction.
+
+As the result of such an origin as that suggested, Mars would possess a
+structure which, in the essential feature of heat-distribution, would be
+the very opposite of that which is believed to characterise the earth,
+yet it might have been produced by a very slight modification of the
+same process. This peculiar heat-distribution, together with a much
+smaller mass and gravitative force, would lead to a very different
+development of the surface and an altogether diverse geological history
+from ours, which has throughout been profoundly influenced by its heated
+interior, its vast supply of water, and the continuous physical and
+chemical reactions between the interior and the crust.
+
+These reactions have, in our case, been of substantially the same
+nature, and very nearly of the same degree of intensity throughout the
+whole vast eons of geological time, and they have resulted in a
+wonderfully 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 or
+depressed, fractured or crushed, inclined or contorted; denuded by rain
+and rivers with the assistance of heat and cold, of frost and ice, in an
+unceasing series of changes, so that however varied the surface may be,
+with hill and dale, plains and uplands, mountain ranges and deep
+intervening valleys, these are as nothing to the diversities of interior
+structure, as exhibited in the sides of every alpine valley or
+precipitous escarpment, and made known to us by the work of the miner
+and 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 its
+interior, is thus seen to be extreme diversity both of form and
+structure, and this is further intensified by the varied texture,
+constitution, hardness, and density of the various rocks and debris of
+which it is composed. It is therefore not surprising that, with such a
+complex outer crust, we should nowhere find examples of those
+geometrical forms and almost world-wide straight lines that give such a
+remarkable, and as Mr. Lowell maintains, 'non-natural' character to the
+surface of Mars, but which, as it seems to me, of themselves afford
+_prima facie_ evidence of a corresponding simplicity and uniformity in
+its internal structure.
+
+Yet we are not ourselves by any means devoid of 'straight lines'
+structurally produced, in spite of every obstacle of diversity of form
+and texture, of softness and hardness, of lamination or crystallisation,
+which are adverse to such developments. Examples of these are the
+numerous 'faults' which occur in the harder rocks, and which often
+extend for great distances in almost perfect straight lines. In our own
+country 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 striking
+is the great Cleveland Dyke--a wall of volcanic rock dipping slightly
+towards the south, but sometimes being almost vertical, and stretching
+across the country, over hill and dale, in an almost perfect straight
+line from a point on the coast ten miles north of Scarborough, in a
+west-by-north direction, passing about two miles south of Stockton and
+terminating about six miles north-by-east of Barnard Castle, a distance
+of very nearly 60 miles. The great fault between the Highlands and
+Lowlands of Scotland extends across the country from Stonehaven to near
+Helensburgh, a distance of 120 miles; and there are very many more of
+less importance.
+
+Much more extensive are some of the great continental dislocations,
+often forming valleys of considerable width and length. The Upper Rhine
+flows in one of these great valleys of subsidence for about 180 miles,
+from Mulhausen to Frankfort, in a generally straight line, though
+modified by denudation. Vaster still is the valley of the Jordan through
+the Sea of Galilee to the Dead Sea, continued by the Wady Arabah to the
+Gulf of Akaba, believed to form one vast geological depression or
+fracture extending in a straight line for 400 miles.
+
+Thousands of such faults, dykes, or depressions exist in every part of
+the world, all believed to be due to the gradual shrinking of the heated
+interior to which the solid crust has to accommodate itself, and they
+are especially interesting and instructive for our present purpose as
+showing the tendency of such fractures of solid rock-material to extend
+to great lengths in straight lines, notwithstanding the extreme
+irregularity both in the surface contour as well as in the internal
+structures of the varied deposits and formations through which they
+pass.
