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+Project Gutenberg's Are the Planets Inhabited?, by E. Walter Maunder
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Are the Planets Inhabited?
+
+Author: E. Walter Maunder
+
+Release Date: April 23, 2011 [EBook #35937]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ARE THE PLANETS INHABITED? ***
+
+
+
+
+Produced by Jonathan Ingram and the Online Distributed
+Proofreading Team at https://www.pgdp.net (This file was
+produced from images generously made available by The
+Internet Archive/American Libraries.)
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+
+
+HARPER'S LIBRARY of LIVING THOUGHT
+
+
+
+
+  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 & BROTHERS
+  LONDON AND NEW YORK
+
+  45 ALBEMARLE STREET, W.
+  1913
+
+
+
+
+  _Published March, 1913_
+
+
+
+
+CONTENTS
+
+
+  CHAPTER                                           PAGE
+
+     I. THE QUESTION STATED                            1
+
+    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             111
+
+     X. THE MAJOR PLANETS                            122
+
+    XI. WHEN THE MAJOR PLANETS COOL                  133
+
+   XII. THE FINAL QUESTION                           143
+
+        INDEX                                        163
+
+
+
+
+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 difference 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 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 ourselves, 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
+richness of vegetation and no fulness and complexity of animal life would
+justify an explorer in describing some land that he had discovered as
+being "inhabited" if no men were there, so we cannot rightly speak of any
+other world as being "inhabited" if it is not the home of intelligent
+life. If the life did not rise above the level of algæ 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 hereafter into manifestations with
+which we could claim a nearer kinship.
+
+On the other hand, of necessity we are precluded from extending our
+enquiry to the case of disembodied intelligences, if such be conceived
+possible. All created existences must be conditioned, 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 subsist. Gnomes, sylphs, elves, and
+fairies, and all similar conceptions, escape the possibility of discussion
+by our ignorance of their properties. As nothing can be asserted of them
+they remain beyond investigation, as they are beyond sight and touch.
+
+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 world 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.
+
+For myself, if there be any theological significance 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 theological 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 concerned with the
+highest form of organism, the one furthest from these two dividing lines.
+
+It suffices that living organisms do exist here, and exist under
+well-defined conditions. Wanting these conditions, they perish. We can, to
+a varying degree, determine the physical conditions prevailing 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
+essentially 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
+particularly 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; surrounding this, a bluish
+zone that is faintly luminous; 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 continuously 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 restricted, 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 which have been produced by the chemical reactions 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 more than twenty to thirty years.
+These experiences 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 production 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 temporarily, really suspended.
+
+"Life is, therefore, quite inseparable from chemical 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."[1]
+
+The essential quality, therefore, of life is continual 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, nitrogen, hydrogen, and sulphur. The
+compounds which these five elements form with each other are most complex
+and varied, and they also admit to combination--but in smaller
+proportions--some of the other elements, of which phosphorus, potassium,
+calcium, magnesium, and iron are the most important.
+
+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 composition 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."[2]
+
+Haeckel, the writer just quoted, describes the plasm, the universal basis
+of all the vital phenomena, 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 distinguishes
+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 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 substance 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 distributed 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."[3] 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 are carried on contemporaneously,
+without one disturbing the other in the slightest degree."[4]
+
+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."[5]
+
+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 undergoing 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.
+
+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 for its
+continuance the presence of one indispensable factor--WATER.
+
+Protoplasm itself, as Czapek 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 protoplasm, its continual movement in the cell, can
+only be carried on in water.
+
+WATER is the compound of oxygen and hydrogen 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 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 0° Centigrade, it boils at 100° Centigrade.
+Necessarily, our experiences are mostly confined within this range, and
+therefore 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
+destroyed, 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 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 dissolved; 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 outside the atmosphere. As the Earth
+condensed from the original nebula, and cooled and solidified, a 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 ceaseless 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."[6]
+
+"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 intermediate 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 spread of intermediate
+being;--which should appease 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."[7]
+
+The leaf and the cloud are the signs of a habitable 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."
+
+
+
+
+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 points 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 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 perpetual
+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
+years 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 introduction to our subject. Further, those conditions
+themselves will bring out certain principles that are of necessary
+application when we come to consider 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 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 deduce 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 describing 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 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.[8]
+
+This relation is one of great importance when we realize that the pressure
+of the Earth's atmosphere is 14·7 lb. 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
+atmosphere 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 × 12 = 408) is 14·7
+lb.
+
+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 atmosphere, 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 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 remaining
+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, 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
+transcends 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
+surface 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
+temperature 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 pressures, on the other hand, apart from the high
+temperatures, would probably solidify every element, 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 appearance 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 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 impressed with the
+difficulty of conceiving that a gaseous body like the Sun could, under the
+conditions 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 incompatible 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 brightness, and
+irregularly scattered. This mottling is sometimes coarsely, sometimes
+finely textured; in some regions it is sharp and well defined, in others
+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 _sunspots_, 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
+_faculae_, 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 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 completely disappeared within
+less than half an hour. The velocity of the gas streams in such eruptions
+often exceeds one hundred miles a second; sometimes, 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 thousand 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 constitution,
+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, 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 negative 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 condition, enclosed by the
+luminous shell which we term the photosphere. This shell Prof. C. A. Young
+and the majority of astronomers regard as consisting of a relatively thin
+layer of glowing clouds, justifying the quaint conceit of R. A. Proctor,
+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 substance 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, refuses
+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 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 of life. 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.[9] 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 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 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 essentially 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
+temperatures 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
+nebulæ. The elemental lines of oxygen are not indeed seen in stellar
+spectra, but that the element is present is shown by the flutings 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.[10]
+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 metabolism of nitrogenous carbon
+compounds in association 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 compounds 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 Mendeléeff's Periodic Law has shown that the
+elements are not to be regarded as disconnected entities. The Law as given
+in Mendeléeff'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 algebraically) 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 acquainted. New elements, no doubt,
+will be discovered in the future, as in the past; and indeed we may be
+able to discover them and learn their atomic weights and properties
+without 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 nebulæ, 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 _Nebulium_,
+will have an atomic weight of 2·1. Prof. Max Wolf, of Heidelberg, has
+recently pointed out[11] 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 abundant 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 discovered between the orbits of Mars and
+Jupiter, 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 planetoidal 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 0° Centigrade, and
+boils at 100° 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 elements 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 0° to 100° Centigrade, have we any reason to
+suppose that protoplasm itself would be able to endure these outlying
+temperatures? Looking through the 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?
+
+
+
+
+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 looking 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 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 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 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 suspected,
+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
+recognized with difficulty. Thus the terse remark of Mädler, "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 metropolis," 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.
+
+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
+triangular 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."[12]
+
+Some observers explain this cycle of changes as due merely to the peculiar
+contour of the two 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
+probably 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 Mädler described the two craters as being
+exactly alike eighty years ago, Messier A is now distinctly the larger;
+but it is very doubtful whether Mädler'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 Linné, 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 Linné 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 brightest formation on the Moon, so bright indeed that
+Sir William Herschel, observing it when illuminated 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 unusual 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 Molesworth 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.
+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."[13]
+
+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 manifestly 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 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 observations. 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 contained 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 because 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 "terminator," 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 perfectly smooth. The grey plains are therefore not expanses of
+water now, nor were they in time past. 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 formations 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
+destroyed 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 reappearance 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 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
+disconcerting; it is hardly less disconcerting to observe 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 spectroscope, 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 1/300th of
+that of the Earth by Neison, and as 1/10000th by W. H. Pickering. If the
+Moon possessed an atmosphere bearing the same proportion 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 Sun as the Earth, and
+therefore, surface for surface, 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 reflection, and is only
+bright by contrast with the darkness 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 Mountain, 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 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 accomplish, 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 wave
+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
+immediately. In 1898, Prof. Very, following up Langley's line of work,
+concluded that the temperature 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 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, therefore, that the
+time of rotation of a planet is an important 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
+inhabited, 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
+tremendous 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
+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 superhuman 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, however, 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 1/70th of the diameter of the Moon to 1/130th. 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 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 telescopes.
+
+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 unmistakable. 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 oppositions 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 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 knowledge of Mars must be
+ascribed to the two German astronomers, Beer and Mädler, who made a series
+of drawings in the years 1830, 1832 and 1837, by means of a telescope of 4
+inches aperture, from which they were able to construct a chart of the
+entire globe. This chart may be 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 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 satisfaction 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 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 astronomers of skill and experience had observed the
+planet in 1877 without detecting the network of 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 observatories, 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 confirmation of not a few of Schiaparelli's "canals"; even
+in 1877 a few of the easiest and most conspicuous 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
+discoveries increased. It has now long been known 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
+objective basis.
+
+As this controversy has passed away, another and a very different one has
+arisen out of an unfortunate 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
+descriptions." 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
+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 believing that the "canals" are
+artificial constructions, or may they be merely natural formations?
+
+In 1894, Mr. Percival Lowell founded at Flagstaff, 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 irrigation, are fighting a losing battle against the gradual
+desiccation of their planet.
+
+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 unlocking 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 curiously 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 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 character: "_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. 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 regularly 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,
+    unconscious 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).
+
+    "That intelligence should thus mutely communicate 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
+achievement 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 existence 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 the evolutionary
+development necessary to its attaining its highest expression,--that where
+the material living organism has become the tabernacle and instrument of
+the conscious intelligent spirit.
+
+
+
+
+CHAPTER VII
+
+THE CONDITION OF MARS
+
+
+The planet Mars is the debatable ground between 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
+living 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
+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.
+
+PLANETARY STATISTICS
+
+  +--------------------------------------------++--------++----------------
+  |                                            || Minor  ||          Inner
+  |                                            ||Planets.||
+  +--------------------------------------------++--------++-------+-------+
+  |                                            || Ceres  || Moon  |Mercury|
+  +--------------------------------------------++--------++-------+-------+
+  |PROPORTIONS OF THE PLANETS:--               ||        ||       |       |
+  | Diameter in miles                          ||   477  || 2163  | 3030  |
+  |     "    [Symbol] = 1                      ||  0·06  || 0·273 | 0·383 |
+  | Surface, [Symbol] = 1                      ||  0·004 || 0·075 | 0·147 |
+  | Volume,  [Symbol] = 1                      ||  0·0002|| 0·02  | 0·06  |
+  | Density, Water = 1                         ||  2·8 ? || 3·39  | 4·72  |
+  |     "    [Symbol] = 1                      ||  0·5 ? || 0·61  | 0·85  |
+  | Mass,    [Symbol] = 1                      ||  0·0001|| 0·012 | 0·048 |
+  | Gravity at surface, [Symbol] = 1           ||  0·028 || 0·17  | 0·33  |
+  | Rate of Fall, Feet in the First Second     ||  0·45  || 2·73  | 5·30  |
+  | Albedo                                     ||  0·14  || 0·17  | 0·14  |
+  |                                            ||        ||       |       |
+  |DETAILS OF ORBIT:--                         ||        ||       |       |
+  | Mean Distance from Sun in millions of miles||257·1   ||92·9   |36·0   |
+  |      "          "   Earth's distance = 1   ||  2·767 || 1·000 | 0·387 |
+  | Period of Revolution, in years             ||  4·60  || 1·00  | 0·24  |
+  | Velocity, in miles per second              || 11·1   ||18·5   | 9·7   |
+  | Eccentricity                               ||  0·0763|| 0·0168| 0·2056|
+  | Aphelion Distance, Perihelion = 1          ||  1·157 || 1·034 | 1·517 |
+  | Inclination of Equator to Orbit            ||  (?)   || 1°·32Ž|  (?)  |
+  |                                            ||        || d h m |   d   |
+  | Rotation period                            ||  (?)   ||27·7·43| 88(?) |
+  |                                            ||        ||       |       |
+  |ATMOSPHERE, assuming the total mass of the  ||        ||       |       |
+  |  atmosphere to be proportional to the mass ||        ||       |       |
+  |  of the planet:--                          ||        ||       |       |
+  | Pressure at the surface in lb. per sq. in. ||  0·014 || 0·40  | 1·6   |
+  |     "       "      "    in "atmospheres"   ||  0·0009|| 0·027 | 0·108 |
+  | Level of half surface pressure in miles    ||119·0   ||19·6   |10·1   |
+  | Boiling point of water at the surface      ||        || 22°C  | 53°C  |
+  |                                            ||        ||       |       |
+  |TEMPERATURE:--                              ||        ||       |       |
+  | Light and heat received from Sun,          ||        ||       |       |
+  |                               [Symbol] = 1 ||  0·13  || 1·00  | 6·67  |
+  | Reciprocal of square-root of distance,     ||        ||       |       |
+  |                               [Symbol] = 1 ||  0·60  || 1·00  | 1·61  |
+  | Equatorial temp. of ideal planet, Absolute ||   188  ||  312° |  502° |
+  |     "       "       "       "    Centigrade||   -65  ||  +39  | +229  |
+  | Average temp. of ideal planet, Absolute    ||   174  ||  290  |  467  |
+  |     "       "       "       "  Centigrade  ||   -99  ||  +17  | +194  |
+  | Upper limit under zenith sun, Absolute     ||   248  ||  412  |  664  |
+  |     "       "       "       " Centigrade   ||   -25  || +139  | +391  |
+  | Average temp. of equivalent disc, Absolute ||   223  ||  371  |  598  |
+  |     "       "       "       "    Centigrade||   -50  ||  +98  | +325  |
+  |                                            ||        ||       |       |
+  +--------------------------------------------++--------++-------+-------+
+
+  ------------------++--------++--------------------------------------+
+   Planets.         ||        ||            Outer Planets.            |
+                    ||        ||                                      |
+  +--------+--------++--------++---------+---------+--------+---------+
+  |  Mars  | Venus  || Earth  || Uranus  | Neptune | Saturn | Jupiter |
+  +--------+--------++--------++---------+---------+--------+---------+
+  |        |        ||        ||         |         |        |         |
+  |  4230  |  7700  ||  7918  ||   31900 |   34800 |  73000 |   86500 |
+  |  0·534 |  0·972 ||  1·000 ||   4·029 |   4·395 |  9·219 |  10·924 |
+  |  0·285 |  0·945 ||  1·000 ||  16·2   |  19·3   | 85·0   | 119·3   |
+  |  0·15  |  0·92  ||  1·00  ||  65·    |  85·    |760·    |1304·    |
+  |  3·92  |  4·94  ||  5·55  ||   1·22  |   1·11  |  0·72  |   1·32  |
+  |  0·71  |  0·89  ||  1·00  ||   0·22  |   0·20  |  0·13  |   0·24  |
+  |  0·107 |  0·820 ||  1·000 ||  14·6   |  17·0   | 94·8   | 317·7   |
+  |  0·38  |  0·87  ||  1·00  ||   0·90  |   0·89  |  1·18  |   2·65  |
+  |  6·11  | 13·99  || 16·08  ||  14·47  |  14·31  | 18·97  |  42·61  |
+  |  0·22  |  0·76  ||  0·50? ||   0·60  |   0·52  |  0·72  |   0·62  |
+  |        |        ||        ||         |         |        |         |
+  |        |        ||        ||         |         |        |         |
+  |141·5   | 67·2   || 92·9   ||1781·9   |2791·6   |886·0   | 483·3   |
+  |  1·524 |  0·723 ||  1·000 ||  19·183 |  30·055 |  9·539 |   5·203 |
+  |  1·88  |  0·62  ||  1·00  ||  84·02  | 164·78  | 29·46  |  11·86  |
+  | 15·0   | 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 |   1·101 |
+  |24°·0Ž  |  (?)   || 23°·27Ž||   (?)   |   (?)   | 26°·49Ž|  3°·5Ž  |
+  |h  m  s |        || h  m  s|| h m     |         | h  m   | h  m    |
+  |24·37·23|  (?)   || 23·56·4|| 9·30(?) |   (?)   |  10·14±|  9·55±  |
+  |        |        ||        ||         |         |        |         |
+  |        |        ||        ||         |         |        |         |
+  |        |        ||        ||         |         |        |         |
+  |        |        ||        ||         |         |        |         |
+  |  2·1   | 11·1   || 14·7   ||  11·9   |  11·5   | 19·4   | 103·8   |
+  |  0·143 |  0·754 ||  1·000 ||   0·81  |   0·78  |  1·32  |   7·06  |
+  |  8·8   |  3·8   ||  3·3   ||   3·7   |   3·8   |  2·8   |   1·3   |
+  |  53°C  |  92°C  || 100°C  ||  94°C   |  93°C   | 108°C  |  166°C  |
+  |        |        ||        ||         |         |        |         |
+  |        |        ||        ||         |         |        |         |
+  |  0·43  |  1·91  ||  1·00  ||   0·003 |   0·001 |  0·011 |   0·037 |
+  |  0·81  |  1·18  ||  1·00  ||   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  ||   +17  ||   -207  |   -221  |  -179  |   -146  |
+  |   337  |   486  ||   412  ||    94   |    74   |   133  |    180  |
+  |   +64  |  +213  ||  +139  ||   -179  |   -199  |  -140  |    -93  |
+  |   300  |   438  ||   371  ||    84   |    67   |   120  |    162  |
+  |   +27  |  +165  ||   +98  ||   -189  |   -206  |  -153  |   -111  |
+  |        |        ||        ||         |         |        |         |
+  +--------+--------++--------++---------+---------+--------+---------+
+
+For in regard to this particular premiss we can do away with hypothesis,
+and deal only with certain 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 dispute 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 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 120 pulsations a minute instead of his normal 66. When Mr.
