UC-NRLF 
 
 ISO 
 
HARPER'S LIBRARY of LIVING THOUGHT 
 

 ARE THE 
 
 PLANETS 
 
 INHABITED? 
 
 BY 
 
 E. iWALTER 
 MAUNDER 
 
 F.R.A.S. 
 
 HARPER 
 BROTHERS 
 
 LONDONXNEJnrORK 
 
ARE THE PLANETS 
 INHABITED? 
 
 BY 
 
 E. WALTER MAUNDER, F.R.A.S. 
 
 SUPERINTENDENT OF THE SOLAR DEPARTMENT, ROYAL OBSERVATORY 
 
 GREENWICH 
 
 AUTHOR OF "ASTRONOMY WITHOUT A TELESCOPE" 
 
 "THE ROYAL OBSERVATORY, GREENWICH, ITS HISTORY AND WORK" 
 
 "THE ASTRONOMY OF THE BIBLE," "THE HEAVENS AND THEIR STORY 
 
 ETC. 
 
 HARPER 6- BROTHERS 
 
 LONDON AND NEW YORK 
 
 45 ALBEMARLE STREET, W. 
 
 1913 
 
Published March 
 
CONTENTS 
 
 CHAPTER PAGE 
 
 I. THE QUESTION STATED . ; . i 
 
 II. THE LIVING ORGANISM ... 6 
 
 III. THE SUN 20 
 
 IV. THE DISTRIBUTION OF THE ELEMENTS 
 
 IN SPACE , ... . 33 
 
 V. THE MOON . . . . . 43 
 
 VI. THE CANALS OF MARS . . -57 
 
 VII. THE CONDITION OF MARS. . . 71 
 
 VIII. THE ILLUSIONS OF MARS ... 96 
 
 IX. VENUS, MERCURY AND THE ASTEROIDS 1 1 1 
 
 X. THE MAJOR PLANETS * . , 122 
 
 XI. WHEN THE MAJOR PLANETS COOL . 133 
 
 XII. THE FINAL QUESTION . . . 143 
 
 INDEX . . . . . 163 
 
 271067 
 
ARE THE PLANETS 
 INHABITED? 
 
 CHAPTER I 
 THE QUESTION STATED 
 
 THE first thought that men had concerning 
 the heavenly bodies was an obvious one : 
 they were lights. There was a greater light to 
 rule the day ; a lesser light to rule the night ; and 
 there were the stars also. 
 
 In those days there seemed an immense differ- 
 ence between the earth upon which men stood, 
 and the bright objects that shone down upon it 
 from the heavens above. The earth seemed to be 
 vast, dark, and motionless ; the celestial lights 
 seemed to be small, and moved, and shone. The 
 earth was then regarded as the fixed centre of the 
 universe, but the Copernican theory has since 
 deprived it of this pride of place. Yet from another 
 point of view the new conception of its position 
 involves a promotion, since the earth itself is now 
 regarded as a heavenly body of the same order as 
 some of those which shine down upon us. It is 
 
ARE THE PLANETS INHABITED? 
 
 amongst them, and it too moves and shines 
 shines, as some of them do, by reflecting the light 
 of the sun. Could we transport ourselves to a 
 neighbouring world, the earth would seem a star, 
 not distinguishable in kind from the rest. 
 
 But as men realized this, they began to ask : 
 " Since this world from a distant standpoint must 
 appear as a star, would not a star, if we could get 
 near enough to it, show itself also as a world ? 
 This world teems with life ; above all, it is the 
 home of human life. Men and women, gifted with 
 feeling, intelligence, and character, look upward 
 from its surface and watch the shining members of 
 the heavenly host. Are none of these the home of 
 beings gifted with like powers, who watch in their 
 turn the movements of that shining point which 
 is our world ? " 
 
 This is the meaning of the controversy on the 
 Plurality of Worlds which excited so much interest 
 some sixty years ago, and has been with us more 
 or less ever since. It is the desire to recognize the 
 presence in the orbs around us of beings like our- 
 selves, possessed of personality and intelligence, 
 lodged in an organic body. 
 
 This is what is meant when we speak of a world 
 being " inhabited." It would not, for example, at 
 all content us if we could ascertain that Jupiter 
 was covered by a shoreless ocean, rich in every 
 variety of fish ; or that the hard rocks of the Moon 
 were delicately veiled by lichens. Just as no rich- 
 
THE QUESTION STATED 
 
 ness of vegetation and no fulness and complexity 
 of animal life would justify an explorer in describing 
 some land that he had discovered as being " in- 
 habited " if no men were there, so we cannot 
 rightly speak of any other world as being " in- 
 habited " if it is not the home of intelligent life. If 
 the life did not rise above the level of algae or 
 oysters, the globe on which they flourish would be 
 uninhabited in our estimation, and its chief interest 
 would lie in the possibility that in the course of 
 ages life might change its forms and develop here- 
 after into manifestations with which we could 
 claim a nearer kinship. 
 
 On the other hand, of necessity we are pre- 
 cluded from extending our enquiry to the case of 
 disembodied intelligences, if such be conceived 
 possible. All created existences must be con- 
 ditioned, but if we have no knowledge of what 
 those conditions may be, or means for attaining 
 such knowledge, we cannot discuss them. Nothing 
 can be affirmed, nothing denied, concerning the 
 possibility of intelligences existing on the Moon or 
 even in the Sun if we are unable to ascertain under 
 what limitations those particular intelligences sub- 
 sist. Gnomes, sylphs, elves, and fairies, and all 
 similar conceptions, escape the possibility of dis- 
 cussion by our ignorance of their properties. As 
 nothing can be asserted of them they remain 
 beyond investigation, as they are beyond sight 
 and touch. 
 
ARE THE PLANETS INHABITED? 
 
 The only beings, then, the presence of which 
 would justify us in regarding another world as 
 " inhabited " are such as would justify us in 
 applying that term to a part of our own world. 
 They must possess intelligence and consciousness 
 on the one hand ; on the other, they must likewise 
 have corporeal form. True, the form might be 
 imagined as different from that we possess ; but, 
 as with ourselves, the intelligent spirit must be 
 lodged in and expressed by a living material body. 
 Our enquiry is thus rendered a physical one ; it is 
 the necessities of the living body that must guide us 
 in it ; a w r orld unsuited for living organisms is not, 
 in the sense of this enquiry, a " habitable " world. 
 
 The discussion, as it was carried on sixty years 
 ago by Dr. Whewell and Sir David Brewster, was 
 essentially a metaphysical, almost a theological one, 
 and it was chiefly considered in its supposed 
 relationship to certain religious conceptions. It 
 was urged that it was derogatory to the wisdom 
 and goodness of the Creator to suppose that He 
 would have created so many great and glorious 
 orbs without having a definite purpose in so doing, 
 and that the only purpose for which a world could 
 be made was that it might be inhabited. So, again, 
 when Dr. A. R. Wallace revived the discussion in 
 1903, he clearly had a theological purpose in his 
 opening paper, though he was taking the opposite 
 view from that held by Brewster half a century 
 earlier. 
 
THE QUESTION STATED 
 
 For myself, if there be any theological signifi- 
 cance attaching to the solving of this problem, I 
 do not know what it is. If we decide that there are 
 very many inhabited worlds, or that there are few, 
 or that there is but one our own I fail to see how 
 it should modify our religious beliefs. For example : 
 explorers have made their way across the Antarctic 
 continent to the South Pole but have found no 
 " inhabitant " there. Has this fact any theological 
 bearing ? or if, on the contrary, a race of men had 
 been discovered there, what change would it have 
 made in the theological position of anyone ? And 
 if this be so with regard to a new continent on this 
 earth, why should it be different with regard to the 
 continents of another planet ? 
 
 The problem therefore seems not to be theo- 
 logical or metaphysical, but purely physical. We 
 have simply to ask with regard to each heavenly 
 body which we pass in review : "Are its physical 
 conditions, so far as we can ascertain them, such 
 as would render the maintenance of life possible 
 upon it ? " The question is not at all as to how life 
 is generated on a world, but as to whether, if once 
 in action on a particular world, its activities could 
 be carried on. 
 
CHAPTER II 
 THE LIVING ORGANISM 
 
 A WORLD for habitation, then, is a world 
 whereon living organisms can exist that are 
 comparable in intelligence with men. But " men " 
 presuppose the existence of living organisms of 
 inferior grades. Therefore a world for habitation 
 must first of all be one upon which it is possible for 
 living organisms, as such, to exist. 
 
 It does not concern us in the present connection 
 how life first came into existence on this planet. 
 It is sufficient that we know from experience that 
 life does exist here ; and in whatsoever way it was 
 first generated here, in that same way we may 
 consider that it could have been generated on 
 another planet. 
 
 Nor need any question trouble us as to the 
 precise line of demarkation to be drawn between 
 inorganic and organic substances, or amongst the 
 latter, between plants and animals. These are 
 important subjects for discussion, but they do 
 not affect us here, for we are essentially con- 
 cerned with the highest form of organism, the one 
 furthest from these two dividing lines. 
 6 
 
THE LIVING ORGANISM 
 
 It suffices that living organisms do exist here, 
 and exist under well-defined conditions. Wanting 
 these conditions, they perish. We can, to a vary- 
 ing degree, determine the physical conditions pre- 
 vailing upon the heavenly bodies, and we can 
 ascertain whether these physical conditions would 
 be favourable, unfavourable, or fatal to the living 
 organism. 
 
 What is a living organism ? A living organism 
 is such that, though it is continually changing its 
 substance, its identity, as a whole, remains essen- 
 tially the same. This definition is incomplete, but 
 it gives us a first essential approximation, it 
 indicates the continuance of the whole, with the 
 unceasing change of the details. Were this 
 definition complete, a river would furnish us with 
 a perfect example of a living organism, because, 
 while the river remains, the individual drops of 
 water are continually changing. There is then 
 something more in the living organism than the 
 continuity of the whole, with the change of the 
 details. 
 
 An analogy, given by Max Verworn, carries us 
 a step further. He likens life to a flame, and takes 
 a gas flame with its butterfly shape as a particu- 
 larly appropriate illustration. Here the shape of 
 the flame remains constant, even in its details. 
 Immediately above the burner, at the base of the 
 flame, there is a completely dark space ; surround- 
 ing this, a bluish zone that is faintly luminous ; 
 
ARE THE PLANETS INHABITED? 
 
 and beyond this again, the broad spread of the 
 two wings that are brightly luminous. The flame, 
 like the river, preserves its identity of form, while 
 its constituent details the gases that feed it are 
 in continual change. But there is not only a change 
 of material in the flame ; there is a change of 
 condition. Everywhere the gas from the burner 
 is entering into energetic combination with the 
 oxygen of the air, with evolution of light and heat. 
 There is change in the constituent particles as well 
 as change of the constituent particles ; there is 
 more than the mere flux of material through the 
 form ; there is change of the material, and in the 
 process of that change energy is developed. 
 
 A steam-engine may afford us a third illustration. 
 Here fresh material is continually being introduced 
 into the engine there to suffer change. Part is 
 supplied as fuel to the fire there to maintain the 
 temperature of the engine ; so far the illustration 
 is analogous to that of the gas flame. But the 
 engine carries us a step further, for part of the 
 material supplied to it is water, which is converted 
 into steam by the heat of the fire, and from the 
 expansion of the steam the energy sought from the 
 machine is derived. Here again we have change in 
 the material with development of energy ; but 
 there is not only work done in the subject, there is 
 work done by it. 
 
 But the living organism differs from artificial 
 machines in that, of itself and by itself, it is con- 
 8 
 
THE LIVING ORGANISM 
 
 tinuously drawing into itself non-living matter, 
 converting it into an integral part of the organism, 
 and so endowing it with the qualities of life. And 
 from this non-living matter it derives fresh energy 
 for the carrying on of the life of the organism. 
 
 The engine and the butterfly gas flame do not 
 give us, any more than the river, a complete 
 picture of the living organism. The form of the 
 river is imposed upon it from without ; the river 
 is defined by its bed, by the contour of the country 
 through which it flows. The form and size of the 
 flame are equally defined by exterior conditions ; 
 they are imposed upon it by the shape of the 
 burner and the pressure of the gas passing through 
 it. The form of the engine is as its designer has 
 made it. But the form of the living organism is 
 imposed upon it from within ; and, as far as we can 
 tell, is inherent in it. Here is the wonder and 
 mystery of life : the power of the living organism 
 to assimilate dead matter, to give it life and bring it 
 into the law and unity of the organism itself. But 
 it cannot do this indiscriminately ; it is not able 
 thus to convert every dead material ; it is re- 
 stricted, narrowly restricted, in its action. " One 
 of the chief characteristics of living matter is found 
 in the continuous range of chemical reactions which 
 take place between living cells and their inorganic 
 surroundings. Without cease certain substances 
 are taken up and disappear in the endless round of 
 chemical reactions in the cell. Other substances 
 
ARE THE PLANETS INHABITED? 
 
 which have been produced by the chemical re- 
 actions in living matter pass out of the cell and 
 reappear in inorganic nature as waste products 
 of the life process. The whole complex of these 
 chemical transformations is generally called 
 Metabolism. Inorganic matter contrasts strikingly 
 with living substance. However long a crystal or 
 a piece of metal is kept in observation, there is no 
 change of the substance, and the molecules remain 
 the same and in the same number. For living 
 matter the continuous change of substances is an 
 indispensable condition of existence. To stop the 
 supply of food material for a certain time is 
 sufficient to cause a serious lesion of the life 
 process or even the death of the cell. But the 
 same happens when we hinder the passing out of 
 the products of chemical transformation from the 
 cell. On the other hand, we may keep a crystal of 
 lifeless matter in a glass tube carefully shut up 
 from all exchange of substance with the external 
 world for as many years as we like. The existence 
 of this crystal will continue without end and 
 without change of any of its properties. There is 
 no known living organism which could remain in a 
 dry resting state for an infinitely long period of 
 time. The longest lived are perhaps the spores of 
 mosses which can exist in a dry state more than a 
 hundred years. As a rule the seeds of higher plants 
 show their vital power already weakened after ten 
 years ; most of them do not germinate if kept 
 
 10 
 
THE LIVING ORGANISM 
 
 more than twenty to thirty years. These experi- 
 ences lead to the opinion that even dry seeds and 
 spores of lower plants in their period of rest of 
 vegetation continue the processes of metabolism to 
 a certain degree. This supposition is confirmed by 
 the fact that a very slight respiration and produc- 
 tion of carbonic acid can be proved when the seeds 
 contain a small percentage of water. It seems as if 
 life were weakened in these plant organs to a quite 
 imperceptible degree, but never, not even tempor- 
 arily, really suspended. 
 
 " Life is, therefore, quite inseparable from chemi- 
 cal reactions, and on the whole what we call life is 
 nothing else but a complex of innumerable chemical 
 reactions in the living substance which we call 
 protoplasm."* 
 
 The essential quality, therefore, of life is con- 
 tinual change, but not mere change in general. It 
 is that special process of the circulation of matter 
 which we call metabolism, and this circulation 
 is always connected with a particular chemical 
 substance protoplasm. 
 
 In this substance five elements are always 
 present and predominant carbon, oxygen, nitro- 
 gen, hydrogen, and sulphur. The compounds 
 which these five elements form with each other are 
 
 * Chemical Phenomena in Life, pp. 62-3, by Dr. Frederick 
 Czapek (Harper's Library of Living Thought). The reader 
 is strongly recommended to study this work in the present 
 connection. 
 
 II 
 
ARE THE PLANETS INHABITED? 
 
 most complex and varied, and they also admit to 
 combination but in smaller porportions some of 
 the other elements, of which phosphorus, potassium, 
 calcium, magnesium, and iron are the most im- 
 portant. 
 
 For protoplasm using the term in the most 
 general sense is a chemical substance, not a mere 
 mixture of a number of chemical elements, nor a 
 mere mechanical structure. " However differently 
 the various plasma substances behave in detail, 
 they always exhibit the same general composi- 
 tion as the other albuminoids out of the five 
 ' organo-genetic elements ' namely in point of 
 weight, 51-54 % carbon, 21-23 % oxygen, 15-17 % 
 nitrogen, 6-7 % hydrogen, and 1-2 % sulphur."* 
 
 Haeckel, the writer just quoted, describes the 
 plasm, the universal basis of all the vital phe- 
 nomena, in the following terms : " In every case 
 where we have with great difficulty succeeded in 
 examining the plasm as far as possible and 
 separating it from the plasma-products, it has the 
 appearance of a colourless, viscous substance, the 
 chief physical property of which is its peculiar 
 thickness and consistency. The physicist dis- 
 tinguishes three conditions of inorganic matter 
 solid, fluid, and gaseous. Active living protoplasm 
 cannot be strictly described as either fluid or solid 
 in the physical sense. It presents an intermediate 
 
 * Wonders of Life, by Ernst Haeckel, Professor at Jena 
 University, p. 130. 
 
 12 
 
THE LIVING ORGANISM 
 
 stage between the two which is best described as 
 viscous ; it is best compared to a cold jelly, or 
 solution of glue. Just as we find the latter sub- 
 stance in all stages between the solid and the fluid, 
 so we find in the case of protoplasm. The cause of 
 this softness is the quantity of water contained in 
 the living matter, which generally amounts to a 
 half of its volume and weight. The water is dis- 
 tributed between the plasma molecules or the 
 ultimate particles of living matter in much the 
 same way as it is in the crystals of salts, but with 
 the important difference that it is very variable 
 in quantity in the plasm. On this depends the 
 capacity for the absorption or imbibition in the 
 plasm, and the mobility of its molecules, which is 
 very important for the performance of the vital 
 actions. However, this capacity of absorption has 
 definite limits in each variety of plasm ; living 
 plasm is not soluble in water, but absolutely 
 resists the penetration of any water beyond this 
 limit."* And Czapek further tells us that " the 
 most striking feature of cell life is the fact that an 
 enormous number of chemical reactions take place 
 within the narrowest space. Most plant cells do 
 not exceed 0*1 to 0-5 millimetres in diameter. 
 Their greatest volume therefore can only be an 
 eighth of a cubic millimetre. Nevertheless, in this 
 minute space we notice in every stage of cell life a 
 considerable number of chemical reactions which 
 * Wonders of Life, pp. 127-8. 
 13 
 
ARE THE PLANETS INHABITED? 
 
 are carried on contemporaneously, without one 
 disturbing the other in the slightest degree."* 
 
 It is clear if organic bodies were built up of 
 chemical compounds of small complexity and great 
 stability that this continuous range of chemical 
 reactions, this unceasing metabolism, could not 
 take place. It is therefore a necessary condition 
 for organic substances that they should be built 
 up of chemical compounds that are most complex 
 and unstable. " Exactly those substances which 
 are most important for life possess a very high 
 molecular weight, and consequently very large 
 molecules, in comparison with inorganic matter. 
 For example : egg- albumin is said to have the 
 molecular weight of at least 15,000, starch more 
 than 30,000, whilst the molecular weight of 
 hydrogen is 2, of sulphuric acid and of potassium 
 nitrate about 100, and the molecular weight of the 
 heaviest metal salts does not exceed about 300. "f 
 
 To sum up : the living organism, whether it be 
 a simple cell, or the ordered community of cells 
 making up the perfect plant or animal, is an 
 entity, a living individual, wherein highly complex 
 and unstable compounds are unceasingly under- 
 going chemical reactions, a metabolism essentially 
 associated with protoplasm. But these complex 
 compounds are, nevertheless, formed by the 
 combinations of but a few of the elements now 
 known to us. 
 
 * Chemical Phenomena in Life, p. 58. f Ibid., p. 22. 
 14 
 
THE LIVING ORGANISM 
 
 Many writers on the subject of the habitability 
 of other worlds, from contemplating the rich and 
 apparently limitless variety of the forms of life, and 
 the diversity of the conditions under which they 
 exist, have been led to assume that the basis of life 
 must itself also in like manner be infinitely broad 
 and infinitely varied. In this they are mistaken. 
 As we have seen, the elements entering into the 
 composition of organic bodies are, in the main, few 
 in number. The temperatures at which they can 
 exist are likewise strictly limited. But, above all, 
 that circulation of matter which we call Life the 
 metabolism of vital processes requires^tor its 
 continuance the presence of op^ indispensable 
 factor WATER. 
 
 Protoplasm itself, as Cz^pek puts it, is practically 
 an albumin sol; that is to say, it is a chemical 
 substance of which the chief constituents are 
 albuminous matter and water, and the protoplasm 
 can only take from without material dissolved in 
 water ; it can only eject matter in the same way. 
 This osmosis is an indispensable condition in the 
 vital process. And the " streaming " of proto- 
 plasm, its continual movement in the cell, can only 
 be carried on in water. 
 
 WATER is the compound of oxygen and hydro- 
 gen in the proportion of two atoms of hydrogen to 
 one of oxygen. It is familiar to us in three states : 
 solid, liquid, and gaseous, or ice, water, and steam. 
 But it is only in the liquid state that water is 
 
ARE THE PLANETS INHABITED? 
 
 available for carrying on the processes of life. This 
 fact limits the temperatures at which the organic 
 functions can be carried on, for water under 
 terrestrial conditions is only liquid for a hundred 
 degrees; it freezes at o Centigrade, it boils at 
 100 Centigrade. Necessarily, our experiences 
 are mostly confined within this range, and there- 
 fore we are apt unconsciously to assume that this 
 range is all the range that is possible, whereas it is 
 but a very small fraction of the range conceivable, 
 and indeed existing, in cosmical space. In its 
 liquid state water is a general solvent, and yet pure 
 water is neutral in its qualities, both characteristics 
 being essential to its usefulness as a vehicle for the 
 protoplasmic actions. Naturally, this function of 
 water as a solvent can only exist when water is in 
 the liquid state ; solid water, that is ice, neither 
 dissolves nor flows ; and water, when heated to 
 boiling point, passes into vapour, and so leaves the 
 organism moistureless, and therefore dead. It is 
 possible to grind a living organism to a pulp so 
 that the structure of the cells is practically de- 
 stroyed, and yet for some reactions which are quite 
 peculiar to life still to show themselves for some 
 appreciable time. But when the cell-pulp is 
 heated to the temperature of boiling water, these 
 chemical processes cannot be longer observed. 
 What is left may then be considered as definitely 
 dead. 
 
 Water is, then, indispensable for the living 
 16 
 
THE LIVING ORGANISM 
 
 organism ; but there are two great divisions of 
 such organisms plants and animals. Animals are 
 generally, but not universally, free to move, and 
 therefore to travel to seek their food. But their 
 food is restricted ; they cannot directly convert 
 inorganic matter to their own use ; they can only 
 assimilate organic material. The plant, on the 
 other hand, unlike the animal, can make use of 
 inorganic material. Plant life, therefore, requires 
 an abundant supply of water in which the various 
 substances necessary for its support can be dis- 
 solved ; it must either be in water, or, if on land, 
 there must be an active circulation of water both 
 through the atmosphere and through the soil, so 
 as to bring to it the food that it requires. Animal 
 life presupposes plant life, for it is always dependent 
 upon it. 
 
 Many writers have assumed that life is very 
 widely distributed in connection with this planet. 
 The assumption is a mistaken one, as has been 
 well pointed out by Garrett P. Serviss, a charming 
 writer on astronomical subjects : "On the Earth 
 we find animated existence confined to the surface 
 of the crust of the globe, to the lower and denser 
 strata of the atmosphere, and to the film of water 
 that constitutes the oceans. It does not exist in 
 the heart of the rocks forming the body of the 
 planet nor in the void of space surrounding it out- 
 side the atmosphere. As the Earth condensed from 
 the original nebula, and cooled and solidified, a 
 c 17 
 
ARE THE PLANETS INHABITED? 
 
 certain quantity of matter remained at its surface 
 in the form of free gases and unstable compounds, 
 and, within the narrow precincts where these things 
 were, lying like a thin shell between the huge inert 
 globe of permanently combined elements below, 
 and the equally unchanging realm of the ether 
 above, life, a phenomenon depending upon cease- 
 less changes, combinations and re-combinations of 
 chemical elements in unstable and temporary union, 
 made its appearance, and there only we find it at 
 the present time."* 
 
 " The huge inert globe of permanently combined 
 elements below, and the equally unchanging realm 
 of the ether above," offer no home for the living 
 organism ; least of all for the highest of such 
 organisms Man. Both must be tempered to a 
 condition which will permit and favour continual 
 change, the metabolism which is the essential 
 feature of life. 
 
 " When the earth had to be prepared for the 
 habitation of man, a veil, as it were, of inter- 
 mediate being was spread between him and its 
 darkness, in which were joined, in a subdued 
 measure, the stability and the insensibility of the 
 earth, and the passion and perishing of mankind. 
 
 " But the heavens, also, had to be prepared for 
 
 his habitation. Between their burning light, their 
 
 deep vacuity, and man, as between the earth's 
 
 gloom of iron substance, and man, a veil had to be 
 
 * Other Worlds, by Garrett P. Serviss, pp. 63-4. 
 
 18 
 
THE LIVING ORGANISM 
 
 spread of intermediate being; which should ap- 
 pease the unendurable glory to the level of human 
 feebleness, and sign the changeless motion of the 
 heavens with the semblance of human vicissitude. 
 Between the earth and man arose the leaf. Between 
 the heaven and man came the cloud. His life 
 being partly as the falling leaf and partly as the 
 flying vapour."* 
 
 The leaf and the cloud are the signs of a habit- 
 able world. The leaf that is to say, plant life, 
 vegetation is necessary because animal life is 
 not capable of building itself up from inorganic 
 material. This step must have been previously 
 taken by the plant. The cloud, that is to say 
 water-vapour, is necessary because the plant in 
 its turn cannot directly assimilate to itself the 
 nitrogen from the atmosphere. The food for the 
 plant is brought to it by water, and it assimilates 
 it by the help of water. It is, therefore, upon the 
 question of the presence of water that the question 
 of the habitability of a given world chiefly turns. 
 In the physical sense, man is " born of water," and 
 any world fitted for his habitation must " stand 
 out of the water and in the water." 
 
 * Modern Painters^ by John Ruskin. 
 
CHAPTER III 
 THE SUN 
 
 THE Sun is, of all the heavenly bodies, the 
 most impressive, and has necessarily, at all 
 times, attracted the chief attention of men. There 
 are only two of the heavenly bodies that appear to 
 be more than joints of light, only two that show a 
 surface to the naked eye, and the Sun, being so 
 much the brighter of the two, and the obvious 
 source of all our light and heat, and the fosterer of 
 vegetation, readily takes the premier place in 
 interest. In the present day we know too much 
 about the Sun for anyone to suppose that it can be 
 the home of organic life ; but it is not many years 
 since its habitability was seriously suggested even 
 by so high an authority as Sir William Herschel. 
 He conceived that it was possible that its stores of 
 light and heat might be confined to a relatively thin 
 shell in its upper atmosphere, and that below this 
 shell a screen of clouds might so check radiation 
 downward that it would be possible for an inner 
 nucleus to exist which should be cool and solid. 
 This fancied inner globe would then necessarily 
 enjoy perpetual daylight, and a climate which 
 knew no variation from pole to pole. To its 
 20 
 
THE SUN 
 
 inhabitants the entire heavens would be generally 
 luminous, the light not being concentrated into 
 any one part of the vault ; and it was supposed that, 
 ignorant of time, a happy race might flourish, 
 cultivating the far-spread solar fields, in per- 
 petual daylight, and in the serenity of a perpetual 
 spring that was distracted by no storm. 
 
 The picture thus conjured up is a pleasing one, 
 though probably, to the restless sons of Earth, it 
 would seem to suffer somewhat from monotony. 
 But we now know that it corresponds in not a 
 single detail to the actual facts. The study of 
 solar conditions carried on through the last hundred 
 3 r ears has revealed to us, not serenity and peace, 
 but storm, stress, and commotion on the most 
 gigantic scale. But though we now can dismiss 
 from our minds the possibility that the Sun can be 
 inhabited, yet it is of such importance to the 
 maintenance of life on this planet, and by parity 
 of reasoning to life on any other planet, that a 
 review of its conditions forms a necessary intro- 
 duction to our subject. Further, those conditions 
 themselves will bring out certain principles that 
 are of necessary application when we come to con- 
 sider the case of particular planets. 
 