+
+_Probable Origin of the Surface-features of Mars._
+
+Returning now to Mars, let us consider the probable course of events
+from the point at which we left it. The heat produced by impact and
+condensation would be likely to release gases which had been in
+combination with some of the solid matter, or perhaps been itself in a
+solid state due to intense cold, and these, escaping outwards to the
+surface, would produce on a small scale a certain amount of upheaval and
+volcanic disturbance; and as an outer crust rapidly formed, a number of
+vents might remain as craters or craterlets in a moderate state of
+activity. Owing to the comparatively small force of gravity, the outer
+crust would become scoriaceous and more or less permeated by the gases,
+which would continue to escape through it, and this would facilitate the
+cooling of the whole of the heated outer crust, and allow it to become
+rather densely compacted. When the greater portion of the gases had thus
+escaped to the outer surface and assisted to form a scanty atmosphere,
+such as now exists, there would be no more internal disturbance and the
+cooling of the heated outer coating would steadily progress, resulting
+at last in a slightly heated, and later in a cold layer of moderate
+thickness and great general uniformity. Owing to the absence of rain and
+rivers, denudation such as we experience would be unknown, though the
+superficial scoriaceous crust might be partially broken up by expansion
+and contraction, and suffer a certain amount of atmospheric erosion.
+
+The final result of this mode of aggregation would be, that the planet
+would consist of an outer layer of moderate thickness as compared with
+the central mass, which outer layer would have cooled from a highly
+heated state to a temperature considerably below the freezing-point, and
+this would have been all the time _contracting upon a previously cold,
+and therefore non-contracting nucleus._ The result would be that very
+early in the process great superficial tensions would be produced, which
+could only be relieved by cracks or fissures, which would initiate at
+points of weakness--probably at the craterlets already referred to--from
+which they would radiate in several directions. Each crack thus formed
+near the surface would, as cooling progressed, develop in length and
+depth; and owing to the general uniformity of the material, and possibly
+some amount of crystalline structure due to slow and continuous cooling
+down to a very low temperature, the cracks would tend to run on in
+straight lines and to extend vertically downwards, which two
+circumstances 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 to
+illustrate one of these two features, the direction of the downward
+fissure. This is, the comparatively common phenomenon of basaltic
+columns and 'Giant's Causeways.' The wonderful regularity of these, and
+especially the not unfrequent upright pillars in serried ranks, as in
+the palisades of the Hudson river, must have always impressed observers
+with their appearance of artificiality. Yet they are undoubtedly the
+result of the very slow cooling and contraction of melted rocks under
+compression by strata _below and above them_, so that, when once
+solidified, the mass was held in position and the tension produced by
+contraction could only be relieved by numerous very small cracks at
+short distances from each other in every direction, resulting in five,
+six, or seven-sided polygons, with sides only a few inches long. This
+contraction began of course at the coolest surface, generally the upper
+one; and observation of these columns in various positions has
+established the rule that their direction lengthways _is always at right
+angles to the cooling surface_, and thus, whenever this surface was
+horizontal, the columns became almost exactly vertical.
+
+_How this applies to Mars._
+
+One of the features of the surface of Mars that Mr. Lowell describes
+with much confidence is, that it is wonderfully uniform and level, which
+of 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 hardened
+outer crust would be vertical; and, in a generally uniform material at a
+very uniform temperature, these cracks would continue almost
+indefinitely in straight lines. The hardened and contracting surface
+being free to move laterally on account of there being a more heated and
+plastic layer below it, the cracks once initiated above would
+continually widen at the surface as they penetrated deeper and deeper
+into the slightly heated substratum. Now, as basalt begins to soften at
+about 1400 deg. F. and the surface of Mars has cooled to at least the
+freezing-point--perhaps very much below it--the contraction would be so
+great that if the fissures produced were 500 miles apart they might be
+three miles wide at the surface, and, if only 100 miles apart, then
+about two-thirds of a mile wide.[15] But as the production of the
+fissures might have occupied perhaps millions of years, a considerable
+amount of atmospheric denudation would result, however slowly it acted.
+Expansion and contraction would wear away the edges and sides of the
+fissures, fill up many of them with the debris, and widen them at the
+surfaces to perhaps double their original size.[16]
+
+[Footnote 15: The coefficient of contraction of basalt is 0.000006 for
+1 deg. 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 is
+partially illuminated. They were at first thought to be mountains; but
+during the opposition of 1894, more than 400 of them were seen at
+Flagstaff during nine months' observation. Usually they are of rare
+occurrence. They are seen to change in form and position from day to
+day, 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 therefore
+capable of producing proportional surface denudation.]