+Glaisher and Mr. Coxwell made their celebrated ascent between 1 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 useless
+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
+attained 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 with moisture, 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 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
+ascending current on Mars would therefore take 1/0·38 × 1/0·38 = 1/0·145,
+or seven times as long to 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 circulation, 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 immense thickness of an obscuring medium. And yet the details of the
+surface are seen with remarkable 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 1/16th,
+and 1/12th, 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 transparent.
+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 practically 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 atmosphere. 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 1/9th of that of the Earth; exactly 0·107. It is
+distributed over a smaller surface, 0·285. Consequently 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 × 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 their great ascent; it is about
+one-half the pressure that is experienced on the top of the very highest
+terrestrial mountains. But the habitable regions of the Earth do not
+extend even so far upward as to the level of a pressure of 7·3 lb. 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 atmosphere 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 0° 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 atmosphere 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 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 determining 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 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 diminished.
+
+These statistics may seem rather dull and tiresome, 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 unchanged. 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 visiting 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 Lilliputians take a truer view of the effect of the
+alteration 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 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 anywhere; 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 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 impossible 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 temperature. 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 computed 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 Law, 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 expenditure 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 reflecting 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 resembling our Moon, and take the Moon's effective average
+temperature as 297° abs., the corresponding 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
+temperature of his surface is everywhere below the freezing-point of
+water."[14] As the atmospheric circulation on Mars must be languid, and
+the atmosphere itself is very rare, the general condition 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 64-1/2°--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, latitude 64-1/2° 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.
+
+Now the mean temperature of latitude 64-1/2°, say the latitude of
+Archangel, is just about freezing-point (0°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 moving 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 between 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
+temperatures of Singapore and Archangel; i.e. of the Earth and Mars; and
+Mars would stand at -40°C. The 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 condition 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 telescope 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--presumed 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 Protonilus 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 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 temperatures has been laid down
+for every 10° of latitude on the Earth, derived as far as possible from
+accepted isothermals in continental countries in the northern hemisphere.
+From this curve ordinates have been drawn at each 10°, upward to show
+average deviation from the mean temperature for the hottest part of the
+day in summer, downward 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 temperature
+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 state.
+
+In constructing a similar diagram for Mars, three modifications have to be
+made. First of all, the mean temperature of the planet must be
+considerably 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 atmosphere, and partly on account of the length of the Martian year.
+
+
+[Illustration: THERMOGRAPHS OF THE EARTH AND MARS]
+
+
+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 temperature 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 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 confined 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 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 substitute 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 warmest 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 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.
+
+
+
+
+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
+improvement 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 objects appear repeatedly on the drawings
+made by Beer and Mädler 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 Mädler undoubtedly portrayed the planet as actually they
+saw it. The one marking was named by Schiaparelli the Lacus Solis, the
+other, the Sinus Sabæus, 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 growth 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 Mädler, 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 attention to these two objects,
+and a mass of complicated structure has been brought out in the regions
+which were so simple in the sight of Beer and Mädler, 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
+Mädler, 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 themselves as uniform spots,
+precisely circular in outline? It is clear that Beer and Mädler 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 Mädler
+was that they were too small to be defined in their instruments; their
+minor irregularities were therefore invisible, and their apparent
+circularity covered detail of an altogether different form.
+
+Beer and Mädler only drew two such spots; Lowell shows about two hundred.
+Beer and Mädler's two spots seemed to them exactly alike; these two spots
+as we see them to-day have no resemblance to each other. Prof. Lowell's
+two hundred oases, with few exceptions, seem all of the same character; is
+it possible to suppose, if telescopes 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 Sabæus? If a novice begins to work upon Mars with a small telescope,
+he will draw the Lacus Solis and the Sinus Sabæus 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 defined vision, as would
+be 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 defined; that is to say for the
+spots to be seen as two separate objects.
+
+It will be seen that there is a wide range between objects that are large
+enough to be quite unmistakably perceived, and objects which are large
+enough to have their true outline really defined. 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 defined, 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;" 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 defined
+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 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 telescope;
+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 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 appearance 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 geometrical 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 described by H. G. Wells in the
+_War of the Worlds_.
+
+There are too many oversights in the canal theory.
+
+Thus no source is indicated for the moisture supposed 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 polar 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 conveyance 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
+conveyed 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 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 significance 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 continent, and its breadth would not be
+uniform from end to end, but the region fertilized would grow narrower
+with increase of distance from the fountain head of the canal.
+
+Under what conditions can we see straight lines, perfectly uniform from
+end to end, spring into existence, 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 defined. 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 experiments; they were keen-sighted,
+well drilled; accustomed to do what they were told without asking
+questions; and they knew nothing whatsoever of astronomy, certainly
+nothing about Mars.
+
+A diagram was hung up, based upon some drawing or other of the planet made
+by Schiaparelli, 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 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 markings 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 marvellous 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. Newcomb 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 Remembrancer,
+sent to me, in his 91st 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 supporting 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 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 disappear. In this, and not in any gigantic engineering 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.
+
+
+
+
+CHAPTER IX
+
+VENUS, MERCURY AND THE ASTEROIDS
+
+
+Of all the planets, Venus appears, to the unassisted 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 satisfying. 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 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 and 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 lb. 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 is 69° C. But water under a
+pressure of 11·2 lb. 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 perpetual 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.
+
+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 neighbour, 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 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 maintained 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.
+
+Since Mercury wears no concealing veil of atmosphere, 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 say, 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 development of the
+theory of tidal friction, puts the condition of Mercury in quite a new
+light. No alternation 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 "debatable land," for, owing
+to the ellipticity 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 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 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 habitability
+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 inhabited
+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 introduced 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 2-1/2 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 proportional change in this one planet's condition than we should
+experience if our Earth were transported 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 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 1/17th 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 distance 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 1/1200th; 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 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 all the rest, known and unknown, put together,
+though something 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 unfavourable
+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 consequence 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 fall
+far below freezing-point, the minimum on the dark side will approach the
+absolute zero.
+
+With Ceres the whole of the asteroidal 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 supports 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.
+
+
+
+
+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 confronted 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, movement 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 uniform tint, the surface being mottled with flocculent 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 background. 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, sometimes 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 colour as the belts, but deeper in tint, as if the fluid
+    medium could be seen to a greater depth."[15]
+
+In other words, Jupiter is a striped or banded planet, the bands lying
+along the direction of turning. 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 in
+different times. They may also alter their form and their colour. Jupiter
+seems, then, to be a planet with a great and rapidly changing atmosphere
+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 1/27th of the light and heat that we receive. But in Chapter VIII, we
+learnt from Mars that as this receives only 3/7ths 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 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 reconcile 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 contraction; 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, therefore, all the
+members of the solar system had at one and the same moment possessed the
+same surface 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, 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 dealing 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 whole cannot be put
+lower than 0·72, or about that of white paper. But many of the "belts" or
+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 lb. 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 probably 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
+portholes 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.
+
+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 movements. The clouds and
+storms of our own atmosphere 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 1/27th 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 inequalities 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 distance 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 spots are exceedingly swift, giving a rotation
+period of 9h. 50m. while the equatorial belt in general gives a period 5m.
+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 impossible, even unthinkable here, but the
+Great Red Spot, for all its long duration, is mobile and inconstant, and
+is therefore no portion of a solid permanent crust.
+
+The giant planet Jupiter, therefore, offers us an 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; Io, the innermost, is midway between the two in its
+dimensions. But Ganymede, the largest, is almost comparable with Mars, its
+diameter being 0·45 that of the Earth instead of the 0·53 of Mars.
+
+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 1/250th as much heat as each unit
+of the Sun's surface. In other words, the absolute surface temperature of
+Jupiter should be 1/4th 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 recipients 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.
+
+
+
+
+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 beautiful 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 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 sufficient. Or we can neglect the question of
+temperature 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 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                [Symbol] = 1    2·44     2·57     4·56    6·82
+    Surface,            [Symbol] = 1    6·0      6·6     20·8    46·6
+    Mass and Volume,    [Symbol] = 1   14·6     17·0     94·8   317·7
+    Gravity at surface, [Symbol] = 1    2·44     2·57     4·56    6·82
+  Rate of Fall, Feet in
+    the First Second                   39·2     41·3     73·3   109·7
+
+  ATMOSPHERE, assuming the
+  total mass of the atmosphere
+  to be proportional to
+  the mass of the planet:--
+
+  Pressure at the surface in lb.
+    per square inch                    88·2     97·0    305·8   685·0
+  Pressure at the surface in
+    "atmospheres"                       6·0      6·6     20·8    46·6
+  Level of half-pressure in miles       1·37     1·30     0·73    0·49
+  Boiling point of water at
+    surface                            127°C    129°C    148°C   164°C
+
+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 proportion. A man who here weighs 150 lb. will there
+weigh over 1000 lb.; 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 proportion. The atmosphere, supposing it to be in proportion
+to the mass of Jupiter, will exercise a pressure of 46-1/2 "atmospheres,"
+or more than 680 lb., 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 upward 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 lb. 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 constructed 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 low range of hills that on the Earth would be considered 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 involve.
+
+The sevenfold gravity of Jupiter, taken in connection with this enormous
+atmospheric pressure, would tend to make the meteorological disturbances
+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 suddenness 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 cataclysmic.
+
+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
+atmospheric pressure would be about 21 "atmospheres," or more than 300 lb.
+on the square inch. The level of half-pressure would be reached at about
+three-quarters of a mile, and the force of gravity be nearly 4-1/2 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 terrestrial 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 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 atmosphere 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, therefore, 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 14th 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 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, 38-1/2°
+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 darkness,
+of unmitigated cold, cannot possibly ever 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 sunlight, but also special lines due to
+their own atmospheres (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 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.
+
+
+
+
+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 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 components 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 brightness, 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, 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 acclimatizing
+themselves to these ever-changing vicissitudes. 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 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 forbids 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 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 observers 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 compared 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, 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
+destroyed. 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 consideration 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, 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 disqualifications 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 telescopes, 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 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 throughout 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 illustrations 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, 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
+
+  2,000,000,000,000,000,000,000,000,000 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 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 we 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, intelligent 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.
+
+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 conditions 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 waterless 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 littérateur 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
+diverse as they are numerous. And everywhere and always, water is found
+essential to protoplasmic 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 unrelated 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 intelligence
+are associated, and we 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 difference 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 supposed 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 associated with the development of one type or 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 possible for the skeleton of creatures from
+other worlds to be brought to us here, then biologists would as
+confidently pronounce on their intelligence 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 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 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 inhabited 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 world of ours. Heavy as the odds are against any
+particular world being an inhabited one, yet when the limitless extent of
+space is considered, and the innumerable numbers of stars and systems of
+stars, it seems but reasonable 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 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 endowed not by the action and intervention of other living
+matter, but without it--we have no knowledge, no experience. And so long
+as this continues 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 speculate 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 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 foreshadowing 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 than to determine how life first came; its
+experience 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 experience,
+we are not left without an answer. A voice has been heard, the voice of
+the Son of God Himself:
+
+"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."