 The distance of the Sun from the Earth is often 
 spoken of as the " astronomical unit " ; it is the 
 fundamental measure of astronomy, and all our 
 information as to the sizes and distances of the 
 various planets rests upon it. And, as we shall 
 
 21 
 
ARE THE PLANETS INHABITED? 
 
 shortly see, the particular problem with which we 
 are engaged the habitability of worlds is directly 
 connected with these two factors : the size of the 
 world in question, and its distance from the Sun. 
 
 The distance of the Sun has been determined by 
 several different methods the principles of which 
 do not concern us here, but they agree in giving 
 the mean distance of the Sun as a little less than 
 93,000,000 miles ; that is to say, it would require 
 11,720 worlds as large as our own to be put side by 
 side in order to bridge the chasm between the two. 
 Or a traveller going round the Earth at its equator 
 would have to repeat the journey 3730 times before 
 he had traversed a space equal to the Sun's 
 distance. 
 
 But knowing the Sun's distance, we are able to 
 aeduce its actual diameter, its superficial extent, 
 and its volume, for its apparent diameter can 
 readily be measured. Its actual diameter then 
 comes out as 866,400 miles, or 109-4 times that of 
 the Earth. Its surface exceeds that of the Earth 
 11,970 times ; its volume, 1,310,000 times. 
 
 But the weight of the Sun is known as well as its 
 size ; this follows as a consequence of gravitation. 
 For the planets move in orbits under the influence 
 of the Sun's attraction ; the dimensions of their 
 orbits are known, and the times taken in de- 
 scribing them ; the amount of the attractive force 
 therefore is also known, that is to say, the mass of 
 the Sun. This is 332,000 times the mass of the 
 
 22 
 
THE SUN 
 
 Earth ; and as the latter has been determined as 
 equal to about 
 
 6,000,000,000,000,000,000,000 tons 
 that of the Sun would be equal to 
 
 2,000,000,000,000,000,000,000,000,000 tons. 
 
 It will be seen that the proportion of the volume 
 of the Sun to that of the Earth is greater than the 
 proportion of its mass to the Earth's mass almost 
 exactly four times greater; so that the mean density 
 of the Sun can be only one-fourth that of the Earth. 
 Yet, if we calculate the force of gravity at the 
 surfaces of both Sun and Earth, we find that the 
 Sun has a great preponderance. Its mass is 332,000 
 times that of the Earth, but to compare it with the 
 attraction of the Earth's surface we must divide by 
 (109-4) 2 , since the distance of the Sun's centre 
 from its surface is 109-4 times as great as the 
 corresponding distance in the case of the Earth, 
 and the force of gravity diminishes as the square 
 of the increased distance. This gives the force of 
 gravity at the solar surface as 27-65 times its 
 power at the surface of the Earth, so that a body 
 weighing one ton here would weigh 27 tons 13 cwt. 
 if it were taken to the Sun.* 
 
 This relation is one of great importance when we 
 realize that the pressure of the Earth's atmosphere 
 
 * If this experiment could be carried out, it would be neces- 
 sary to use a spring balance. If the object were weighed in a 
 pair of scales or by a steelyard, the counterbalancing weights 
 would be likewise affected in the same proportion, so that the 
 equilibrium would be undisturbed. 
 
 23 
 
ARE THE PLANETS INHABITED? 
 
 is 147 Ib. on the square inch at the sea level ; that 
 is to say, if we could take a column of air one 
 square inch in section, extending from the surface 
 of the Earth upwards to the very limit of the 
 atmosphere, we should find that it would have this 
 weight. If we construct a water barometer, the 
 column of water required to balance the atmo- 
 sphere must be 34 feet high, while the height of 
 the column of mercury in a mercurial barometer is 
 30 inches high, for the weight of 30 cubic inches of 
 mercury or of 408 cubic inches of water (34 X 12 = 
 408) is 14-7 Ib. 
 
 If, now, we ascend a mountain, carrying a 
 mercurial barometer with us we should find that it 
 would fall about one inch for the first 900 feet of 
 our ascent ; that is to say, we should have left one- 
 thirtieth of the atmosphere below us by ascending 
 900 feet. As we went up higher we should find 
 that we should have to climb more than 900 feet 
 further in order that the barometer might fall 
 another inch ; and each successive inch, as we went 
 upward, would mean a longer climb. At the height 
 of 2760 feet the barometer would have fallen three 
 inches ; we should have passed through one-tenth 
 of the atmosphere. At the height of 5800 feet, we 
 should have passed through one-fifth of the atmo- 
 sphere, the barometer would have dropped six 
 inches ; and so on, until at about three and a third 
 miles above sea level the barometer would read 
 fifteen inches, showing that we had passed through 
 24 
 
THE SUN 
 
 half the atmosphere. Mont Blanc is not quite 
 three miles high, so that in Europe we cannot 
 climb to the height where half the atmosphere is 
 left below us, and there is no terrestrial mountain 
 anywhere which would enable us to double the 
 climb ; that is to say, to ascend six and two-third 
 miles. Could we do so, however, we should find 
 that the barometer had fallen to seven and a half 
 inches ; that the second ascent of three and a 
 third miles had brought us through half the re- 
 maining atmosphere, so that only one-fourth still 
 remained above us. In the celebrated balloon 
 ascent made by Mr. Coxwell and Mr. Glaisher on 
 September 5, 1861, an even greater height was 
 attained, and it was estimated that the barometer 
 fell at its lowest reading to seven inches, which 
 would correspond to a height of 39,000 feet. 
 
 But on the Sun, where the force of gravity is 
 27-65 times as great as at the surface of the Earth, 
 it would, if all the other conditions were similar, 
 only be necessary to ascend one furlong, instead 
 of three and a third miles, in order to reach the 
 level of half the surface pressure, and an ascent of 
 two furlongs would bring us to the level of quarter 
 pressure, and so on. If then the solar atmosphere 
 extends inwards, below the apparent surface, it 
 should approximately double in density with each 
 furlong of descent. These considerations, if taken 
 alone, would point to a mean density of the Sun 
 not as we know it to be, less than that of the Earth, 
 
 25 
 
ARE THE PLANETS INHABITED? 
 
 but immeasurably greater ; but the discordance is 
 sufficiently explained when we come to another 
 class of facts. 
 
 , These relate to the temperature of the Sun, and 
 to the enormous amount of light and heat which 
 it radiates forth continually. This entirely tran- 
 scends our power to understand or appreciate. 
 Nevertheless, the astonishing figures which the 
 best authorities give us may, by their vastness, 
 convey some rough general impression that may be 
 of service. Thus Prof. C. A. Young puts the total 
 quantity of sunlight as equivalent to 
 
 1,575,000,000,000,000,000,000,000,000 
 
 standard candles. 
 
 The intensity of sunlight at each point of the Sun's 
 surface is variously expressed as 
 
 190,000 times that of a standard candle, 
 5300 times that of the metal in a Bessemer 
 
 converter, 
 
 146 times that of a calcium light, 
 or, 3-4 times that of an electric arc. 
 The same authority estimates at 30 calories the 
 value of the Solar Constant ; that is to say, the heat 
 which, if our atmosphere were removed, would be 
 received from the Sun in a minute of time upon a 
 square metre of the Earth's surface that had the 
 Sun in its zenith, would be sufficient to raise the 
 temperature of a kilogram of water 30 degrees 
 Centigrade. This would involve that the heat 
 radiation from each square metre of the Sun's sur- 
 26 
 
THE SUN 
 
 face would equal 1,340,000 calories ; or sufficient 
 to melt through in each minute of time a shell of 
 ice surrounding the Sun to the thickness of 58-2 
 feet. Prof. Abbot's most recent determination of 
 the solar constant diminishes these estimates by 
 one third ; but he still gives the probable tempera- 
 ture of the solar surface as not far short of 7000 
 degrees Centigrade, or about 12,000 degrees 
 Fahrenheit. 
 
 The Sun, then, presents us with temperatures and 
 pressures which entirely surpass our experience on 
 the Earth. The temperatures, on the one hand, are 
 sufficient to convert into a permanent gas every 
 substance with which we are acquainted ; the pres- 
 sures, on the other hand, apart from the high 
 temperatures, would probably solidify every ele- 
 ment, and the Sun, as a whole, would present itself 
 to us as a comparatively small solid globe, with a 
 density like that of platinum. With both factors 
 in operation, we have the result already given : a 
 huge globe, more than one hundred times the 
 diameter of the Earth, yet only one-fourth its 
 density, and gaseous probably throughout the 
 whole of its enormous bulk. 
 
 What effect have these two factors, so stupendous 
 in scale, upon its visible surface ? What is the ap- 
 pearance of the Sun ? 
 
 It appears to be a large glowing disc, sensibly 
 circular in outline, with its edge fairly well-defined 
 both as seen in the telescope and as registered on 
 27 
 
ARE THE PLANETS INHABITED? 
 
 photographs. In the spectroscope, or when in an 
 eclipse of the Sun the Moon covers the whole disc, 
 a narrow serrated ring is seen surrounding the rim, 
 like a velvet pile of a bright rose colour. This 
 crimson rim, the sierra or chromosphere as it is 
 usually called, is always to be found edging the 
 entire Sun, and therefore must carpet the surface 
 everywhere. But under ordinary conditions, we do 
 not see the chromosphere itself, but look down 
 through it on the photosphere, or general radiating 
 surface. This, to the eye, certainly looks like a 
 definite shell, but some theorists have been so im- 
 pressed with the difficulty of conceiving that a 
 gaseous body like the Sun could, under the condi- 
 tions of such stupendous temperatures as there 
 exist, have any defined limit at all, that they deny 
 that what we see on the Sun is a real boundary, and 
 argue that it only appears so to us through the 
 effects of the anomalous refraction or dispersion of 
 light. Such theories introduce difficulties greater 
 and more numerous than those that they clear 
 away, and they are not generally accepted by 
 practical observers of the Sun. They seem in- 
 compatible with the apparent structure of the 
 photosphere, which is everywhere made up of a 
 complicated mottling : minute grains somewhat 
 resembling those of rice in shape, of intense bright- 
 ness, and irregularly scattered. This mottling is 
 sometimes coarsely, sometimes finely textured ; in 
 some regions it is sharp and well defined, in others 
 28 
 
THE SUN 
 
 misty or blurred, and in both cases they are often 
 arranged in large elaborate patterns, the figures of 
 the pattern sometimes extending for a hundred 
 thousand miles or more in any direction. The rice- 
 like grains or granules of which these figures are 
 built up, and the darker pores between them, are, 
 on the other hand, comparatively small, and do not, 
 on the average, exceed two to four hundred miles in 
 diameter. 
 
 But the Sun shows us other objects of quite a 
 different order in their dimensions. Here and there 
 the bright granules of the photosphere become 
 disturbed and torn apart, and broad areas are 
 exposed which are relatively dark. These are sun- 
 spots, and in the early stages of their development 
 they are usually arranged in groups which tend to 
 be stretched out parallel to the Sun's equator. A 
 group of spots in its later stages of development is 
 more commonly reduced to a single round, well- 
 defined, dark spot. These groups, when near the 
 edge of the Sun, are usually seen to be accompanied 
 by very bright markings, arranged in long irregular 
 lines, like the foam on an incoming tide. These 
 markings are known as the fac^llae, from their 
 brightness. In the spectroscope, when the serrated 
 edges of the chromosphere are under observation, 
 every now and then great prominences, or tongues 
 and clouds of flame, are seen to rise up from them, 
 sometimes changing their form and appearance so 
 rapidly that the motion can almost be followed by 
 
 29 
 
ARE THE PLANETS INHABITED? 
 
 the eye. An interval of fifteen or twenty minutes 
 has frequently been sufficient to transform, quite 
 beyond recognition, a mass of flame fifty thousand 
 miles in height. Sometimes a prominence of these, 
 or even greater, dimensions has formed, developed, 
 risen to a great distance from the Sun, and com- 
 pletely disappeared within less than half an hour. 
 The velocity of the gas streams in such eruptions 
 often exceeds one hundred miles a second ; some- 
 times, though only rarely, it reaches a speed twice 
 as great. 
 
 Sunspots do not offer us examples of motions of 
 this order of rapidity, but the areas which they 
 affect are not less astonishing. Many spot groups 
 have been seen to extend over a length of one 
 hundred thousand, or one hundred and fifty thou- 
 sand miles, and to cover a total area of a thousand 
 million square miles. Indeed, the great group of 
 February, 1905, at its greatest extent, covered an 
 area four times as great as this. Again, in the 
 normal course of the development of a spot group, 
 the different members of the group frequently show 
 a kind of repulsion for each other in the early stages 
 of the group's history, and the usual speed with 
 which they move away from each other is three 
 hundred miles an hour. 
 
 The spots, the faculae, the prominences, are all, 
 in different ways, of the nature of storms in an 
 atmosphere ; that is to say, that, in the great gaseous 
 bulk of the Sun, certain local differences of constitu- 
 
 30 
 
THE SUN 
 
 tion, temperature, and pressure are marked by 
 these different phenomena. From this point of 
 view it is most significant that many spots are 
 known to last for more than a month ; some have 
 been known to endure for even half a year. The 
 nearest analogy which the Earth supplies to these 
 disturbances may be found in tropical cyclones, 
 but these are relatively of far smaller area, and only 
 last a few days at the utmost, while a hundred miles 
 an hour is the greatest velocity they ever exhibit, 
 and this, fortunately, only under exceptional 
 circumstances. For a wind of such violence mows 
 down buildings and trees as a scythe the blades of 
 grass ; and were tornadoes moving at a rate of 300 
 miles an hour as common upon the Earth as spots 
 are upon the Sun, it would be stripped bare of 
 plants and animals, as well as of men and of all 
 their works. 
 
 It is not an accident that the Sun, when storm- 
 swept, shows this violence of commotion, but a 
 necessary consequence of its enormous temperature 
 and pressures. As we have seen, the force of 
 gravity at its surface is 27-65 times that at the 
 surface of the Earth, where a body falls 16-1 feet in 
 the first second of time ; on the Sun, therefore, a 
 body would fall 445 feet in the first second ; and 
 the atmospheric motions generally would be 
 accelerated in the same proportion. 
 
 The high temperatures, the great pressures, the 
 violent commotions which prevail on the Sun are, 
 
 31 
 
ARE THE PLANETS INHABITED? 
 
 therefore, the direct consequence of its enormous 
 mass. The Sun is, then, not merely the type and 
 example of the chief source of light and heat in a 
 given planetary system ; it indicates to us that size 
 and mass are the primary tokens by which we may 
 judge the temperature of a world, and the activity 
 to be expected in its changes. 
 
CHAPTER IV 
 
 THE DISTRIBUTION OF THE ELEMENTS 
 IN SPACE 
 
 IT is now an old story, but still possessing its 
 interest, how Fraunhofer analysed the light 
 of the Sun by making it pass through a narrow 
 slit and a prism, and found that the broad rainbow- 
 tinted band of light so obtained was interrupted 
 by hundreds of narrow dark lines, images in nega- 
 tive of the slit ; and how Kirchhoff succeeded in 
 proving that two of these dark lines were caused by 
 the white light of the solar photosphere having 
 suffered absorption at the Sun by passing through 
 a stratum of glowing sodium vapour. From that 
 time forward it has been known that the Sun is 
 surrounded by an atmosphere of intensely heated 
 gases, among which figure many of those elements 
 familiar to us in the solid form on the Earth, such 
 as iron, cobalt, nickel, copper, manganese, and the 
 like. These metals, here the very types of solid 
 bodies, are permanent gases on the Sun. 
 
 The Sun, then, is in an essentially gaseous con- 
 dition, enclosed by the luminous shell which we 
 term the photosphere. This shell Prof. C. A. 
 Young and the majority of astronomers regard as 
 
 D 33 
 
ARE THE PLANETS INHABITED? 
 
 consisting of a relatively thin layer of glowing 
 clouds, justifying the quaint conceit of R. A. Proc- 
 tor, who spoke of the Sun as a " Bubble" ; that 
 is, a globe of gas surrounded by an envelope so 
 thin in comparison as to be a mere film. There 
 has been much difference of opinion as to the sub- 
 stance forming these clouds, but the theory is still 
 widely held which was first put forward by Dr. 
 Johnstone Stoney in 1867, that they are due to 
 the condensation of carbon, the most refractory of 
 all known elements. Prof. Abbot, however, re- 
 fuses to believe in a surface of this nature, 
 holding that the temperature of the Sun is too 
 high even at the surface to permit any such 
 condensation. 
 
 The application of the spectroscope to astronomy 
 is not confined to the Sun, but reaches much 
 further. The stars also yield their spectra, and we 
 are compelled to recognize that they also are 
 suns ; intensely heated globes of glowing gas, rich 
 in the same elements as those familiar to us on the 
 Earth and known by their spectral lines to be 
 present on the Sun. The stars, therefore, cannot 
 themselves be inhabited worlds any more than the 
 Sun, and at a stroke the whole of the celestial 
 luminaries within the furthest range of our most 
 powerful telescopes are removed from our present 
 search. Only those members of our solar system 
 that shine by reflecting the light of the Sun can be 
 cool enough for habitation ; the true stars cannot 
 
 34 
 
DISTRIBUTION OF ELEMENTS 
 
 be inhabited, for, whatever their quality and order, 
 they are all suns, and must necessarily be in far too 
 highly heated a condition to be the abode jDiJlife. 
 Many of them may, perhaps, be a source of light 
 and heat to attendant planets, but there is no 
 single instance in which such a planet has been 
 directly observed ; no dark, non-luminous body 
 has ever been actually seen in attendance on a 
 star. Many double or multiple stars are known, 
 but these are all instances in which one sun-like 
 body is revolving round another of the same order.* 
 We see no body shining by reflected light outside 
 the limits of the solar system. Planets to the 
 various stars may exist in countless numbers, 
 but they are invisible to us, and we cannot 
 discuss conditions where everything is unknown. 
 Enquiry in such a case is useless, and speculation 
 vain. 
 
 The stars, as revealed to us by the spectroscope 
 are all of the same order as the Sun, but they are 
 not all of the same species. Quite a large number 
 of stars, of which Arcturus is one of the best-known 
 examples, show spectra that are essentially the 
 same as that of the Sun, but there are other stars 
 of which the spectra bear little or no semblance to 
 
 * The movements of not a few double stars point to pertur- 
 bations caused by the attraction of unseen bodies. There are 
 also a number of instances known of "Eclipse" or "Algol-type " 
 variable stars, in which the presence of a dark companion is 
 indicated by the diminution of the light of the star at regular 
 intervals. 
 
 35 
 
ARE THE PLANETS INHABITED? 
 
 it. Nevertheless, it remains true that, on the whole, 
 stellar spectra bear witness to the presence of just 
 the same elements as we recognize in the Sun, 
 though not always in the same proportions or in 
 the same conditions hydrogen, calcium, sodium, 
 magnesium, iron, titanium, and many more are 
 recognized in nearly all. It is true that not all the 
 known terrestrial elements have yet been identified 
 in either Sun or stars ; but, in general, those missing 
 are either " negative " elements like the halogens, 
 or elements of great atomic weight like mercury 
 and platinum. That elements of one class should, 
 as a rule, reveal their presence in Sun and stars 
 wherever these are placed, and, correspondingly, 
 that other classes should as generally fail to show 
 themselves, indicate that such absence is more 
 likely to be due to the general structure of the 
 stellar photospheres and reversing layers than to 
 any irregularity in the distribution of matter in 
 the universe. It is easy, for example, to conceive 
 that the heavy metals may lie somewhat deeper 
 down within the Sun or star than those of low 
 atomic weight. In the case of the Sun, there 
 seems a clear connection between atomic weight 
 and the distinctness with which the element is 
 recognized in the spectrum of the photosphere, 
 the lower atomic weights showing themselves 
 more conspicuously. 
 
 It is clear that not all elements present in a Sun 
 or star show themselves in its spectrum. Oxygen 
 
 36 
 
DISTRIBUTION OF ELEMENTS 
 
 is very feebly represented by its elemental lines, 
 but the flutings of titanium oxide are found in 
 sunspots, and with great distinctness in a certain 
 type of stars. Nitrogen, too, though not directly 
 recognized, proves its presence by the lines of 
 cyanogen. The case of helium is one of particular 
 interest ; this element was recognized by a very 
 bright yellow line in the solar prominences before 
 it was known to exist on the Earth ; indeed, it 
 received the name helium because it then seemed 
 to be a purely solar constituent. Now it is seen as 
 a strong absorption line in the spectrum of many 
 stars ; but for some reason it is not in general seen 
 as an absorption line over the Sun's disc, and if our 
 Sun were removed to such distance so as to appear 
 to us only as a star, we should have no evidence 
 that it contained any helium at all. So far, then, as 
 the evidence of the spectroscope goes, the elements 
 present in the Earth are present throughout the 
 whole extent of the universe within our view : the 
 same elements and with the same qualities. For 
 the lines of the spectrum of an element are the 
 revelation of its innermost molecular structure, so 
 that we can confidently affirm that hydrogen and 
 oxygen on Sirius, Arcturus, or the Sun, are essen- 
 tially the same elements as hydrogen and oxygen 
 on the Earth. On a planet attached to any of 
 these stars, the two gases would combine together 
 to form water under just the same conditions as 
 they do here on the Earth ; and at suitable tem- 
 
 37 
 
ARE THE PLANETS INHABITED? 
 
 peratures that water would be a neutral liquid, 
 capable of dissolving just the same chemical 
 substances that it does here. It would freeze as 
 it does here ; it would evaporate as it does 
 here ; it would be water as completely in all 
 its qualities and conditions as earthly water is. 
 And what applies to one element or compound 
 applies to all. Throughout the whole extent of 
 space, the same building materials have been 
 employed, and throughout they retain the same 
 qualities. 
 
 Hydrogen is seen in the spectra of nearly all 
 stars, and also in those of nebulae. The elemental 
 lines of oxygen are not indeed seen in stellar 
 spectra, but that the element is present is shown 
 by the flu tings of titanium oxide which distinguish 
 stars like Antares. Nitrogen and carbon again are 
 not recognized by their elemental lines, but the 
 lines of cyanogen are seen in the spectra of comets 
 and of sunspots, and hydrocarbon flutings in the 
 spectra of comets and red stars ; while in a few 
 of the hottest stars even sulphur has recently been 
 identified.* All the five organo-genetic elements 
 are therefore abundantly diffused through space ; 
 the materials for protoplasm, " the albuminous 
 substance with water," are at hand everywhere. 
 This being so, it is reasonable to infer that if 
 organic life exists elsewhere than on this Earth, 
 its essential feature, there as here, is the meta- 
 * Proc. R. Soc., LXXX, 50, 1907. 
 38 
 
DISTRIBUTION OF ELEMENTS 
 
 holism of nitrogenous carbon compounds in asso- 
 ciation with protoplasm. 
 
 But it is objected that " we are not yet able to 
 identify all the lines in solar or stellar spectra ; 
 may not some of these lines be due to elements of 
 which we know nothing here, and may not such 
 new elements form complex and unstable com- 
 pounds with each other, or with some of those 
 familiar to us, that would take the place of the 
 five organo-generators, and so give rise to a 
 physical basis of life, different from that we know 
 on this Earth ? " 
 
 But the development of MendeleefFs Periodic 
 Law has shown that the elements are not to be 
 regarded as disconnected entities. The Law as 
 given in Mendeleeff's own words, runs : " The 
 properties of the elements as well as the forms and 
 properties of their compounds are in periodic 
 dependence on, or (expressing ourselves algebra- 
 ically) form a periodic function of the atomic 
 weights of the elements." In other words, they 
 form a series, not only as it regards their atomic 
 weights, but also as it regards their own properties 
 and the forms and properties of their compounds. 
 We are no longer at liberty, as we might have been 
 many years ago, to call into fancied existence new 
 elements having no relation in their properties 
 and compounds to those with which we are ac- 
 quainted. New elements, no doubt, will be dis- 
 covered in the future, as in the past ; and indeed we 
 
 39 
 
ARE THE PLANETS INHABITED? 
 
 may be able to discover them and learn their 
 atomic weights and properties withput ever being 
 able to handle them in a terrestrial laboratory. 
 
 In a series of remarkable papers communicated 
 to the Royal Astronomical Society during the past 
 year (1911-1912), Dr. J. W. Nicholson has given 
 the result of his computation of the positions of the 
 spectral lines of two elements of simple structure, 
 and has found that the resulting lines correspond, 
 for one dynamical system, to the chief unidentified 
 lines observed in the spectra of nebulae, and for the 
 other, to the chief unidentified lines in the spectrum 
 of the corona. The latter element is probably 
 associated with the halogens, but of much lower 
 atomic weight (namely, 1-3), than fluorine ; he 
 therefore gives it the name of Protofluorine. The 
 other element, to which he gives the name Nebu- 
 liuwt, will have an atomic weight of 2*1. Prof. Max 
 Wolf, of Heidelberg, has recently pointed out* the 
 evidence of the presence of two other unknown gases 
 in the Ring nebula in Lyra, and there is no reason 
 to suppose that the process of discovery has come 
 to an end. But we cannot imagine that we shall 
 discover any new elements that are more abun- 
 dant and more universally diffused than the five 
 which give us protoplasm" the physical basis of 
 life." To take an analogy from the solar system : 
 many hundreds of planetoids have now been dis- 
 covered between the orbits of Mars and Jupiter, 
 * Nature, LXXX, 158 (April 8th, 1909). 
 40 
 
DISTRIBUTION OF ELEMENTS 
 
 and probably many hundreds more remain to be 
 discovered ; but of one thing we are certain, that 
 none of the planetoids yet to be discovered will be 
 of the same rank as either of those two guardians, 
 Mars and Jupiter, who revolve on the confines of 
 the planet oidal zone. Indeed, Ceres, the planetoid 
 first discovered, has a greater mass than the 
 aggregate of all discovered since, and probably of 
 all that exist in the zone. 
 
 Water is essential for life here, but the quality in 
 water which restricts the range of terrestrial life 
 is that it freezes at o Centigrade, and boils at 
 1 00 Centigrade; it is only in the liquid state 
 during the intermediate range of 100 degrees. In 
 order to extend the range for living organisms, we 
 should have, therefore, to discover a new vehicle, 
 that, possessing all the other qualities of water, is 
 not restricted to the liquid state within the same 
 limits. But we are at once met with the difficulty 
 that the first essential for the vehicle is that it 
 should be abundant, and there are no other ele- 
 ments more abundant than hydrogen and oxygen. 
 This new vehicle must, like water, be both neutral 
 and stable, or it would itself interfere with the 
 highly unstable compounds that are a necessity for 
 metabolism. And, if we could find this new vehicle, 
 liquid at temperatures outside the o to 100 
 Centigrade, have we any reason to suppose that 
 protoplasm itself would be able to endure these 
 outlying temperatures ? Looking through the 
 
 41 
 
ARE THE PLANETS INHABITED? 
 
 range of substances available, we can only say 
 that none other presents itself as approaching 
 water in suitability for its essential office. If we, 
 ourselves, were able to create a vehicle, could we 
 imagine one more perfectly suited ? 
 
 42 
 
CHAPTER V 
 THE MOON 
 
 THE Sun and Moon offer to our sight almost 
 exactly the same apparent diameters ; to 
 the eye, they look the same size. But as we know 
 the Sun to be 400 times as distant as the Moon, it 
 is necessarily 400 times as large ; its surface must 
 exceed that of the Moon by the square of 400, or 
 160,000 ; its volume by the cube of 400, or 
 64,000,000. As the Sun is of low mean density, 
 its mass does not exceed that of the Moon in quite 
 the same high ratio ; but it is equal in mass to 
 27,000,000 moons. 
 
 Compared with the Sun, the Moon is therefore an 
 insignificant little ball a mere particle ; but as a 
 world for habitation it possesses some advantages 
 over the Sun. The first glance at it in a telescope 
 is sufficient to assure the observer that he is look- 
 ing at a solid, substantial globe. It is not only 
 substantial, it is rugged ; its surface is broken up 
 into mountains, hills, valleys, and plains ; the 
 mountains stand out in sensible relief ; it looks 
 like a ball of solid silver boldly embossed and 
 chased. 
 