+
+_Suggested Explanation of the 'Oases.'_
+
+The numerous round dots seen upon the 'canals,' and especially at points
+from which several canals radiate and where they intersect--termed
+'oases' by Mr. Lowell and 'craterlets' by Mr. Pickering may be explained
+in two ways. Those from which several canals radiate may be true craters
+from which the gases imprisoned in the heated surface layers have
+gradually escaped. They would be situated at points of weakness in the
+crust, and become centres from which cracks would start during
+contraction. Those dots which occur at the crossing of two straight
+canals or cracks may have originated from the fact that at such
+intersections there would be four sharply-projecting angles, which,
+being exposed to the influence of alternate heat and cold (during day
+and night) on the two opposite surfaces, would inevitably in time become
+fractured and crumbled away, resulting in the formation of a roughly
+circular chasm which would become partly filled up by the debris. Those
+formed by cracks radiating from craterlets would also be subject to the
+same process of rounding off to an even greater extent; and thus would
+be produced the 'oases' of various sizes up to 50 miles or more in
+diameter recorded by Mr. Lowell and other observers.
+
+_Probable Function of the Great Fissures._
+
+Mr. Pickering, as we have seen, supposes that these fissures give out
+the gases which, overflowing on each side, favour the growth of the
+supposed vegetation which renders the course of the canals visible, and
+this no doubt may have been the case during the remote periods when
+these cracks gave access to the heated portions of the surface layer.
+But it seems more probable that Mars has now cooled down to the almost
+uniform mean temperature it derives from solar heat, and that the
+fissures--now for the most part broad shallow valleys--serve merely as
+channels along which the liquids and heavy gases derived from the
+melting of the polar snows naturally flow, and, owing to their nearly
+level 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 which
+Mr. Lowell has erroneously assumed to be itself a proof of the presence
+of liquid water. Just as the blue of our sky is undoubtedly due to
+reflection from the ultra-minute dust particles in our higher
+atmosphere, similar particles brought down by the 'snow' from the higher
+Martian atmosphere might produce the blue tinge in the great volumes of
+heavy gas produced by its evaporation or liquefaction.
+
+It may be noted that Mr. Lowell objects to the carbon-dioxide theory of
+the formation of the snow-caps, that this gas at low pressures does not
+liquefy, but passes at once from the solid to the gaseous state, and
+that only water remains liquid sufficiently long to produce the blue
+colour' which plays so large a part in his argument for the mild climate
+essential for an inhabited planet. But this argument, as I have already
+shown, is valueless. For only very deep water can possibly show a blue
+colour by reflected light, while a dust-laden atmosphere--especially
+with a layer of very dense gas at the bottom of it, as would be the case
+with the newly evaporated carbon-dioxide from the diminishing snow-cap
+--would provide the very conditions likely to produce this blue tinge of
+colour.
+
+It may be considered a support to this view that carbonic-acid gas
+becomes liquid at--140 deg. F. and solid at--162 deg. F., temperatures far
+higher than we should expect to prevail in the polar and north temperate
+regions of Mars during a considerable part of the year, but such as
+might be reached there during the summer solstice when the `snows' so
+rapidly disappear, to be re-formed a few months later.
+
+_The Double Canals._
+
+The curious phenomena of the 'double canals' are undoubtedly the most
+difficult to explain satisfactorily on any theory that has yet been
+suggested. They vary in distance apart from about 100 to 400 miles. In
+many cases they appear perfectly parallel, and Mr. Lowell gives us the
+impression that they are almost always so. But his maps show, in some
+cases, decided differences of width at the two extremities, indicating
+considerable want of parallelism. A few of the curved canals are also
+double.