+
+
+
+
+INDEX
+
+
+  Abbot, C. G., 27, 34
+
+  Albedo of Earth, 54, 81
+    ---- Jupiter, 127
+    ---- Mars, 81
+    ---- 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, 11, 34, 38, 39
+
+  Carbonic acid, 11
+
+  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, 1
+
+  Copper, 33
+
+  Corona, 40
+
+  Coxwell, 25, 75, 79
+
+  Cyanogen, 37, 38
+
+  Czapek, F., 11, 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
+
+  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, 81
+
+
+  "Inhabitant," 5
+
+  "Inhabited" Worlds, 2, 3, 4
+
+  Io, satellite of Jupiter, 131
+
+  Iron, 12, 33, 36
+
+
+  Jupiter, 122-32
+
+  ----, belts, 127, 129
+
+  ----, great red spot, 130
+
+  ----, proper motion of spots, 129
+
+  ----, 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
+
+  Linné, lunar crater, 48
+
+  Lockyer, J. N., 60
+
+  Lowell, P., 65, 66, 67, 69, 71, 81, 97, 98, 99, 101, 103, 104, 105, 106,
+  108, 109, 110
+
+  Lucifer, 111
+
+
+  Mädler, 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, 57-70, 78, 101, 102
+
+  ----, conditions of, 71-95
+
+  ----, illusions of, 96-110
+
+  ----, meteorology of, 93-4
+
+  ----, oases of, 65, 98, 99, 101
+
+  ----, thermograph of, 91, 92
+
+  ----, winds of, 77
+
+  Mendeléeff, 39
+
+  Mercury, 114-18
+
+  Messier, lunar crater, 47, 48
+
+  Metabolism, 10, 11, 14, 15, 38
+
+  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
+
+  Nicholson, J. W., 40
+
+  Nickel, 33
+
+  Nilosyrtis, "canal" on Mars, 89
+
+  Nitrogen, 11, 37, 38, 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, 11, 36, 37, 38, 41
+
+
+  Periodic Law, Mendeléeff's, 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, 115
+
+  Proctor, R. A., 34, 77
+
+  Prominences, 29, 30, 37
+
+  Protofluorine, 40
+
+  Protonilus, "canal" on Mars, 89
+
+  Protoplasm, 11, 12, 13, 15, 38, 40, 154
+
+  Pyramid, Great, 45
+
+
+  Refraction, anomalous, 28
+
+  Reversing layer, 36
+
+  "Rice-grains," of Sun's surface, 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, T. 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, 11, 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
+
+  Tornadoes, 31, 137
+
+  "Twinkler," 114
+
+  Tycho, lunar crater, 46
+
+
+  Uranus, 132, 140
+
+
+  Venus, 57, 111-18
+
+  Verworn, Max, 7
+
+  Very, F. W., 55
+
+  Vesper, 111
+
+  "Victoria," hypothetical planet, 83
+
+
+  Wallace, A. R., 4
+
+  "War of the Worlds," 104, 156
+
+  Waste, 151, 152
+
+  Water, indispensable factor, 15, 41
+
+  Wells, H. G., 104, 156
+
+  Whewell, 4
+
+  Williams, A. Stanley, 104
+
+  Wolf, Max, 40
+
+
+  Young, C. A., 26, 33
+
+
+WILLIAM BRENDON AND SON, LTD.
+
+PRINTERS, PLYMOUTH
+
+
+
+
+Footnotes:
+
+[1] _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.
+
+[2] _Wonders of Life_, by Ernst Haeckel, Professor at Jena University, p.
+130.
+
+[3] _Wonders of Life_, pp. 127-8.
+
+[4] _Chemical Phenomena in Life_, p. 58.
+
+[5] _Ibid._, p. 22.
+
+[6] _Other Worlds_, by Garrett P. Serviss, pp. 63-4.
+
+[7] _Modern Painters_, by John Ruskin.
+
+[8] If this experiment could be carried out, it would be necessary 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.
+
+[9] The movements of not a few double stars point to perturbations 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.
+
+[10] _Proc. R. Soc._, LXXX, 50, 1907.
+
+[11] _Nature_, LXXX, 158 (April 8th, 1909).
+
+[12] "Periodic Changes upon the Moon," _Memoirs_, British Astronomical
+Association, Vol. XIII, p. 88.
+
+[13] _The Moon_, by Philip Fauth, p. 156.
+
+[14] _Radiation in the Solar System: Its Effects on Temperature, and its
+Pressure on Small Bodies_, by Dr. J. H. Poynting (_Phil. Trans. of the
+Royal Society_, Vol. 202 A).
+
+[15] _Publ. of the Astron. Soc. of the Pacific_, Vol. II, pp. 286-8.
+
+
+
+
+Harper's Library of Living Though
+
+
+ARTHUR HOLMES
+
+THE AGE OF THE EARTH
+
+And Associated Problems. _Illustrated_
+
+Gives us the result of the latest research into this field of enquiry. The
+radioactive minerals are shown to be recording their own age with the
+exquisite accuracy of a chronometer--their records checking physical,
+astronomical, and geological methods of computation.
+
+
+PROF. A. W. BICKERTON
+
+THE BIRTH OF WORLDS AND SYSTEMS
+
+_Illustrated_
+
+_Preface by Prof. Ernest Rutherford, F.R.S._
+
+A graphic account of the formation of new stars from the collision of dead
+suns or other celestial bodies. The theory throws light on many
+astronomical problems, and with its conception of an immortal cosmos, is
+of great philosophical importance.
+
+
+PROF. SVANTE ARRHENIUS
+
+THE LIFE OF THE UNIVERSE
+
+_2 Vols. Illustrated_
+
+"We can thoroughly recommend these volumes. The information is accurate,
+useful, and most suggestive. There are many for whom the first chapters of
+Genesis are a subtle allegory covering the profoundest truths, and we are
+grateful to the author for having set out this mass of facts."--_The
+Globe._
+
+
+SIR OLIVER LODGE, F.R.S.
+
+THE ETHER OF SPACE
+
+_Illustrated_
+
+"This work by the great physicist will be found to possess an abiding
+charm and an intellectual stimulation."-_Observer._
+
+"Opens up new views into the nature of the universe. Precise and lucid, it
+summarises our knowledge of the substance which fills all space and
+penetrates all matter--the substratum of matter itself."--_Birmingham
+Post._
+
+_Please write for announcements and descriptive list:_
+
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+
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+
+
+PROF. ARTHUR KEITH, M.D.
+
+(Hunterian Professor Royal College of Surgeons)
+
+ANCIENT TYPES OF MAN
+
+_Illustrated_
+
+"The kind of book that only a master of his subject could write. It must
+interest every thinking person."_--British Medical Journal._
+
+
+PROF. FREDERICK CZAPEK
+
+CHEMICAL PHENOMENA IN LIFE
+
+Discusses in clear, concise terms the great question--"Can life be
+explained by physics and chemistry?" It deals with the life-processes of
+plants, the molecular structure of protoplasm, organic synthesis in the
+cell, the nature of ferments, and the subject of inheritance.
+
+
+SIR A. TILDEN, F.R.S.
+
+THE ELEMENTS
+
+Speculations as to their Nature and Origin
+
+_Diagrams, &c._
+
+Points to the conclusion that the elements resulted from a change in some
+primal essence, and discusses "whether all may not be suffering a slow
+waste, which, in the long run, must lead back to the primal chaos."
+
+
+SIR WILLIAM RAMSAY, F.R.S.
+
+ELEMENTS AND ELECTRONS
+
+_Diagrams_
+
+The electron--"the atom of electricity"--is shown to be separable from
+matter, and to be capable under certain circumstances of independent
+existence. The book shows that the electron must be regarded as a kind of
+"element" itself, with much stronger claims to "elementary" or
+undecomposable characters than the bodies hitherto ranked as elements.
+
+
+
+
+Transcriber's Notes:
+
+Passages in italics are indicated by _italics_.
+
+Superscripted characters are indicated by {superscript}.
+
+The original text includes symbols which are represented by [Symbol] in
+this text version.
+
+
+
+
+
+
+End of Project Gutenberg's Are the Planets Inhabited?, by E. Walter Maunder
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+
+Project Gutenberg's Are the Planets Inhabited?, by E. Walter Maunder
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Are the Planets Inhabited?
+
+Author: E. Walter Maunder
+
+Release Date: April 23, 2011 [EBook #35937]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ARE THE PLANETS INHABITED? ***
+
+
+
+
+Produced by Jonathan Ingram and the Online Distributed
+Proofreading Team at https://www.pgdp.net (This file was
+produced from images generously made available by The
+Internet Archive/American Libraries.)
+
+
+
+
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+</pre>
+
+
+
+
+<p class="center"><span class="huge"><span class="smcap">Harper&#8217;s Library</span> <i>of</i> <span class="smcap">Living Thought</span></span></p>
+<p>&nbsp;</p>
+<div class="figcenter"><img src="images/circle.jpg" alt="" /></div>
+
+<p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p>
+<div class="figcenter"><img src="images/title.jpg" alt="" /></div>
+<p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p>
+
+<p class="center"><span class="giant">ARE THE PLANETS<br />INHABITED?</span></p>
+<p>&nbsp;</p>
+<p class="center">BY<br />
+<span class="big">E. WALTER MAUNDER, F.R.A.S.</span><br />
+<small>SUPERINTENDENT OF THE SOLAR DEPARTMENT, ROYAL OBSERVATORY<br />GREENWICH<br /><br />
+AUTHOR OF &#8220;ASTRONOMY WITHOUT A TELESCOPE&#8221;<br />
+&#8220;THE ROYAL OBSERVATORY, GREENWICH, ITS HISTORY AND WORK&#8221;<br />
+&#8220;THE ASTRONOMY OF THE BIBLE,&#8221; &#8220;THE HEAVENS AND THEIR STORY&#8221;<br />ETC.</small></p>
+<p>&nbsp;</p>
+<p class="center">HARPER &amp; BROTHERS<br />
+LONDON AND NEW YORK</p>
+<p class="center">45 ALBEMARLE STREET, W.<br />1913</p>
+<p>&nbsp;</p><p>&nbsp;</p>
+
+<p class="center"><i>Published March, 1913</i></p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<h2>CONTENTS</h2>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td><small>CHAPTER</small></td><td>&nbsp;</td><td align="right"><small>PAGE</small></td></tr>
+<tr><td align="right"><a href="#CHAPTER_I">I.</a></td><td><span class="smcap">The Question Stated</span></td><td align="right"><a href="#Page_1">1</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_II">II.</a></td><td><span class="smcap">The Living Organism</span></td><td align="right"><a href="#Page_6">6</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_III">III.</a></td><td><span class="smcap">The Sun</span></td><td align="right"><a href="#Page_20">20</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_IV">IV.</a></td><td><span class="smcap">The Distribution of the Elements in Space</span></td><td align="right"><a href="#Page_33">33</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_V">V.</a></td><td><span class="smcap">The Moon</span></td><td align="right"><a href="#Page_43">43</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_VI">VI.</a></td><td><span class="smcap">The Canals of Mars</span></td><td align="right"><a href="#Page_57">57</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_VII">VII.</a></td><td><span class="smcap">The Condition of Mars</span></td><td align="right"><a href="#Page_71">71</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_VIII">VIII.</a></td><td><span class="smcap">The Illusions of Mars</span></td><td align="right"><a href="#Page_96">96</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_IX">IX.</a></td><td><span class="smcap">Venus, Mercury and the Asteroids</span></td><td align="right"><a href="#Page_111">111</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_X">X.</a></td><td><span class="smcap">The Major Planets</span></td><td align="right"><a href="#Page_122">122</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_XI">XI.</a></td><td><span class="smcap">When the Major Planets Cool</span></td><td align="right"><a href="#Page_133">133</a></td></tr>
+<tr><td align="right"><a href="#CHAPTER_XII">XII.</a></td><td><span class="smcap">The Final Question</span></td><td align="right"><a href="#Page_143">143</a></td></tr>
+<tr><td>&nbsp;</td><td><span class="smcap">Index</span></td><td align="right"><a href="#Page_163">163</a></td></tr></table>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></span></p>
+<p class="center"><span class="giant">ARE THE PLANETS INHABITED?</span></p>
+<p>&nbsp;</p>
+<h2><a name="CHAPTER_I" id="CHAPTER_I"></a>CHAPTER I</h2>
+<p class="center"><span class="big">THE QUESTION STATED</span></p>
+
+<p class="dropcap"><span class="caps">The</span> 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.</p>
+
+<p>In those days there seemed an immense difference 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<span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span> amongst them, and
+it too moves and shines&mdash;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.</p>
+
+<p>But as men realized this, they began to ask: &#8220;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?&#8221;</p>
+
+<p>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 ourselves, possessed of personality and
+intelligence, lodged in an organic body.</p>
+
+<p>This is what is meant when we speak of a world being &#8220;inhabited.&#8221; 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
+richness<span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span> of vegetation and no fulness and complexity of animal life would
+justify an explorer in describing some land that he had discovered as
+being &#8220;inhabited&#8221; if no men were there, so we cannot rightly speak of any
+other world as being &#8220;inhabited&#8221; if it is not the home of intelligent
+life. If the life did not rise above the level of alg&aelig; 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 hereafter into manifestations with
+which we could claim a nearer kinship.</p>
+
+<p>On the other hand, of necessity we are precluded from extending our
+enquiry to the case of disembodied intelligences, if such be conceived
+possible. All created existences must be conditioned, 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 subsist. Gnomes, sylphs, elves, and
+fairies, and all similar conceptions, escape the possibility of discussion
+by our ignorance of their properties. As nothing can be asserted of them
+they remain beyond investigation, as they are beyond sight and touch.</p>
+
+<p><span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span>The only beings, then, the presence of which would justify us in regarding
+another world as &#8220;inhabited&#8221; 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 world unsuited for living organisms is not, in the sense of this
+enquiry, a &#8220;habitable&#8221; world.</p>
+
+<p>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.</p>
+
+<p><span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span>For myself, if there be any theological significance 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&mdash;our own&mdash;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 &#8220;inhabitant&#8221; 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?</p>
+
+<p>The problem therefore seems not to be theological or metaphysical, but
+purely physical. We have simply to ask with regard to each heavenly body
+which we pass in review: &#8220;Are its physical conditions, so far as we can
+ascertain them, such as would render the maintenance of life possible upon
+it?&#8221; 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.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span></p>
+<h2><a name="CHAPTER_II" id="CHAPTER_II"></a>CHAPTER II</h2>
+<p class="center"><span class="big">THE LIVING ORGANISM</span></p>
+
+<p class="dropcap"><span class="caps">A world</span> for habitation, then, is a world whereon living organisms can
+exist that are comparable in intelligence with men. But &#8220;men&#8221; 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.</p>
+
+<p>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.</p>
+
+<p>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 concerned with the
+highest form of organism, the one furthest from these two dividing lines.</p>
+
+<p><span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span>It suffices that living organisms do exist here, and exist under
+well-defined conditions. Wanting these conditions, they perish. We can, to
+a varying degree, determine the physical conditions prevailing upon the
+heavenly bodies, and we can ascertain whether these physical conditions
+would be favourable, unfavourable, or fatal to the living organism.</p>
+
+<p>What is a living organism? A living organism is such that, though it is
+continually changing its substance, its identity, as a whole, remains
+essentially 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.</p>
+
+<p>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
+particularly 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; surrounding this, a bluish
+zone that is faintly luminous;<span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span> 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&mdash;the gases
+that feed it&mdash;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.</p>
+
+<p>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.</p>
+
+<p>But the living organism differs from artificial machines in that, of
+itself and by itself, it is <span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span>continuously 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.</p>
+
+<p>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 restricted, narrowly restricted, in its action. &#8220;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<span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span> which have been produced by the chemical reactions 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 <i>Metabolism</i>. 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<span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span> more than twenty to thirty years.