 So far all is to the good for the purpose of 
 
 43 
 
ARE THE PLANETS INHABITED? 
 
 habitation. Wherever men are, they must have 
 a solid platform on which to stand ; they must 
 have a stable terrene whereon their food may grow, 
 and this the Moon could supply. " The Earth's 
 gloom of iron substance " is necessary for man here, 
 and the Moon appears to offer a like stability. 
 
 Another favourable condition is that we know 
 that the Moon receives from the Sun a sufficient 
 supply of light and heat. Each square yard of its 
 surface receives, on the average, the same amount 
 of light and heat that would fall upon a square 
 yard on the Earth that was presented towards 
 the Sun at the same inclination ; and we know 
 from our own experience that this is sufficient for 
 the maintenance of life. 
 
 And the Moon is near enough for us to subject 
 her to a searching scrutiny. Every part of the 
 hemisphere turned toward us has been repeatedly 
 examined, measured, and photographed ; to that 
 extent our knowledge of its topography is more 
 complete than of the world on which we live. 
 There are no unexplored regions on our side of the 
 Moon. The great photographs taken in recent 
 years at the observatories of Paris and of the 
 University of Chicago have shown thousands of 
 " crater-pits," not more than a mile across ; and 
 narrow lines on the Moon's surface have been 
 detected with a breadth less than one-tenth of 
 this. An elevation on the Moon, if it rose up 
 abruptly from an open plain, would make its 
 
 44 
 
THE MOON 
 
 presence apparent by the shadow which it would 
 cast soon after sunrise or near sunset ; in this way 
 an isolated building, if it were as large as the great 
 pyramid of Ghizeh, would also show itself, and 
 all our great towns and cities would be apparent 
 as areas of indistinct mottling, though the details 
 of the cities would not be made out. 
 
 But if vegetation took the same forms on the 
 Moon as on the Earth, and passed through the 
 same changes, we should have no difficulty in 
 perceiving the evidence of its presence. If we were 
 transported to the Moon and turned our eyes 
 earthward, we should not need the assistance of 
 any telescope in order to detect terrestrial changes 
 which would be plainly connected with the seasonal 
 changes of vegetation. The Earth would present 
 to us a disc four times the apparent diameter of 
 the Moon, and on that disc Canada would offer as 
 great an area as the whole of the Moon does to us. 
 We could easily follow with the naked eye the 
 change from the glittering whiteness of the aspect 
 of Canada when snow-covered in winter, to the 
 brown, green and gold which would succeed each 
 other during the brighter months of the year. And 
 this type of change would alternate between the 
 northern and southern hemispheres, for the winter 
 of Canada is the summer of the Argentine, and 
 conversely. 
 
 We ought, therefore, to have no difficulty in 
 observing seasonal changes on the Moon, if such 
 
 45 
 
ARE THE PLANETS INHABITED? 
 
 take place. But nothing of the kind has ever been 
 remarked ; no changes sufficiently pronounced for 
 us to be sure of them are ever witnessed. Here 
 and there some slight mutations have been sus- 
 pected, nearly all accomplishing their cycle in the 
 course of a lunar day; so that it is difficult to 
 separate them from changes purely apparent, 
 brought about by the change in the incidence of 
 the illumination. 
 
 The difference in appearance of a given area on 
 the Moon when viewed under a low Sun and when 
 the Sun is on the meridian is very striking. In the 
 first case everything is in the boldest relief ; the 
 shadows are long and intensely black ; the whole 
 area under examination in the telescope seems as 
 if it might be handled. Under the high Sun, the 
 contrasts are gone ; the scenery appears flat, many 
 of the large conspicuous markings are only recog- 
 nized with difficulty. Thus the terse remark of 
 Madler, " The full Moon knows no Maginus," has 
 become a proverb amongst selenographers ; yet 
 Maginus is a fine walled plain some eighty miles in 
 diameter, and its rampart attains a height in 
 parts of 14,000 feet. Maginus lies near Tycho, 
 which has been well named " the lunar metro- 
 polis," for from it radiates the principal system 
 of bright streaks conspicuous on the full Moon. 
 These white streaks appear when the shadows 
 have vanished or are growing short ; they are not 
 seen under a low Sun. 
 
 4 6 
 
THE MOON 
 
 The changes which appear to take place in the 
 lunar formations owing to the change in their 
 illumination are much more striking and varied 
 than would be anticipated. But the question 
 arises whether all the changes that are associated 
 with the progress of the lunar day can be ascribed 
 to this effect. Thus, Prof. W. H. Pickering writes 
 concerning a well-known pair of little craters of 
 about nine miles in diameter, " known as Messier 
 and Messier A, situated side by side not far from 
 the centre of the Mare Fecunditatis. When the 
 Sun rises first on them, the eastern one, A, is tri- 
 angular and larger than Messier, which latter is 
 somewhat pear-shaped. About three days after 
 sunrise they both suddenly turn white, Messier 
 rapidly grows in size, soon surpasses A, and also 
 becomes triangular in shape. Six days after 
 sunrise the craters are again nearly of the same 
 size, owing to the diminution of Messier. The 
 shape of A has become irregular, and differs in 
 different lunations. At nine days after sunrise the 
 craters are exactly alike in size and shape, both 
 now being elliptical, with their major axes lying in 
 a nearly N. and S. direction. Just before sunset 
 A is again the larger, being almost twice the size 
 of Messier." * 
 
 Some observers explain this cycle of changes as 
 due merely to the peculiar contour of the two 
 
 * " Periodic Changes upon the Moon," Memoirs, British 
 Astronomical Association, Vol. XIII, p. 83. 
 
 47 
 
ARE THE PLANETS INHABITED? 
 
 objects, the change in the lighting during the lunar 
 day altering their apparent figures. Prof. W. H. 
 Pickering, on the other hand, while recognizing 
 that some portion of the change of shape is prob- 
 ably due to the contour of the ground, conceives 
 that, in order to explain the whole phenomenon, it 
 is necessary to suppose that a white layer of hoar 
 frost is formed periodically round the two craters. 
 It is also alleged that whereas Madler described the 
 two craters as being exactly alike eighty years ago, 
 Messier A is now distinctly the larger ; but it is very 
 doubtful whether Madler's description can be 
 trusted to this degree of nicety. If it could, this 
 would establish a permanent change in the actual 
 structure of the lunar surface at this point. 
 
 There are several other cases of the same order 
 of ambiguity. The most celebrated is Linne, a 
 white spot about six miles in diameter on the Mare 
 Serentatis. This object appears to change in size 
 during the progress of the lunar day, and, as with 
 Messier, some selenographers consider that it has 
 also suffered an actual permanent change in shape 
 within the last sixty or seventy years. Here again 
 the evidence is not decisive ; Neison is by no means 
 convinced that a change has taken place, yet does 
 not think it impossible that Linne may once have 
 been a crater with steep walls which have collapsed 
 into its interior through the force of gravity. 
 
 Another type of suspected change is associated 
 with the neighbourhood of Aristarchus, the bright- 
 48 
 
THE MOON 
 
 est formation on the Moon, so bright indeed that 
 Sir William Herschel, observing it when illu- 
 minated by earthshine in the dark portion of the 
 Moon, thought that he was watching a lunar 
 volcano in eruption. In 1897, on September 21, 
 the late Major Molesworth noticed that the crater 
 was at that time under the rays of the setting Sun, 
 and filled with shadow, and the inner terraces, 
 which should have been invisible, were seen as 
 faint, knotted, glimmering streaks under both the 
 eastern and western walls, and the central peak 
 was also dimly discernible. He thought this un- 
 usual lighting up of rocks on which the Sun had 
 already set might be due either to phosphorescence 
 produced by long exposure to the Sun's rays, or to 
 inherent heat, or to reflected glare from the western 
 rampart. Still more important, both Major Moles- 
 worth and Mr. Walter Goodacre, each on more 
 than one occasion, observed what seemed to be a 
 faint bluish mist on the inner slope of the east wall, 
 soon after sunrise, but this was visible only for a 
 short time. Other selenographers too, on rare 
 occasions, have made observations accordant with 
 these, relating to various regions on the Moon. 
 
 These, and a few other similar instances, are all 
 that selenography has to offer by way of evidence 
 of actual lunar change. Of seeming change there 
 is abundance, but beyond that we have only cases 
 for controversy, and one of the most industrious 
 of the present-day observers of the Moon, M. 
 E 49 
 
ARE THE PLANETS INHABITED? 
 
 Philip Fauth, declares that " as a student of the 
 Moon for the last twenty years, and as probably 
 one of the few living investigators who have kept 
 in practical touch with the results of selenography, 
 he is bound to express his conviction that no eye 
 has ever seen a physical change in the plastic 
 features of the Moon's surface." * 
 
 In this matter of change, then, the Earth and 
 Moon stand in the greatest contrast to each other. 
 As we have seen, from the view-point of the Moon, 
 the appearance of the Earth would change so mani- 
 festly with the progress of the seasons that no one 
 could fail to remark the difference, even though 
 observing with the naked eye. But from the view- 
 point of the Earth, the Moon when examined by 
 our most experienced observers, armed with our 
 most powerful telescopes, offers us only a few 
 doubtful enigmatical instances of possible change 
 confined to small isolated localities ; we see no 
 evidence that the "gloom of iron substance" 
 below is ever concealed by a veil of changing 
 vegetation, or that " between the burning light and 
 deep vacuity " of the heavens above, the veil of 
 the flying vapour has ever been spread out. We 
 see the Moon so clearly that we are assured it 
 holds no water to nourish plant life ; we see it so 
 clearly because there is no air to carry the vapour 
 that might dim our view. 
 
 Life is change, and a planet where there is no 
 * The Moon, by Philip Fauth, p. 156. 
 50 
 
THE MOON 
 
 change, or where that change is very small, can be 
 no home for life. The " stability and insensibility " 
 are indeed required in the platform upon which 
 life is to appear, but there must be the presence of 
 " the passion and the perishing," or life will be 
 unable to find a home. 
 
 We infer the absence of water and air from the 
 Moon not only from the unchanging character of 
 its features and the distinctness with which we 
 see them ; we are able to make direct observa- 
 tions. Galileo, the first man to observe the Moon 
 to better advantage than with the naked eye, was 
 not long before he decided that the Moon con- 
 tained no water, for though Milton, in a well- 
 known passage, makes Galileo discover 
 
 " Rivers or mountains on her spotty globe," 
 
 Galileo himself wrote : " I do not believe that the 
 body of the Moon is composed of earth and water." 
 The name of maria was given to the great grey 
 plains of the Moon by Hevelius, but this was 
 simply for convenience of nomenclature, not be- 
 cause he actually believed them to be seas. One 
 observation is, in itself, sufficient to prove that the 
 maria are not water surfaces. The Moon's " termi- 
 nator," that is to say, the line dividing the part in 
 sunlight from that in darkness, is clearly irregular 
 when it passes over the great plains ; were they 
 actually sea it would be a bright line and per- 
 fectly smooth. The grey plains are therefore not 
 expanses of water now, nor were they in time past. 
 
ARE THE PLANETS INHABITED? 
 
 It is obvious that in some remote antiquity their 
 surface was in a fluid condition, but it was the 
 fluidity of molten rock. This is seen by the way in 
 which the maria have invaded, breached, broken 
 down, and submerged many of the circular forma- 
 tions on their margins. Thus the Mare Humorum 
 has swept away half the wall of the rings, Hippalus 
 and Doppelmayer, and far out in the open plain of 
 the Mare Nubium, great circles like Kies, and that 
 immediately north of Flamsteed, stand up in faint 
 relief as of half -submerged rings. Clearly there 
 was a period after the age in which the great ring 
 mountains and walled plains came into existence, 
 when an invasive flood attacked and partially de- 
 stroyed a large proportion of them. And the flood 
 itself evidently became more viscous and less fluid 
 the further it spread from its original centre of 
 action, for the ridges and crumpling of the surface 
 indicate that the material found more and more 
 difficulty in its flow. 
 
 We have evidence just as direct that there is no 
 atmosphere. This is very strikingly shown when 
 the Moon, in its monthly progress among the stars, 
 passes before one of them and occults it. Such an 
 occultation is instantaneous, and is particularly 
 impressive when either a disappearance or a re- 
 appearance occurs at the defective limb ; that is 
 to say, at the limb which is not illuminated by the 
 Sun, and is therefore invisible. The observer may 
 have a bright star in the field of view, showing 
 
 52 
 
THE MOON 
 
 steadily in a cloudless sky ; there is not a hint of a 
 weakening in its light ; suddenly it is gone. The 
 first experience of such an observation is most dis- 
 concerting ; it is hardly less disconcerting to ob- 
 serve the reappearance at the dark limb. One 
 moment the field of view of the telescope is empty ; 
 the next, without any sort of dawning, a bright 
 star is shining steadily in the void, and it almost 
 seems to the observer as if an explosion had taken 
 place. If the Moon had an atmosphere extending 
 upwards from its surface in all directions and of 
 any appreciable density, an occultation would not 
 be so exceedingly abrupt ; and, in particular, if 
 the occultation were watched through a spectro- 
 scope, then, at the disappearance, the spectrum of 
 the star would not vanish as a whole, but the red 
 end would go first, and the rest of the spectrum 
 would be swept out of sight successively, from 
 orange to the violet. This does not happen ; the 
 whole spectrum goes out together, and it is clear 
 that no appreciable atmosphere can exist on the 
 Moon. In actual observation so inappreciable is it 
 that its density at the Moon's surface is variously 
 estimated as nJ^th of that of the Earth by Neison, 
 and as T ooo-th by W. H. Pickering. If the Moon 
 possessed an atmosphere bearing the same propor- 
 tion to her total mass as we find in the case of the 
 Earth, she would have a density of one-fortieth of 
 our atmosphere at the sea level. 
 The Moon is at the same mean distance from the 
 
 S3 
 
ARE THE PLANETS INHABITED? 
 
 Sun as the Earth, and therefore, surface for sur- 
 face, receives from it on the average the same 
 amount of light and heat. But it makes a very 
 different use of these supplies. Bright as the Moon 
 appears when seen at the full on some winter 
 night, it has really but a very low power of reflec- 
 tion, and is only bright by contrast with the dark- 
 ness of the midnight sky. If the full Moon is seen 
 in broad daylight, it is pale and ghost-like. Sir 
 John Herschel has put it on record that when in 
 South Africa he often had the opportunity of 
 comparing the Moon with the face of Table Moun- 
 tain, the Sun shining full upon both, and the Moon 
 appeared no brighter than the weathered rock. 
 The best determinations of the albedo of the Moon, 
 that is to say, of its reflective power, give it as 0-17, 
 so that only one-sixth of the incident light is 
 reflected, the other five-sixths being absorbed. It 
 is difficult to obtain a good determination of the 
 Earth's albedo, but the most probable estimate 
 puts it as about 0-50, or three times as great as 
 that of the Moon. This high reflective power is 
 partly to be accounted for by the great extent of 
 the terrestrial polar caps, but chiefly by the clouds 
 and dust layer always present in its atmosphere. 
 
 A larger proportion, therefore, of the solar rays 
 are employed in heating the soil of the Moon than 
 in heating that of the Earth, and in this connection 
 the effect of an important difference between the 
 two worlds must be noted. The Earth rotates on 
 
 54 
 
THE MOON 
 
 its axis in 23 hours 56 minutes 4 seconds, the mean 
 length of its rotation as referred to the Sun being 
 24 hours. The rotation of the Moon, on the other 
 hand, takes 27 days 7 hours 43 minutes to accom- 
 plish, giving a mean rotation, as referred to the 
 Sun, of 29 days 12 hours 44 minutes. The lunar 
 surface is therefore exposed uninterruptedly to the 
 solar scorching for very nearly fifteen of our days 
 at a time, and it is, in turn, exposed to the intense 
 cold of outer space for an equal period. As the 
 surface absorbs heat so readily, it must radiate it 
 as quickly ; hence radiation must go on with great 
 rapidity during the long lunar night. Lord Rosse 
 and Prof. Very have both obtained measures of 
 the change in the lunar heat radiation during the 
 progress of a total eclipse of the Moon, with the 
 result that the heat disappeared almost completely, 
 though not quite at the same time as the light. 
 Prof. Langley succeeded in obtaining from the 
 Moon, far down in the long w r ave lengths of the 
 infra-red, a heat spectrum which was only partly 
 due to reflection from the Sun ; part coming from 
 the lunar soil itself, which, having absorbed heat 
 from the Sun, radiated it out again almost im- 
 mediately. In 1898, Prof. Very, following up 
 Langley's line of work, concluded that the tem- 
 perature of the lunar soil must range through 
 about 350 Centigrade, considerably exceeding 
 100 at the height of the lunar day, and falling to 
 about the temperature of liquid air during the 
 
 55 
 
ARE THE PLANETS INHABITED? 
 
 lunar night. So wide a range of temperature must 
 be fatal to living organisms, particularly when the 
 range is repeated at short, regular intervals of 
 time. But this range of temperature comes 
 directly from the length of the Moon's rotation 
 period ; for the longer the day of the Moon, the 
 higher the temperature which may be attained in 
 it ; the longer the night, the greater the cold 
 which will in turn be experienced. We learn, there- 
 fore, that the time of rotation of a planet is an im- 
 portant factor in its habitability. 
 
CHAPTER VI 
 THE CANALS OF MARS 
 
 BOTH of the two worlds best placed for our 
 study are thus, for different reasons, ruled 
 out of court as worlds for habitation. The Sun 
 by its vastness, its intolerable heat and the violence 
 of its changes, has to be rejected on the one 
 hand, while the Moon, so small, and therefore so 
 rigid, unchanging and bare, is rejected on the 
 other. 
 
 Of the other heavenly bodies, the planet Mars 
 is the one that we see to best advantage. Two 
 other planets, Eros and Venus, at times come 
 nearer to us, but neither offers us on such occasions 
 equal facilities for their examination. But of 
 Mars it has been asserted not only that it is in- 
 habited, but that we know it to be the case, since 
 the evidence of the handiwork of intelligent 
 beings is manifest to us, even across the tre- 
 mendous gulf of forty or more million miles of 
 space. 
 
 A claim so remarkable almost captures the 
 position by its audacity. There is a natural desire 
 among men to believe the marvellous, and the very 
 
 57 
 
ARE THE PLANETS INHABITED? 
 
 boldness of the assertion goes no small way to 
 overcome incredulity. And when we consider how 
 puny are men as we see them on this our planet, 
 how minute their greatest works, how super- 
 human any undertaking would be which could 
 demonstrate our existence to observers on another 
 planet, we must admit that it is a marvel that 
 there should be any evidence forthcoming that 
 could bear one way or another on the solution of a 
 problem so difficult. 
 
 The first fact that we have to remember with 
 regard to the planet Mars is the smallness of its 
 apparent size. To the eye it is nearly a star a 
 point of light without visible surface. It is almost 
 twice the size of the Moon in actual diameter, 
 but as its mean distance from the Earth is 
 600 times that of the Moon, its mean apparent 
 diameter is 300 times smaller. We cannot, how- 
 ever, watch Mars in all parts of its orbit ; it is 
 best placed for observation, and, therefore, most 
 observed, when in opposition, and oppositions may 
 be favourable or unfavourable. At the most 
 favourable opposition, Mars is 140 times as distant 
 as the Moon ; at the least favourable, 260 times ; 
 so that on such occasions its apparent size varies 
 from 7 Vh of the diameter of the Moon to i^th. 
 But a telescope with a magnifying power of 70 
 could never, under the most perfect conditions, 
 show Mars, even in the closest opposition, as well 
 as the Moon is seen with the naked eye, for the 
 
 53 
 
THE CANALS OF MARS 
 
 practical magnifying power of a telescope is never 
 as great as the theoretical. In practice, a child's 
 spy-glass magnifying some six diameters will show 
 the full Moon to better advantage than Mars has 
 ever been seen, even in our most powerful tele- 
 scopes. 
 
 The small apparent size of the planet explains 
 how it was that Galileo does not seem to have 
 been able to detect any markings upon it. In 1659, 
 Huyghens laid the foundation stone of areography 
 by observing some dark spots, and determining 
 from their apparent movements that the planet had 
 a rotation on its axis, which it accomplished in 
 about the same time as the Earth. Small and 
 rough as are the drawings that Huyghens made, 
 the identification of one or two of his spots is un- 
 mistakable. Seven years later, in 1666, both 
 Cassini and Hooke made a number of sketches, 
 and those by Hooke have been repeatedly used in 
 modern determinations of the rotation period of 
 the planet. The next great advance was made 
 by Sir William Herschel, who, during the opposi- 
 tions of 1777, 1779, 1781, and 1783, determined 
 the inclination of the axis of Mars to the plane 
 of its orbit, measured its polar and equatorial 
 diameters, and ascertained the amount of the 
 polar flattening. He paid also special attention 
 to two bright white spots upon the planet, and he 
 showed that these formed round the planet's 
 poles and increased in size as the winter of 
 
 59 
 
ARE THE PLANETS INHABITED? 
 
 each several hemisphere drew on and diminished 
 again with the advance of summer, behaving 
 therefore as do the snow caps of our own polar 
 regions. 
 
 The next stage in the development of our know- 
 ledge of Mars must be ascribed to the two German 
 astronomers, Beer and Madler, who made a series of 
 drawings in the years 1830, 1832 and 1837, by m eans 
 of a telescope of 4 inches aperture, from which they 
 were able to construct a chart of the entire globe. 
 This chart maybe considered classic, for the features 
 which it represents have been observed afresh 
 at each succeeding opposition. Mars, therefore, 
 possesses a permanent topography, and some of the 
 markings in question can be identified, not only in 
 the rough sketches made by Sir William Herschel, 
 but even in those made by Hooke and Cassini as 
 far back as the year 1666. In the forty years that 
 followed, the planet was studied by many of the 
 most skilled observers, particularly by Mr. J. N. 
 Lockyer in 1862, and the Rev. W. R. Dawes in 
 1864. In 1877, the late Mr. N. E. Green, drawing- 
 master to Queen Victoria, and a distinguished 
 painter in water colours, made a series of sketches 
 of the planet from a station in the island of 
 Madeira 2000 feet above sea-level. When the 
 opposition was over, Mr. Green collected together 
 a large number of drawings, and formed a chart of 
 the planet, much richer in detail than any that 
 had preceded it, and from his skill, experience and 
 60 
 
THE CANALS OF MARS 
 
 training as an artist he reproduced the appearance 
 of the planet with a fidelity that had never been 
 equalled before and has never been surpassed 
 since. At this time it was generally assumed that 
 Mars was a miniature of our own world. The 
 brighter districts of its surface were supposed to 
 be continents, the darker, seas. As Sir William 
 Herschel had already pointed out long before, the 
 little world evidently had its seasons, its axis 
 being inclined to the plane of its orbit at much the 
 same angle as is the case with the Earth ; it had 
 its polar caps, presumably of ice and snow ; its 
 day was but very little longer than that of the 
 Earth ; and the only important difference seemed 
 to be that it had a longer year, and was a little 
 further off the Sun. But the general conclusion 
 was that it was so like the Earth in its conditions 
 that we had practically found out all that there 
 was to know ; all that seemed to be reserved for 
 future research was that a few minor details of 
 the surface might be filled in as the power of our 
 telescopes was increased. 
 
 But fortunately for progress, this sense of satis- 
 faction was to be rudely disturbed. As Mars, in 
 its progress round the Sun, receded from the Earth, 
 or rather as the Earth moved away from it, the 
 astronomers who observed so diligently during 
 the autumn of 1877 turned their attention to other 
 objects. One of them, however, Schiaparelli, the 
 most distinguished astronomer on the continent 
 61 
 
ARE THE PLANETS INHABITED? 
 
 of Europe, still continued to watch the planet, and, 
 as the result of his labours, he published some 
 months later the first of a magnificent series of 
 Memoirs, bringing to light what appeared to be a 
 new feature. His drawings not only showed the 
 " lands " and " seas," that is to say the bright 
 and dark areas, that Green and his predecessors 
 had drawn, but also a number of fine, narrow, 
 dark lines crossing the " lands " in every direction. 
 These narrow lines are the markings which have 
 since been so celebrated as the " canals of Mars," 
 and the discussion as to the real nature of these 
 canals has focussed attention upon Mars in a way 
 that, perhaps, nothing else could have done. 
 Before 1877 the study of planetary markings was 
 left almost entirely to the desultory labours of 
 amateurs, skilled though many of them were ; 
 since 1877, the most powerful telescopes of the 
 great public observatories of the world have 
 been turned upon Mars, and the most skilful 
 and experienced of professional astronomers 
 have not been ashamed to devote their time 
 to it. 
 
 There is no need to pass in review the whole of 
 the immense mass of observations that have been 
 accumulated since Schiaparelli brought out the 
 first of his great Memoirs. That Memoir gave rise 
 to an immediate controversy, for many astrono- 
 mers of skill and experience had observed the 
 planet in 1877 without detecting the network of 
 62 
 
THE CANALS OF MARS 
 
 lines which Schiaparelli had revealed, and it was 
 natural that they should feel some reluctance in 
 accepting results so strange and novel. But little 
 by little this controversy has passed. We now 
 know that the " canals " vary much in their 
 visibility, and " curiously enough the canals are 
 most conspicuous, not at the time the planet is 
 nearest to the Earth and its general features are 
 in consequence best seen, but as the planet goes 
 away the canals come out. The fact is that the 
 orbital position and the seasonal epoch conspire to 
 a masking of the phenomena." This was the chief 
 reason why Schiaparelli 's discoveries seemed at 
 first to stand so entirely without corroboration ; 
 the " canals " did not become conspicuous until 
 after most observers had desisted from following 
 the planet. Another reason was that, in 1877, 
 Mars was low down in the sky for northern obser- 
 vatories, and good definition is an essential for 
 their recognition. But the careful examination 
 of drawings made in earlier oppositions, especially 
 those made by Dawes and Green, afforded con- 
 firmation of not a few of Schiaparelli 's " canals " ; 
 even in 1877 a few of the easiest and most con- 
 spicuous had been delineated by other astronomers 
 before any rumour of Schiaparelli's work had come 
 abroad, and as Mars came under observation again 
 and again at successive oppositions, the number of 
 those who were able to verify Schiaparelli's dis- 
 coveries increased. It has now long been known 
 
 63 
 
ARE THE PLANETS INHABITED? 
 
 that the great Italian astronomer was not the 
 victim of a mere optical illusion ; there were 
 actual markings on the planet Mars where he had 
 represented them ; markings which, when seen 
 under like conditions and with equal instrumental 
 equipment, did present the appearance of straight, 
 narrow lines. The " canals of Mars " are not mere 
 figments of the imagination, but have a real ob- 
 jective basis. 
 
 As this controversy has passed away, another 
 and a very different one has arisen out of an un- 
 fortunate mistranslation of the term chosen by 
 Schiaparelli to indicate these linear streaks. In 
 conformity with the type of nomenclature adopted 
 by previous areographers who had divided Mars 
 into "seas," "continents," "islands," "isthmuses," 
 " straits " and the like, Schiaparelli had called 
 the narrow lines he detected " canali," that is 
 to say " channels," but without intending to 
 convey the idea of artificial construction. Indeed, 
 he himself was careful to point out that these 
 designations " were not intended to prejudge the 
 nature of the spot, and were nothing but an artifice 
 for helping the memory and for shortening de- 
 scriptions." And he added, "We speak in the same 
 way of the lunar seas, although we well know 
 that there are no true seas on the Moon." But 
 " canali " was unhappily rendered in English as 
 " canals," instead of " channels." " Channel " 
 would have left the nature of the marking an open 
 
 64 
 
THE CANALS OF MARS 
 
 question, but, in English, "canal" means an 
 artificial waterway. Here then the question as to 
 whether or no Mars is inhabited comes definitely 
 before us. Have we sufficient grounds for be- 
 lieving that the " canals " are artificial construc- 
 tions, or may they be merely natural forma- 
 tions ? 
 