+
+There is one drawing in Mr. Lowell's book (p. 219) of the mouths, or
+starting points, of the Euphrates and Phison, two widely separated
+double canals diverging at an angle of about 40 deg. from the same two
+oases, so that the two inner canals cross each other. Now this suggests
+two wide bands of weakness in the planet's crust radiating probably from
+within the dark tract called the 'Mare Icarium,' and that some
+widespread volcanic outburst initiated diverging cracks on either side
+of these bands. Something of this kind may have been the cause of most
+of the double canals, or they may have been started from two or more
+craterlets not far apart, the direction being at first decided by some
+local peculiarity of structure; and where begun continuing in straight
+lines owing to homogeneity or uniform density of material. This is very
+vague, but the phenomena are so remarkable, and so very imperfectly
+known 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, a
+probable explanation of the surface-features of Mars, I have
+endeavoured to be guided by known facts or accepted theories both
+astronomical and geological. I think I may claim to have shown that
+there are some analogous features of terrestrial rock-structure to
+serve as guides towards a natural and intelligible explanation of the
+strange geometric markings discovered during the last thirty years, and
+which have raised this planet from comparative obscurity into a position
+of the very first rank both in astronomical and popular interest.
+
+This wide-spread interest is very largely due to Mr. Lowell's devotion
+to its study, both in seeking out so admirable a position as regards
+altitude and climate, and in establishing there a first-class
+observatory; and also in bringing his discoveries before the public in
+connection with a theory so startling as to compel attention. I venture
+to think that his merit as one of our first astronomical observers will
+in no way be diminished by the rejection of his theory, and the
+substitution 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--that
+is, if they were produced by the contraction of a heated outward crust
+upon a cold, and therefore non-contracting interior, the result of such
+a condition might be shown experimentally.
+
+Several baked clay balls might be formed to serve as cores, say of 8 to
+10 inches in diameter. These being fixed within moulds of say half an
+inch to an inch greater diameter, the outer layer would be formed by
+pouring in some suitable heated liquid material, and releasing it from
+the mould as soon as consolidation occurs, so that it may cool rapidly
+from the _outside._ Some kinds of impure glass, or the brittle metals
+bismuth or antimony or alloys of these might be used, in order to see
+what form the resulting fractures would take. It would be well to have
+several duplicates of each ball, and, as soon as tension through
+contraction manifests itself, to try the effect of firing very small
+charges of small shot to ascertain whether such impacts would start
+radiating fractures. When taken from the moulds, the balls should be
+suspended in a slight current of air, and kept rotating, to reproduce
+the planetary condition as nearly as possible.
+
+The exact size and material of the cores, the thickness of the heated
+outer crust, the material best suited to show fracture by contraction,
+and the details of their treatment, might be modified in various ways as
+suggested by the results first obtained. Such a series of experiments
+would probably throw further light on the physical conditions which have
+produced the gigantic system of fissures or channels we see upon the
+surface of Mars, though it would not, of course, prove that such
+conditions actually existed there. In such a speculative matter we can
+only be guided by probabilities, based upon whatever evidence is
+available.
+
+
+
+CHAPTER VIII.
+
+
+SUMMARY AND CONCLUSION.
+
+This little volume has necessarily touched upon a great variety of
+subjects, in order to deal in a tolerably complete manner with the very
+extraordinary theories by which Mr. Lowell attempts to explain the
+unique features of the surface of the planet, which, by long-continued
+study, he has almost made his own. It may therefore be well to sum up
+the main points of the arguments against his view, introducing a few
+other facts and considerations which greatly strengthen my argument.