+These experiences 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 production 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 temporarily, really suspended.</p>
+
+<p>&#8220;Life is, therefore, quite inseparable from chemical 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.&#8221;<small><a name="f1.1" id="f1.1" href="#f1">[1]</a></small></p>
+
+<p>The essential quality, therefore, of life is continual 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&mdash;protoplasm.</p>
+
+<p>In this substance five elements are always present and
+predominant&mdash;carbon, oxygen, nitrogen, hydrogen, and sulphur. The
+compounds which these five elements form with each other are<span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span> most complex
+and varied, and they also admit to combination&mdash;but in smaller
+proportions&mdash;some of the other elements, of which phosphorus, potassium,
+calcium, magnesium, and iron are the most important.</p>
+
+<p>For protoplasm&mdash;using the term in the most general sense&mdash;is a chemical
+substance, not a mere mixture of a number of chemical elements, nor a mere
+mechanical structure. &#8220;However differently the various plasma substances
+behave in detail, they always exhibit the same general composition as the
+other albuminoids out of the five &#8216;organo-genetic elements&#8217;&mdash;namely in
+point of weight, 51-54% carbon, 21-23% oxygen, 15-17% nitrogen, 6-7%
+hydrogen, and 1-2% sulphur.&#8221;<small><a name="f2.1" id="f2.1" href="#f2">[2]</a></small></p>
+
+<p>Haeckel, the writer just quoted, describes the plasm, the universal basis
+of all the vital phenomena, in the following terms: &#8220;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 distinguishes
+three conditions of inorganic matter&mdash;solid, fluid, and gaseous. Active
+living protoplasm cannot be strictly described as either fluid or solid in
+the physical sense. It presents an intermediate<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span> 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 substance 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 distributed 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.&#8221;<small><a name="f3.1" id="f3.1" href="#f3">[3]</a></small> And Czapek further tells us that &#8220;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&middot;1 to 0&middot;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<span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span> are carried on contemporaneously,
+without one disturbing the other in the slightest degree.&#8221;<small><a name="f4.1" id="f4.1" href="#f4">[4]</a></small></p>
+
+<p>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.
+&#8220;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.&#8221;<small><a name="f5.1" id="f5.1" href="#f5">[5]</a></small></p>
+
+<p>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 undergoing 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.</p>
+
+<p><span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span>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&mdash;the metabolism of vital processes&mdash;requires for its
+continuance the presence of one indispensable factor&mdash;WATER.</p>
+
+<p>Protoplasm itself, as Czapek puts it, is practically an <i>albumin sol</i>;
+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 <i>osmosis</i> is an indispensable condition in the vital process.
+And the &#8220;streaming&#8221; of protoplasm, its continual movement in the cell, can
+only be carried on in water.</p>
+
+<p>WATER is the compound of oxygen and hydrogen 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<span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span> 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 0&deg; Centigrade, it boils at 100&deg; Centigrade.
+Necessarily, our experiences are mostly confined within this range, and
+therefore 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
+destroyed, 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.</p>
+
+<p>Water is, then, indispensable for the living<span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span> organism; but there are two
+great divisions of such organisms&mdash;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 dissolved; 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.</p>
+
+<p>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: &#8220;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 outside the atmosphere. As the Earth
+condensed from the original nebula, and cooled and solidified, a<span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span> 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 ceaseless 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.&#8221;<small><a name="f6.1" id="f6.1" href="#f6">[6]</a></small></p>
+
+<p>&#8220;The huge inert globe of permanently combined elements below, and the
+equally unchanging realm of the ether above,&#8221; offer no home for the living
+organism; least of all for the highest of such organisms&mdash;Man. Both must
+be tempered to a condition which will permit and favour continual change,
+the metabolism which is the essential feature of life.</p>
+
+<p>&#8220;When the earth had to be prepared for the habitation of man, a veil, as
+it were, of intermediate 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.</p>
+
+<p>&#8220;But the heavens, also, had to be prepared for his habitation. Between
+their burning light,&mdash;their deep vacuity, and man, as between the earth&#8217;s
+gloom of iron substance, and man, a veil had to be<span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span> spread of intermediate
+being;&mdash;which should appease 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.&#8221;<small><a name="f7.1" id="f7.1" href="#f7">[7]</a></small></p>
+
+<p>The leaf and the cloud are the signs of a habitable world. The leaf&mdash;that
+is to say, plant life, vegetation&mdash;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 &#8220;born of water,&#8221; and any world fitted for his
+habitation must &#8220;stand out of the water and in the water.&#8221;</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span></p>
+<h2><a name="CHAPTER_III" id="CHAPTER_III"></a>CHAPTER III</h2>
+<p class="center"><span class="big">THE SUN</span></p>
+
+<p class="dropcap"><span class="caps">The</span> 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 points 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<span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span> 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 perpetual
+daylight, and in the serenity of a perpetual spring that was distracted by
+no storm.</p>
+
+<p>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
+years 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 introduction to our subject. Further, those conditions
+themselves will bring out certain principles that are of necessary
+application when we come to consider the case of particular planets.</p>
+
+<p>The distance of the Sun from the Earth is often spoken of as the
+&#8220;astronomical unit&#8221;; 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<span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span> shortly see, the particular problem with which
+we are engaged&mdash;the habitability of worlds&mdash;is directly connected with
+these two factors: the size of the world in question, and its distance
+from the Sun.</p>
+
+<p>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&#8217;s distance.</p>
+
+<p>But knowing the Sun&#8217;s distance, we are able to deduce 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&middot;4 times that of the Earth. Its surface exceeds that of the Earth
+11,970 times; its volume, 1,310,000 times.</p>
+
+<p>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&#8217;s attraction; the dimensions of their orbits are
+known, and the times taken in describing 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<span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span> Earth; and as the latter has
+been determined as equal to about</p>
+
+<p class="center">6,000,000,000,000,000,000,000 tons</p>
+
+<p>that of the Sun would be equal to</p>
+
+<p class="center">2,000,000,000,000,000,000,000,000,000 tons.</p>
+
+<p>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&#8217;s
+mass&mdash;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&#8217;s surface we
+must divide by (109&middot;4)<sup>2</sup>, since the distance of the Sun&#8217;s centre from its
+surface is 109&middot;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&middot;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.<small><a name="f8.1" id="f8.1" href="#f8">[8]</a></small></p>
+
+<p>This relation is one of great importance when we realize that the pressure
+of the Earth&#8217;s atmosphere<span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span> is 14&middot;7 lb. 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
+atmosphere 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 &times; 12 = 408) is 14&middot;7
+lb.</p>
+
+<p>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 atmosphere, 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<span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span> 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 remaining
+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.</p>
+
+<p>But on the Sun, where the force of gravity is 27&middot;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,<span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span> but immeasurably greater; but the discordance is sufficiently
+explained when we come to another class of facts.</p>
+
+<p>These relate to the temperature of the Sun, and to the enormous amount of
+light and heat which it radiates forth continually. This entirely
+transcends 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</p>
+
+<p class="center">1,575,000,000,000,000,000,000,000,000 standard candles.</p>
+
+<p>The intensity of sunlight at each point of the Sun&#8217;s surface is variously
+expressed as</p>
+
+<p class="blockquot">190,000 times that of a standard candle,<br />
+5300 times that of the metal in a Bessemer converter,<br />
+146 times that of a calcium light,<br />
+or, 3&middot;4 times that of an electric arc.</p>
+
+<p>The same authority estimates at 30 <i>calories</i> the value of the <i>Solar
+Constant</i>; 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&#8217;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&#8217;s
+<span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span>surface 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&middot;2 feet. Prof. Abbot&#8217;s most recent determination of the solar constant
+diminishes these estimates by one third; but he still gives the probable
+temperature of the solar surface as not far short of 7000 degrees
+Centigrade, or about 12,000 degrees Fahrenheit.</p>
+
+<p>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 pressures, on the other hand, apart from the high
+temperatures, would probably solidify every element, 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.</p>
+
+<p>What effect have these two factors, so stupendous in scale, upon its
+visible surface? What is the appearance of the Sun?</p>
+
+<p>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<span class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></span> 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 <i>chromosphere</i> 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 <i>photosphere</i>, or
+general radiating surface. This, to the eye, certainly looks like a
+definite shell, but some theorists have been so impressed with the
+difficulty of conceiving that a gaseous body like the Sun could, under the
+conditions 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 incompatible 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 brightness, and
+irregularly scattered. This mottling is sometimes coarsely, sometimes
+finely textured; in some regions it is sharp and well defined, in others<span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span>
+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.</p>
+
+<p>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 <i>sunspots</i>, and in the early stages of their development
+they are usually arranged in groups which tend to be stretched out
+parallel to the Sun&#8217;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
+<i>faculae</i>, from their brightness. In the spectroscope, when the serrated
+edges of the chromosphere are under observation, every now and then great
+<i>prominences</i>, 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<span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span> 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 completely disappeared within
+less than half an hour. The velocity of the gas streams in such eruptions
+often exceeds one hundred miles a second; sometimes, though only rarely,
+it reaches a speed twice as great.</p>
+
+<p>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 thousand 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&#8217;s history, and the usual speed with which
+they move away from each other is three hundred miles an hour.</p>
+
+<p>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 constitution,<span class="pagenum"><a name="Page_31" id="Page_31">[Pg 31]</a></span>
+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.</p>
+
+<p>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&middot;65
+times that at the surface of the Earth, where a body falls 16&middot;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.</p>
+
+<p>The high temperatures, the great pressures, the violent commotions which
+prevail on the Sun are,<span class="pagenum"><a name="Page_32" id="Page_32">[Pg 32]</a></span> 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.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_33" id="Page_33">[Pg 33]</a></span></p>
+<h2><a name="CHAPTER_IV" id="CHAPTER_IV"></a>CHAPTER IV</h2>
+<p class="center"><span class="big">THE DISTRIBUTION OF THE ELEMENTS IN SPACE</span></p>
+
+<p class="dropcap"><span class="caps">It</span> 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 negative 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.</p>
+
+<p>The Sun, then, is in an essentially gaseous condition, enclosed by the
+luminous shell which we term the photosphere. This shell Prof. C. A. Young
+and the majority of astronomers regard as<span class="pagenum"><a name="Page_34" id="Page_34">[Pg 34]</a></span> consisting of a relatively thin
+layer of glowing clouds, justifying the quaint conceit of R. A. Proctor,
+who spoke of the Sun as a &#8220;Bubble&#8221;; 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 substance 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, refuses
+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.</p>
+
+<p>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<span class="pagenum"><a name="Page_35" id="Page_35">[Pg 35]</a></span> 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 of life. 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.<small><a name="f9.1" id="f9.1" href="#f9">[9]</a></small> 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.</p>
+
+<p>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<span class="pagenum"><a name="Page_36" id="Page_36">[Pg 36]</a></span> 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&mdash;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 &#8220;negative&#8221; 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.</p>
+
+<p>It is clear that not all elements present in a Sun or star show themselves
+in its spectrum. Oxygen<span class="pagenum"><a name="Page_37" id="Page_37">[Pg 37]</a></span> 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 <i>helium</i> 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&#8217;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 essentially 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
+<span class="pagenum"><a name="Page_38" id="Page_38">[Pg 38]</a></span>temperatures 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.</p>
+
+<p>Hydrogen is seen in the spectra of nearly all stars, and also in those of
+nebul&aelig;. The elemental lines of oxygen are not indeed seen in stellar
+spectra, but that the element is present is shown by the flutings 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.<small><a name="f10.1" id="f10.1" href="#f10">[10]</a></small>
+All the five organo-genetic elements are therefore abundantly diffused
+through space; the materials for protoplasm, &#8220;the albuminous substance
+with water,&#8221; 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 <span class="pagenum"><a name="Page_39" id="Page_39">[Pg 39]</a></span>metabolism of nitrogenous carbon
+compounds in association with protoplasm.</p>
+
+<p>But it is objected that &#8220;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 compounds 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?&#8221;</p>
+
+<p>But the development of Mendel&eacute;eff&#8217;s Periodic Law has shown that the
+elements are not to be regarded as disconnected entities. The Law as given
+in Mendel&eacute;eff&#8217;s own words, runs: &#8220;The properties of the elements as well
+as the forms and properties of their compounds are in periodic dependence
+on, or (expressing ourselves algebraically) form a periodic function of
+the atomic weights of the elements.&#8221; 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 acquainted. New elements, no doubt,
+will be discovered in the future, as in the past; and indeed we<span class="pagenum"><a name="Page_40" id="Page_40">[Pg 40]</a></span> may be
+able to discover them and learn their atomic weights and properties
+without ever being able to handle them in a terrestrial laboratory.</p>
+
+<p>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 nebul&aelig;, 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&middot;3), than fluorine; he therefore gives it the name of
+<i>Protofluorine</i>. The other element, to which he gives the name <i>Nebulium</i>,
+will have an atomic weight of 2&middot;1. Prof. Max Wolf, of Heidelberg, has
+recently pointed out<small><a name="f11.1" id="f11.1" href="#f11">[11]</a></small> 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 abundant and more
+universally diffused than the five which give us protoplasm&mdash;&#8220;the physical
+basis of life.&#8221; To take an analogy from the solar system: many hundreds of
+planetoids have now been discovered between the orbits of Mars and
+Jupiter,<span class="pagenum"><a name="Page_41" id="Page_41">[Pg 41]</a></span> 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 planetoidal 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.</p>
+
+<p>Water is essential for life here, but the quality in water which restricts
+the range of terrestrial life is that it freezes at 0&deg; Centigrade, and
+boils at 100&deg; 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 elements 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 0&deg; to 100&deg; Centigrade, have we any reason to
+suppose that protoplasm itself would be able to endure these outlying
+temperatures? Looking through the<span class="pagenum"><a name="Page_42" id="Page_42">[Pg 42]</a></span> 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?</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_43" id="Page_43">[Pg 43]</a></span></p>
+<h2><a name="CHAPTER_V" id="CHAPTER_V"></a>CHAPTER V</h2>
+<p class="center"><span class="big">THE MOON</span></p>
+
+<p class="dropcap"><span class="caps">The</span> 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</p>
+
+<p class="center">27,000,000 moons.</p>
+
+<p>Compared with the Sun, the Moon is therefore an insignificant little
+ball&mdash;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 looking 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.</p>
+
+<p>So far all is to the good for the purpose of<span class="pagenum"><a name="Page_44" id="Page_44">[Pg 44]</a></span> 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. &#8220;The Earth&#8217;s gloom of iron substance&#8221; is necessary for man here,
+and the Moon appears to offer a like stability.</p>
+
+<p>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.</p>
+
+<p>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 &#8220;crater-pits,&#8221; not more than a mile
+across; and narrow lines on the Moon&#8217;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<span class="pagenum"><a name="Page_45" id="Page_45">[Pg 45]</a></span> 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.</p>
+
+<p>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.</p>
+
+<p>We ought, therefore, to have no difficulty in observing seasonal changes
+on the Moon, if such<span class="pagenum"><a name="Page_46" id="Page_46">[Pg 46]</a></span> 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 suspected,
+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.</p>
+
+<p>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
+recognized with difficulty. Thus the terse remark of M&auml;dler, &#8220;The full
+Moon knows no Maginus,&#8221; 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 &#8220;the lunar metropolis,&#8221; 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.</p>
+
+<p><span class="pagenum"><a name="Page_47" id="Page_47">[Pg 47]</a></span>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, &#8220;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
+triangular 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.&#8221;<small><a name="f12.1" id="f12.1" href="#f12">[12]</a></small></p>
+
+<p>Some observers explain this cycle of changes as due merely to the peculiar
+contour of the two<span class="pagenum"><a name="Page_48" id="Page_48">[Pg 48]</a></span> 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
+probably 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 M&auml;dler described the two craters as being
+exactly alike eighty years ago, Messier A is now distinctly the larger;
+but it is very doubtful whether M&auml;dler&#8217;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.</p>
+
+<p>There are several other cases of the same order of ambiguity. The most
+celebrated is Linn&eacute;, 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 Linn&eacute; may once have been a crater with steep walls which
+have collapsed into its interior through the force of gravity.</p>
+
+<p>Another type of suspected change is associated with the neighbourhood of
+Aristarchus, the brightest<span class="pagenum"><a name="Page_49" id="Page_49">[Pg 49]</a></span> formation on the Moon, so bright indeed that
+Sir William Herschel, observing it when illuminated 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 unusual lighting up of rocks on which the Sun had already set might
+be due either to phosphorescence produced by long exposure to the Sun&#8217;s
+rays, or to inherent heat, or to reflected glare from the western rampart.