 In 1894, Mr. Percival Lowell founded at Flag- 
 staff , Arizona, U.S.A., a well-equipped observatory 
 for the special study of Mars, and he has continued 
 his scrutiny of the planet from that time to the 
 present with the most unrelaxing perseverance. 
 The chief results that he has obtained have been 
 the detection of many new "canals "; the discovery 
 of a number of dark, round dots, termed by him 
 "oases," at the junctions of the "canals"; and 
 the demonstration that the " canals " and certain 
 of the dusky regions are subject to strictly seasonal 
 change, as really as the polar caps themselves. 
 In addition, he has formed the conclusion, which 
 he has supported with much ingenuity and skill, 
 that the regularity of the " canals " and " oases " 
 quite precludes the possibility of their being 
 natural formations. Hence there has arisen the 
 second controversy: that on the nature of the 
 " canals " ; for Mr. Lowell considers that their 
 presence proves the existence of inhabitants on 
 Mars, who, by means of a Titanic system of irriga- 
 tion, are fighting a losing battle against the gradual 
 desiccation of their planet. 
 F 65 
 
ARE THE PLANETS INHABITED? 
 
 In a paper published in the International 
 Scientific Review, " Scientia," in January, 1910, 
 Mr. Lowell gave a summary of his argument. 
 
 " Organic life needs water for its existence. This 
 water we see exists on Mars, but in very scant 
 amount, so that if life of any sort exists there, it 
 must be chiefly dependent on the semi-annual un- 
 locking of the polar snows for its supply, inasmuch 
 as there are no surface bodies of it over the rest of 
 the planet. Now the last few years, beginning 
 with Schiaparelli in 1877, and much extended 
 since at Flagstaff, have shown : 
 
 " The surface of the planet to be very curi- 
 ously meshed by a fine network of lines and 
 spots. 
 
 " Now if one considers first the appearance of 
 this network of lines and spots, and then its 
 regular behaviour, he will note that its geometrism 
 precludes its causation on such a scale by any 
 natural process and, on the other hand, that 
 such is precisely the aspect which an artificial 
 irrigating system, dependent upon the melting of 
 the polar snows, would assume. Since water is 
 only to be had at the time it is there unlocked, 
 and since for any organic life it must be got, it 
 would be by tapping the disintegrated cap, and 
 only so, that it could be obtained. If Mars be 
 inhabited, therefore, it is precisely such a curious 
 system we should expect to see, and only by such 
 explanation does it seem possible to account for 
 the facts. 
 
 " These lines are the so-called canals of Mars. It 
 is not supposed that what we see is the conduit 
 itself. On the contrary, the behaviour of these 
 lines indicates that what we are looking at is 
 
 66 
 
THE CANALS OF MARS 
 
 vegetation. Now, vegetation can only be induced 
 by a water-supply. What we see resembles the 
 yearly inundation of the Nile, of which to a 
 spectator in space the river itself might be too 
 narrow to be seen, and only the verdured country 
 on its banks be visible. This is what we suppose 
 to be the case with Mars. However the water be 
 conducted, whether in covered conduits, which 
 seems probable, or not, science is not able to 
 state, but the effects of it are so palpable and so 
 exactly in accord with what such a system of 
 irrigation would show, that we are compelled to 
 believe that such is indeed its vera causa.' ' 
 
 Beside the bulky Memoirs in which Prof. Lowell 
 has published the scientific results obtained at his 
 observatory at Flagstaff, and papers and articles 
 appearing in various scientific journals, he has 
 brought out three books of a more popular charac- 
 ter : " Mars ";" Mars and its Canals "; and " Mars 
 as the Abode of Life." In these he shows that to 
 the assiduity of the astronomer he adds the 
 missionary's zeal and eagerness for converts as 
 he pleads most skilfully for the acceptance of his 
 chosen doctrine of the presence of men on Mars. 
 In the last of the three books mentioned, he deals 
 directly with " Proofs of Life on Mars." The 
 presence of vegetation may be inferred from 
 seasonal changes of tint, just as an observer on the 
 Moon might with the naked eye watch effects on 
 the Earth. But though " vegetable life could 
 thus reveal itself directly, animal life could not. 
 
 6? 
 
ARE THE PLANETS INHABITED? 
 
 Not by its body but by its mind would it be 
 known. Across the gulf of space it could be 
 recognized only by the imprint it had made on the 
 face of Mars." 
 
 " Confronting the observer are lines and spots 
 that but impress him the more, as his study goes 
 on, with their non-natural look. So uncommonly 
 regular are they, and on such a scale as to raise 
 suspicions whether they can be by nature regu- 
 larly produced" (p. 188). 
 
 "... Unnatural regularity, the observations 
 showed, betrays itself in everything to do with the 
 lines : in their surprising straightness, their 
 amazing uniformity throughout, their exceeding 
 tenuity, and their immense length" (p. 189). 
 
 "As a planet ages, its surface water grows scarce. 
 Its oceans in time dry up, its rivers cease to flow, 
 its lakes evaporate (p. 203). . . . Now, in the 
 struggle for existence, water must be got. . . . 
 Its procuring depends on the intelligence of the 
 organisms that stand in need of it. ... As a 
 planet ages, any organisms upon it will share in 
 its development. They must evolve with it, 
 indeed, or perish. At first they change only, as 
 environment offers opportunity, in a lowly, un- 
 conscious way. But, as brain develops, they rise 
 superior to such occasioning. . . . The last stage 
 in the expression of life upon a planet's surface 
 must be that just antecedent to its dying of 
 thirst. . . . With an intelligent population this 
 inevitable end would be long foreseen. . . . Both 
 polar caps would be pressed into service in order 
 to utilize the whole available supply and also 
 to accommodate most easily the inhabitants of 
 each hemisphere" (pp. 204-11). 
 
 68 
 
THE CANALS OF MARS 
 
 " That intelligence should thus mutely communi- 
 cate^its existence to us across the far reaches of 
 space, itself remaining hid, appeals to all that is 
 highest and most far-reaching in man himself. 
 More satisfactory than strange this; for in no other 
 way could the habitation of the planet have been 
 revealed. It simply shows again the supremacy of 
 mind. . . . Thus, not only do the observations we 
 have scanned lead us to the conclusion that Mars 
 at this moment is inhabited, but they land us at 
 the further one that these denizens are of an 
 order whose acquaintance was worth the making " 
 (p. 215). 
 
 For the moment, let us leave Prof. Lowell's 
 argument as he puts it. Whether we accept it or 
 not, it remains that it is a marvellous achieve- 
 ment of the optician's skill and the observer's 
 devotion that from a planet so small and so distant 
 as Mars any evidence should be forthcoming at 
 all that could bear upon the question of the ex- 
 istence of intelligent organisms upon its surface. 
 But it is of the utmost significance to note that the 
 whole question turns upon the presence of water 
 of water in the liquid state, of water in a sufficient 
 quantity ; and the final decision, for Mr. Lowell's 
 contention, or against it, must turn on that one 
 point. The search for Life on Mars is essentially a 
 search for Water ; a search for water, not only in 
 the present state of Mars, but in its past as well. 
 For, without water in sufficient quantities in the 
 past, life on Mars could not have passed through 
 69 
 
ARE THE PLANETS INHABITED? 
 
 the evolutionary development necessary to its 
 attaining its highest expression, that where the 
 material living organism has become the taber- 
 nacle and instrument of the conscious intelligent 
 spirit. 
 
 70 
 
CHAPTER VII 
 THE CONDITION OF MARS 
 
 THE planet Mars is the debatable ground be- 
 tween two opinions. Here, the two opposing 
 views join issue ; the controversy comes to a 
 focus. The point in debate is whether certain 
 markings some linear, some circular are natural 
 or artificial. If, it is argued, some are truly like a 
 line, without curve or break, as if drawn with pen, 
 ink, and ruler ; or others, so truly circular, without 
 deviation or break, as if drawn with pen, ink, and 
 compass ; if, moreover, when we obtain more 
 powerful telescopes, erected in better climates for 
 observing, these markings become more truly lines 
 and circles the better we see them ; then they 
 are artificial, not natural structures. 
 
 But artificial structures imply artificers. And 
 if the structures are so designed as to meet the 
 needs of a living organism, it implies that the liv- 
 ing organism that designed them must have a 
 reasonable mind lodged in a natural body. If, 
 then, the " lines " and " circles " that Prof. Lowell 
 and his disciples assert to be artificial canals and 
 oases are really such, they premise the order of 
 being that we call Man. But these canals and 
 
PLANETARY 
 
 PROPORTIONS OF THE PLANETS : 
 
 Minor 
 Planets. 
 
 Inner 
 
 Ceres 
 
 Moon 
 
 Mercury 
 
 477 
 0*06 
 0-004 
 
 O-OOO2 
 2-8? 
 
 0-5? 
 
 O-OOOI 
 
 0-028 
 o-45 
 0-14 
 
 257-1 
 
 2-767 
 4-60 
 n-i 
 0-0763 
 
 w 
 
 (?) 
 
 0-014 
 o'ooog 
 119-0 
 
 2163 
 0-273 
 0-075 
 0-02 
 3-39 
 0-61 
 
 O-OI2 
 
 0-17 
 2-73 
 0-17 
 
 92-9 
 
 I-OOO 
 I-OO 
 
 18-5 
 0-0168 
 1-034 
 i-32' 
 d h m 
 27-7-43 
 
 0-40 
 
 0-027 
 19-6 
 
 22C 
 
 I-OO 
 I-OO 
 
 312 
 
 +39 
 290 
 + 17 
 412 
 + 139 
 371 
 +93 
 
 3030 
 0-383 
 0-147 
 0-06 
 4-72 
 0-85 
 0-048 
 o-33 
 5'30 
 0-14 
 
 36-0 
 0-387 
 0-24 
 29-7 
 0-2056 
 
 F 
 
 88(?) 
 
 1-6 
 
 0-108 
 
 IO-I 
 
 53C 
 
 6-67 
 1-61 
 
 502 
 +229 
 
 467 
 
 +194 
 664 
 +391 
 598 
 +325 
 
 ,, = i 
 
 Surface, = i 
 
 Volume, i 
 
 
 M i 
 
 
 Gravity at surface, i . 
 
 Rate of Fall, Feet in the Firt Second . 
 
 Albedo 
 
 DETAILS OF ORBIT: 
 Mean Distance from Sun in millions of miles . . 
 ,, ,, Earth's distance = i .. 
 Period of Revolution, in years ... .... 
 
 
 Eccentricitv . . 
 
 Aphelion Distance, Perihelion i 
 
 Inclination of Equator to Orbit . 
 
 Rotation period 
 
 ATMOSPHERE, assuming the total mass of the 
 atmosphere to be proportional to the mass of 
 the planet : 
 Pressure at the surface in Ib. per sq. inch 
 
 ,, ,, ,, in " atmospheres " .... 
 Level of half surface pressure in miles 
 
 Boiling point of water at the surface 
 
 TEMPERATURE : 
 Light and heat received from Sun, i 
 
 0-13 
 0-60 
 188 
 -65 
 174 
 -99 
 248 
 -25 
 223 
 -50 
 
 Reciprocal of square-root of distance, = i ... 
 Equatorial temp, of ideal planet, Absolute .... 
 ,, ,, Centigrade . . . 
 Average temp, of ideal planet, Absolute 
 
 ,, ,, ,, ,, Centigrade 
 Upper limit under zenith sun, Absolute 
 
 ,, ,, Centigrade 
 Average temp, of equivalent disc, Absolute . . . 
 M Centigrade .. 
 
STATISTICS 
 
 Planets. 
 
 
 Outer Planets. 
 
 Mars 
 
 Venus 
 
 Earth 
 
 Uranus 
 
 Neptune 
 
 Saturn 
 
 Jupiter 
 
 4230 
 
 7700 
 
 7918 
 
 31900 
 
 34800 
 
 73000 
 
 86500 
 
 Q-534 
 
 O-2h5 
 
 0-972 
 0-945 
 
 I-OOO 
 I -000 
 
 4-029 
 16-2 
 
 4-395 
 19-3 
 
 9-219 
 85-0 
 
 10-924 
 119-3 
 
 0-15 
 
 0-92 
 
 I-OO 
 
 65' 
 
 85- 
 
 760- 
 
 1304- 
 
 3.92 
 
 4'94 
 
 5-55 
 
 1-22 
 
 i -i i 
 
 0-72 
 
 1.32 
 
 0-71 
 
 0-89 
 
 I-OO 
 
 O-22 
 
 O-2O 
 
 0-13 
 
 0-24 
 
 0-107 
 0-38 
 
 0-820 
 0-87 
 
 I -000 
 I-OO 
 
 14-6 
 0-90 
 
 17-0 
 0-89 
 
 94-8 
 1-18 
 
 3177 
 2-65 
 
 6-n 
 
 I3-99 
 
 16-08 
 
 14-47 
 
 I4-3I 
 
 18-97 
 
 42-61 
 
 0-22 
 
 0-76 
 
 0-50 ? 
 
 0-60 
 
 0-52 
 
 0-72 
 
 0-62 
 
 I4I-5 
 
 67-2 
 
 92-9 
 
 1781-9 
 
 2791-6 
 
 886-0 
 
 483-3 
 
 I-524 
 
 0-723 
 
 I-OOO 
 
 19-183 
 
 30-055 
 
 9-539 
 
 5-203 
 
 1-88 
 
 0-62 
 
 I-OO 
 
 84-02 
 
 164-78 
 
 29-46 
 
 n-86 
 
 I5'0 
 
 21-9 
 
 18-5 
 
 4-2 
 
 3-4 
 
 6-0 
 
 8-1 
 
 0-0933 
 
 0-0068 
 
 0-0168 
 
 0-0463 
 
 0-0090 
 
 0-0561 
 
 0-0483 
 
 1-207 
 
 1-013 
 
 1-034 
 
 1-097 
 
 1-018 
 
 1-107 
 
 I-IOI 
 
 2 4 -o' 
 
 (?) 
 
 23'27' 
 
 (?) 
 
 (?) 
 
 26-49' 
 
 3-5' 
 
 h m s 
 
 
 h m s 
 
 h m 
 
 
 h m 
 
 h m 
 
 24-37-23 
 
 (?) 
 
 23.56.4 
 
 9-3o(?) 
 
 (?) 
 
 10.14 + 
 
 9-55 
 
 2-1 
 
 ii-i 
 
 14-7 
 
 n-9 
 
 "5 
 
 19-4 
 
 103.8 
 
 0-143 
 
 o-754 
 
 I-OOO 
 
 0-81 
 
 0-78 
 
 1-32 
 
 7-06 
 
 8-8 
 53 C 
 
 3-8 
 92 C 
 
 ,: 
 
 9 3 & 
 
 3-8 
 93 C 
 
 2-8 
 
 io8C 
 
 i66C 
 
 o-43 
 
 I -91 
 
 I -00 
 
 O-OO3 
 
 o-ooi 
 
 O-OII 
 
 0-037 
 
 0-81 
 
 1-18 
 
 I-OO 
 
 0-23 
 
 0-18 
 
 0-32 
 
 0-44 
 
 253 
 
 368 
 
 312 
 
 71 
 
 56 
 
 101 
 
 137 
 
 20 
 
 +95 
 
 + 39 
 
 202 
 
 -217 
 
 -172 
 
 -136 
 
 235 
 
 342 
 
 290 
 
 66 
 
 52 
 
 94 
 
 127 
 
 -38 
 
 +69 
 
 + i7 
 
 -207 
 
 221 
 
 -179 
 
 -146 
 
 337 
 
 486 
 
 412 
 
 94 
 
 74 
 
 133 
 
 1 80 
 
 + 64 
 
 + 213 
 
 + 139 
 
 -179 
 
 -199 
 
 -140 
 
 -93 
 
 300 
 
 438 
 
 37i 
 
 84 
 
 67 
 
 120 
 
 162 
 
 +27 
 
 + 165 
 
 +98 
 
 189 
 
 206 
 
 -153 
 
 in 
 
ARE THE PLANETS INHABITED? 
 
 oases also premise the liquid that we call Water 
 water that flows and water utilized in cultivation. 
 In this chapter we will leave out of count the first 
 premiss Man and only deal with what concerns 
 the second premiss Water ; with water that 
 flows and is utilized in vegetation. 
 
 For in regard to this particular premiss we can 
 do away with hypothesis, and deal only with cer- 
 tain physical facts that are not controversial and 
 are not in dispute. 
 
 The first of this series of facts concerning Mars 
 about which there can be no controversy or dis- 
 pute relates to its size and mass. As the foregoing 
 Table shows, it comes between the Moon and the 
 Earth in these respects. 
 
 The figures show at a glance that Mars ranks in 
 its dimensions between the Moon and the Earth, 
 and that, on the whole, it is more like to the Moon 
 than it is to the Earth. 
 
 But in what way would this affect Mars as a 
 suitable home for life ? In many ways ; and 
 amongst these the distribution of its atmosphere 
 and the sluggishness of its atmospheric circulation 
 are not the least important. 
 
 It was mentioned in Chapter III that at 
 a height of about three and a third miles the 
 barometer will stand at 15 inches, or half its 
 mean height at sea level, showing that one 
 half the atmosphere has been passed through. 
 Mont Blanc, the highest mountain in Europe, is 
 
 74 
 
THE CONDITION OF MARS 
 
 under 3 miles in height, so that it is not possible, 
 in Europe, to climb to the level of half-pressure ; 
 Mt. Everest, the highest mountain in the world, 
 is not quite six miles high, so that no part of 
 the solid substance of our planet reaches up to 
 the level of the quarter pressure. On a very few 
 occasions daring aeronauts have soared into the 
 empyrean higher than the summits of even our 
 loftiest mountains, but the excursion has been a 
 dangerous one, and they have with difficulty 
 brought their life back from so rare and cold, so 
 inhospitable a region. When Gay-Lussac, in 1804, 
 attained a height of 23,000 feet above sea level, 
 the thermometer, which on the ground read 31 C., 
 sank to 9 below zero, and the rare atmosphere 
 was so dry that paper crumpled up as if it had 
 been placed near the fire, and his pulse rose to 
 1 20 pulsations a minute instead of his normal 66. 
 When Mr. Glaisher and Mr. Coxwell made their 
 celebrated ascent between i and 2 o'clock on the 
 afternoon of September 5, 1861, they found that 
 at a height of 21,000 feet the temperature sank to 
 -10-4; at 26,000 feet to -15*2; and at 39,000 feet 
 the temperature was down to -16-0 C. At this 
 height the rarefaction of the air was so great and 
 the cold so intense that Mr. Glaisher fainted, and 
 Mr. Coxwell's hands being rendered numb and use- 
 less by the cold, he was only able to bring about 
 their descent in time by pulling the string of the 
 safety valve with his teeth. Yet when they at- 
 
 75 
 
ARE THE PLANETS INHABITED? 
 
 tained this height they were far above all cloud or 
 mist, and the Sun's rays fell full upon them. The 
 Sun's rays had all the force that they had at the 
 surface of the Earth, but in the rare atmosphere of 
 seven miles above the Earth, the radiation from 
 every particle not in direct sunlight was so great 
 that while the right hand, exposed to the Sun, 
 might burn, the left hand, protected from his 
 direct rays, might freeze. 
 
 But gravity at the surface of Mars is much 
 feebler than at the surface of the Earth, and in 
 order to reach the level of half-pressure a Martian 
 mountaineer would have to climb, not three and 
 a third miles, but eight and three-quarter miles ; 
 that is to say, the distance to be ascended is in the 
 inverse proportion of the force of gravity at the 
 surface of the planet. The atmosphere of Mars, 
 therefore, is much deeper than that of the Earth, 
 and one great cause of precipitation here is much 
 weakened there. A current of air heavily laden 
 withmoisture,if it encounters a range of mountains, 
 is forced upwards, and consequently expands, 
 owing to the diminished pressure. The expansion 
 brings about a cooling, and from both causes the 
 atmosphere is unable to retain as much water- 
 vapour as it carried before. On Mars, the same 
 relative expansion and cooling would only follow 
 if the ascent were nearly three times as great, and 
 the feeble force of gravity has its effect in another 
 way ; for just as a weight on Mars will only fall 
 76 
 
THE CONDITION OF MARS 
 
 six feet in the first second as against sixteen on the 
 Earth, so a dense and heavy column of air will fall 
 with proportionate slowness and a light column 
 ascend in the same languid manner. An as- 
 cending current on Mars would therefore take 
 
 5-X ^r = ~ , or seven times as long to 
 
 0-38 0-38 0-145 
 
 attain the same relative expansion as on the Earth. 
 
 The winds of Mars are therefore sluggish, and 
 
 precipitation is slight. So far at least it resembles 
 
 " The island valley of Avilion ; 
 Where falls not hail, or rain, or any snow, 
 Nor ever wind blows loudly ; " 
 
 and R. A. Proctor, acute and accurate writer on 
 planetary physics as he was, fell into a mistake 
 when he referred to Mars as being " hurricane- 
 swept." There are no hurricanes on Mars ; its 
 fiercest winds can never exceed in violence what 
 a sailor would call a " capful." 
 
 This holds good for Mars, but it also holds good 
 for every planet where the force of gravity at the 
 surface is relatively feeble. The greater the force 
 of gravity the more active the atmospheric circu- 
 lation, and more violent its disturbances ; the 
 feebler the action of gravity the more languid 
 the circulation, and the slighter the disturbances. 
 
 The atmosphere of Mars is relatively deeper 
 than that of the Earth, so that we, in observing the 
 details of its surface, are looking down through an 
 
 77 
 
ARE THE PLANETS INHABITED? 
 
 immense thickness of an obscuring medium. And 
 yet the details of the surface are seen with remark- 
 able distinctness ; not as clearly indeed as we can 
 see those of the Moon, but nearly so. For instance, 
 the " canals " appear to have a breadth of from 
 15 to 20 miles, corresponding to yVth, and tVth, of 
 a second of arc, at an average opposition. The 
 oases, as a rule, are about 120 miles in diameter, 
 that is to say about half a second of arc. These 
 are extraordinarily fine details to be perceived and 
 held, even if Mars had no atmosphere at all ; it 
 would certainly be impossible to detect them unless 
 the atmosphere were exceedingly thin and trans- 
 parent. For we must remember that, though our 
 own atmosphere is a hindrance to our observing, 
 yet the atmosphere of the planet into which we are 
 looking is a greater hindrance still. Like the lace 
 curtains of the window of a house, it is a much 
 greater obstacle to looking inward than to looking 
 outward, and as the perfect distinctness with 
 which we see the Moon is a proof that it is practi- 
 cally without an atmosphere, so the great detail 
 visible on Mars bears unmistakable testimony to 
 the slightness of the atmospheric veil around that 
 planet. 
 
 And when we turn again to the statistics of 
 Mars, we see that this must inevitably be the case. 
 Of two planets, one heavier than the other, it is 
 not possible to suppose that the lighter should 
 secure the greater proportional amount of atmo- 
 
 78 
 
THE CONDITION OF MARS 
 
 sphere. With planets, as with persons, it is the 
 most powerful that gets the lion's share : "to 
 him that hath it is given, and from him that hath 
 not is taken away even that which he seemeth to 
 have." But if we assume that Mars has acquired 
 an atmosphere proportional to its mass, then we 
 see from the Table that this must be a little less 
 than ^th of that of the Earth ; exactly 0-107. ^ 
 is distributed over a smaller surface, 0-285. Con- 
 sequently the amount of air above each square 
 inch of Martian surface is 0-107-^0-285 =0-38. 
 But since the force of gravity at the surface of 
 Mars is less than on the Earth, this column of air 
 will only weigh 0-38 X 0-38 =0-145 ; or one-seventh 
 of the column of air resting on a square inch of the 
 Earth's surface. The pressure at the surface of 
 Mars will therefore be 2-1 lb.; and the aneroid 
 barometer would read 4-3 inches. (In order to 
 express the diminished pressure of the Martian 
 atmosphere, it is necessary to refer it to the aneroid 
 barometer. The mercury in a mercurial barometer, 
 or the water in a water barometer would lose in 
 weight in consequence of the diminished force of 
 gravity in the same proportion as the air would, 
 and the mercurial barometer would read 11-4 inches .) 
 But a pressure of 2-1 lb. on the square inch is 
 far less than that experienced by Coxwell and 
 Glaisher in tl^eir great ascent; it is about one-half 
 the pressure that is experienced on the top of the 
 very highest terrestrial mountains. But the habit- 
 
 79 
 
ARE THE PLANETS INHABITED? 
 
 able regions of the Earth do not extend even so 
 far upward as to the level of a pressure of 7-3 Ib. 
 on the square inch ; that is, of half the terrestrial 
 surface pressure. Plant life dies out before we 
 reach that point, and though birds or men may 
 occasionally attain greater heights, they cannot 
 domicile there, and are, indeed, only able thus to 
 ascend in virtue of nourishment which they have 
 procured in more favoured regions. If we could 
 suppose the conditions of the whole Earth changed 
 to correspond with those prevailing at the summit 
 of Mt. Everest, or even at the summit of Mont 
 Blanc, it is clear that the life now present on this 
 planet would be extinguished, and that speedily. 
 Much more would this be the case if the atmo- 
 sphere were diminished to one half the pressure 
 on the summit of the highest earthly mountain. 
 
 The tenuity of the atmosphere on Mars has 
 another consequence. Here water freezes at o C. 
 and boils at 100 C. ; so that for one hundred 
 degrees it remains in a liquid condition. On Mars, 
 under the assumed conditions, water would boil at 
 53 C., and the range of temperature within which 
 it would be liquid would be much curtailed. But 
 it is only water in the liquid state that is useful for 
 sustaining life. 
 
 The above estimate of the density of the atmo- 
 sphere of Mars is an outside limit, for it assumes 
 that Mars has retained an atmosphere to the full 
 proportion of its mass. But as the molecules of a 
 80 
 
THE CONDITION OF MARS 
 
 gas are in continual motion, and in every direction, 
 the lighter, most swiftly moving molecules must 
 occasionally be moving directly outwards from the 
 planet at the top of their speed, and in this case, if 
 the speed of recession should exceed that which 
 the gravity of the planet can control, the particle 
 is lost to the planet for ever. A small planet 
 therefore is subject to a continual drain upon its 
 atmosphere, a drain of the lightest constituents. 
 Hence it is, no doubt, that free hydrogen is not a 
 constituent of the atmosphere of the Earth. 
 
 To what extent, then, has the atmosphere of 
 Mars fallen below its full proportion ? Mr. Lowell 
 has adopted an ingenious method of obtaining 
 some light on this question, by comparing the 
 relative albedoes of the Earth and Mars ; that is 
 to say the relative power of reflection possessed by 
 the two planets. Of course the method is rough ; 
 we have first of all no satisfactory means of deter- 
 mining the albedo of the Earth itself, and Mr. 
 Lowell puts it higher than most astronomers would 
 do ; then there is the difficulty of determining 
 what portion of the total albedo is to be referred 
 to the atmosphere and what to the actual soil or 
 surface of the planet. But, on the whole, Mr. 
 Lowell concludes that the amount of atmosphere 
 above the unit of surface of Mars is 0'222 of 
 that above the unit of surface of the Earth. This 
 would bring down the pressure on each square inch 
 of Mars to 1-2 lb., and the aneroid barometer would 
 G Si 
 
ARE THE PLANETS INHABITED? 
 
 read 2-5 inches ; and water would boil at 44 C. 
 The range of temperature from day to night, from 
 summer to winter, at any place on the planet 
 would be increased, while the range within which 
 water could retain its liquid form would be dimin- 
 ished. 
 