+
+The one great feature of Mars which led Mr. Lowell to adopt the view of
+its being inhabited by a race of highly intelligent beings, and, with
+ever-increasing discovery to uphold this theory to the present time, is
+undoubtedly that of the so-called 'canals'--their straightness, their
+enormous length, their great abundance, and their extension over the
+planet's whole surface from one polar snow-cap to the other. The very
+immensity of this system, and its constant growth and extension during
+fifteen years of persistent observation, have so completely taken
+possession of his mind, that, after a very hasty glance at analogous
+facts and possibilities, he has declared them to be 'non-natural'--
+therefore to be works of art--therefore to necessitate the
+presence of highly intelligent beings who have designed and constructed
+them. This idea has coloured or governed all his writings on the
+subject. The innumerable difficulties which it raises have been either
+ignored, or brushed aside on the flimsiest evidence. As examples, he
+never even discusses the totally inadequate water-supply for such
+worldwide irrigation, or the extreme irrationality of constructing so
+vast a canal-system the waste from which, by evaporation, when exposed
+to such desert conditions as he himself describes, would use up ten
+times the probable supply.
+
+Again, he urges the 'purpose' displayed in these 'canals.' Their being
+_all_ so straight, _all_ describing great circles of the 'sphere,' all
+being so evidently arranged (as he thinks) either to carry water to some
+'oasis' 2000 miles away, or to reach some arid region far over the
+equator in the opposite hemisphere! But he never considers the
+difficulties this implies. Everywhere these canals run for thousands of
+miles across waterless deserts, forming a system and indicating a
+purpose, the wonderful perfection of which he is never tired of dwelling
+upon (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 the
+limits of the polar snows--why the method of irrigation did not, as with
+all human arts, begin gradually, at home, with terraces and channels to
+irrigate the land close to the source of the water. How, with such a
+desert as he describes three-fourths of Mars to be, did the inhabitants
+ever get to _know_ anything of the equatorial regions and its needs, so
+as to start right away to supply those needs? All this, to my mind, is
+quite opposed to the idea of their being works of art, and altogether in
+favour of their being natural features of a globe as peculiar in origin
+and internal structure as it is in its surface-features. The explanation
+I have given, though of course hypothetical, is founded on known
+cosmical and terrestrial facts, and is, I suggest, far more scientific
+as well as more satisfactory than Mr. Lowell's wholly unsupported
+speculation. This view I have explained in some detail in the preceding
+chapter.
+
+Mr. Lowell never even refers to the important question of loss by
+evaporation in these enormous open canals, or considers the undoubted
+fact that the only intelligent and practical way to convey a limited
+quantity of water such great distances would be by a system of
+water-tight and air-tight tubes laid _under the ground._ The mere
+attempt to use open canals for such a purpose shows complete ignorance
+and stupidity in these alleged very superior beings; while it is certain
+that, long before half of them were completed their failure to be of any
+use would have led any rational beings to cease constructing them.
+
+He also fails to consider the difficulty, that, if these canals are
+necessary for existence in Mars, how did the inhabitants ever reach a
+sufficiently large population with surplus food and leisure enabling
+them to rise from the low condition of savages to one of civilisation,
+and ultimately to scientific knowledge? Here again is a dilemma which is
+hard to overcome. Only a _dense_ population with _ample_ means of
+subsistence could possibly have constructed such gigantic works; but,
+given these two conditions, no adequate motive existed for the
+conception and execution of them--even if they were likely to be of any
+use, 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 that
+must _necessarily_ result from an atmospheric envelope one-twelfth (or
+at most one-seventh) the density of our own; in either case
+corresponding to an altitude far greater than that of our highest
+mountains.[17] Surely this phenomenon, everywhere manifested on the
+earth even under the equator, of a regular decrease of temperature with
+altitude, the only cause of which is a less dense atmosphere, should
+have been fairly grappled with, and some attempt made to show why it
+should not apply to Mars, except the weak remark that on a level surface
+it 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 plateaux
+often hundreds of miles in extent, in proportion to their altitude.
+Quito, at 9350 ft. above the sea, has a mean temperature of about 57 deg.
+F., giving a lowering of 23 deg. from that of Manaos at the mouth of the Rio
+Negro. This is about a degree for each 400 feet, while the general fall
+for isolated mountains is about one degree in 340 feet according to
+Humboldt, who notes the above difference between the rate of cooling for
+altitude of the plains--or more usually sheltered valleys in which the
+towns are situated--and the exposed mountain sides. It will be seen that
+this lower rate would bring the temperature of Mars at the equator down
+to 20 deg. F. below the freezing point of water from this cause alone.