+Still more important, both Major Molesworth 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.</p>
+
+<p>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.<span class="pagenum"><a name="Page_50" id="Page_50">[Pg 50]</a></span>
+Philip Fauth, declares that &#8220;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&#8217;s surface.&#8221;<small><a name="f13.1" id="f13.1" href="#f13">[13]</a></small></p>
+
+<p>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 manifestly 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 &#8220;gloom of iron substance&#8221; below is ever concealed by
+a veil of changing vegetation, or that &#8220;between the burning light and deep
+vacuity&#8221; 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.</p>
+
+<p>Life is change, and a planet where there is no<span class="pagenum"><a name="Page_51" id="Page_51">[Pg 51]</a></span> change, or where that
+change is very small, can be no home for life. The &#8220;stability and
+insensibility&#8221; are indeed required in the platform upon which life is to
+appear, but there must be the presence of &#8220;the passion and the perishing,&#8221;
+or life will be unable to find a home.</p>
+
+<p>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 observations. 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 contained no water, for though
+Milton, in a well-known passage, makes Galileo discover</p>
+
+<p class="center">&#8220;Rivers or mountains on her spotty globe,&#8221;</p>
+
+<p>Galileo himself wrote: &#8220;I do not believe that the body of the Moon is
+composed of earth and water.&#8221; The name of <i>maria</i> was given to the great
+grey plains of the Moon by Hevelius, but this was simply for convenience
+of nomenclature, not because he actually believed them to be seas. One
+observation is, in itself, sufficient to prove that the maria are not
+water surfaces. The Moon&#8217;s &#8220;terminator,&#8221; 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 perfectly smooth. The grey plains are therefore not expanses of
+water now, nor were they in time past.<span class="pagenum"><a name="Page_52" id="Page_52">[Pg 52]</a></span> 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 formations 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
+destroyed 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.</p>
+
+<p>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 reappearance 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<span class="pagenum"><a name="Page_53" id="Page_53">[Pg 53]</a></span> 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
+disconcerting; it is hardly less disconcerting to observe 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 spectroscope, 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&#8217;s surface is variously estimated as <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">300</span>th of
+that of the Earth by Neison, and as <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">10000</span>th by W. H. Pickering. If the
+Moon possessed an atmosphere bearing the same proportion 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.</p>
+
+<p>The Moon is at the same mean distance from the<span class="pagenum"><a name="Page_54" id="Page_54">[Pg 54]</a></span> Sun as the Earth, and
+therefore, surface for surface, 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 reflection, and is only
+bright by contrast with the darkness 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 Mountain, the Sun
+shining full upon both, and the Moon appeared no brighter than the
+weathered rock. The best determinations of the <i>albedo</i> of the Moon, that
+is to say, of its reflective power, give it as 0&middot;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&#8217;s
+<i>albedo</i>, but the most probable estimate puts it as about 0&middot;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.</p>
+
+<p>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<span class="pagenum"><a name="Page_55" id="Page_55">[Pg 55]</a></span> 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 accomplish, 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 wave
+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
+immediately. In 1898, Prof. Very, following up Langley&#8217;s line of work,
+concluded that the temperature of the lunar soil must range through about
+350&deg; Centigrade, considerably exceeding 100&deg; at the height of the lunar
+day, and falling to about the temperature of liquid air during the<span class="pagenum"><a name="Page_56" id="Page_56">[Pg 56]</a></span> 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&#8217;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, therefore, that the
+time of rotation of a planet is an important factor in its habitability.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_57" id="Page_57">[Pg 57]</a></span></p>
+<h2><a name="CHAPTER_VI" id="CHAPTER_VI"></a>CHAPTER VI</h2>
+<p class="center"><span class="big">THE CANALS OF MARS</span></p>
+
+<p class="dropcap"><span class="caps">Both</span> 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.</p>
+
+<p>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
+inhabited, 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
+tremendous gulf of forty or more million miles of space.</p>
+
+<p>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<span class="pagenum"><a name="Page_58" id="Page_58">[Pg 58]</a></span>
+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 superhuman 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.</p>
+
+<p>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&mdash;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, however, 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 <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">70</span>th of the
+diameter of the Moon to <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">130</span>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<span class="pagenum"><a name="Page_59" id="Page_59">[Pg 59]</a></span> practical magnifying power of a telescope is
+never as great as the theoretical. In practice, a child&#8217;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 telescopes.</p>
+
+<p>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 unmistakable. 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 oppositions 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&#8217;s poles and increased in size as the winter of<span class="pagenum"><a name="Page_60" id="Page_60">[Pg 60]</a></span> 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.</p>
+
+<p>The next stage in the development of our knowledge of Mars must be
+ascribed to the two German astronomers, Beer and M&auml;dler, who made a series
+of drawings in the years 1830, 1832 and 1837, by means of a telescope of 4
+inches aperture, from which they were able to construct a chart of the
+entire globe. This chart may be 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<span class="pagenum"><a name="Page_61" id="Page_61">[Pg 61]</a></span> 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.</p>
+
+<p>But fortunately for progress, this sense of satisfaction 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<span class="pagenum"><a name="Page_62" id="Page_62">[Pg 62]</a></span> 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 <i>Memoirs</i>, bringing to light what
+appeared to be a new feature. His drawings not only showed the &#8220;lands&#8221; and
+&#8220;seas,&#8221; 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 &#8220;lands&#8221; in every direction. These narrow lines are the
+markings which have since been so celebrated as the &#8220;canals of Mars,&#8221; 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.</p>
+
+<p>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 astronomers of skill and experience had observed the
+planet in 1877 without detecting the network of<span class="pagenum"><a name="Page_63" id="Page_63">[Pg 63]</a></span> 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 &#8220;canals&#8221; vary much in their
+visibility, and &#8220;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.&#8221; This was the chief reason why Schiaparelli&#8217;s
+discoveries seemed at first to stand so entirely without corroboration;
+the &#8220;canals&#8221; 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 observatories, 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 confirmation of not a few of Schiaparelli&#8217;s &#8220;canals&#8221;; even
+in 1877 a few of the easiest and most conspicuous had been delineated by
+other astronomers before any rumour of Schiaparelli&#8217;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&#8217;s
+discoveries increased. It has now long been known<span class="pagenum"><a name="Page_64" id="Page_64">[Pg 64]</a></span> 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
+&#8220;canals of Mars&#8221; are not mere figments of the imagination, but have a real
+objective basis.</p>
+
+<p>As this controversy has passed away, another and a very different one has
+arisen out of an unfortunate 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
+&#8220;seas,&#8221; &#8220;continents,&#8221; &#8220;islands,&#8221; &#8220;isthmuses,&#8221; &#8220;straits&#8221; and the like,
+Schiaparelli had called the narrow lines he detected &#8220;<i>canali</i>&#8221;, that is
+to say &#8220;channels,&#8221; but without intending to convey the idea of artificial
+construction. Indeed, he himself was careful to point out that these
+designations &#8220;were not intended to prejudge the nature of the spot, and
+were nothing but an artifice for helping the memory and for shortening
+descriptions.&#8221; And he added, &#8220;We speak in the same way of the lunar seas,
+although we well know that there are no true seas on the Moon.&#8221; But
+&#8220;<i>canali</i>&#8221; was unhappily rendered in English as &#8220;canals,&#8221; instead of
+&#8220;channels.&#8221; &#8220;Channel&#8221; would have left the nature of the marking an open<span class="pagenum"><a name="Page_65" id="Page_65">[Pg 65]</a></span>
+question, but, in English, &#8220;canal&#8221; 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 believing that the &#8220;canals&#8221; are
+artificial constructions, or may they be merely natural formations?</p>
+
+<p>In 1894, Mr. Percival Lowell founded at Flagstaff, 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 &#8220;canals&#8221;; the discovery of a number of
+dark, round dots, termed by him &#8220;oases,&#8221; at the junctions of the &#8220;canals&#8221;;
+and the demonstration that the &#8220;canals&#8221; 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
+&#8220;canals&#8221; and &#8220;oases&#8221; quite precludes the possibility of their being
+natural formations. Hence there has arisen the second controversy: that on
+the nature of the &#8220;canals&#8221;; for Mr. Lowell considers that their presence
+proves the existence of inhabitants on Mars, who, by means of a Titanic
+system of irrigation, are fighting a losing battle against the gradual
+desiccation of their planet.</p>
+
+<p><span class="pagenum"><a name="Page_66" id="Page_66">[Pg 66]</a></span>In a paper published in the <i>International Scientific Review</i>, &#8220;Scientia,&#8221;
+in January, 1910, Mr. Lowell gave a summary of his argument.</p>
+
+<div class="blockquot"><p>&#8220;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 unlocking 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:</p>
+
+<p>&#8220;The surface of the planet to be very curiously meshed by a fine
+network of lines and spots.</p>
+
+<p>&#8220;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.</p>
+
+<p>&#8220;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<span class="pagenum"><a name="Page_67" id="Page_67">[Pg 67]</a></span> 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 <i>vera causa</i>.&#8221;</p></div>
+
+<p>Beside the bulky <i>Memoirs</i> 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 character: &#8220;<i>Mars</i>&#8221;; &#8220;<i>Mars and its
+Canals</i>&#8221;; and &#8220;<i>Mars as the Abode of Life</i>.&#8221; In these he shows that to the
+assiduity of the astronomer he adds the missionary&#8217;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 &#8220;Proofs of Life on Mars.&#8221; 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 &#8220;vegetable life could thus reveal itself directly, animal life
+could not.<span class="pagenum"><a name="Page_68" id="Page_68">[Pg 68]</a></span> 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.&#8221;</p>
+
+<div class="blockquot"><p>&#8220;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 regularly produced&#8221; (p. 188).</p>
+
+<p>&#8220;... 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&#8221; (p. 189).</p>
+
+<p>&#8220;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,
+unconscious way. But, as brain develops, they rise superior to such
+occasioning.... The last stage in the expression of life upon a
+planet&#8217;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&#8221; (pp. 204-11).</p>
+
+<p><span class="pagenum"><a name="Page_69" id="Page_69">[Pg 69]</a></span>&#8220;That intelligence should thus mutely communicate 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&#8221; (p. 215).</p></div>
+
+<p>For the moment, let us leave Prof. Lowell&#8217;s argument as he puts it.