 These statistics may seem rather dull and tire- 
 some, but if we are to deal with the problem before 
 us at all, it is important to understand that one 
 factor in the condition of a planet cannot be 
 altered and all the other factors retained un- 
 changed. It will be seen that in computing the 
 density of the atmosphere of Mars, we had to take 
 into consideration not only the diameter of the 
 planet, but the surface, which varies as the square 
 of the diameter ; the volume, which varies as the 
 cube ; the mass, which varies in a higher power 
 still ; and various combinations of these numbers. 
 Novelists who write tales of journeys to other 
 worlds or of the inhabitants of other worlds visit- 
 ing this one, usually assume that the atmosphere 
 is of the same density on all planets, and the action 
 of gravity unchanged. In their view it is only that 
 men would have a little less ground to walk upon 
 on Mars, and a good deal more on Jupiter. Dean 
 Swift, in Gulliver's Travels, made the Lillipu- 
 tians take a truer view of the effect of the altera- 
 tion of one dimension, for, finding that Gulliver 
 was twelve times as tall as the average Lilliputian, 
 they did not appoint him the rations of twelve 
 82 
 
THE CONDITION OF MARS 
 
 Lilliputians, which would have been rather poor 
 feeding for that veracious mariner, but allotted 
 him the cube of twelve, viz. seventeen hundred and 
 twenty-eight rations. Mr. J. Holt Schooling, in 
 one of his ingenious and interesting statistical 
 papers, tried to bring home the vast extent of the 
 British Empire by supposing that it seceded, and 
 taking the portion of Earth that has fallen to it, 
 set up a world of its own the planet " Victoria." 
 He allots to the British Empire 21 per cent of 
 the land surface of the world. If the Earth were 
 divided so as to form two globes with surfaces 
 in proportion of 21 to 79, the smaller globe, which 
 would correspond to Mr. Schooling's new planet 
 " Victoria," would be less than half the present 
 Earth in diameter ; it would be considerably 
 smaller than Mars. But " the rest of the world " 
 would be 0*96 of the present Earth in diameter, 
 or very nearly the size of Venus, and it would 
 contain just eight-ninths of the substance of the 
 Earth, leaving only one-ninth for " Victoria." 
 The statistics given above will suggest to the 
 reader that, could such a secession be carried out, 
 the inhabitants of the British Empire would not 
 be happier for the change during the very short 
 continued existence that remained to them. The 
 " rest of the world " could spare our fraction of 
 the planet much better than we could spare 
 theirs. 
 
 This is a principle which applies to worlds any- 
 
 83 
 
ARE THE PLANETS INHABITED? 
 
 where ; not merely within the limits of the solar 
 system but wherever they exist. Everywhere the 
 surface must vary with the square of the diameter ; 
 the volume with the cube ; everywhere the smaller 
 planet must have the rarer atmosphere, and with a 
 rare atmosphere the extreme range of temperature 
 must be great, while the range of temperature 
 within which water will flow will be restricted. 
 Our Earth stands as the model of a world of the 
 right size for the maintenance of life ; much 
 smaller than our Earth would be too small ; much 
 larger, as we shall see later, would be too 
 large. 
 
 So far we have dealt with Mars as if it received 
 the same amount of light and heat from the Sun 
 that the Earth does. But, as the Table shows, from 
 its greater distance from the Sun, Mars receives 
 per unit of surface only about three-sevenths of 
 the light and heat of that received by the Earth. 
 
 The inclination of the axis of Mars is almost the 
 same as that of the Earth, so that the general 
 character of the seasons is not very different on 
 the two planets, and the torrid, temperate, and 
 frigid zones have almost the same proportions. 
 The length of the day is also nearly the same for 
 both, the Martian day being slightly longer ; but 
 the most serious factor is the greater distance of 
 Mars, and the consequent diminution in the light 
 and heat received from the Sun. The light and 
 heat received by the Earth are not so excessive that 
 84 
 
THE CONDITION OF MARS 
 
 we could be content to see them diminished, even 
 by 5 per cent, but for Mars they are diminished by 
 57 per cent. How can we judge the effect of so 
 important a difference ? 
 
 The mean temperature of our Earth is supposed 
 to be about 60 F., or 16 C. Three-sevenths of 
 this would give us 7 C. as the mean temperature 
 of Mars, which would signify a planet not im- 
 possible for life. But the zero of the Centigrade 
 scale is not the absolute zero ; it only marks the 
 freezing-point of water. The absolute zero is 
 computed to be 273 on the Centigrade scale ; the 
 temperature of the Earth on the absolute scale 
 therefore should be taken as 289, and three- 
 sevenths of this would give 124 of absolute tem- 
 perature. But this is 149 below freezing-point, 
 and no life could exist on a planet under such 
 conditions. 
 
 But the mean temperature of Mars cannot be com- 
 puted quite so easily. The hotter a body is the more 
 rapidly it radiates heat ; the cooler it is the slower 
 its radiation. According to Stefan's Lav/, the 
 radiation varies for a perfect radiator with the 4th 
 power of the absolute temperature ; so that if Mars 
 were at 124 abs., while the Earth were at 289 abs., 
 the Earth would be radiating its heat nearly 30 
 times faster than Mars. The heat income of Mars 
 would therefore be in a much higher proportion 
 than its expenditure ; and necessarily its heat 
 capital would increase until income and expendi- 
 
 85 
 
ARE THE PLANETS INHABITED? 
 
 ture balanced. Prof. Poynting has made the 
 temperature of the planets under the 4th power 
 law of radiation the subject of an interesting 
 enquiry, and the figures which he has obtained 
 for Mars and other planets are included in the 
 Table. 
 
 The equatorial and average temperatures are 
 given under the assumption that Mars possesses an 
 atmosphere as efficient as our own in equalizing 
 the temperature of the whole planet. If, on the 
 other hand, its atmosphere has no such regulating 
 power, then under the zenith Sun the upper limit 
 of the temperature of a portion of its surface re- 
 flecting one-eighth would be, as shown in the Table, 
 64 C. This would imply that the temperature 
 on the dark side of the planet was very nearly at 
 the absolute zero. " If we regard Mars as re- 
 sembling our Moon, and take the Moon's effective 
 average temperature as 297 abs., the correspond- 
 ing temperature for Mars is 240 abs., and the 
 highest temperature is four-fifths of 337 270 abs. 
 But the surface of Mars has probably a higher 
 coefficient of absorption than the surface of the 
 Moon it certainly has for light so that we may 
 put his effective average temperature, on this 
 supposition, some few degrees above 240 abs., and 
 his equatorial temperature some degrees higher 
 still. It appears as exceedingly probable, then, that 
 whether we regard Mars as like the Earth or, 
 going to the other extreme, as like the Moon, the 
 86 
 
THE CONDITION OF MARS 
 
 temperature of his surface is everywhere below the 
 freezing-point of water."* As the atmospheric 
 circulation on Mars must be languid, and the 
 atmosphere itself is very rare, the general condi- 
 tion of the planet will approximate rather to the 
 lunar type than to the terrestrial, and the extremes, 
 both of heat and cold, will approach those which 
 would prevail on a planet without a regulating 
 atmosphere. 
 
 There is another way of considering the effect on 
 the climate of Mars and its great distance from the 
 Sun, which, though only rough and crude, may be 
 helpful to some readers. If we take the Earth at 
 noonday at the time of the equinox, then a square 
 yard at the equator has the Sun in its zenith, and 
 is fully presented to its light and heat. But, as 
 we move away from the equator, we find that 
 each higher latitude is less fully presented to the 
 Sun, until, when we reach latitude 64^ in other 
 words just outside the Arctic Circle 7 square 
 yards are presented to the Sun so as to receive only 
 as much of the solar radiation as 3 square yards 
 receive at the equator. We may take, then, lati- 
 tude 64^ as representing Mars, while the equator 
 represents the Earth. Or, we may take it that we 
 should compare the climate of Archangel with the 
 climate of Singapore. 
 
 * Radiation in the Solar System : Its Effects on Temperature^ 
 and its Pressure on Small Bodies, by Dr. J. II. Poynting (Phil. 
 Trans, of the Royal Society, Vol. 202 A). 
 
 87 
 
ARE THE PLANETS INHABITED? 
 
 Now the mean temperature of latitude 64 J, say 
 the latitude of Archangel, is just about freezing- 
 point (o C.), while that of the equator is about 
 28 C. We should therefore expect from this a 
 difference between the mean temperatures of the 
 Earth and Mars of 28 ; that is to say, as the Earth 
 stands at 16 C., Mars would be at -12 C. But, on 
 the Earth, the evaporation and precipitation is 
 great, and the atmospheric circulation vigorous. 
 Evaporation is always going on in equatorial regions, 
 and the moisture-laden winds are continually mov- 
 ing polewards, carrying with them vast stores of 
 heat to be liberated as the rain falls. The oceanic 
 currents have the same effect, and how great the 
 modification which they introduce may be seen by 
 comparing the climates of Labrador and Scotland. 
 There appear to be no great oceans on Mars. The 
 difference of 28 which we find on the Earth be- 
 tween the equator and the edge of the Arctic Circle 
 is a difference which remains after the convection 
 currents of air and sea have done much to reduce 
 the temperature of the equator and to raise that 
 of high latitudes. If we suppose that their effect 
 has been to reduce this difference to one half of 
 what it would have been were each latitude 
 isolated from the rest, we shall not be far wrong, 
 and we should get a range of 56 as the true 
 equivalent difference between the mean tempera- 
 tures of Singapore and Archangel ; i.e. of the Earth 
 and Mars ; and Mars would stand at -40 C. The 
 88 
 
THE CONDITION OF MARS 
 
 closeness with which this figure agrees with that 
 reached by Prof. Poynting suggests that it is a 
 fair approximation to the correct figure. 
 
 The size of Mars taught us that we have in it a 
 planet with an atmosphere of but one half the 
 density of that prevailing on the top of our highest 
 mountain ; the distance of Mars from the Sun 
 showed us that it must have a mean temperature 
 close to that of freezing mercury. What chance 
 would there be for life on a world the average con- 
 dition of which would correspond to that of a 
 terrestrial mountain top, ten miles high and in the 
 heart of the polar regions ? But Mars in the tele- 
 scope does not look like a cold planet. As we look 
 at it, and note its bright colour, the small extent 
 of the white caps presumed to be snow, and the 
 high latitudes in which the dark markings pre- 
 sumed to be water or vegetation are seen, it seems 
 difficult to suppose that the mean temperature of 
 the planet is lower than that of the Earth. Thus 
 on the wonderful photographs taken by Prof. 
 Barnard in 1909, the Nilosyrtis with the Protoni- 
 lus is seen as a dark canal. Now the Protonilus is 
 in North Lat. 42, and on the date of observation 
 September 28, 1909 the winter solstice of the 
 northern hemisphere of Mars was just past. There 
 would be nothing unusual for the ground to be 
 covered with snow and the water to be frozen in a 
 corresponding latitude if in a continental situation 
 on the Earth. Then, again, in the summer, the 
 
 89 
 
ARE THE PLANETS INHABITED? 
 
 white polar caps of Mars diminish to a far greater 
 extent than the snow and ice caps of the Earth ; 
 indeed, one of the Martian caps has been known 
 to disappear completely. 
 
 Yet, as the accompanying diagram will show, 
 something of this kind is precisely what we ought 
 to expect to see. The diagram has been constructed 
 in the following manner: A curve of mean tem- 
 peratures has been laid down for every 10 of lati- 
 tude on the Earth, derived as far as possible from 
 accepted isothermals in continental countries in 
 the northern hemisphere. From this curve ordi- 
 nates have been drawn at each 10, upward to 
 show average deviation from the mean tempera- 
 ture for the hot test part of the day in summer, down- 
 ward for the deviation for the coldest part of the 
 night in winter. Obviously, on the average, the 
 range from maximum to minimum will increase 
 from the equator to the poles. The mean tempera- 
 ture of the Earth has been taken as 16 C., and as 
 representing that prevailing in about 42 lat. The 
 diagram shows that the maximum temperature of 
 no place upon the Earth's surface approaches the 
 boiling-point of water, and that it is only within 
 the polar circle that the mean temperature is below 
 freezing-point. Water, therefore, on the Earth 
 must be normally in the liquid states 
 
 In constructing a similar diagram for Mars, 
 three modifications have to be made. First of all, 
 the mean temperature of the planet must be con- 
 go 
 
POILIMC POINT OF WATER 
 
 EARTH 
 
 UNT.CRAOE 
 - +90 
 
 -no* 
 
 MO* 
 + 60* 
 + 30' 
 
 MARS 
 
 BOILING POINT OF 
 
 MO' 
 f 30' 
 
 + 20' 
 + 10' 
 
 at 
 -isi 
 
 -274' 
 -232 
 -510' 
 Itf 
 
 \ 
 
 \ 
 
 THERMOGRAPHS OK THE EARTH AND MARS 
 
ARE THE PLANETS INHABITED? 
 
 siderably lower than that of the Earth. Next, 
 since the atmospheric circulation is languid and 
 there are no great oceans, the temperatures of 
 different latitudes cannot be equalized to the same 
 extent as on the Earth. It follows, therefore, that 
 the range in mean temperature from equator to 
 pole must be considerably greater on Mars than 
 on the Earth. Thirdly, the range in temperature 
 in any latitude, from the hottest part of the day in 
 summer to the coldest part of the night in winter, 
 must be much greater than with us ; partly on 
 account of the very slight density of the atmo- 
 sphere, and partly on account of the length of the 
 Martian year. 
 
 . We cannot know the exact figures to adopt, but 
 the general type of the thermograph for Mars as 
 compared with that of the Earth will remain. The 
 mean temperature of Mars will be lower, the range 
 of temperature from equator to pole will be greater, 
 and the extremes of temperature in any given 
 latitude more pronounced than upon the Earth. 
 And the general lesson of the diagram may be 
 summed up in a sentence. The maximum tempera- 
 ture on the planet is well above freezing-point, and 
 the part of the planet at maximum temperature 
 is precisely the part that we see the best. But 
 while this is so, it is clear that water on Mars 
 must normally be in the state of ice ; Mars is 
 essentially a frozen planet ; and the extremes of 
 cold experienced there, not only every year but 
 92 
 
THE CONDITION OF MARS 
 
 every night, far transcend the bitterest extremes 
 of our own polar regions. 
 
 The above considerations do not appear to 
 render it likely that there is any vegetation on 
 Mars. A planet ice-bound every night and with 
 its mean temperature considerably below freezing- 
 point does not seem promising for vegetation. If 
 vegetation exists, it must be of a kind that can pass 
 through all the stages of its life-history during the 
 few bright hours of the Martian day. Every night 
 will be for it a winter, a winter of undescribable 
 frost, which it could only endure in the form of 
 spores. So if there be vegetation it must be con- 
 fined to some hardy forms of a low type. At a 
 distance of forty millions of miles it is not easy to 
 discriminate between the darkness of sheets of 
 water and the darkness of stretches of vegetation. 
 Some of the so-called " seas " may possibly be 
 really of the latter class, but that there must be 
 expanses of water on the planet is clear, for if 
 there were no water surfaces there would be no 
 evaporation ; and if there were no evaporation 
 from whence could come the supply of moisture 
 that builds up the winter pole cap ? 
 
 The great American astronomer, Prof. Newcomb, 
 gave in Harper 's Weekly for July 25, 1908, an 
 admirable summary of the verdict of science as to 
 the character of the meteorology of Mars. " The 
 most careful calculation shows that if there are any 
 considerable bodies of water on our neighbouring 
 
 93 
 
ARE THE PLANETS INHABITED? 
 
 planet they exist in the form of ice, and can never 
 be liquid to a depth of more than one or two 
 inches, and that only within the torrid zone and 
 during a few hours each day. . . . There is no 
 evidence that snow like ours ever forms around the 
 poles of Mars. It does not seem possible that any 
 considerable fall of such snow could ever take 
 place, nor is there any necessity of supposing actual 
 snow or ice to account for the white caps. At a 
 temperature vastly below any ever felt in Siberia, 
 the smallest particles of moisture will be condensed 
 into what we call hoar frost, and will glisten with 
 as much whiteness as actual snow. . . . Thus we 
 have a kind of Martian meteorological changes, 
 very slight indeed and seemingly very different 
 from those of our earth, but yet following similar 
 lines on their small scale. For snowfall substitute 
 frostfall ; instead of feet or inches say fractions of 
 a millimetre, and instead of storms or wind sub- 
 stitute little motions of an air thinner than that 
 on the top of the Himalayas, and we shall have a 
 general description of Martian meteorology." 
 
 What we know of Mars, then, shows us a planet, 
 icebound every night, but with a day temperature 
 somewhat above freezing-point. As we see it, we 
 look upon its w r armest regions, and the rapidity 
 with which it is cleared of ice, snow, and cloud 
 shows the atmosphere to be rare and the moisture 
 little in amount and readily evaporated. The 
 seas are probably shallow depressions, filled with 
 
 94 
 
THE CONDITION OF MARS 
 
 ice to the bottom, but melted as to their surfaces 
 by day. From the variety of tints noted in the 
 seas, and the recurrent changes in their outlines, 
 they are composed of congeries of shallow pools, fed 
 by small sluggish streams ; great ocean basins 
 into which great rivers discharge themselves are 
 quite unknown. 
 
 95 
 
CHAPTER VIII 
 THE ILLUSIONS OF MARS 
 
 THE two preceding chapters have led to two 
 opposing, two incompatible conclusions. In 
 Chapter VI, a summary was given of Prof. Lowell's 
 claim to have had ocular demonstration of the 
 handiwork of intelligent organisms on Mars. In 
 Chapter VII, it was shown that the indispensable 
 condition for living organisms, water in the liquid 
 state, is only occasionally present there, the general 
 temperature being much below freezing-point, so 
 that living organisms of high development and 
 more than ephemeral existence are impossible. 
 
 Prof. Lowell argues that the appearance of the 
 network of lines and spots formed by the canals 
 and oases, and its regular behaviour, " preclude its 
 causation on such a scale by any natural process," 
 his assumption being that he has obtained finality 
 in his seeing of the planet, and that no improve- 
 ment in telescopes, no increase in experience, no 
 better eyesight will ever break up the perfect 
 regularity of form and position, which he gives to 
 the canals, into finer and more complex detail. 
 
 But the history of our knowledge of the planet's 
 surface teaches us a different lesson. Two small 
 96 
 
THE ILLUSIONS OF MARS 
 
 objects appear repeatedly on the drawings made 
 by Beer and Madler in 1830 ; these are two 
 similar dark spots, the one isolated, the other at 
 the end of a gently curved line. Both spots 
 resemble in form and character the oases of Prof. 
 Lowell, and the curved line, at the termination of 
 which one of the spots appears, represents closely 
 the appearance presented by several of the canals. 
 In the year 1830 no better drawings of Mars had 
 appeared ; and in representing these two spots as 
 truly circular and the curved line as narrow, sharp, 
 and uniform, Beer and Madler undoubtedly por- 
 trayed the planet as actually they saw it. The 
 one marking was named by Schiaparelli the Lacus 
 Solis, the other, the Sinus Sabaeus, and they are 
 two of the best known and most easily recognized 
 of the planet's features ; so that it is easy to trace 
 the gro\vth of our knowledge of both of them 
 from 1830 up to the present time. They were 
 drawn by Dawes in 1864, by Schiaparelli in 1877 
 and the succeeding years, by Lowell in 1894 and 
 since, and by Antoniadi in 1909 and 1911. But 
 whereas the drawings of Beer and Madler, made by 
 the aid of a telescope of 4 inches aperture, show 
 the two spots as exactly alike, in those of Dawes, 
 made with a telescope of 8 inches, the resemblance 
 between the two has entirely vanished, and neither 
 is shown as a plain circular dot. Since then, 
 observers of greater experience and equipped with 
 more powerful instruments have directed their 
 H 97 
 
ARE THE PLANETS INHABITED? 
 
 attention to these two objects, and a mass of com- 
 plicated structure has been brought out in the 
 regions which were so simple in the sight of Beer 
 and Madler, so that not a trace of resemblance 
 remains between the two objects that to them 
 appeared indistinguishable. 
 
 Now the gradation in size, from the Lacus Solis 
 down to the smallest oasis of Lowell, is a complete 
 one. If a future development in the power of 
 telescopes should equal the advance made from 
 the 4-inch of Beer and Madler, to the 33-inch which 
 Antoniadi used in 1909, is it reasonable to suppose 
 that Prof. Lowell's oases will refuse to yield to 
 such improvement, and will all still show them- 
 selves as uniform spots, precisely circular in outline? 
 It is clear that Beer and Madler would have been 
 mistaken if they had argued that the apparently 
 perfect circularity of the two oases which they 
 observed proved them to be artificial, because the 
 increase in telescopic power has since shown us 
 that neither is circular. The obvious reason why 
 they appeared so round to Beer and Madler was 
 that they were too small to be denned in their 
 instruments ; their minor irregularities were there- 
 fore invisible, and their apparent circularity 
 covered detail of an altogether different form. 
 
 Beer and Madler only drew two such spots ; 
 Lowell shows about two hundred. Beer and Mad- 
 ler's two spots seemed to them exactly alike ; these 
 two spots as we see them to-day have no resem- 
 
THE ILLUSIONS OF MARS 
 
 blance to each other. Prof. Lowell's two hundred 
 oases, with few exceptions, seem all of the same 
 character ; is it possible to suppose, if tele- 
 scopes develop in the future as they have done in 
 the past, that the two hundred oases will preserve 
 their uniformity of appearance any more than the 
 Lacus Solis and the head of the Sinus Sabaeus ? 
 If a novice begins to work upon Mars with a small 
 telescope, he will draw the Lacus Solis and the 
 Sinus Sabaeus as two round, uniform spots, and as 
 he gains experience, and his instrumental power 
 is increased, he will begin to detect detail in 
 them, and draw them as Dawes and Schiaparelli 
 and others have shown them later. It is no question 
 of planetary change ; it is a question of experience 
 and of " seeing." 
 
 There is a much simpler explanation of the 
 regularity of the canals and oases than to suppose 
 that an industrious population of geometers have 
 dug them out or planted them ; it is connected 
 with the nature of vision. 
 
 A telegraph wire seen against a background of 
 a bright cloud can be discerned at an amazing 
 distance in fact, at 200,000 times the breadth of 
 the wire ; a distance at which the wire subtends 
 a breadth of a second of arc. For average normal 
 sight the perception of the wire will be quite 
 unmistakable, but at the same time it would be 
 quite untrue to say that the perception of the wire 
 was of the nature of denned vision, as would be 
 
 99 
 
ARE THE PLANETS INHABITED? 
 
 seen at once if small objects of irregular shape were 
 threaded on the wire ; these would have to be 
 many times the breadth of the wire in order to be 
 detected. Again, if instead of a wire of very great 
 length extending right across the field of view of 
 both eyes, a short, black line be drawn on a white 
 ground, it will be found that as the length of the 
 line is diminished below a certain point so its 
 breadth must be increased. If the observer is 
 distant from the line 6000 times its length, then 
 the breadth must be increased to be equal to the 
 length, and the object, whatever its actual shape, 
 can be just recognized as a small circular spot, 
 which will subtend about 34 seconds of arc. 
 
 But though a black spot, 34 seconds in diameter, 
 can be perceived on a white ground, we have not 
 yet attained to defined vision. For if we place 
 two black spots each 34 seconds of arc in diameter, 
 near each other, they will not be seen as separate 
 spots unless there is a clear space between them of 
 six times that amount. Nearer than that they will 
 give the impression that they form one circular 
 spot, or an oval one, or even a uniform straight 
 line, according to the amount of separation. If 
 two equal round spots be placed so that the distance 
 between their centres is equal to two diameters, 
 then the diameter of each spot must be, at least, 
 70 seconds of arc for them to be distinctly de- 
 fined ; that is to say for the spots to be seen as 
 two separate objects. 
 
 100 
 
THE ILLUSIONS OF MARS 
 
 It will be seen that there is a wide range between 
 objects that are large enough to be quite unmis- 
 takably perceived, and objects which are large 
 enough to have their true outline really denned. 
 It is a question of seconds of arc in the one case 
 and of minutes of arc in the other. Within this 
 range, between the limit at which objects can be 
 just perceived and that where they can be just 
 defined, objects must all appear as of one of two 
 forms the straight line and the circular dot. 
 
 This depends upon the structure of the eye and 
 of the retina ; the eye being essentially a lens with 
 its defining power necessarily limited by its aperture, 
 and the retina a sensitive screen built up of an 
 immense number of separate elements each of 
 which can only transmit a single sensation. 
 Different eyes will have different limits, both for 
 the smallest objects which can be discerned and 
 for the smallest objects that can be denned, but for 
 any sight the range between the two will be of the 
 order just indicated. 
 
 Prof. Lowell has drawn attention to the 
 " strangely economic character of both the 
 canals and oases in the matter of form." It is 
 true that straight lines and circles are economic 
 forms, but they are economic not only in the 
 construction of irrigation works but also in vision. 
 " The circle is the figure which encloses the 
 maximum area for the minimum average distance 
 from its centre to any point situated within it ; " 
 101 
 
ARE THE PLANETS INHABITED? 
 
 therefore, if a small spot be perceived by the sight 
 but be too small to have its actual outline defined, 
 it will be recognized by the eye as being truly 
 circular, on the principle of economy of effort. 
 So, again, a straight line is the shortest that 
 can be drawn between two points; and a 
 straight line can be perceived as such when 
 of an angular breadth quite 40 times less than 
 that of the smallest spot. A straight line is that 
 which gives the least total excitement in order to 
 produce an appreciable impression, and therefore 
 the smallest appreciable impression produces the 
 effect of a straight line. 
 
 It is sufficient, then, for us to suppose that the 
 surface of Mars is dotted over with minute irregular 
 markings, with a tendency to aggregate in certain 
 directions, such as would naturally arise in the 
 process of the cooling of a planet when the outer 
 crust was contracting above an unyielding nucleus. 
 If these markings are fairly near each other it is 
 not necessary, in order to produce the effect of 
 " canals," that they should be individually large 
 enough to be seen. They may be of any conceivable 
 shape, provided that they are separately below 
 the limit of denned vision, and are sufficiently 
 sparsely scattered. In this case the eye inevitably 
 sums up the details (which it recognizes but cannot 
 resolve) into lines essentially "canal-like" in 
 character. Wherever there is a small aggregation 
 of these minute markings, an impression will be 
 
 102 
 
THE ILLUSIONS OF MARS 
 
 given of a circular spot, or, to use Prof. Lowell's 
 nomenclature, an " oasis/' If the aggregation be 
 greater still and more extended, we shall have a 
 shaded area a " sea." 
 
 The above remarks apply to observation with 
 the unaided eye, but the same principle applies 
 yet more strongly to telescopic vision. No star 
 is near enough or sufficiently large to give the 
 least impression of a true disc ; its diameter is 
 indistinguishable ; it is for us a mathematical 
 point, " without parts or magnitude." But the 
 image of a star formed by a telescope is not a 
 point but a minute disc, surrounded by a series of 
 diffraction rings. This disc is " spurious," for the 
 greater the aperture of the telescope the smaller 
 the apparent disc. 
 
 That which holds good for a bright point like a 
 star holds good for every individual point of a 
 planetary surface when viewed through the tele- 
 scope ; that is to say, each point is represented by 
 a minute disc ; all lines and outlines therefore are 
 slightly blurred, so that minute irregularities are 
 inevitably smoothed out. 
 
 When we come to photographs, the process is 
 carried to a third stage. The image is formed by 
 the telescope, subject to all the limitations of 
 telescopic images, and is received on a plate 
 essentially granular in structure, and is finally 
 examined by the eye. The granular structure of 
 the plate acts as the third factor in concealing 
 103 
 
ARE THE PLANETS INHABITED? 
 
 irregularities and simplifying details ; a third 
 factor in producing the two simplest types of form 
 the straight line and the circular dot. 
 
 Prof. Lowell describes the canals as like lines 
 drawn with pen, ink and ruler, but not a few of 
 our best observers have advanced much beyond 
 this stage. Even as far back as 1884, some of the 
 canals were losing their strict rectilinear appear- 
 ance to Schiaparelli, and the observers of the 
 planet who have been best favoured by the power of 
 the telescope at their disposal, by the atmospheric 
 conditions under which they worked, and by their 
 own skill and experience such as Antoniadi, 
 Barnard, Cerulli, Denning, Millochau, Molesworth, 
 Phillips, Stanley Williams and others have found 
 them to show evident signs of resolution. Thus, in 
 1909, Antoniadi found that of 50 canals, 14 were 
 resolved into disconnected knots of diffused 
 shadings, 4 were seen as irregular lines, 10 as more 
 or less dark bands; and he found that, in good 
 seeing, there was no trace whatever of the geo- 
 metrical network. 
 
 The progress of observation, therefore, has left 
 Prof. Lowell behind, and has dispelled the fable 
 which he has defended with so much ingenuity. 
 But, indeed, there never was any more reason for 
 taking seriously his theory as to the presence of 
 artificial waterways on Mars than for believing in 
 the actual existence of the weird creatures de- 
 scribed by H. G. Wells in the War of the Worlds. 
 104 
 
THE ILLUSIONS OF MARS 
 
 There are too many oversights in the canal 
 theory. 
 
 Thus no source is indicated for the moisture sup- 
 posed to be locked up in the winter pole cap. Prof. 
 Lowell holds that there are no large bodies of water 
 on the planet ; that the so-called seas are really 
 cultivated land. In this case there could be little 
 or no evaporation, and so no means by which the 
 pokr deposits could be recruited. 
 