+
+[Footnote 17: A four inches barometer is equivalent to a height of
+40,000 feet above sea-level with us.]
+
+But all enquirers have admitted, that if conditions as to atmosphere
+were the same as on the earth, its greater distance from the sun would
+reduce the temperature to-31 deg. F., equal to 63 deg. below the freezing
+point. It is therefore certain that the combined effect of both causes
+must bring the temperature of Mars down to at least 70 deg. or 80 deg.below the
+freezing point.
+
+The cause of this absolute dependence of terrestrial temperatures upon
+density of the air-envelope is seldom discussed in text-books either of
+geography or of physics, and there seems to be still some uncertainty
+about it. Some impute it wholly to the thinner air being unable to
+absorb and retain so much heat as that which is more dense; but if this
+were the case the soil at great altitudes not having so much of its heat
+taken up by the air should be warmer than below, since it undoubtedly
+_receives_ more heat owing to the greater transparency of the air above
+it; but it certainly does not become warmer. The more correct view seems
+to be that the loss of heat by radiation is increased so much through
+the rarity of the air above it as to _more_ than counterbalance the
+increased insolation, so that though the surface of the earth at a given
+altitude may receive 10 per cent. more direct sun-heat it loses by
+direct radiation, combined with diminished air and cloud-radiation,
+perhaps 20 or 25 per cent. more, whence there is a resultant cooling
+effect of 10 or 15 per cent. This acts by day as well as by night, so
+that the greater heat received at high altitudes does not warm the soil
+so much as a less amount of heat with a denser atmosphere.
+
+This effect is further intensified by the fact that a less dense cannot
+absorb and transmit so much heat as a more dense atmosphere. Here then
+we have an absolute law of nature to be observed operating everywhere on
+the earth, and the mode of action of which is fairly well understood.
+This law is, that reduced atmospheric pressure increases radiation, or
+loss of heat, _more rapidly_ than it increases insolation or gain of
+heat, so that the result is _always_ a considerable _lowering_ of
+temperature. What this lowering is can be seen in the universal fact,
+that even within the tropics perpetual snow covers the higher mountain
+summits, while on the high plains of the Andes, at 15,000 or 16,000 feet
+altitude, where there is very little or no snow, travellers are often
+frozen to death when delayed by storms; yet at this elevation the
+atmosphere has much more than double the density of that of Mars!
+
+The error in Mr. Lowell's argument is, that he claims for the scanty
+atmosphere 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 by
+direct radiation, as well as by the diminution of air-radiation, which
+together necessarily produce a great reduction of temperature.
+
+It is this great principle of the prepotency of radiation over
+absorption with a diminishing atmosphere that explains the excessively
+low temperature of the moon's surface, a fact which also serves to
+indicate a very low temperature for Mars, as I have shown in Chapter VI.
+These two independent arguments--from alpine temperatures and from those
+of the moon--support and enforce each other, and afford a conclusive
+proof (as against anything advanced by Mr. Lowell) that the temperature
+of Mars must be far too low to support animal life.
+
+A third independent argument leading to the same result is Dr. Johnstone
+Stoney's proof that aqueous vapour cannot exist on Mars; and this fact
+Mr. 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 the
+sun, it would have a mean temperature of about-35 deg. F. (= 456 deg. 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 a
+height where the barometer stands at about three times as high as on
+Mars, proves, that from scantiness of atmosphere alone Mars cannot
+possibly have a temperature as high as the freezing point of water; and
+this 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 of
+Mars 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--is
+non-existent.
+
+The conclusion from these three independent proofs, which enforce each
+other in the multiple ratio of their respective weights, is therefore
+irresistible--that animal life, especially in its higher forms, cannot
+exist on the planet.
+
+Mars, therefore, is not only uninhabited by intelligent beings such as
+Mr. Lowell postulates, but is absolutely UNINHABITABLE.
+
+
+
+
+
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