+Whether we accept it or not, it remains that it is a marvellous
+achievement of the optician&#8217;s skill and the observer&#8217;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 existence 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&mdash;of water in the liquid state, of water in a sufficient quantity;
+and the final decision, for Mr. Lowell&#8217;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<span class="pagenum"><a name="Page_70" id="Page_70">[Pg 70]</a></span> the evolutionary
+development necessary to its attaining its highest expression,&mdash;that where
+the material living organism has become the tabernacle and instrument of
+the conscious intelligent spirit.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_71" id="Page_71">[Pg 71]</a></span></p>
+<h2><a name="CHAPTER_VII" id="CHAPTER_VII"></a>CHAPTER VII</h2>
+<p class="center"><span class="big">THE CONDITION OF MARS</span></p>
+
+<p class="dropcap"><span class="caps">The</span> planet Mars is the debatable ground between two opinions. Here, the
+two opposing views join issue; the controversy comes to a focus. The point
+in debate is whether certain markings&mdash;some linear, some circular&mdash;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 <i>artificial</i>, not
+natural structures.</p>
+
+<p>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
+living organism that designed them must have a reasonable mind lodged in a
+natural body. If, then, the &#8220;lines&#8221; and &#8220;circles&#8221; 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
+oases also premise the liquid that we call Water&mdash;water that flows and
+water utilized in cultivation. In this chapter we will leave out of count
+the first premiss&mdash;Man&mdash;and only deal with what concerns the second
+premiss&mdash;Water; with water that flows and is utilized in vegetation.</p>
+
+<p><span class="pagenum"><a name="Page_72" id="Page_72">[Pg 72 &amp; 73]</a></span></p>
+<p class="center">PLANETARY STATISTICS</p>
+
+<table border="0" cellpadding="0" cellspacing="0" summary="table">
+<tr><td class="btlrd">&nbsp;</td>
+    <td class="btrd" align="center">Minor<br />Planets.</td>
+    <td class="btrd" colspan="4" align="center">Inner Planets.</td>
+    <td class="btrd">&nbsp;</td>
+    <td class="btr" colspan="4" align="center">Outer Planets.</td></tr>
+<tr><td class="blrd">&nbsp;</td>
+    <td class="btrd" align="center">Ceres</td>
+    <td class="btr" align="center">Moon</td>
+    <td class="btr" align="center">Mercury</td>
+    <td class="btr" align="center">Mars</td>
+    <td class="btrd" align="center">Venus</td>
+    <td class="btrd" align="center">Earth</td>
+    <td class="btr" align="center">Uranus</td>
+    <td class="btr" align="center">Neptune</td>
+    <td class="btr" align="center">Saturn</td>
+    <td class="btr" align="center">Jupiter</td></tr>
+<tr><td class="btlrd"><span class="smcap">Proportions of the Planets</span>:&mdash;</td>
+    <td class="btrd" align="center">&nbsp;</td>
+    <td class="btr" align="center">&nbsp;</td>
+    <td class="btr" align="center">&nbsp;</td>
+    <td class="btr" align="center">&nbsp;</td>
+    <td class="btrd" align="center">&nbsp;</td>
+    <td class="btrd" align="center">&nbsp;</td>
+    <td class="btr" align="center">&nbsp;</td>
+    <td class="btr" align="center">&nbsp;</td>
+    <td class="btr" align="center">&nbsp;</td>
+    <td class="btr" align="center">&nbsp;</td></tr>
+<tr><td class="blrd">Diameter in miles</td>
+    <td class="brd" align="center">477</td>
+    <td class="br" align="center">2163</td>
+    <td class="br" align="center">3030</td>
+    <td class="br" align="center">4230</td>
+    <td class="brd" align="center">7700</td>
+    <td class="brd" align="center">7918</td>
+    <td class="br" align="center">31900</td>
+    <td class="br" align="center">34800</td>
+    <td class="br" align="center">73000</td>
+    <td class="br" align="center">86500</td></tr>
+<tr><td class="blrd"><span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp; &nbsp;</span>&oplus; = 1</td>
+    <td class="brd" align="center">0&middot;06</td>
+    <td class="br" align="center">0&middot;273</td>
+    <td class="br" align="center">0&middot;383</td>
+    <td class="br" align="center">0&middot;534</td>
+    <td class="brd" align="center">0&middot;972</td>
+    <td class="brd" align="center">1&middot;000</td>
+    <td class="br" align="center">4&middot;029</td>
+    <td class="br" align="center">4&middot;395</td>
+    <td class="br" align="center">9&middot;219</td>
+    <td class="br" align="center">10&middot;924</td></tr>
+<tr><td class="blrd">Surface,<span class="spacer">&nbsp;</span>&oplus; = 1</td>
+    <td class="brd" align="center">0&middot;004</td>
+    <td class="br" align="center">0&middot;075</td>
+    <td class="br" align="center">0&middot;147</td>
+    <td class="br" align="center">0&middot;285</td>
+    <td class="brd" align="center">0&middot;945</td>
+    <td class="brd" align="center">1&middot;000</td>
+    <td class="br" align="center">16&middot;2</td>
+    <td class="br" align="center">19&middot;3</td>
+    <td class="br" align="center">85&middot;0</td>
+    <td class="br" align="center">119&middot;3</td></tr>
+<tr><td class="blrd">Volume,<span class="spacer">&nbsp;</span>&oplus; = 1</td>
+    <td class="brd" align="center">0&middot;0002</td>
+    <td class="br" align="center">0&middot;02</td>
+    <td class="br" align="center">0&middot;06</td>
+    <td class="br" align="center">0&middot;15</td>
+    <td class="brd" align="center">0&middot;92</td>
+    <td class="brd" align="center">1&middot;00</td>
+    <td class="br" align="center">65&middot;</td>
+    <td class="br" align="center">85&middot;</td>
+    <td class="br" align="center">760&middot;</td>
+    <td class="br" align="center">1304&middot;</td></tr>
+<tr><td class="blrd">Density, Water = 1</td>
+    <td class="brd" align="center">2&middot;8 ?</td>
+    <td class="br" align="center">3&middot;39</td>
+    <td class="br" align="center">4&middot;72</td>
+    <td class="br" align="center">3&middot;92</td>
+    <td class="brd" align="center">4&middot;94</td>
+    <td class="brd" align="center">5&middot;55</td>
+    <td class="br" align="center">1&middot;22</td>
+    <td class="br" align="center">1&middot;11</td>
+    <td class="br" align="center">0&middot;72</td>
+    <td class="br" align="center">1&middot;32</td></tr>
+<tr><td class="blrd"><span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp; &nbsp;</span>&oplus; = 1</td>
+    <td class="brd" align="center">0&middot;5 ?</td>
+    <td class="br" align="center">0&middot;61</td>
+    <td class="br" align="center">0&middot;85</td>
+    <td class="br" align="center">0&middot;71</td>
+    <td class="brd" align="center">0&middot;89</td>
+    <td class="brd" align="center">1&middot;00</td>
+    <td class="br" align="center">0&middot;22</td>
+    <td class="br" align="center">0&middot;20</td>
+    <td class="br" align="center">0&middot;13</td>
+    <td class="br" align="center">0&middot;24</td></tr>
+<tr><td class="blrd">Mass,<span class="spacer">&nbsp; &nbsp;&nbsp;</span>&oplus;= 1</td>
+    <td class="brd" align="center">0&middot;0001</td>
+    <td class="br" align="center">0&middot;012</td>
+    <td class="br" align="center">0&middot;048</td>
+    <td class="br" align="center">0&middot;107</td>
+    <td class="brd" align="center">0&middot;820</td>
+    <td class="brd" align="center">1&middot;000</td>
+    <td class="br" align="center">14&middot;6</td>
+    <td class="br" align="center">17&middot;0</td>
+    <td class="br" align="center">94&middot;8</td>
+    <td class="br" align="center">317&middot;7</td></tr>
+<tr><td class="blrd">Gravity at surface, &oplus; = 1</td>
+    <td class="brd" align="center">0&middot;028</td>
+    <td class="br" align="center">0&middot;17</td>
+    <td class="br" align="center">0&middot;33</td>
+    <td class="br" align="center">0&middot;38</td>
+    <td class="brd" align="center">0&middot;87</td>
+    <td class="brd" align="center">1&middot;00</td>
+    <td class="br" align="center">0&middot;90</td>
+    <td class="br" align="center">0&middot;89</td>
+    <td class="br" align="center">1&middot;18</td>
+    <td class="br" align="center">2&middot;65</td></tr>
+<tr><td class="blrd">Rate of Fall, Feet in the First Second</td>
+    <td class="brd" align="center">0&middot;45</td>
+    <td class="br" align="center">2&middot;73</td>
+    <td class="br" align="center">5&middot;30</td>
+    <td class="br" align="center">6&middot;11</td>
+    <td class="brd" align="center">13&middot;99</td>
+    <td class="brd" align="center">16&middot;08</td>
+    <td class="br" align="center">14&middot;47</td>
+    <td class="br" align="center">14&middot;31</td>
+    <td class="br" align="center">18&middot;97</td>
+    <td class="br" align="center">42&middot;61</td></tr>
+<tr><td class="blrd">Albedo</td>
+    <td class="brd" align="center">0&middot;14</td>
+    <td class="br" align="center">0&middot;17</td>
+    <td class="br" align="center">0&middot;14</td>
+    <td class="br" align="center">0&middot;22</td>
+    <td class="brd" align="center">0&middot;76</td>
+    <td class="brd" align="center">0&middot;50 ?</td>
+    <td class="br" align="center">0&middot;60</td>
+    <td class="br" align="center">0&middot;52</td>
+    <td class="br" align="center">0&middot;72</td>
+    <td class="br" align="center">0&middot;62</td></tr>
+<tr><td class="blrd">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td></tr>
+<tr><td class="blrd"><span class="smcap">Details of Orbit</span>:&mdash;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td></tr>
+<tr><td class="blrd">Mean Distance from Sun in millions of miles</td>
+    <td class="brd" align="center">257&middot;1</td>
+    <td class="br" align="center">92&middot;9</td>
+    <td class="br" align="center">36&middot;0</td>
+    <td class="br" align="center">141&middot;5</td>
+    <td class="brd" align="center">67&middot;2</td>
+    <td class="brd" align="center">92&middot;9</td>
+    <td class="br" align="center">1781&middot;9</td>
+    <td class="br" align="center">2791&middot;6</td>
+    <td class="br" align="center">886&middot;0</td>
+    <td class="br" align="center">483&middot;3</td></tr>
+<tr><td class="blrd"><span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>Earth&#8217;s distance = 1</td>
+    <td class="brd" align="center">2&middot;767</td>
+    <td class="br" align="center">1&middot;000</td>
+    <td class="br" align="center">0&middot;387</td>
+    <td class="br" align="center">1&middot;524</td>
+    <td class="brd" align="center">0&middot;723</td>
+    <td class="brd" align="center">1&middot;000</td>
+    <td class="br" align="center">19&middot;183</td>
+    <td class="br" align="center">30&middot;055</td>
+    <td class="br" align="center">9&middot;539</td>
+    <td class="br" align="center">5&middot;203</td></tr>
+<tr><td class="blrd">Period of Revolution, in years</td>
+    <td class="brd" align="center">4&middot;60</td>
+    <td class="br" align="center">1&middot;00</td>
+    <td class="br" align="center">0&middot;24</td>
+    <td class="br" align="center">1&middot;88</td>
+    <td class="brd" align="center">0&middot;62</td>
+    <td class="brd" align="center">1&middot;00</td>
+    <td class="br" align="center">84&middot;02</td>
+    <td class="br" align="center">164&middot;78</td>
+    <td class="br" align="center">29&middot;46</td>
+    <td class="br" align="center">11&middot;86</td></tr>
+<tr><td class="blrd">Velocity, in miles per second</td>
+    <td class="brd" align="center">11&middot;1</td>
+    <td class="br" align="center">18&middot;5</td>
+    <td class="br" align="center">9&middot;7</td>
+    <td class="br" align="center">15&middot;0</td>
+    <td class="brd" align="center">21&middot;9</td>
+    <td class="brd" align="center">18&middot;5</td>
+    <td class="br" align="center">4&middot;2</td>
+    <td class="br" align="center">3&middot;4</td>
+    <td class="br" align="center">6&middot;0</td>
+    <td class="br" align="center">8&middot;1</td></tr>
+<tr><td class="blrd">Eccentricity</td>
+    <td class="brd" align="center">0&middot;0763</td>
+    <td class="br" align="center">0&middot;0168</td>
+    <td class="br" align="center">0&middot;2056</td>
+    <td class="br" align="center">0&middot;0933</td>
+    <td class="brd" align="center">0&middot;0068</td>
+    <td class="brd" align="center">0&middot;0168</td>
+    <td class="br" align="center">0&middot;0463</td>
+    <td class="br" align="center">0&middot;0090</td>
+    <td class="br" align="center">0&middot;0561</td>
+    <td class="br" align="center">0&middot;0483</td></tr>
+<tr><td class="blrd">Aphelion Distance, Perihelion = 1</td>
+    <td class="brd" align="center">1&middot;157</td>
+    <td class="br" align="center">1&middot;034</td>
+    <td class="br" align="center">1&middot;517</td>
+    <td class="br" align="center">1&middot;207</td>
+    <td class="brd" align="center">1&middot;013</td>
+    <td class="brd" align="center">1&middot;034</td>
+    <td class="br" align="center">1&middot;097</td>
+    <td class="br" align="center">1&middot;018</td>
+    <td class="br" align="center">1&middot;107</td>
+    <td class="br" align="center">1&middot;101</td></tr>
+<tr><td class="blrd">Inclination of Equator to Orbit</td>
+    <td class="brd" align="center">(?)</td>
+    <td class="br" align="center">1&deg;&middot;32&acute;</td>
+    <td class="br" align="center">(?)</td>
+    <td class="br" align="center">24&deg;&middot;0&acute;</td>
+    <td class="brd" align="center">(?)</td>
+    <td class="brd" align="center">23&deg;&middot;27&acute;</td>
+    <td class="br" align="center">(?)</td>
+    <td class="br" align="center">(?)</td>
+    <td class="br" align="center">26&deg;&middot;49&acute;</td>
+    <td class="br" align="center">3&deg;&middot;5&acute;</td></tr>
+<tr><td class="blrd">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">d h m</td>
+    <td class="br" align="center">d</td>
+    <td class="br" align="center">h  m  s</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="brd" align="center">h  m  s</td>
+    <td class="br" align="center">h  m</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">h  m</td>
+    <td class="br" align="center">h  m</td></tr>
+<tr><td class="blrd">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td></tr>
+<tr><td class="blrd">Rotation period</td>
+    <td class="brd" align="center">(?)</td>
+    <td class="br" align="center">27&middot;7&middot;43</td>
+    <td class="br" align="center">88(?)</td>
+    <td class="br" align="center">24&middot;37&middot;23</td>
+    <td class="brd" align="center">(?)</td>
+    <td class="brd" align="center">23&middot;56&middot;4</td>
+    <td class="br" align="center">9&middot;30(?)</td>
+    <td class="br" align="center">(?)</td>
+    <td class="br" align="center">10&middot;14&plusmn;</td>
+    <td class="br" align="center">9&middot;55&plusmn;</td></tr>
+<tr><td class="blrd">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td></tr>
+<tr><td class="blrd"><span class="smcap">Atmosphere</span>, assuming the total mass of the atmosphere<br />
+<span style="margin-left: 1em;">to be proportional to the mass of the planet:&mdash;</span></td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td></tr>
+<tr><td class="blrd">Pressure at the surface in lb. per sq. inch.</td>
+    <td class="brd" align="center">0&middot;014</td>
+    <td class="br" align="center">0&middot;40</td>
+    <td class="br" align="center">1&middot;6</td>
+    <td class="br" align="center">2&middot;1</td>
+    <td class="brd" align="center">11&middot;1</td>
+    <td class="brd" align="center">14&middot;7</td>
+    <td class="br" align="center">11&middot;9</td>
+    <td class="br" align="center">11&middot;5</td>
+    <td class="br" align="center">19&middot;4</td>