 Yet it is certain that the supply of the winter 
 pole cap must come from the evaporation of water 
 in some region or other. And here is another 
 oversight of the artificial canal theory. The 
 canals are supposed to be necessary for the con- 
 veyance of water from the pole towards the 
 equator; although, as this was "uphill," vast 
 pumping stations at short intervals had to be 
 predicated. But it is not supposed that the 
 water needed to travel by way of the canals to 
 the poles. If, however, the moisture is con- 
 veyed as vapour through the atmosphere to 
 the pole as winter approaches, it cannot be 
 impossible that it should be conveyed in the 
 same manner from the pole as summer draws 
 on, and in that case the artificial canals 
 would not be needed. If the canals are necessary 
 for conveying the water in one direction, they 
 would be necessary for the opposite direction. 
 But there would be something too farcical in the 
 idea of the careful Martians dispatching their 
 
 105 
 
ARE THE PLANETS INHABITED? 
 
 water first to the pole to be frozen there, and then, 
 after it had been duly frozen and melted again, 
 fetching it back along thousands of miles and 
 through numerous pumping stations for use in 
 irrigating their fields. 
 
 Of all the many hundreds of canals only a few 
 actually touch the polar caps. But on the theory 
 that the entire canal system is fed by the polar 
 cap in summer, the carrying capacity of the 
 polar canals should be equal to, if not greater than, 
 that of the entire system outside the polar circle. 
 A glance at the charts of the planet shows that the 
 polar canals could not supply a twentieth part of 
 the water needed for those in the equatorial 
 regions. Another oversight is that of the signifi- 
 cance of the alleged uniformity and breadth of the 
 canals. Prof. Lowell repeatedly insists that the 
 canals are of even breadth from end to end, and 
 spring into existence at once throughout their 
 whole length. This statement is in itself a proof 
 that the canals cannot be what he supposes 
 them to be. An irrigation system could not 
 have these characteristics ; the region fertilized 
 would take time to develop ; we should see the 
 canal extending itself gradually across the con- 
 tinent, and its breadth would not be uniform from 
 end to end, but the region fertilized would grow 
 narrower with increase of distance from the foun- 
 tain head of the canal. 
 
 Under what conditions can we see straight lines, 
 106 
 
THE ILLUSIONS OF MARS 
 
 perfectly uniform from end to end, spring into exist- 
 ence, in their entirety, without going through any 
 stages of growth ? When the lines are not actual 
 images, but are suggested by markings perceived, 
 but not perfectly denned. In 1902 and 1903, in 
 conjunction with Mr. Evans, the headmaster of 
 Greenwich Hospital School, I tried a number of 
 experiments on this point, with the aid of about 
 two hundred of the boys of the school. They had 
 several qualifications in respect of these experi- 
 ments ; they were keen-sighted, well drilled ; 
 accustomed to do what they were told without 
 asking questions ; and they knew nothing what- 
 soever of astronomy, certainly nothing about 
 Mars. 
 
 A diagram was hung up, based upon some 
 drawing or other of the planet made by Schia- 
 parelli, Lowell or other Martian observer, but the 
 canals were not inserted ; only a few dots or 
 irregular markings were put in here and there. 
 And the boys were arranged at different distances 
 from the diagram and told to draw exactly what 
 they saw. Those nearest the diagram were able to 
 detect the little irregular markings and represented 
 them under their true forms. Those at the back 
 of the room could not see anything of them, and 
 only represented the broadest features of the 
 diagram, the continents and seas. Those in the 
 middle of the room were too far off to define the 
 minute markings, but were near enough for those 
 107 
 
ARE THE PLANETS INHABITED? 
 
 markings to produce some impression upon them ; 
 and that impression always was of a network of 
 straight lines, sometimes with dots at the points of 
 meeting. Advancing from a distance toward the 
 diagram the process of development became 
 quite clear. At the back of the room no straight 
 lines were seen ; as the observer came slowly 
 forward, first one straight line would appear 
 completely, then another, and so on, until all the 
 chief canals drawn by Schiaparelli and Lowell in 
 the region represented had come into evidence in 
 their proper places. Advancing still further, the 
 canals disappeared, and the little irregular mark- 
 ings which had given rise to them were perceived 
 in their true forms. 
 
 These experiments at the Greenwich Hospital 
 School were merely the repetition of similar ones 
 that I had myself made privately twelve years 
 earlier, leading me to the conclusion, published in 
 1894, that the canals of Mars were simply the 
 summation of a complexity of detail too minute 
 to be separately discerned. 
 
 A little later, in his work " Marte nel 1896-7," 
 Dr. Cerulli independently arrived at the same 
 conclusion, and wrote: "These lines are formed 
 by the eye . . . which utilizes . . . the dark 
 elements which it finds along certain directions " ; 
 and " a large number of these elements forms a 
 broad band"; and "a smaller number of them 
 gives rise to a narrow line." Also, " the mar- 
 108 
 
THE ILLUSIONS OF MARS 
 
 vellous appearance of the lines in question has 
 its origin, not in the reality of the thing, but in 
 the inability of the present telescope to show 
 faithfully such a reality." In 1907, Prof. New- 
 comb made some experiments in the same direction 
 and reached the same general conclusion. More 
 recently still, Prof. W. H. Pickering has worked on 
 the same lines and with the same result. The 
 venerable George Pollock, formerly the Senior 
 Master of the Supreme Court and King's Remem- 
 brancer, sent to me, in his gist year, the following 
 note as affording an apt illustration of the true 
 nature of the canaliform markings on Mars : 
 
 " On Saturday last, journeying in a motor-car, 
 I came into a broad road bounded by a dark 
 wood. Looking up I was amazed to see distinct, 
 well-defined, vertical, parallel white lines, the 
 wood forming the dark background. On getting 
 nearer, these lines resolved themselves into spots, 
 and they proved to be the white insulators sup- 
 porting the telegraph wires." 
 
 Prof. Lowell has objected that all experiments 
 and illustrations of this kind are irrelevant ; only 
 observations upon the planet itself ought to be 
 taken into account. 
 
 But such observations have been made upon the 
 planet itself with just the same result. Observers 
 have seen streaks upon Mars knotted, broken, 
 irregular, full of detail and when the planet has 
 receded to a greater distance, the very same 
 109 
 
ARE THE PLANETS INHABITED? 
 
 marking has shown itself as a narrow straight line, 
 uniform from end to end, as if drawn with pen, 
 ink and ruler. The greater distance has caused 
 the irregularities, seen when nearer at hand, to dis- 
 appear. In this, and not in any gigantic engineer- 
 ing works, is the explanation of the artificiality 
 of the markings on Mars as Prof. Lowell sees them. 
 That artificiality has already disappeared under 
 better seeing with more powerful telescopes. 
 
 This chapter is entitled " The Illusions of Mars." 
 Yet the illusions of Mars are not the straight lines 
 and round dots of the canal system, but the forced 
 and curious interpretation which has been put upon 
 them. If the planet be within a certain range of 
 distance and under examination with a certain 
 telescopic power, the straight lines and round 
 dots are inevitable. Their artificiality is not a 
 function of the actual Martian details themselves, 
 but of the mode in which, under given conditions, 
 we are obliged to see them. 
 
 no 
 
CHAPTER IX 
 VENUS, MERCURY AND THE ASTEROIDS 
 
 OF all the planets, Venus appears, to the un- 
 assisted eye, by far the loveliest. When 
 seen in the early morning before sunrise its 
 "western elongation" or after sundown in the 
 evening its "eastern elongation " and still more 
 as it attains its greatest brilliancy, it has attracted 
 attention everywhere and in all ages. It then 
 shines with brilliance ten times as great as Jupiter 
 in opposition, and the brightest members of the 
 heavenly host look pale and dim beside it. It is 
 emphatically the morning or the evening star, 
 Lucifer, or Vesper, herald or follower of the Sun ; 
 it can even assert itself in the presence of the Lord 
 of Day, for it has often been seen at noonday by 
 watchers who knew where to look ; sometimes by 
 the general crowd. 
 
 But in the telescope Venus appears less satis- 
 fying. It is a pretty spectacle indeed to watch the 
 phases of the gleaming little globe of silver, for, 
 like the Moon under varying illumination from 
 the Sun, it undergoes change of apparent shape. 
 But the surface of the planet yields little detail, 
 and that little is illusive and ill-defined. The 
 in 
 
ARE THE PLANETS INHABITED? 
 
 clear-cut outlines and black shadows of the Moon 
 have no place here, nor do the ruddy plains and 
 blue-grey " seas " of Mars find any analogues. 
 All that can be observed beyond the changes of 
 phase are a few faint, ill-defined patches, where the 
 molten silver of the general surface is slightly 
 dimmed and tarnished, and perhaps one or two 
 spots, not less evasive and difficult to fix, that 
 exceed the rest of the surface in brightness. 
 
 This very difficulty in making out the markings 
 on Venus is hopeful for our search ; it points to a 
 veiling over the planet, a veiling by an atmosphere. 
 And the statistics of the Table show that Venus 
 closely resembles our Earth in size arid mass, and 
 therefore probably in atmospheric equipment. 
 If we assume that the atmosphere of any planet is 
 in direct proportion to its mass and as Venus is 
 so nearly the twin of the Earth there is no reason 
 to expect any great difference between the two in 
 this respect the atmosphere of Venus would have 
 a pressure of about 11*2 Ib. on the square inch, 
 and the level of half pressure would be nearly 
 four miles above the surface. In other words the 
 atmosphere would be both thinner and deeper 
 than that of the Earth, but the difference would not 
 be important in amount. 
 
 But Venus is nearer to the Sun than the Earth, 
 
 and receives nearly double the light and heat. Its 
 
 theoretical equatorial temperature is 368 abs., 
 
 or 95 C., and its corresponding mean temperature 
 
 112 
 
VENUS, MERCURY AND ASTEROIDS 
 
 is 69 C. But water under a pressure of 11-2 Ib. 
 will boil at 93 C., so that at the equator of Venus 
 the upper limit for water as a liquid is just passed, 
 but, for the planet in general, a fairly safe margin 
 is maintained. Here then is sufficient explanation 
 why the topography of Venus is concealed. The 
 atmosphere will always be abundantly charged 
 with water- vapour, and an almost unbroken screen 
 of clouds be spread throughout its upper regions. 
 Such a screen will greatly protect the planet from 
 the full scorching of the Sun, and tend to equalize 
 the temperature of day and night, of summer and 
 winter, of equator and poles. The temperature 
 range will be slight, and there will be no wide 
 expanses of polar ice. Water that flows will 
 be abundant everywhere. 
 
 So far all the facts connected with Venus are 
 favourable for life, even though the picture called 
 up to the mind may not seem inviting to us. 
 For views of the heavens must be rare ; the Sun 
 must seldom pierce through the cloud veil ; there 
 is no moon and the stars must be almost always 
 hidden. The Earth with its Moon might form a 
 beautiful ornament at times in the midnight sky 
 if the cloud-shell should occasionally open, but on 
 the whole, the planet is shut up to itself in a per- 
 petual vapour-bath, and its condition will approach 
 that of some of the most humid countries in the 
 terrestrial tropics during the height of their rainy 
 seasons. 
 
 i 113 
 
ARE THE PLANETS INHABITED? 
 
 But it would seem that life both of plants and 
 animals, under such conditions, might flourish and 
 be abundant. The mean temperature would not, 
 in general, be high enough to drive off the water as 
 steam, nor low enough to congeal it into ice ; it 
 would remain water water that flows. 
 
 But there is still a possible hindrance to life on 
 Venus, a hindrance that actually exists in the case 
 of Mercury. 
 
 Mercury, the " Twinkler," is not an easy object 
 in our Northern latitudes, but, in countries near 
 the tropics, is often quite conspicuous, a little 
 scintillating gem of light in the bright sky, before 
 sunrise or after sunset. In the telescope it is not so 
 attractive as Venus, partly because it is smaller, 
 partly because, though it receives more than three 
 times as much light from the Sun, it is duller in 
 hue. Yet it is not quite so secretive as its neigh- 
 bour, and a certain number of markings have been 
 detected upon its disc, markings which, like those 
 of the Moon, appear to be permanent. 
 
 A glance at the Table will show that this was to 
 be expected. In size, Mercury comes between 
 the Moon and Mars, and the atmospheric veil 
 ought therefore to be, as it evidently is, very slight 
 and transparent ; offering little or no hindrance to 
 an observer scanning it from another world. The 
 other necessary consequences of small size and 
 mass will follow ; the feeble force of gravitation, 
 the languid atmospheric circulation, the extreme 
 114 
 
VENUS, MERCURY AND ASTEROIDS 
 
 range of temperatures, the low temperature at 
 which water will boil. 
 
 But the heat to which Mercury is exposed far 
 transcends our terrestrial experience. In the mean 
 it receives nearly seven times as much heat from the 
 Sun as the Earth does, but this supply is not main- 
 tained uniformly, for Mercury moves round the Sun 
 in a very eccentric orbit, so that when in aphelion it 
 receives, surface for surface, only about four times 
 as much heat as the Earth, but some six weeks later 
 when in perihelion it receives more than eleven 
 times. The great range of temperature due to the 
 thinness of the atmosphere must therefore be 
 further increased by the varying distance of the 
 planet from the Sun. 
 
 A reference to Prof. Poynting's figures shows 
 that the mean temperature of Mercury must 
 approximate to 194 C., while water will boil at 
 40 C. or even lower. Here, then, is a condition 
 the exact reverse of Mars. Water as a liquid will 
 be rare on Mercury, not because it is congealed, 
 but because it is evaporated ; on the dark side of 
 the planet it may, indeed, pass into ice, but on the 
 side exposed to the Sun it must exist normally as 
 a constituent of the atmosphere. Water in a liquid 
 state, water that flows, must be almost unknown. 
 
 But we have good reason to believe that that 
 which is the dark side of Mercury at one time is 
 always dark ; that which is exposed to the Sun is 
 always exposed to it. 
 
 "5 
 
ARE THE PLANETS INHABITED? 
 
 Since Mercury wears no concealing veil of atmo- 
 sphere, and displays markings that can be identified 
 and followed, a surprising circumstance has come 
 to light. In 1889, Schiaparelli discovered that 
 Mercury, instead of rotating on its axis in about 
 24 hours like the Earth and Mars, rotates in 88 
 days ; that is to sajr, it always turns the same face 
 towards the Sun, just as the Moon turns the same 
 face towards the Earth. This fact, confirmed 
 theoretically by Prof. G. H. Darwin in his develop- 
 ment of the theory of tidal friction, puts the con- 
 dition of Mercury in quite a new light. No alter- 
 nation of day or night refreshes and restores the 
 little world ; one hemisphere is for ever exposed 
 to the blasting heat of the Sun, seven times hotter 
 for it than for the Earth ; the other hemisphere is 
 for ever exposed to the darkness and cold of outer 
 space, a range from something like 390 C. above 
 freezing-point, to 270 C. below. It is true that 
 between the two hemispheres there is a " debat- 
 able land," for, owing to the elliptic! ty of the orbit, 
 the face turned to the Sun is not exactly the same 
 at all times, and a region about 47 in width on 
 each side of the planet, that is to say, rather more 
 than a quarter of its entire surface, has one day 
 and one night in each period of 88 days, but these 
 more favoured sections can scarcely be considered 
 more habitable than the rest. 
 
 The conditions of Mercury are so unfavourable 
 for life that, even if this remarkable relation of 
 116 
 
VENUS, MERCURY AND ASTEROIDS 
 
 rotation period to revolution did not hold good, it 
 would still be impossible to regard it as a world 
 for habitation. But its case shows that a further 
 condition of habitability has to be satisfied by a 
 planet. Size and distance from the Sun afford the 
 first two conditions ; a suitable rotation period is 
 now seen to be a third. 
 
 And it is possible that in this very particular 
 Venus fails to qualify. Schiaparelli, the first 
 observer of his time, assisted by the clear Italian 
 sky, believed that he had demonstrated that 
 Venus, like Mercury, rotates once in her year ; 
 her day being thus equal in length to 225 of ours, 
 and the face that she turns to the Sun being always 
 the same. 
 
 And in her case this statement requires practically 
 no qualification, for, her orbit being nearly circular, 
 there is hardly any libration ; a place that has 
 the Sun in its zenith has it so for ever ; one on the 
 night side of Venus can never have a sunrise, or 
 gladden in the daylight. The side exposed to the 
 Sun will wither in a temperature of about 227 C., 
 in which all moisture will be evaporated ; the side 
 remote from it will be bound in eternal ice. In 
 neither hemisphere will water exist in the liquid 
 state ; in neither hemisphere will life be possible. 
 
 But as yet the evidence is not conclusive that 
 
 Venus has this long rotation period. Several 
 
 observers of high rank believe that our neighbour 
 
 rotates in nearly the same time as the Earth, but 
 
 117 
 
ARE THE PLANETS INHABITED? 
 
 its markings are so faint and elusive that the 
 problem is a difficult one. The spectroscopic 
 method of determining the speed of rotation has 
 been equally indecisive. Until, therefore, the 
 rotation period has been decided, the habit- 
 ability of Venus must remain in question. If it 
 always turns the same face to the Sun, there can 
 be no more life upon it than upon Mercury ; if on 
 the contrary it rotates in much the same time as 
 the Earth, then, so far as we know, it may well be 
 a habitable world. Whether it is actually in- 
 habited is a matter at present entirely beyond our 
 knowledge. 
 
 A page or two back we touched lightly on the 
 eccentricity of the orbit of Mercury lightly, 
 because it was not the chief factor in disabling the 
 planet for habitation. But the condition intro- 
 duced by this eccentricity is one which of itself 
 would be sufficient to put it out of court. In the 
 six weeks in which Mercury moves from aphelion 
 to perihelion, it approaches the Sun by fourteen 
 millions of miles, and the heat received by it is 
 increased 2j times. Then, in the next six weeks, 
 it recedes as far, and there is a like diminution. In 
 other words, six weeks makes a greater propor- 
 tional change in this one planet's condition than 
 we should experience if our Earth were trans- 
 ported from its own orbit to that of Mars. 
 
 But there are other members of the solar system 
 whose orbits are so elongated that that of Mercury 
 118 
 
VENUS, MERCURY AND ASTEROIDS 
 
 seems in comparison almost circular. These are 
 the comets, some of which all but graze the surface 
 of the Sun at perihelion, and then recede from him 
 for periods that it takes even thousands of years to 
 complete. But without dwelling on such extreme 
 cases, two of the best known of the periodic comets 
 may be taken as examples of the rest. Encke's is 
 the comet of shortest period, returning in about 
 3*3 years. At perihelion it is 31 millions of miles 
 from the Sun ; one- third the distance of the Earth. 
 It receives, therefore, at this part of its orbit, 9 
 times as much light and heat as the Earth. But at 
 aphelion it retreats deep into the region of the 
 asteroids, and is much more than four times the 
 mean distance of the Earth. At this part of its 
 orbit it receives but T Vth as much heat as the 
 Earth. By far the most famous of all the comets is 
 that known by the name of Halley, and its mean 
 period is 76 years. At perihelion it comes within 
 the orbit of Venus; indeed, nearly halfway 
 between that and the orbit of Mercury. At 
 aphelion it recedes to thirty-five times the dis- 
 tance of the Earth, far beyond the orbit of 
 Neptune. The range in its light and heat from 
 the Sun is from 3 times that of the Earth to less 
 than T^wth ; or, in other words, the supply of 
 heat at one time is nearly 4000 times that at 
 another, and of the 76 years of its period, only 
 80 days are spent within the orbit of the Earth. 
 Comets cannot be homes of life ; they are not 
 119 
 
ARE THE PLANETS INHABITED? 
 
 sufficiently condensed ; indeed, they are probably 
 but loose congeries of small stones. But even 
 if comets were of planetary size it is clear that life 
 could not be supported on them ; water could not 
 remain in the liquid state on a world that rushed 
 from one such extreme of temperature to another. 
 
 Between the orbits of Mars and Jupiter there 
 are scattered an untold number of little planets 
 commonly known as asteroids or minor planets. 
 Minor planets indeed they are, for the one first 
 discovered Ceres probably outweighs allthe rest, 
 known and unknown, put together, though some- 
 thing like 700 have already been detected, and the 
 list grows at the rate of about one a week. 
 
 As the Table shows, Ceres is so small that the 
 Earth exceeds it in volume 5000 times ; even the 
 Moon is 90 times as large. The mass of Ceres is 
 not known ; being so small, its density is probably 
 less than that of the Moon, so that the Earth may 
 easily outweigh it 10,000 times. The unfavour- 
 able conditions resulting from smallness of size 
 that the Moon presents are therefore exaggerated 
 exceedingly in the case of Ceres ; its atmosphere 
 must approach in tenuity what we should regard 
 as a vacuum in a terrestrial laboratory, and water 
 as a liquid be entirely unknown. Its distance from 
 the Sun is another hostile factor ; for in conse- 
 quence it receives per unit of surface only 13 per 
 cent of the light and heat that falls on the Earth ; 
 its maximum temperature under a zenith Sun will 
 120 
 
VENUS, MERCURY AND ASTEROIDS 
 
 fall far below freezing-point, the minimum on the 
 dark side will approach the absolute zero. 
 
 With Ceres the whole of the aster oidal family 
 can be dismissed as possible abodes of life. No 
 astronomer can regard them as such. Yet they 
 have their lesson to teach. Life can exist on the 
 Earth only on the upper face of its crust, and in 
 a very thin film of air and water ; but the enormous 
 solid bulk within, inert though it be, that sup- 
 ports the stage on which the great drama of life 
 is played, is as really essential as air and water 
 themselves. If that bulk were much smaller and 
 less massive life could find no place upon its 
 surface. 
 
 121 
 
CHAPTER X 
 THE MAJOR PLANETS 
 
 IT is a striking change to pass from Ceres, the 
 giant of the minor planets, to Jupiter, the giant 
 of the major planets. Instead of a world that the 
 Earth exceeds in volume 5000 times, we are con- 
 fronted by one that exceeds the Earth 1400 times. 
 Ceres, when viewed through a large telescope, is 
 just able to present a perceptible disc ; Jupiter 
 offers the largest shown by any heavenly body 
 after the Sun and Moon. 
 
 And that disc is one that never fails to charm the 
 attentive student, for it abounds in colour, move- 
 ment and change. The late Prof. James Keeler, 
 an observer of the first rank, having the advantage 
 of observing the planet from the summit of Mt. 
 Hamilton and with the great 36-inch telescope of 
 the Lick Observatory, thus describes the aspect of 
 the planet in 1889. 
 
 " Seen with this instrument on a fine night, the 
 disc of Jupiter was a most beautiful object, 
 covered with a wealth of detail which could not 
 possibly be accurately represented in a drawing. . . . 
 Scarcely any portion of Jupiter, except the Red 
 Spot and the extreme polar regions, was of a uni- 
 
 122 
 
THE MAJOR PLANETS 
 
 form tint, the surface being mottled with floccu- 
 lent and more or less irregular cloud masses. . . . 
 The equatorial zone, occupying the space between 
 the red belts, was marked in the centre by a 
 salmon-coloured stripe, which was occasionally 
 interrupted by an extension of the white clouds 
 on the sides of the zone. The edges were brilliant 
 white, and were formed of rounded cloud -like 
 masses, which at certain places extended into 
 the red belts as long streamers. . . . Near 
 their junction with the equatorial zone, the 
 streamers were white and definite in outline, 
 but they became redder in tint toward their 
 outer extremities, and more diffuse, until they 
 were lost in the general red colour of the back- 
 ground. When the seeing was good they were 
 seen to be formed of irregular rounded or feathery 
 clouds, fading toward the outer ends, until the 
 structure could no longer be distinguished. . . . 
 The portions of the equatorial zone surrounding 
 the roots of well-marked streamers were somewhat 
 brighter than at other places, and it is a curious 
 circumstance that they were almost invariably 
 suffused with a pale olive-green colour, which 
 seemed to be associated with great disturbance, 
 and which was rarely seen elsewhere. . . . The 
 red belts presented on all occasions the appearance 
 of a passive medium, in which the phenomena 
 of the streamers and other forms . . . were 
 manifested. The phenomena would be exactly 
 reproduced by streamers of cloudy white matter 
 floating in a semi-transparent reddish fluid, some- 
 times submerged and sometimes rising to the 
 surface. ... The dark spots frequently seen on 
 the red belts usually occupied spaces left by sharp 
 turns in the streamers, and they were of the same 
 
 123 
 
ARE THE PLANETS INHABITED? 
 
 colour as the belts, but deeper in tint, as if the 
 fluid medium could be seen to a greater depth."* 
 
 In other words, Jupiter is a striped or banded 
 planet, the bands lying along the direction of turn- 
 ing. These bands are coloured in varying tints, 
 and the planet rotates very rapidly, for the details 
 in the bands pass quickly from one limb to the 
 other. And not only is the speed of rotation of 
 the whole very rapid Jupiter turns about its 
 axis in a little less than ten hours, so that a particle 
 at its equator moves through 466 miles in each 
 minute but the various items that form the bands 
 rotate indifferent times. They may also alter their 
 form and their colour. Jupiter seems, then, to be 
 a planet with a great and rapidly changing atmo- 
 sphere that extends above a shoreless sea formed 
 of some liquified substance or substances the 
 whole in a state of flux. 
 
 But if we turn back to the Table, we see that 
 Jupiter at its mean distance from the Sun is 5-2 
 times that of the Earth ; that is to say, it receives 
 only ^Vth of the light and heat that we receive. 
 But in Chapter VIII, we learnt from Mars that as 
 this receives only fths of the Earth's light and 
 heat, its mean temperature would sink to -30 C. ; 
 the Earth's being 16 C. Mars is therefore 
 almost always a frozen planet ; frozen except 
 on its mere surface when this is exposed to the 
 full rays of the Sun. No sea there would ever be 
 * Publ. of the Aslron. Soc. of the Pacific, Vol. II, pp. 286-8. 
 124 
 
THE MAJOR PLANETS 
 
 melted to a depth of more than a few inches, even 
 at noonday in midsummer. And yet Mars has at 
 least ten times the advantages of Jupiter. Jupiter, 
 then, must be a frozen planet through and through ; 
 no liquid of any sort can exist on its surface ; no 
 vapour of any substance can exist in its atmosphere. 
 It must be icebound even at its summer noonday. 
 
 Yet, from the description given by Prof. Keeler, 
 it is manifestly not so ; and another item in the 
 Table emphasizes that it cannot be so. The density 
 of the Sun is 1-4 that of water, Jupiter's is 1-33, 
 showing that but a very small proportion (if 
 any) of its bulk can be solid ; the rest must be 
 vaporous, or at least fluid. How then can we re- 
 concile these inconsistencies ? 
 
 It is in the dimensions of Jupiter that we find 
 the answer. The mass of the planet is 317 times 
 that of the Earth ; it is indeed nearly three times as 
 great as that of all the other planets put together. 
 But the aggregation of so vast an amount of 
 material is of itself a source of heat ; the chief 
 source at the present time of the enormous output 
 of heat from the Sun is ascribed to its gradual con- 
 traction ; the slow falling of its substance, if we 
 may so express it, a little nearer to its centre. The 
 great mass of Jupiter points to its inherent store 
 of heat being much greater than that of any other 
 planet. And of two bodies equally hot, the larger 
 must cool more slowly than the smaller. If, there- 
 fore, all the members of the solar system had at 
 125 
 
ARE THE PLANETS INHABITED? 
 
 one and the same moment possessed the same sur- 
 face temperature, that equality would have ceased 
 directly they began to radiate their heat into 
 space ; the temperature of the smaller bodies 
 falling more rapidly than those of the larger. This 
 is another example of the principle that has already 
 been noted, that the properties of a small world are 
 not those of a large world divided by a constant 
 factor. It is not possible to conceive a model of 
 the solar system in which all the significant factors 
 should be true to the same scale. If the diameters 
 and distances were all made on a one-tenth scale, 
 the surfaces would be one-hundredth of reality, the 
 volumes one-thousandth. 
 
 But a radiating body radiates from its surface, 
 while the store of heat from which that radiation 
 is kept up is supplied by its volume. It follows, 
 therefore, that a large and heavy world must differ 
 from a small light world, not merely in scale, but 
 also in kind. 
 
 The surface of a world is all that we see of it ; it is, 
 therefore, very commonly all that we consider. But 
 unseen, and hence often unconsidered, beneath 
 the surface lies its substance or mass, and it is this 
 that determines the state and condition of the 
 surface ; it is the underlying power. Two men may 
 be contending in a financial struggle ; to the eye 
 they may look alike, equally prosperous ; both 
 may have the same amount of money actually in 
 their pockets ; but the one has nothing else, 
 126 
 
THE MAJOR PLANETS 
 
 the other has a great banking account and vast 
 investments, and is, in fact, a millionaire; and it is 
 his unseen power and resources that will make 
 themselves felt. 
 
 Jupiter therefore introduces us to a new factor in 
 world-condition ; not all its heat is derived from 
 the Sun ; much is inherent to it. And though it is 
 not possible at present to say that the mass of 
 Jupiter being so much its inherent heat must be 
 this or that quantity as a function of that mass, 
 yet in general, and neglecting other considerations, 
 we can say that of two worlds the one with the 
 greater mass will be that with the higher inherent 
 temperature. This factor of inherent temperature 
 was one that did not require to be noticed in deal- 
 ing with the Moon, or Venus, or Mars, for these and 
 all the planets yet noticed are less in size, surface, 
 volume, and mass than the Earth, and hence possess 
 less inherent heat. It is only now that the greater 
 planets are being considered that the question of 
 a source of heat, other than the Sun, can arise. 
 
 But the evidence of such heat on Jupiter is not 
 to be disputed. The albedo or reflective index of 
 Jupiter has been put by the late Prof. G. Bond, 
 of Harvard College Observatory, as higher than 
 unity ; in other words, that it emits more light 
 than it receives. This is now generally regarded 
 as an excessive estimate, but the albedo of the disc 
 as a w r hole cannot be put lower than 0-72, or about 
 that of white paper. But many of the " belts " or 
 127 
 
ARE THE PLANETS INHABITED? 
 
 dark regions are of a dull copper tint, and the polar 
 caps are dusky, so that Bond's estimate must be 
 realized for the most brilliant " zones," as the 
 brighter regions are called ; certainly for the 
 whitest of the white spots. 
 
 No direct evidence of inherent luminosity has 
 been obtained, for the satellites disappear entirely 
 in eclipse. But though their shadows in transit 
 appear very dark, it is clear that they are not 
 absolutely black, since sometimes such a shadow is 
 not distinguishable in darkness from the satellite 
 that casts it ; a delicate proof that the background 
 on which it falls has some intrinsic luminosity. 
 
 Unless there is the counteracting effect of a high 
 temperature, the atmosphere of Jupiter would have 
 a pressure at the surface of 104 Ib. to the square 
 inch, and the level of half pressure be attained 
 at a mile and a quarter ; the reverse condition 
 to that on Mars would obtain, and the atmosphere 
 of Jupiter would be much denser and much 
 shallower than that of the Earth. Denser it prob- 
 ably is ; shallower it cannot be, for the great 
 white spots, each often five or six thousand miles 
 in diameter, that range themselves at times along 
 the equatorial regions till they look like the port- 
 holes of a ship, evidently rise from depths great 
 even as compared with their size. But it is only 
 by intense heat that the effect of the great mass 
 of Jupiter in constricting its atmosphere within 
 shallow depths can be overcome. 
 128 
 
THE MAJOR PLANETS 
 
 Again, the extraordinary lightness of the planet, 
 so little above the density of water, points in the 
 same direction. So, not less unmistakably, do the 
 magnitude and rapidity of the atmospheric move- 
 ments. The clouds and storms of our own atmo- 
 sphere are worked by solar heat ; solar heat it is 
 that draws up the vapours and provides the chief 
 part of the energy manifested in the speed and 
 strength of the air-current. But solar heat can 
 only give ^yth the amount of that energy at the 
 distance of Jupiter, so that, if they were entirely 
 dependent on solar radiation, the winds of Jupiter 
 should be very feeble. 
 
 Further, the difference of presentment due to 
 the difference of latitude is a fruitful cause of in- 
 equalities of temperature and pressure in the 
 terrestrial atmosphere. But as a degree of latitude 
 on Jupiter is eleven times as wide as on the Earth, 
 such inequalities connected with a given difference 
 in latitude are spread over eleven times the dis- 
 tance that they would be on the Earth, and are, 
 therefore, so much the less pronounced. Yet, 
 across a gulf of 400 millions of miles we can clearly 
 discern the bright zones of Jupiter now narrowing 
 down and constricting the red belts, now thrust 
 apart by them, and can detect changes taking 
 place in an hour of time over areas equal to that 
 of a terrestrial hemisphere. 
 
 A notable peculiarity of Jupiter is found in the 
 proper motions of its spots. Many of the white 
 K 129 
 
ARE THE PLANETS INHABITED? 
 
 spots are exceedingly swift, giving a rotation 
 period of gh. 5om. while the equatorial belt in 
 general gives a period 5111. longer ; so that in 
 119 rotations (nearly 49 days) a white spot will 
 have passed entirely round the belt, gaining upon 
 it at a rate of nearly 240 miles an hour. 
 
 The most famous of all the markings in Jupiter 
 is the Great Red Spot, which became conspicuous 
 in 1878, since when the spot itself, or at least the 
 nest in which it lay, has always been visible. It 
 has been identified with a great red spot observed 
 by Hooke and Cassini in 1664-6, that appeared and 
 vanished again eight times between 1665 and 1708. 
 It therefore has had a history practically as long 
 as our telescopic knowledge of the planet, and 
 may be looked upon as in some sort a permanent 
 feature. Yet that it is not in the nature of a 
 portion of a solid crust is clear. It occupies on 
 Jupiter much the position and relative area of 
 Australia on the Earth, but whereas Australia of 
 necessity rotates in one piece with all the other 
 continents, the Great Red Spot has a rotation 
 period which is neither that of the equatorial belt, 
 nor of the quickly moving white spots, and is not 
 itself stable. An "Australia on the loose" is im- 
 possible, even unthinkable here, but the Great Red 
 Spot, for all its long duration, is mobile and in- 
 constant, and is therefore no portion of a solid 
 permanent crust. 
 
 The giant planet Jupiter, therefore, offers us an 
 
 130 
 
THE MAJOR PLANETS 
 
 example of what we may call a " semi-sun " ; a 
 world still bubbling with tremendous energies of 
 its own, still pulsing with its own inherent heat, 
 still without a solid crust ; probably without a 
 solid nucleus, liquid or vaporous throughout. 
 Whatever the future may hold for such an orb, it 
 is clearly no world for habitation at present. Full 
 of colour, and movement, and change as it is, it 
 lacks the Earth's " gloom of iron substance," which 
 is necessary, no less than its veiling by the plant, as 
 a stage for " the passion and perishing of mankind." 
 
 But if Jupiter be a semi-Sun, still a source of 
 heat, perhaps even of light, can it yield the means 
 of life to its satellites ? For Jupiter is sun-like, not 
 merely in its own condition, but also in that it is 
 the centre and ruler of a system of its own. We 
 know already of eight satellites revolving round it. 
 
 Of these eight, only four the four discovered 
 by Galileo, in the first days of his possession of a 
 telescope need be considered ; the other four 
 are of the same order of size as the asteroids, and 
 are indeed much smaller than Ceres. 
 
 But the Galilean satellites are of a higher rank. 
 Europa, the smallest, is in size a twin to the Moon ; 
 Callisto, the outermost, is almost exactly the size 
 of Mercury ; lo, the innermost, is midway between 
 the two in its dimensions. But Ganymede, the 
 largest, is almost comparable with Mars, its dia- 
 meter being 0-45 that of the Earth instead of the 
 0-53 of Mars. 
 
ARE THE PLANETS INHABITED? 
 
 But the Moon, Mercury, and Mars have all been 
 shown, on the ground of their small size, to be 
 worlds unfit for habitation ; the satellites of 
 Jupiter are, therefore, all rejected on the same score. 
 Nor can the greater nearness of their immediate 
 primary compensate for their remoteness from the 
 Sun. It is true that Jupiter presents to Ganymede 
 a disc with more than 200 times the apparent area 
 that the Sun presents to the Earth, but to make 
 up for the falling-off of the solar radiation, each 
 unit of this area should radiate about zluth as much 
 heat as each unit of the Sun's surface. In other 
 words, the absolute surface temperature of Jupiter 
 should be Jth that of the Sun, or about 1550 C., 
 and this is higher than can be admitted. The 
 Sun and Jupiter together cannot put Ganymede 
 in as favourable a position as Mars, much less as 
 favourable as the Earth. 
 
 The case of Jupiter carries with it those of 
 Saturn, Uranus, and Neptune. All three, from 
 their high albedoes and low densities, are still in a 
 vaporous condition ; still in some sort, semi- Suns ; 
 sources of a certain amount of heat, and not recipi- 
 ents merely. The days are yet far distant when a 
 solid crust can form on any one of them, and the 
 water condense from the steamy atmosphere to 
 form oceans, seas, and rivers. Not till then, if at 
 all, when water as a liquid, water that flows, is 
 present, can life begin to appear and enter on its 
 long course of change. 
 
 132 
 
CHAPTER XI 
 WHEN THE MAJOR PLANETS COOL 
 
 THE question has been asked : "It is evident 
 that life cannot exist at the present time on 
 the outer planets, since they are in a highly heated 
 and quasi-solar condition ; but when they cool 
 down, as cool they must, and a solid crust is formed, 
 may not a time come when they will be habitable ? 
 It seems impossible to think that worlds so beau- 
 tiful to our eyes and so vast in scale are destined 
 never to be peopled by intelligent beings." 
 
 It is clearly difficult to answer satisfactorily a 
 question that requires so deep a plunge into the 
 recesses of the unknown future ; yet, so far as our 
 knowledge goes, there is no reason to think that 
 Jupiter will be more habitable then than it is now. 
 The difficulty of the small supply of light and heat 
 received from the Sun would apparently still 
 remain, if indeed, the cooling of the Sun itself 
 would not increase it. We do not know of any 
 means by which our Sun could so increase its 
 radiation as to supply to Jupiter from 25 to 30 
 times as much heat as it now receives, and this 
 would be necessary to place it in the same favoured 
 condition as the Earth. If so great a change were 
 
 133 
 
ARE THE PLANETS INHABITED? 
 
 to take place in the Sun, life would be scorched 
 out of existence on all planets nearer than Jupiter, 
 and, similarly, if the solar emission were increased 
 to meet the necessities of Uranus or Neptune, even 
 Jupiter would fall a victim. 
 
 But we may consider it as a conceivable case 
 that a planet of the exact dimensions of Jupiter 
 may be revolving in an annual period of the same 
 length as his, round some star that is capable of 
 affording it adequate nourishment ; and so with 
 the three other giant planets. The actual Jupiter 
 and Saturn of the solar system have, so far as we 
 can tell, neither present nor future as habitable 
 worlds, but we can consider what would be the 
 case of imaginary bodies of similar dimensions in 
 systems where the supply of heat would be suffi- 
 cient. Or we can neglect the question of tempera- 
 ture altogether, as we did at first in the case of 
 Mars. 
 
 All the four planets must shrink much in volume 
 before their solidification will take place. Their 
 average density at present but little exceeds that 
 of water ; indeed, Saturn is not so dense as water ; 
 yet we must suppose that the same elements are in 
 general common to the Earth and to them all. If 
 we assume, then, that the four planets all cool to 
 the point of solidification, their densities must be 
 much increased, and their volumes correspondingly 
 diminished. Since all four greatly exceed the Earth 
 in mass, it is but natural to expect that, when they 
 
WHEN THE MAJOR PLANETS COOL 
 
 have assumed the terrestrial condition, they will 
 be more closely compacted than the Earth, and 
 their densities in consequence will be greater. It 
 will, however, be simpler if we assume exactly 
 the same density for them as for the Earth. 
 Jupiter will then have shrunk to about one-fourth 
 of its present volume, and the statistics for the 
 four planets will run as in the following Table : 
 
 STATISTICS OF THE FOUR OUTER PLANETS IF 
 WITH THE SAME DENSITY AS THE EARTH 
 
 PROPORTIONS OF THE PLANETS : 
 
 Uranus Neptune Saturn Jupiter 
 
 Diameter in miles 19300 20400 36000 54000 
 
 do = i 2-44 2-57 4-56 6-82 
 
 Surface, = i 6'o 6 '6 20 '8 46-6 
 
 Mass and Volume, = i 14 '6 17' 94 '8 3 r 77 
 
 Gravity at surface, = I 2*44 2*57 4-56 6*82 
 Rate of Fall, Feet in the First 
 
 Second 39'2 41 "3 73*3 1097 
 
 ATMOSPHERE, assuming the 
 total mass of the atmo- 
 sphere to be proportional to 
 the mass of the planet : 
 
 Pressure at the surface in Ib. 
 
 per square inch 88'2 97*0 35'8 685 - o 
 Pressure at the surface in 
 
 "atmospheres" 6'o 6'6 20 '8 46-6 
 
 Level of half-pressure in miles 1*37 1*30 073 0^49 
 Boiling point of water at 
 
 surface I27C I29C I48C i64C 
 
 135 
 
ARE THE PLANETS INHABITED? 
 
 Jupiter offers two peculiarities. In its shrunken 
 condition, its diameter, instead of being eleven 
 times that of the Earth, will be not quite seven, 
 and the force of gravity at the surface will be 
 greater than that of the Earth in the same pro- 
 portion. A man who here weighs 150 Ib. will 
 there weigh over 1000 Ib. ; and the muscular 
 effort of movement will be increased in the same 
 ratio. The athlete who here can clear a height 
 5 ft. 8 in. will there, with like pains, surmount 10 
 inches ; and other efforts will be in the same pro- 
 portion. The atmosphere, supposing it to be in 
 proportion to the mass of Jupiter, will exercise a 
 pressure of 46 J "atmospheres, "or more than 680 Ib., 
 to the square inch. Following on this enormous 
 pressure at the surface would be the rapidity with 
 which the atmosphere would thin out in the up- 
 ward direction. The level of half-pressure would 
 be attained by ascending less than half a mile in 
 height ; that is to say, there would be a difference 
 of pressure of 340 Ib. on the square inch from that 
 experienced at the sea-level. We know from the 
 fact that fishes live at enormous depths in the 
 ocean, that living organisms can be constructed to 
 endure great pressures, but they are not con- 
 structed to endure great alterations of pressure. 
 The deep-sea fishes are as instantly killed by being 
 brought up to the surface, as the surface fishes or 
 the land animals would be if they were plunged 
 into the depths. And it is clear that on Jupiter a 
 136 
 
WHEN THE MAJOR PLANETS COOL 
 
 low range of hills that on the Earth would be con- 
 sidered only an easy climb, would be an impassable 
 barrier, not only from the immense exertion of 
 mounting it, but chiefly from the unendurable 
 change of pressure which the ascent would in- 
 volve. 
 
 The sevenfold gravity of Jupiter, taken in con- 
 nection with this enormous atmospheric pressure, 
 would tend to make the meteorological disturb- 
 ances of the planet violent far beyond anything of 
 which the Earth can furnish an example. The 
 atmosphere would possess a high viscosity, and 
 differences in condition, pressure and saturation 
 would tend to accumulate, until at length the 
 balance would be restored with explosive sudden- 
 ness and force. Here our most violent tornadoes 
 may reach a speed of 100 miles an hour ; on 
 Jupiter, gales of five or six times that velocity 
 would be common. We cannot conceive that 
 living organisms would be able to grow, flourish 
 and multiply where the conditions were so cata- 
 clysmic. 
 
 This difficulty must always exist where the 
 planet is great in mass, and the force of gravity 
 high at the surface. The case of Saturn is not so 
 extreme as that of Jupiter, though it is probably 
 sufficiently severe to exclude it from the ranks of 
 worlds that could ever be dwelt in. The atmo- 
 spheric pressure would be about 21 " atmospheres," 
 or more than 300 Ib. on the square inch. The 
 
 i37 
 
ARE THE PLANETS INHABITED? 
 
 level of half-pressure would be reached at about 
 three-quarters of a mile, and the force of gravity 
 be nearly 4^ times that of the Earth. 
 
 But the serious condition for Saturn would come 
 from that feature which renders it by far the most 
 attractive of all the planets seen in the telescope, 
 the presence of the wonderful Ring system. 
 
 To us, viewing Saturn from afar, and from 
 practically the same direction as the Sun, the Rings 
 are seen lit up ; but to a dweller on Saturn, the 
 Rings during the day are between his world and the 
 Sun, and hence turn their dark side toward him. 
 More than that, the telescope shows us that the 
 Rings cast a shadow on the planet ; in other words, 
 they eclipse part of it ; and this shadow changes 
 its position with the progress of the Saturnian 
 year. Proctor computed that if the Rings were 
 a hundred miles in thickness, the equator would 
 suffer, in consequence, total eclipse for nearly ten 
 days at each equinox, and partial eclipse for about 
 forty days more. Moving away from the equator, 
 each higher latitude would have a longer and longer 
 period of eclipse in the winter half of its year ; the 
 higher the latitude, the later after the autumnal 
 equinox the eclipse would begin, and the longer it 
 would last, until about latitude 40 was reached. 
 Here the eclipses would begin nearly three terres- 
 trial years after the time of the autumnal equinox. 
 At first the Sun would be eclipsed only in the 
 morning and evening of each day, but the length of 
 
 138 
 
WHEN THE MAJOR PLANETS COOL 
 
 the daily eclipse would increase, until the Sun was 
 hidden the whole day long. This period of total 
 eclipse would last for about 6 years 8 months, 
 terrestrial reckoning, or with the periods of partial 
 eclipse, 8 years and nearly 10 months. Whatever 
 the efficiency of the Sun that afforded light and 
 heat to such a planet, it is clear that such eclipses 
 must be fatal to life in two ways : light and heat 
 would be cut off from wide regions of the planet 
 for long periods of time, and terrible meteorological 
 convulsions must follow in the train. Here on the 
 Earth, though a total eclipse generally lasts only 
 two or three minutes, the atmospheric disturbance 
 is perceptible, and the fall of temperature very 
 marked, and it does not require much reflection to 
 see that the analogous disturbance in an atmo- 
 sphere twenty times as dense must be terrific 
 indeed during an eclipse that lasts not a few 
 minutes only, but for more than six of our years. 
 The case of Uranus introduces us to another 
 class of conditions fatal to habitability. The 
 equator of Jupiter is inclined only 3 to the plane 
 of its orbit ; the difference in its seasons is, there- 
 fore, almost imperceptible ; there is hardly any 
 alteration in the incidence of the solar rays ; it is, 
 as if on the Earth, the height of the Sun at noon in 
 mid-winter were what it actually is on the I4th of 
 March, and its height at midsummer the same as 
 we observe on March 28. The inclination of the 
 equator of Saturn is considerably greater than 
 
ARE THE PLANETS INHABITED? 
 
 that of Mars or the Earth, so that its seasons are 
 more pronounced, but not to an extent that would 
 introduce any radical difference. But for Uranus, 
 the inclination of the equator to the plane of the 
 orbit is 82. If this were the case for the Earth, 
 the noonday sun for London would be, at the 
 spring equinox, 38^ high as at present, but its 
 altitude day by day would increase with great 
 rapidity, and before the end of April, the Sun at 
 noon would be right in the zenith, and 13 above 
 the horizon at midnight. At midsummer, indeed, 
 it would be only 59 high at noonday, but it would 
 be north of the zenith instead of south, and at 
 technical midnight, it would still be 44 in altitude, 
 thus moving round in a very small circle, only 
 15 in diameter. From about April 18 to August 25 
 that is to say, for 129 days the Sun would never 
 set, and unlike the summer day of our own polar 
 regions now, wherein the Sun, though always 
 present, is always low down in the sky, for much 
 of that period it would pass the meridian quite 
 close to the zenith. 
 
 As the year of Uranus is 84 times the length of 
 our year, the London of Uranus would have to 
 endure not far short of 30 years continuous 
 scorching. 
 
 And the winter would be as long ; the perpetual 
 day of summer would be replaced by a night as 
 enduring. More than 29 years of unbroken dark- 
 ness, of unmitigated cold, cannot possibly ever 
 140 
 
WHEN THE MAJOR PLANETS COOL 
 
 consist with the conditions necessary for life upon 
 a planet. Whatever the brightness of the imagined 
 sun of Uranus, if for 29 years at a time that sun 
 were below the horizon, the water on the planet 
 must be congealed, and during the 29 years of 
 unbroken day all the water would be as certainly 
 evaporated. 
 
 Thus, though Uranus is not burdened by the 
 enormous mass of Jupiter, nor overshadowed, like 
 Saturn, by a system of rings, the extraordinary 
 inclination of its axis introduces a condition which 
 is as fatal to it, as a world to dwell in, as any of 
 the disabilities of the other planets. 
 
 It is curious that these four outer planets, that 
 resemble each other so strikingly in many of their 
 conditions in their vast size, high albedo, low 
 density, and vaporous envelopes, that show, in 
 their spectra, not merely the lines of reflected sun- 
 light, but also special lines due to their own at- 
 mospheres (the chief of these being common to all 
 the four planets) should yet, in the inclination 
 of their axes to the plane of their orbits, display 
 every possible variety. The axis of Jupiter is 
 almost normal to its orbit, that of Uranus lies 
 almost in the plane of its orbit. The axes of Saturn 
 and Neptune have a mean inclination, but it 
 would appear that the rotation of Neptune is in 
 the reverse direction to that of planets in general, 
 so that the true inclination is usually taken as 
 being the complement of the observed angle, as 
 141 
 
ARE THE PLANETS INHABITED? 
 
 if the axis were turned right over. It is uncertain 
 whether this would have any important effect 
 upon the habitability of the planet, but it supplies 
 the fourth possible case for the position of the 
 axis. 
 
 142 
 
CHAPTER XII 
 THE FINAL QUESTION 
 
 IN passing in review the various members of the 
 solar system, it has been seen that there are 
 many conditions that have to be fulfilled before a 
 planet can be regarded as the possible abode of 
 life, because there are many conditions necessary 
 in order that water may exist on its surface in 
 the liquid state. The size and mass of the planet 
 are restricted within quite narrow limits ; and 
 a world much larger or much smaller than our 
 own is necessarily excluded. The supply of light 
 and heat received from the Sun must not fall much 
 below that received by the Earth, nor greatly 
 exceed it ; in other words, the distance of the 
 planet from its Sun is somewhat precisely fixed, 
 since the light and heat vary inversely not as 
 the distance, but as its square. Of course, in 
 different systems, with suns of different power, the 
 most favourable distance will not be the same in 
 each ; but in any system there will be one most 
 advantageous distance, and no great departure 
 from it will be possible. This condition further 
 implies that the planetary orbits must be nearly 
 circular ; pronounced eccentricity, such as the 
 
 143 
 
ARE THE PLANETS INHABITED? 
 
 orbits of even our short-period comets display, 
 would be fatal to the persistence of water in the 
 liquid state, and hence to the continuance of life. 
 A wide discordance between the planes of the 
 planet's equator and of its orbit, by rendering the 
 seasons extravagantly diverse, would act as 
 prejudicially as an eccentric orbit, and a rotation 
 period equal to that of revolution would mean 
 that one hemisphere was eternally frozen while 
 the other was exposed to perpetual heat. 
 
 It follows that in any given system there can be 
 at most only one or two planets upon which life can 
 find a home, and this only where the right conditions 
 of size and mass, of rotation period, inclination of 
 axis, and shape of orbit, all co-exist in a globe at 
 the proper distance. But the type of system 
 offered by our Sun and his planets is not the only 
 one that exists. A very large proportion of stars 
 are binaries two suns revolve round their common 
 centre of gravity. In many cases the two suns are 
 separable in the telescope, and their relative 
 movements can be measured ; in other cases, 
 termed " spectroscopic binaries," we only learn 
 that a star which appears absolutely single has 
 two components from the evidence of its spectrum ; 
 the spectroscope revealing two sets of lines that 
 vibrate to and fro with respect to each other. 
 Yet, again, a third class of double stars has made 
 itself known in the " Algol variables." The 
 optical double stars are cases where the two com- 
 
 144 
 
THE FINAL QUESTION 
 
 ponents are far distant from each other, and hence 
 can be distinguished in our telescopes as separate 
 points of light. The " spectroscopic binaries " are 
 cases where the two components are too close to 
 be separately perceived, but where the two are 
 not greatly unequal in brightness, so that the 
 spectrum of the one does not overpower that of 
 the other. The " Algol variables " are cases where 
 the two components are of very unequal bright- 
 ness, and, being very close to each other, are so 
 placed with respect to the Earth that the fainter 
 partly eclipses the brighter in its revolution 
 round it, and so causes a temporary diminution 
 in its light at regular intervals. All these three 
 classes of binary systems are now known to be 
 very numerous. Prof. Campbell estimates that 
 fully one star in six is a spectroscopic binary. 
 But there must be many binary systems that do 
 not reveal themselves double stars where the 
 companion is too faint or too close to be detected, 
 Algol systems where the companion does not pass 
 before its primary and it seems almost certain 
 that simple systems, like that of which our Sun is 
 the unchallenged autocrat, must be comparatively 
 rare. 
 
 But the problem of the movements of a planet 
 attendant upon two or more suns is one of amazing 
 complexity, and our greatest mathematicians have 
 as yet only been able to deal with the approximate 
 solution of a few very special cases. These are, 
 
 L 145 
 
ARE THE PLANETS INHABITED? 
 
 however, sufficient to show that the orbit of a 
 planet so placed would be most irregular ; the 
 variations in the supplies of light and heat received 
 would be as great as even comets experience 
 within the solar system, and, what would be more 
 disastrous still, these variations would not be 
 periodic but irregular. One year would be unlike 
 that which preceded it, and would be followed 
 by changed conditions in the next. Plants and 
 animals would never have the chance of acclima- 
 tizing themselves to these ever-changing vicissi- 
 tudes. The stability of condition essential for the 
 maintenance of water in a liquid state would be 
 wanting ; and, in consequence, Life could neither 
 come into existence, nor persist if it once appeared. 
 
 So far, therefore, our line of thought has led us 
 to recognize that Life can exist in comparatively 
 few of the innumerable stellar systems strewn 
 through infinite space, and in any given system 
 it can at best find only one or two homes. The 
 conditions for a Life-bearing planet are thus both 
 numerous and stringent there is no elasticity 
 about them. It is not sufficient that a planet 
 might fulfil many or even most of these conditions ; 
 failure in one is failure altogether ; " one black 
 ball excludes ; " the candidate who fails in a single 
 subject is " ploughed " without mercy. And in 
 most cases the failure is final ; no opportunity is 
 given to the candidate to " sit " again. 
 