+    <td class="br" align="center">103&middot;8</td></tr>
+<tr><td class="blrd"><span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>" &nbsp;&nbsp; in &#8220;atmospheres&#8221;</td>
+    <td class="brd" align="center">0&middot;0009</td>
+    <td class="br" align="center">0&middot;027</td>
+    <td class="br" align="center">0&middot;108</td>
+    <td class="br" align="center">0&middot;143</td>
+    <td class="brd" align="center">0&middot;754</td>
+    <td class="brd" align="center">1&middot;000</td>
+    <td class="br" align="center">0&middot;81</td>
+    <td class="br" align="center">0&middot;78</td>
+    <td class="br" align="center">1&middot;32</td>
+    <td class="br" align="center">7&middot;06</td></tr>
+<tr><td class="blrd">Level of half surface pressure in miles</td>
+    <td class="brd" align="center">119&middot;0</td>
+    <td class="br" align="center">19&middot;6</td>
+    <td class="br" align="center">10&middot;1</td>
+    <td class="br" align="center">8&middot;8</td>
+    <td class="brd" align="center">3&middot;8</td>
+    <td class="brd" align="center">3&middot;3</td>
+    <td class="br" align="center">3&middot;7</td>
+    <td class="br" align="center">3&middot;8</td>
+    <td class="br" align="center">2&middot;8</td>
+    <td class="br" align="center">1&middot;3</td></tr>
+<tr><td class="blrd">Boiling point of water at the surface</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">22&deg;C</td>
+    <td class="br" align="center">53&deg;C</td>
+    <td class="br" align="center">53&deg;C</td>
+    <td class="brd" align="center">92&deg;C</td>
+    <td class="brd" align="center">100&deg;C</td>
+    <td class="br" align="center">94&deg;C</td>
+    <td class="br" align="center">93&deg;C</td>
+    <td class="br" align="center">108&deg;C</td>
+    <td class="br" align="center">166&deg;C</td></tr>
+<tr><td class="blrd">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td></tr>
+<tr><td class="blrd"><span class="smcap">Temperature</span>:&mdash;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="brd" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td>
+    <td class="br" align="center">&nbsp;</td></tr>
+<tr><td class="blrd">Light and heat received from Sun, &oplus; = 1</td>
+    <td class="brd" align="center">0&middot;13</td>
+    <td class="br" align="center">1&middot;00</td>
+    <td class="br" align="center">6&middot;67</td>
+    <td class="br" align="center">0&middot;43</td>
+    <td class="brd" align="center">1&middot;91</td>
+    <td class="brd" align="center">1&middot;00</td>
+    <td class="br" align="center">0&middot;003</td>
+    <td class="br" align="center">0&middot;001</td>
+    <td class="br" align="center">0&middot;011</td>
+    <td class="br" align="center">0&middot;037</td></tr>
+<tr><td class="blrd">Reciprocal of square-root of distance, &oplus; = 1</td>
+    <td class="brd" align="center">0&middot;60</td>
+    <td class="br" align="center">1&middot;00</td>
+    <td class="br" align="center">1&middot;61</td>
+    <td class="br" align="center">0&middot;81</td>
+    <td class="brd" align="center">1&middot;18</td>
+    <td class="brd" align="center">1&middot;00</td>
+    <td class="br" align="center">0&middot;23</td>
+    <td class="br" align="center">0&middot;18</td>
+    <td class="br" align="center">0&middot;32</td>
+    <td class="br" align="center">0&middot;44</td></tr>
+<tr><td class="blrd">Equatorial temp. of ideal planet, Absolute</td>
+    <td class="brd" align="center">188</td>
+    <td class="br" align="center">312&deg;</td>
+    <td class="br" align="center">502&deg;</td>
+    <td class="br" align="center">253&deg;</td>
+    <td class="brd" align="center">368&deg;</td>
+    <td class="brd" align="center">312&deg;</td>
+    <td class="br" align="center">71&deg;</td>
+    <td class="br" align="center">56&deg;</td>
+    <td class="br" align="center">101&deg;</td>
+    <td class="br" align="center">137&deg;</td></tr>
+<tr><td class="blrd"><span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>Centigrade</td>
+    <td class="brd" align="center">-65</td>
+    <td class="br" align="center">+39</td>
+    <td class="br" align="center">+229</td>
+    <td class="br" align="center">-20</td>
+    <td class="brd" align="center">+95</td>
+    <td class="brd" align="center">+39</td>
+    <td class="br" align="center">-202</td>
+    <td class="br" align="center">-217</td>
+    <td class="br" align="center">-172</td>
+    <td class="br" align="center">-136</td></tr>
+<tr><td class="blrd">Average temp. of ideal planet, Absolute</td>
+    <td class="brd" align="center">174</td>
+    <td class="br" align="center">290</td>
+    <td class="br" align="center">467</td>
+    <td class="br" align="center">235</td>
+    <td class="brd" align="center">342</td>
+    <td class="brd" align="center">290</td>
+    <td class="br" align="center">66</td>
+    <td class="br" align="center">52</td>
+    <td class="br" align="center">94</td>
+    <td class="br" align="center">127</td></tr>
+<tr><td class="blrd"><span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>Centigrade</td>
+    <td class="brd" align="center">-99</td>
+    <td class="br" align="center">+17</td>
+    <td class="br" align="center">+194</td>
+    <td class="br" align="center">-38</td>
+    <td class="brd" align="center">+69</td>
+    <td class="brd" align="center">+17</td>
+    <td class="br" align="center">-207</td>
+    <td class="br" align="center">-221</td>
+    <td class="br" align="center">-179</td>
+    <td class="br" align="center">-146</td></tr>
+<tr><td class="blrd">Upper limit under zenith sun, Absolute</td>
+    <td class="brd" align="center">248</td>
+    <td class="br" align="center">412</td>
+    <td class="br" align="center">664</td>
+    <td class="br" align="center">337</td>
+    <td class="brd" align="center">486</td>
+    <td class="brd" align="center">412</td>
+    <td class="br" align="center">94</td>
+    <td class="br" align="center">74</td>
+    <td class="br" align="center">133</td>
+    <td class="br" align="center">180</td></tr>
+<tr><td class="blrd"><span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>Centigrade</td>
+    <td class="brd" align="center">-25</td>
+    <td class="br" align="center">+139</td>
+    <td class="br" align="center">+391</td>
+    <td class="br" align="center">+64</td>
+    <td class="brd" align="center">+213</td>
+    <td class="brd" align="center">+139</td>
+    <td class="br" align="center">-179</td>
+    <td class="br" align="center">-199</td>
+    <td class="br" align="center">-140</td>
+    <td class="br" align="center">-93</td></tr>
+<tr><td class="blrd">Average temp. of equivalent disc, Absolute</td>
+    <td class="brd" align="center">223</td>
+    <td class="br" align="center">371</td>
+    <td class="br" align="center">598</td>
+    <td class="br" align="center">300</td>
+    <td class="brd" align="center">438</td>
+    <td class="brd" align="center">371</td>
+    <td class="br" align="center">84</td>
+    <td class="br" align="center">67</td>
+    <td class="br" align="center">120</td>
+    <td class="br" align="center">162</td></tr>
+<tr><td class="bblrd"><span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>"<span class="spacer">&nbsp;</span>Centigrade</td>
+    <td class="bbrd" align="center">-50</td>
+    <td class="bbr" align="center">+98</td>
+    <td class="bbr" align="center">+325</td>
+    <td class="bbr" align="center">+27</td>
+    <td class="bbrd" align="center">+165</td>
+    <td class="bbrd" align="center">+98</td>
+    <td class="bbr" align="center">-189</td>
+    <td class="bbr" align="center">-206</td>
+    <td class="bbr" align="center">-153</td>
+    <td class="bbr" align="center">-111</td></tr></table>
+
+<p><span class="pagenum"><a name="Page_74" id="Page_74">[Pg 74]</a></span>For in regard to this particular premiss we can do away with hypothesis,
+and deal only with certain physical facts that are not controversial and
+are not in dispute.</p>
+
+<p>The first of this series of facts concerning Mars about which there can be
+no controversy or dispute relates to its size and mass. As the foregoing
+Table shows, it comes between the Moon and the Earth in these respects.</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
+<p>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<span class="pagenum"><a name="Page_75" id="Page_75">[Pg 75]</a></span> 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&deg; C., sank to 9&deg; 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 120 pulsations a minute instead of his normal 66. When Mr.
+Glaisher and Mr. Coxwell made their celebrated ascent between 1 and 2
+o&#8217;clock on the afternoon of September 5, 1861, they found that at a height
+of 21,000 feet the temperature sank to -10&middot;4&deg;; at 26,000 feet to -15&middot;2&deg;;
+and at 39,000 feet the temperature was down to -16&middot;0&deg; C. At this height
+the rarefaction of the air was so great and the cold so intense that Mr.
+Glaisher fainted, and Mr. Coxwell&#8217;s hands being rendered numb and useless
+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
+<span class="pagenum"><a name="Page_76" id="Page_76">[Pg 76]</a></span>attained this height they were far above all cloud or mist, and the Sun&#8217;s
+rays fell full upon them. The Sun&#8217;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.</p>
+
+<p>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 with moisture, 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<span class="pagenum"><a name="Page_77" id="Page_77">[Pg 77]</a></span> 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
+ascending current on Mars would therefore take <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">0&middot;38</span> &times;
+<span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">0&middot;38</span> =
+<span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">0&middot;145</span>,
+or seven times as long to attain the same relative expansion as on the Earth.</p>
+
+<p>The winds of Mars are therefore sluggish, and precipitation is slight. So
+far at least it resembles</p>
+
+<p class="poem">&#8220;The island valley of Avilion;<br />
+Where falls not hail, or rain, or any snow,<br />
+Nor ever wind blows loudly;&#8221;</p>
+
+<p>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
+&#8220;hurricane-swept.&#8221; There are no hurricanes on Mars; its fiercest winds can
+never exceed in violence what a sailor would call a &#8220;capful.&#8221;</p>
+
+<p>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 circulation, and more
+violent its disturbances; the feebler the action of gravity the more
+languid the circulation, and the slighter the disturbances.</p>
+
+<p>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<span class="pagenum"><a name="Page_78" id="Page_78">[Pg 78]</a></span> immense thickness of an obscuring medium. And yet the details of the
+surface are seen with remarkable distinctness; not as clearly indeed as we
+can see those of the Moon, but nearly so. For instance, the &#8220;canals&#8221;
+appear to have a breadth of from 15 to 20 miles, corresponding to <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">16</span>th,
+and <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">12</span>th, 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 transparent.
+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 practically without an atmosphere, so the great detail visible on
+Mars bears unmistakable testimony to the slightness of the atmospheric
+veil around that planet.</p>
+
+<p>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 <span class="pagenum"><a name="Page_79" id="Page_79">[Pg 79]</a></span>atmosphere. With planets, as with persons, it is
+the most powerful that gets the lion&#8217;s share: &#8220;to him that hath it is
+given, and from him that hath not is taken away even that which he seemeth
+to have.&#8221; 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 <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">9</span>th of that of the Earth; exactly 0&middot;107. It is
+distributed over a smaller surface, 0&middot;285. Consequently the amount of air
+above each square inch of Martian surface is 0&middot;107 &divide; 0&middot;285 = 0&middot;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&middot;38 &times; 0&middot;38 = 0&middot;145; or
+one-seventh of the column of air resting on a square inch of the Earth&#8217;s
+surface. The pressure at the surface of Mars will therefore be 2&middot;1 lb.;
+and the aneroid barometer would read 4&middot;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&middot;4 inches.)</p>
+
+<p>But a pressure of 2&middot;1 lb. on the square inch is far less than that
+experienced by Coxwell and Glaisher in their great ascent; it is about
+one-half the pressure that is experienced on the top of the very highest
+terrestrial mountains. But the habitable<span class="pagenum"><a name="Page_80" id="Page_80">[Pg 80]</a></span> regions of the Earth do not
+extend even so far upward as to the level of a pressure of 7&middot;3 lb. 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 atmosphere were diminished to one
+half the pressure on the summit of the highest earthly mountain.</p>
+
+<p>The tenuity of the atmosphere on Mars has another consequence. Here water
+freezes at 0&deg; C. and boils at 100&deg; C.; so that for one hundred degrees it
+remains in a liquid condition. On Mars, under the assumed conditions,
+water would boil at 53&deg; 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.</p>
+
+<p>The above estimate of the density of the atmosphere 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<span class="pagenum"><a name="Page_81" id="Page_81">[Pg 81]</a></span> 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.</p>
+
+<p>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 determining 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&middot;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&middot;2 lb., and the aneroid barometer would<span class="pagenum"><a name="Page_82" id="Page_82">[Pg 82]</a></span> read 2&middot;5
+inches; and water would boil at 44&deg; 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 diminished.</p>
+
+<p>These statistics may seem rather dull and tiresome, 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 unchanged. 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 visiting 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 <i>Gulliver&#8217;s
+Travels</i>, made the Lilliputians take a truer view of the effect of the
+alteration 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<span class="pagenum"><a name="Page_83" id="Page_83">[Pg 83]</a></span> 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&mdash;the planet &#8220;Victoria.&#8221; 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&#8217;s new planet &#8220;Victoria,&#8221; would be less
+than half the present Earth in diameter; it would be considerably smaller
+than Mars. But &#8220;the rest of the world&#8221; would be 0&middot;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
+&#8220;Victoria.&#8221; 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 &#8220;rest of the world&#8221; could spare our
+fraction of the planet much better than we could spare theirs.</p>
+
+<p>This is a principle which applies to worlds <span class="pagenum"><a name="Page_84" id="Page_84">[Pg 84]</a></span>anywhere; 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.</p>
+
+<p>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.</p>
+
+<p>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<span class="pagenum"><a name="Page_85" id="Page_85">[Pg 85]</a></span> 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?</p>
+
+<p>The mean temperature of our Earth is supposed to be about 60&deg;F., or 16&deg;C.