 But Space is not the only horizon along which 
 146 
 
THE FINAL QUESTION 
 
 our thought must be directed ; there is also the 
 horizon of Time. Every world must have its 
 Past and its Future, as well as its Present. For 
 some worlds the conditions are so fixed that, like 
 Jupiter and Saturn, they are not now worlds that 
 can be dwelt in, they never were in that condition, 
 and they never can be ; their enormous mass for- 
 bids it. Mercury and the Moon at the other end of 
 the planetary scale are also permanently disabled ; 
 their insignificant size excludes them. There was 
 also a time when the Earth was not a world of 
 habitation ; it was " without form and void " ; hot 
 and vaporous, even as the four outer planets are 
 now. Now it is inhabited, but there may come a 
 time when this phase of its history has run its 
 course, and either from a falling off in the tribute 
 of light and heat rendered to it by the Sun, or 
 from the gradual desiccation of the surface, or, 
 perchance, from the slow loss of its atmosphere, 
 it may approach the condition of Mars, and in its 
 turn be no longer an abode of life. Many planets 
 are essentially debarred from ever entering on the 
 vital stage ; but of those to which such a stage 
 is possible, it can only form an incident in the entire 
 duration of the orb. And if our Earth is any type 
 or example of the vital stage in general, vast aeons 
 must run their course from the first appearance of 
 the humblest germs of life up to the bringing forth 
 of Life in conscious Intelligence. One hundred 
 million years are freely spoken of in this connection 
 
ARE THE PLANETS INHABITED? 
 
 by those who study the crust of the Earth and 
 those who are occupied with the relations of the 
 varied forms of life. Man is the latest arrival on 
 this planet, and however far back we try to push 
 the time of his earliest appearance, it is beyond 
 question that that time, relatively to the entire 
 duration of the Earth since a solid crust began to 
 form, is but as yesterday. If, from some other 
 globe in the depths of space, this world of ours 
 could have been watched during the long aeons 
 that elapsed from its first separation from the solar 
 nebula down to the time when it first possessed a 
 surface of land and water, and from that time, 
 again, throughout the hypothetical one hundred 
 million years that preceded the advent of man, 
 then, during all those aeons, those imagined ob- 
 servers would have had under their scrutiny a 
 world as yet without inhabitant. The Earth now 
 is in the inhabited condition, but science gives us 
 no clue as to how long that condition will endure ; 
 rather such hints as are afforded us would seem to 
 point to its lasting but for a brief season as com- 
 pared with the indefinite duration which preceded 
 it, and the indefinite duration which shall follow. 
 If this thought be sound, it places before us an 
 entirely new and most serious consideration. The 
 world predestined for habitation must not only 
 have its size within certain narrow limits, its 
 distance from its central sun in a certain narrow 
 zone, its rotation period, the inclination of its axis, 
 148 
 
THE FINAL QUESTION 
 
 the eccentricity of its orbit, all suitable alike, but 
 even if in these and in all other necessaries it is 
 perfectly adapted for habitation, yet it will be 
 only during a relatively small fraction of its entire 
 duration that Intelligent Life, clothed in material 
 form, will find a place upon it. 
 
 Let us sum shortly what we know and what we 
 conclude. We know that this, our Earth, is a 
 habitable globe, for we ourselves are living upon it. 
 We know what constitutes the physical basis of 
 our life, and under what conditions on this Earth 
 it flourishes, and under what conditions it is de- 
 stroyed. If we turn our eyes from this, our Earth, 
 and look out upon the starry skies, we see the 
 other planets of our system, and the suns which 
 are the centres of other systems. From the con- 
 sideration of the planets in our own system, we 
 have seen how stringent and how many are the 
 conditions imposed for Life to be possible. Round 
 our Sun there is but a narrow zone in which a 
 habitable world may circle ; in this zone there is 
 room for but few worlds, and we actually know 
 of three alone, the Earth, the Moon, and Venus. 
 We know that the Earth can be and is inhabited ; 
 that the Moon is not and cannot be inhabited ; 
 and that Venus, though of habitable size, may yet 
 be subject to the fatal disqualification of always 
 turning the same face to the Sun. Of other 
 planetary systems than our own, we actually know 
 of none, but we assume that there are such, 
 149 
 
ARE THE PLANETS INHABITED? 
 
 and as numerous as there are suns in the 
 starry depths. But of these planetary systems 
 we can rule out, as containing no habitable member, 
 all such as circle round double or multiple suns or, 
 indeed, round any single star that, from whatever 
 cause, is largely variable and, therefore, much 
 less stable than our own. Mira Ceti, which in 5 
 months increases its brightness 1000 times, may 
 stand as an example. Probably these disquali- 
 fications rule out of court the great proportion of 
 the stellar systems. Of the few, comparatively 
 speaking, single and stable suns that remain in 
 the heavenly abyss, we must conclude, from what 
 we know of our solar system, that they, too, have 
 but a narrow zone, outside of which no world would 
 be fit to dwell in ; whilst in the zone the few worlds 
 which might exist must violate no one of many 
 strict conditions. If we assume that there are a 
 hundred million stars within the ken of our tele- 
 scopes, we may well believe that not more than one 
 in a hundred of these would fulfil the condition of 
 being a single and stable sun, such as ours. Of 
 the planets revolving round these million suns 
 stable and efficient suns can we expect that in 
 more cases than one in a hundred there will 
 be a planet in the habitable zone fulfilling all the 
 other conditions of habitability, of size, mass, 
 inclination of axis, circular orbit, and rotation ? 
 Of these ten thousand earths which may be made 
 fit for the habitation of Man, can we assume that 
 
 150 
 
THE FINAL QUESTION 
 
 even one in a hundred is now at that epoch in its 
 history when it is no longer " without form and 
 void," when a division has been made between the 
 waters under the firmament and those that are 
 above the firmament ; when the waters under the 
 heaven have been gathered into one place, and the 
 dry land has appeared, and when the earth and the 
 waters have brought forth life abundantly } Out 
 of a hundred million of planetary systems through- 
 out the depths of space, can we suppose that there 
 are even one hundred worlds that are actually 
 inhabited at the present moment ? These numbers 
 and proportions certainly are not, and cannot be, 
 based on knowledge ; they are given as illustra- 
 tions only ; but, vague as they are, they suggest 
 that our Earth may be neither one of many 
 inhabited earths, nor yet unique, but one of a few 
 indeed of a very few. 
 
 And then the objection is raised : "If our own 
 Earth is but one of, perhaps, two inhabited worlds 
 in the solar system ; and of perhaps one or two 
 hundred inhabited worlds throughout the furthest 
 space that we can scan ; why is all this waste ? " 
 Of all the countless millions of stellar systems 
 without living organisms as inhabitants, we cannot 
 tell the purpose for the simple reason that we 
 do not know it ; but of " waste " in the solar 
 system, there is no question. Relatively speaking, 
 this is quite insignificant, for we cannot consider 
 that as " waste material " which is useful and, 
 
\ 
 
 ARE THE PLANETS INHABITED? 
 
 indeed, essential to existence. For, consider first 
 the material in the Earth itself. Its total volume 
 is 260,613,000,000 cubic miles, but man only 
 lives upon its surface of less than 200 million 
 square miles in extent, and he can not probe down 
 as far as ten miles below it, through the depths 
 of ocean or by his deepest mine. Thus we are left 
 with over 258 thousand million of cubic miles that 
 man, or plant, or beast can never make direct use of. 
 But without this 258 thousand million cubic miles 
 that he can never sow nor reap, the overlying 
 platform on which he dwells would be useless for 
 retaining the air or the water by which he lives. 
 No less essential is the Sun ; its vast bulk of 
 
 2,000,000,000,000,000,000,006,000,000 tons 
 can, in no single unit, be counted " waste," for it 
 is from this that the heat and light necessary for 
 life on the Earth is derived. But the tonnage of all 
 the planets combined is but 0-13 per cent of the 
 Sun alone ; and a wastage, if such it is, like this is 
 insignificant from a material point of view. 
 
 There is a type of politician at the present day 
 who is convinced that the highest purpose to 
 which land can be put is to build upon it ; that 
 being, in general, the use giving the highest money 
 return per square foot, though the return does not 
 always fall to the builder. It has taken not a 
 little agitation and popular pressure to enforce the 
 truth that cultivated land is also of use. But 
 there are few who realize that land that is neither 
 
 152 
 
THE FINAL QUESTION 
 
 built upon nor cultivated is also essential. Our 
 barren moors and bleak hillsides, " wastelands " 
 as we call them, are absolutely necessary as 
 collectors of the water by which w r e live. From 
 them our springs take their source ; and they 
 supply our cities with the first necessity of life. 
 
 We find, then, in this universe so far as we can 
 know it, that Space is lavishly provided, Matter 
 is lavishly scattered, Time is unsparingly drawn 
 upon, but Life in any form, and especially in its 
 highest form, is, relatively speaking, very sparsely 
 given. That very circumstance surely points to 
 the overwhelming importance of conscious, in- 
 telligent Life, and the insignificance of lifeless 
 matter in comparison with it. We have to exhaust 
 arithmetic in computing the size, the mass, the 
 output of heat and light of our Sun, yet it is but 
 the hearth-fire and lamp of terrestrial life ; and 
 its amazing agglomeration of matter and energy 
 is ungrudgingly devoted to this humble purpose. 
 Whatever view we hold as to the scheme of the 
 universe ; whether with the unthinking we fail 
 to recognize Thought and Purpose behind its 
 marvellous manifestations, or, with the thoughtful, 
 realize that only Infinite Thought could provide so 
 wonderfully for the bringing forth of thought in 
 living material organisms, the conclusion still 
 remains: living intelligences are, by the direct 
 testimony of the universe itself, its noblest and 
 most precious product. 
 
 153 
 
ARE THE PLANETS INHABITED? 
 
 The plea is often made that as we find life 
 adapting itself to a great variety of conditions on 
 this Earth, we must not set limits to its power of 
 adaption to the conditions of other worlds. But 
 this plea is an unthinking one. The range of con- 
 ditions through which we find life on this Earth 
 is as nothing to the range given by the varied 
 sizes and positions of the different planets ; and 
 even on our Earth, life in the unfavoured regions 
 the tops of mountains, the polar snows, the water- 
 less deserts, the ocean depths is only possible 
 because there are more favoured regions close at 
 hand, and there are, as it were, " crumbs that fall 
 from the rich man's table." A well-known littera- 
 teur in setting forth "a hundred ways of making 
 money " gave great prominence to the method 
 of living as caretaker in an empty house. But 
 residing in an empty house does not, in itself, 
 supply the means of sustenance; these have to be 
 furnished by the wealthier man who employs the 
 caretaker. 
 
 Another plea for vague sentiment in this matter 
 is that we cannot expect that intelligent beings 
 on other worlds would have the same form as 
 man, and if not the same form, then, that the 
 same conditions of existence would not hold good 
 for them as for us. Both contentions are unsound. 
 Protoplasm is the physical basis of all the life 
 that we know, whatever its form ; though these 
 forms are to be counted by the million, and are as 
 
 i54 
 
THE FINAL QUESTION 
 
 diverse as they are numerous. And everywhere 
 and always, water is found essential to protoplas- 
 mic life. Of life of any other kind we do not know 
 any examples ; we have no instance ; if such 
 exist, then they are beyond our ken. 
 
 And neither anthropologist nor biologist would 
 admit that the form of intelligent life was an un- 
 related accident. Whether the form brought the 
 intelligence, or the intelligence the form, or both 
 were evolved together, the one reacting on the 
 other, the human form and the human intelli- 
 gence are associated, and \ve feel this to be so of 
 necessity. In 1891, Dr. Eugene Dubois found in Java 
 a molar tooth and a portion of a skull, and later 
 the thigh bone of the left leg, and two more teeth. 
 Such as they were, these relics appeared nearer in 
 form to the corresponding fragments of an average 
 Australian than to those of an ape, and on this 
 ground intelligence was claimed for the creature 
 of which they were the remains, and it was given 
 the name of Pithecanthropus, or Ape-Man. The 
 discovery aroused much discussion, but on all 
 sides it was unhesitatingly assumed that the dif- 
 ference between the form of Pithecanthropus and 
 that of the most similar ape was an index of its 
 superior intelligence over the ape, just in so far as 
 that difference was in the direction of the modern 
 human form. The same remark applies to the 
 recent discovery of very ancient human remains 
 in Sussex. Never at any time has it been sup- 
 
ARE THE PLANETS INHABITED? 
 
 posed that the physical frame has followed any 
 other path in the evolution of intelligence than 
 that which brought forth man. The flesh-eating 
 animals have attained efficiency in hunting and 
 warfare by variation along many types of form ; 
 the herbivora have been not less varied in the 
 forms by which as races they secured themselves 
 from destruction ; but Thought has been asso- 
 ciated with the development of one type of form 
 only, and the entire future of Thought on this 
 planet rested neither with mammoth nor cave- 
 bear, but with the possessor of the erect stature, the 
 upward look, the differentiation of hand and foot, 
 even in their crudest and earliest stages. 
 
 Swift, in Gulliver's Travels, conceived of a 
 land where the intelligence and conscience of Man 
 dwelt in the form of the horse, and the human form 
 tabernacled the instincts of the beast. H. G. Wells, 
 in his War of the Worlds, attributed intelligence to 
 monsters half-cuttlefish and half-anemone, and 
 the human form to their helpless, unresisting prey. 
 Both conceptions are as scientifically absurd as 
 they are gross and revolting ; and if it were pos- 
 sible for the skeleton of creatures from other 
 worlds to be brought to us here, then biologists 
 would as confidently pronounce on their intelli- 
 gence as they do on the extinct forms of bygone 
 ages the nearer to the human form, the nearer to 
 the human mind. We have found the figures of 
 reindeer, horse, and mammoth scratched in outline 
 156 
 
THE FINAL QUESTION 
 
 on a mammoth tusk ; but though the artist has 
 left no other trace, we need no further evidence 
 of his bodily form. Neither horse, nor reindeer, nor 
 mammoth made those rough outlines ; they were 
 drawn by a man. More striking still, France yields 
 us chipped flints by the million, flints so slightly 
 shaped that it is in dispute whether they may not 
 have been so broken by the action of torrents. 
 But there are only two theories about them ; 
 either they were so chipped by natural action, 
 or they were designedly so chipped by creatures 
 resembling ourselves in head and hand. 
 
 The question that has been dealt with in this 
 volume is a scientific one, and the attempt has 
 been made to treat it as such, and to argue from 
 known physical facts as to the conditions of worlds 
 which we cannot visit. But by many the question 
 is generally discussed wholly apart from physical 
 facts at all, and it becomes one of sentiment and 
 of religious sympathy. Yet, curiously enough, the 
 division between those who think that all worlds 
 must be inhabited and those who think that our 
 own world stands alone is not coincident with any 
 line of theological divisions, but rather cuts across 
 all such. Some believers in Christianity argue that 
 since God has filled this world with Life, Life has 
 been His purpose in the world, and must therefore 
 have been His purpose in all other worlds they 
 too must be filled with Life in like manner. Other 
 believers argue that this world was the scene of the 
 
ARE THE PLANETS INHABITED? 
 
 Incarnation of Our Lord, and is therefore unique in 
 that respect ; and that this uniqueness sets its 
 stamp upon this world in all respects. Opponents to 
 Christianity are divided into the same two classes, 
 the one arguing that wherever there is matter the 
 inevitable course of evolution will produce life, 
 and eventually intelligent life. The other class 
 are equally clear that all forms of life are special, 
 the result of the particular environment, and that 
 it is unreasonable to expect that any other world 
 has had the same history as our own, or that the 
 same special conditions have prevailed elsewhere. 
 In other words the belief that there are other in- 
 habited worlds has depended chiefly neither on 
 science nor on religious belief, but upon sentiment. 
 There are some who like to think themselves, 
 and the race to which they belong, altogether 
 exceptional ; others delight in finding themselves 
 reflected wherever they look. So far as Science 
 has progressed and can return an answer to an 
 enquiry that exceeds so far the bounds of our 
 direct observation, it dissents from both orders of 
 thought. The conditions of life are indeed narrow, 
 special, restricted ; intelligent, organic life must, 
 relatively speaking, be a rarity in the universe, but 
 we lack the information that would enable us to 
 affirm with any confidence that such life is only 
 to be found upon this \vorld of ours. Heavy as 
 the odds are against any particular world being an 
 inhabited one, yet when the limitless extent of 
 158 
 
THE FINAL QUESTION 
 
 space is considered, and the innumerable numbers 
 of stars and systems of stars, it seems but reason- 
 able to conclude that though inhabited worlds 
 are relatively rare, the absolute number of them 
 may be considerable ; considerable, if not at one 
 particular moment of time, yet when the whole 
 duration of the universe is admitted. 
 
 But there is a religious question connected with 
 this enquiry ; one that goes down to the very roots 
 of man's deepest thoughts and aspirations. As 
 individuals our days on the Earth are as a shadow, 
 and there is none abiding ; as individuals we pass 
 and disappear ; and though the race remains, yet 
 as far as science can guide us and enable us to 
 penetrate the future, the same lot awaits the race 
 as well. Slowly but surely the water of a planet 
 will combine with its substance or disappear into 
 its crust. The cooling of the Sun, though it may 
 be long delayed, would seem to be inevitable in the 
 sequel. 
 
 " Oh, life as futile then as frail, 
 
 What hope of answer or redress ? 
 Behind the veil, behind the veil." 
 
 It is to this veil that we are now brought. It seems 
 impossible to believe that Life, so rare a fruit of the 
 universe, intelligent Life, conscious Life, to which 
 the long course of evolution has been so manifestly 
 leading up all through the long ages, should have 
 
ARE THE PLANETS INHABITED? 
 
 no better destiny than a final and hopeless 
 extinction ; that this Earth and all the efforts 
 and aspirations of the long generations of men 
 should have no worthier end than to swing, 
 throughout the eternal ages, an empty, frozen heap 
 of dust, circling round the extinct cinder that was 
 once its Sun. If we look backward, we seem to 
 discern clear signs of progress ; if we look forward, 
 we discern nothing but the veil. Science is but 
 organized experience, and experience of the future 
 we have none. 
 
 There was a time when on this world there was 
 no life ; a time when life began. How did it 
 begin ? Under what conditions ? 
 
 Of that great change when non-living matter 
 first became endowed with life, became so en- 
 dowed not by the action and intervention of other 
 living matter, but without it we have no know* 
 ledge, no experience. And so long as this con- 
 tinues to be the case, that change, the greatest 
 physical change that has yet taken place in the 
 history of the universe, the first change of the non- 
 living into the living, is outside the reach of science ; 
 it lies beyond its border. We may guess and specu- 
 late about it, but speculation is not science ; we 
 may spin words about it with the utmost skill of 
 the dialectician, but metaphysics is not science ; 
 it can never come within the scope of science until 
 it has first come within the scope of experience. 
 
 There is, therefore, a veil behind us as well as 
 160 
 
THE FINAL QUESTION 
 
 the one that encloses us in front ; and as hitherto 
 Science has failed to pierce the veil of the past, it is 
 even less able to pierce the veil of the future ; for 
 of the future we have no experience. 
 
 Here, then, our enquiry must end, for it is an 
 enquiry of physical science ; the search for living 
 material organisms endowed with intelligence. 
 How life first came upon this Earth, or when, or 
 where, is beyond the power of science to determine. 
 Yet it did come. There was a time when there 
 was no life here ; none, not even the humblest 
 form of it ; nor was there any hint or fore- 
 shadowing of it, still less of all its infinities of 
 form, and possibilities of development. 
 
 Once Life was not, yet Life came, and now, 
 life is abundant, but abundant only in worlds 
 quite exceptional in their conditions, and therefore 
 few in number; it is even conceivable that this 
 Earth of ours may be unique. But life as we know- 
 it, protoplasmic life, life dependent upon water, 
 the life of intelligence united to the material 
 organism, is under sentence of death. Has it any 
 future beyond that veil ? Is there any kind of 
 life not subject to these narrow limitations ; not 
 under the inexorable decree ? 
 
 To questions such as these Science has no reply 
 to give ; it is even more helpless to answer them 
 
 M 161 
 
 p 
 
ARE THE PLANETS INHABITED? 
 
 than to determine how life first came ; its experi- 
 ence does not reach so far. Science can examine 
 the present conditions of physical life, but whether 
 or no that life can undergo a change greater than 
 that which passed upon the old inorganic world, 
 it cannot determine. It has no experience. 
 
 But if Science is dumb, if the utmost exertion 
 of human energy and power of research can throw 
 no light on a future of which we have no experi- 
 ence, we are not left without an answer. A voice 
 has been heard, the voice of the Son of God Him- 
 self : 
 
 " I am the Resurrection and the Life. He that 
 believeth on Me, though he were dead, yet shall he 
 live." 
 
 And accepting His word, the Church in all ages, 
 and among all nations, peoples, and tongues, has 
 made reply : 
 
 " I LOOK FOR THE RESURRECTION OF THE DEAD 
 AND THE LIFE OF THE WORLD TO COME." 
 
 162 
 
INDEX 
 
 Abbot, C. G., 27, 34 
 Albedo of Earth, 54, 81 
 
 Jupiter, 127 
 
 Mars, 8 1 
 
 Moon, 54 
 Albumin sol, 15 
 Algol-type stars, 35, 144, 145 
 Antares, 38 
 
 Antoniadi, E. M., 97, 104 
 Archangel, climate of, 87, 88 
 Arcturus, 35, 37 
 Aristarchus, lunar crater, 48 
 " Astronomical unit," 21 
 Atmosphere of, Mars, 76 
 , Moon, 53 
 , Sun, 25 
 , Venus, 112 
 
 Barnard, E. E., 89, 104 
 Beer, 60, 97, 98 
 Bond, G. P., 127, 128 
 Brewster, Sir David, 4 
 
 Calcium, 12, 36 
 
 Callisto, satellite of Jupiter, 
 
 131 
 
 Calory, 26 
 
 Campbell, W. W., 145 
 Carbon, II, 34, 38, 39 
 Carbonic acid, n 
 Cassini, 59, 60, 130 
 Ceres, minor planet, 41, 120, 
 
 121, 122, 131 
 
 Cerulli, V., 104, 108 
 
 Chromosphere, 28, 29 
 Cobalt, 33 
 Comet, Encke's, 119 
 , Halley's, 119 
 , spectra, 38 
 Copernican theory, I 
 Copper, 33 
 Corona, 40 
 Coxwell, 25, 75, 79 
 Cyanogen, 37, 38 
 Czapek, F., II, 13 
 
 Darwin, Sir G. H., 116 
 Dawes, W. R., 60, 63, 97, 99 
 Denning, W. F., 104 
 Dispersion, anomalous, 28 
 Doppelmayer, lunar crater, 52 
 Dubois, Eugene, 155 
 
 Eros, minor planet, 57 
 Europa, satellite of Jupiter, 131 
 Evans, J. E., 107 
 
 Faculae, 29, 30 
 Fauth, P., 50 
 Flamsteed, lunar crater, 52 
 Fluorine, 40 
 Fraunhofer, 33 
 
 Galileo, 51, 59, 131 
 Ganymede, satellite of Jupiter, 
 
 131* 132 
 Gay-Lussac, 75 
 Glaisher, J., 25, 75, 79 
 
 163 
 
INDEX 
 
 Goodacre, W., 49 
 Green, N. E., 60, 62, 63 
 Greenwich Hospital School, 
 
 107, 108 
 "Gulliver's Travels," 82, 156 
 
 Haeckel, E., 12 
 
 Halogens, 36, 40 
 
 " Harper's Weekly," 93 
 
 Helium, 37, 
 
 Herschel, Sir J., 54 
 
 Sir W., 20, 49, 59, 60, 
 
 61 
 
 Hevelius, 51 
 
 Hippalus, lunar crater, 52 
 Hooke, R., 59, 60, 130 
 Huyghens, 59 
 Hydrocarbons, 38 
 Hydrogen, 11, 36, 37, 38,41, 
 
 "Inhabitant, "5 
 " Inhabited " Worlds, 2, 3, 4 
 lo, satellite of Jupiter, 131 
 Iron, 12, 33, 36 
 
 Jupiter, 122-32 
 
 , belts, 127, 129 
 
 , great red spot, 130 
 
 , proper motion of spots, 
 
 , satellites of, 128, 131 
 
 , white spots, 128, 130 
 
 Keeler, J. E., 122, 125 
 Kies, lunar crater, 52 
 Kirchhoff, 33 
 
 Lacus Solis, 97, 98, 99 
 Langley, S. P., 55 
 Lilliputians, 82, 83 
 Linne, lunar crater, 48 
 Lockyer, J. N. , 60 
 
 129 
 
 Lowell, P., 65,66,67,69,71, 
 81, 97, 98, 99, 101, 103, 
 104, 105, 106, 108, 109, no 
 
 Lucifer, in 
 
 Madler, 46, 48, 60, 97, 98 
 Maginus, lunar crater, 46 
 Magnesium, 12, 36 
 Manganese, 33 
 Mare Fecunditatis, 47 
 
 Humerum, 52 
 
 Nubium, 52 
 
 Serenitatis, 48 
 
 Mars, canals of, 57-7i 7^, 
 
 101, 102 
 
 , conditions of, 7 I- 95 
 , illusions of, 96-110 
 , meteorology of, 93-4 
 , oases of, 65, 98, 99, 101 
 , thermograph of, 91, 92 
 , winds of, 77 
 Mendeleeff, 39 
 Mercury, 114-18 
 Messier, lunar crater, 47, 48 
 Metabolism, 10, n, 14, 15, 
 
 3 8 
 Millechau, 104 
 
 Milton, 51 
 
 Mira Ceti, 150 
 
 Molesworth, P. B., 49, 104 
 
 Moon, 43-56 
 
 , "terminator" of, 51 
 
 Mont Blanc, 25, 74, 80 
 
 Mount Everest, 75, 80 
 
 Nature of Vision, 99 
 Nebulae, spectrum of, 38, 40 
 Nebulium, 40 
 Negative elements, 36 
 Neison, E., 48, 53 
 Neptune, 132, 141 
 Newcomb, S., 93, 109 
 
 164 
 
INDEX 
 
 Nicholson, J. W., 40 
 
 Nickel, 33 
 
 Nilosyrtis, " canal " on Mars, 
 
 89 
 Nitrogen, 11, 37, 3 8 , 39 
 
 Observatory, Chicago, 44 
 , Harvard College, 127 
 , Lick, 122 
 , Paris, 44 
 Occultation, 52, 53 
 Organic Life, definition of, 15 
 Organism, living, 6-19 
 Organo-genetic elements, 12, 
 
 38,39 
 Osmosis, 15 
 Oxygen, n, 36, 37, 38, 41 
 
 Periodic Law, Mendeleeffs, 39 
 Phillips, T. E. R., 104 
 Phosphorus, 12 
 Photosphere, 28, 33, 36 
 Pickering, W. H., 47, 48, 
 
 .53, 109 
 
 Pithecanthropus, 155 
 Planetary statistics, table of, 
 
 72,73, 135 
 Platinum, 36 
 " Plurality of Worlds," 2 
 Pollock, Master, 109 
 Potassium, 12 
 
 Poynting, J. H. , 86, 87, 89, 1 1 5 
 Proctor, R. A., 34, 77 
 Prominences, 29, 30, 37 
 Protofluorine, 40 
 Protonilus, "canal "on Mars, 
 
 89 
 Protoplasm, II, 12, 13, 15, 
 
 38, 40, 154 
 Pyramid, Great, 45 
 
 Refraction, anomalous, 28 
 
 Reversing layer, 36 
 " Rice-grains," of Sun's sur- 
 face, 28, 29 
 
 Ring Nebula in Lyra, 40 
 Rosse, Lord, 55 
 Ruskin, J., 19 
 
 Saturn, 132 
 
 , Rings of, 138 
 
 Schiaparelli, G. V., 61, 62, 63, 
 
 64, 66, 97, 99, 107, 108, 
 
 116, 117 
 
 Schooling, J. Holt, 83 
 " Scientia," 66 
 "Semi-suns," 131, 132 
 Serviss, Garrett P., 17 
 Singapore, climate of, 87, 88 
 Sinus Sabaeus, marking on 
 
 Mars, 97, 99 
 Sirius, 37 
 Sodium, 33, 36 
 "Solar Constant," 26 
 Spectroscopic binaries, 144,145 
 Spectrum, 53 
 , heat, 55 
 "Spurious" disc, 103 
 Stars, double, 35 
 , multiple, 35 
 -, red, 38 
 
 , spectra of, 34, 38, 39 
 Stefan's Law, 85 
 Stoney, G. Johnstone, 34 
 " Streaming," 15 
 Sulphur, II, 38 
 Sun, 20-32 
 
 Sunspots, 29, 30, 31, 38 
 , spectra of, 37 
 Swift, Dean, 82, 156 
 
 Table Mountain, 54 
 Thermograph of Mars, 91, 92 
 Titanium, 36, 37, 38 
 
 165 
 
INDEX 
 
 Tornadoes, 31, 137 
 "Twinkler," 114 
 Tycho, lunar crater, 46 
 
 Uranus, 132, 140 
 
 Venus, 57, 111-18 
 Verworn, Max, 7 
 Very, F. W., 55 
 Vesper, HI 
 
 "Victoria," hypothetical pla- 
 net, 83 
 
 Wallace, A. R., 4 
 
 "War of the Worlds," 104, 
 
 156 
 
 Waste, 151, 152 
 Water, indispensable factor, 
 
 IS, 41 
 
 Wells, H. G., 104, 156 
 Whewell, 4 
 
 Williams, A. Stanley, 104 
 Wolf, Max, 40 
 
 Young, C. A., 26, 33 
 
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