+Three-sevenths of this would give us 7&deg;C. as the mean temperature of Mars,
+which would signify a planet not impossible 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&deg; on the
+Centigrade scale; the temperature of the Earth on the absolute scale
+therefore should be taken as 289&deg;, and three-sevenths of this would give
+124&deg; of absolute temperature. But this is 149&deg; below freezing-point, and
+no life could exist on a planet under such conditions.</p>
+
+<p>But the mean temperature of Mars cannot be computed 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&#8217;s Law, the radiation varies for
+a perfect radiator with the 4th power of the absolute temperature; so that
+if Mars were at 124&deg; abs., while the Earth were at 289&deg; 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 expenditure<span class="pagenum"><a name="Page_86" id="Page_86">[Pg 86]</a></span> 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.</p>
+
+<p>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 reflecting one-eighth would
+be, as shown in the Table, 64&deg;C. This would imply that the temperature on
+the dark side of the planet was very nearly at the absolute zero. &#8220;If we
+regard Mars as resembling our Moon, and take the Moon&#8217;s effective average
+temperature as 297&deg; abs., the corresponding temperature for Mars is 240&deg;
+abs., and the highest temperature is four-fifths of 337&deg; = 270&deg; abs. But
+the surface of Mars has probably a higher coefficient of absorption than
+the surface of the Moon&mdash;it certainly has for light&mdash;so that we may put
+his effective average temperature, on this supposition, some few degrees
+above 240&deg; 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<span class="pagenum"><a name="Page_87" id="Page_87">[Pg 87]</a></span>
+temperature of his surface is everywhere below the freezing-point of
+water.&#8221;<small><a name="f14.1" id="f14.1" href="#f14">[14]</a></small> As the atmospheric circulation on Mars must be languid, and
+the atmosphere itself is very rare, the general condition 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.</p>
+
+<p>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&#189;&deg;&mdash;in other words just
+outside the Arctic Circle&mdash;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, latitude 64&#189;&deg; 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.</p>
+
+<p><span class="pagenum"><a name="Page_88" id="Page_88">[Pg 88]</a></span>Now the mean temperature of latitude 64&#189;&deg;, say the latitude of
+Archangel, is just about freezing-point (0&deg;C.), while that of the equator
+is about 28&deg;C. We should therefore expect from this a difference between
+the mean temperatures of the Earth and Mars of 28&deg;; that is to say, as the
+Earth stands at 16&deg;C, Mars would be at -12&deg;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 moving 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&deg; which we
+find on the Earth between 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&deg; as the true equivalent difference between the mean
+temperatures of Singapore and Archangel; i.e. of the Earth and Mars; and
+Mars would stand at -40&deg;C. The<span class="pagenum"><a name="Page_89" id="Page_89">[Pg 89]</a></span> closeness with which this figure agrees
+with that reached by Prof. Poynting suggests that it is a fair
+approximation to the correct figure.</p>
+
+<p>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 condition 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 telescope 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&mdash;presumed to be water or vegetation&mdash;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 Protonilus is seen as a
+dark canal. Now the Protonilus is in North Lat. 42&deg;, and on the date of
+observation&mdash;September 28, 1909&mdash;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<span class="pagenum"><a name="Page_90" id="Page_90">[Pg 90]</a></span> 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.</p>
+
+<p>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 temperatures has been laid down
+for every 10&deg; of latitude on the Earth, derived as far as possible from
+accepted isothermals in continental countries in the northern hemisphere.
+From this curve ordinates have been drawn at each 10&deg;, upward to show
+average deviation from the mean temperature for the hottest part of the
+day in summer, downward 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 temperature
+of the Earth has been taken as 16&deg;C, and as representing that prevailing
+in about 42&deg; lat. The diagram shows that the maximum temperature of no
+place upon the Earth&#8217;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 state.</p>
+
+<p>&nbsp;<span class="pagenum"><a name="Page_91" id="Page_91">[Pg 91]</a></span></p>
+<div class="figcenter"><img src="images/thermo_tmb.jpg" alt="" /><br />
+<a href="images/thermo.jpg"><small>Larger Image</small></a></div>
+<p class="center"><span class="smcap">Thermographs of the Earth and Mars</span></p>
+<p>&nbsp;</p>
+
+<p><span class="pagenum"><a name="Page_92" id="Page_92">[Pg 92]</a></span>In constructing a similar diagram for Mars, three modifications have to be
+made. First of all, the mean temperature of the planet must be
+considerably 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 atmosphere, and partly on account of the length of the Martian year.</p>
+
+<p>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 temperature 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<span class="pagenum"><a name="Page_93" id="Page_93">[Pg 93]</a></span> every night, far transcend the
+bitterest extremes of our own polar regions.</p>
+
+<p>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 confined 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 &#8220;seas&#8221; 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?</p>
+
+<p>The great American astronomer, Prof. Newcomb, gave in <i>Harper&#8217;s Weekly</i>
+for July 25, 1908, an admirable summary of the verdict of science as to
+the character of the meteorology of Mars. &#8220;The most careful calculation
+shows that if there are any considerable bodies of water on our
+neighbouring<span class="pagenum"><a name="Page_94" id="Page_94">[Pg 94]</a></span> 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 substitute little motions of an air thinner than that on
+the top of the Himalayas, and we shall have a general description of
+Martian meteorology.&#8221;</p>
+
+<p>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 warmest 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<span class="pagenum"><a name="Page_95" id="Page_95">[Pg 95]</a></span> 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.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_96" id="Page_96">[Pg 96]</a></span></p>
+<h2><a name="CHAPTER_VIII" id="CHAPTER_VIII"></a>CHAPTER VIII</h2>
+<p class="center"><span class="big">THE ILLUSIONS OF MARS</span></p>
+
+<p class="dropcap"><span class="caps">The</span> two preceding chapters have led to two opposing, two incompatible
+conclusions. In Chapter VI, a summary was given of Prof. Lowell&#8217;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.</p>
+
+<p>Prof. Lowell argues that the appearance of the network of lines and spots
+formed by the canals and oases, and its regular behaviour, &#8220;preclude its
+causation on such a scale by any natural process,&#8221; his assumption being
+that he has obtained finality in his seeing of the planet, and that no
+improvement 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.</p>
+
+<p>But the history of our knowledge of the planet&#8217;s surface teaches us a
+different lesson. Two small<span class="pagenum"><a name="Page_97" id="Page_97">[Pg 97]</a></span> objects appear repeatedly on the drawings
+made by Beer and M&auml;dler 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 M&auml;dler undoubtedly portrayed the planet as actually they
+saw it. The one marking was named by Schiaparelli the Lacus Solis, the
+other, the Sinus Sab&aelig;us, and they are two of the best known and most
+easily recognized of the planet&#8217;s features; so that it is easy to trace
+the growth 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 M&auml;dler, 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<span class="pagenum"><a name="Page_98" id="Page_98">[Pg 98]</a></span> attention to these two objects,
+and a mass of complicated structure has been brought out in the regions
+which were so simple in the sight of Beer and M&auml;dler, so that not a trace
+of resemblance remains between the two objects that to them appeared
+indistinguishable.</p>
+
+<p>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
+M&auml;dler, to the 33-inch which Antoniadi used in 1909, is it reasonable to
+suppose that Prof. Lowell&#8217;s oases will refuse to yield to such
+improvement, and will all still show themselves as uniform spots,
+precisely circular in outline? It is clear that Beer and M&auml;dler 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 M&auml;dler
+was that they were too small to be defined in their instruments; their
+minor irregularities were therefore invisible, and their apparent
+circularity covered detail of an altogether different form.</p>
+
+<p>Beer and M&auml;dler only drew two such spots; Lowell shows about two hundred.
+Beer and M&auml;dler&#8217;s two spots seemed to them exactly alike; these two spots
+as we see them to-day have no <span class="pagenum"><a name="Page_99" id="Page_99">[Pg 99]</a></span>resemblance to each other. Prof. Lowell&#8217;s
+two hundred oases, with few exceptions, seem all of the same character; is
+it possible to suppose, if telescopes 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 Sab&aelig;us? If a novice begins to work upon Mars with a small telescope,
+he will draw the Lacus Solis and the Sinus Sab&aelig;us 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 &#8220;seeing.&#8221;</p>
+
+<p>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.</p>
+
+<p>A telegraph wire seen against a background of a bright cloud can be
+discerned at an amazing distance&mdash;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 defined vision, as would
+be<span class="pagenum"><a name="Page_100" id="Page_100">[Pg 100]</a></span> 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.</p>
+
+<p>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 defined; that is to say for the
+spots to be seen as two separate objects.</p>
+
+<p><span class="pagenum"><a name="Page_101" id="Page_101">[Pg 101]</a></span>It will be seen that there is a wide range between objects that are large
+enough to be quite unmistakably perceived, and objects which are large
+enough to have their true outline really defined. 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&mdash;the straight line and the circular dot.</p>
+
+<p>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 defined, but for any sight the range between the two will be of the
+order just indicated.</p>
+
+<p>Prof. Lowell has drawn attention to the &#8220;strangely economic character of
+both the canals and oases in the matter of form.&#8221; 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. &#8220;The circle is
+the figure which encloses the maximum area for the minimum average
+distance from its centre to any point situated within it;&#8221;<span class="pagenum"><a name="Page_102" id="Page_102">[Pg 102]</a></span> 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.</p>
+
+<p>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 &#8220;canals,&#8221; 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 defined
+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 &#8220;canal-like&#8221; in character. Wherever there is a
+small aggregation of these minute markings, an impression will be<span class="pagenum"><a name="Page_103" id="Page_103">[Pg 103]</a></span> given
+of a circular spot, or, to use Prof. Lowell&#8217;s nomenclature, an &#8220;oasis.&#8221; If
+the aggregation be greater still and more extended, we shall have a shaded
+area&mdash;a &#8220;sea.&#8221;</p>
+
+<p>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,
+&#8220;without parts or magnitude.&#8221; 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 &#8220;spurious,&#8221; for the greater the aperture
+of the telescope the smaller the apparent disc.</p>
+
+<p>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 telescope;
+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.</p>
+
+<p>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<span class="pagenum"><a name="Page_104" id="Page_104">[Pg 104]</a></span> irregularities and
+simplifying details; a third factor in producing the two simplest types of
+form&mdash;the straight line and the circular dot.</p>
+
+<p>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 appearance 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&mdash;such as Antoniadi, Barnard, Cerulli,
+Denning, Millochau, Molesworth, Phillips, Stanley Williams and
+others&mdash;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 geometrical network.</p>
+
+<p>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 described by H. G. Wells in the
+<i>War of the Worlds</i>.</p>
+
+<p><span class="pagenum"><a name="Page_105" id="Page_105">[Pg 105]</a></span>There are too many oversights in the canal theory.</p>
+
+<p>Thus no source is indicated for the moisture supposed 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 polar deposits could be recruited.</p>
+
+<p>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 conveyance of water from the pole towards the equator;
+although, as this was &#8220;uphill,&#8221; 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
+conveyed 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<span class="pagenum"><a name="Page_106" id="Page_106">[Pg 106]</a></span> 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.</p>
+
+<p>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 significance 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 continent, and its breadth would not be
+uniform from end to end, but the region fertilized would grow narrower
+with increase of distance from the fountain head of the canal.</p>
+
+<p>Under what conditions can we see straight lines,<span class="pagenum"><a name="Page_107" id="Page_107">[Pg 107]</a></span> perfectly uniform from
+end to end, spring into existence, 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 defined. 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 experiments; they were keen-sighted,
+well drilled; accustomed to do what they were told without asking
+questions; and they knew nothing whatsoever of astronomy, certainly
+nothing about Mars.</p>
+
+<p>A diagram was hung up, based upon some drawing or other of the planet made
+by Schiaparelli, 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<span class="pagenum"><a name="Page_108" id="Page_108">[Pg 108]</a></span> 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 markings which had given rise to them were
+perceived in their true forms.</p>
+
+<p>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.</p>
+
+<p>A little later, in his work &#8220;<i>Marte nel 1896-7</i>,&#8221; Dr. Cerulli
+independently arrived at the same conclusion, and wrote: &#8220;These lines are
+formed by the eye ... which utilizes ... the dark elements which it finds
+along certain directions&#8221;; and &#8220;a large number of these elements forms a
+broad band&#8221;; and &#8220;a smaller number of them gives rise to a narrow line.&#8221;
+Also, &#8220;the <span class="pagenum"><a name="Page_109" id="Page_109">[Pg 109]</a></span>marvellous 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.&#8221; In 1907, Prof. Newcomb 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&#8217;s Remembrancer,
+sent to me, in his 91st year, the following note as affording an apt
+illustration of the true nature of the canaliform markings on Mars:</p>
+
+<p>&#8220;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 supporting the telegraph
+wires.&#8221;</p>
+
+<p>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.</p>
+
+<p>But such observations have been made upon the planet itself with just the
+same result. Observers have seen streaks upon Mars&mdash;knotted, broken,
+irregular, full of detail&mdash;and when the planet has receded to a greater
+distance, the very same<span class="pagenum"><a name="Page_110" id="Page_110">[Pg 110]</a></span> 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 disappear. In this, and not in any gigantic engineering 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.</p>
+
+<p>This chapter is entitled &#8220;The Illusions of Mars.&#8221; 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.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_111" id="Page_111">[Pg 111]</a></span></p>
+<h2><a name="CHAPTER_IX" id="CHAPTER_IX"></a>CHAPTER IX</h2>
+<p class="center"><span class="big">VENUS, MERCURY AND THE ASTEROIDS</span></p>
+
+<p class="dropcap"><span class="caps">Of</span> all the planets, Venus appears, to the unassisted eye, by far the
+loveliest. When seen in the early morning before sunrise&mdash;its &#8220;western
+elongation&#8221;&mdash;or after sundown in the evening&mdash;its &#8220;eastern
+elongation&#8221;&mdash;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.</p>
+
+<p>But in the telescope Venus appears less satisfying. 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<span class="pagenum"><a name="Page_112" id="Page_112">[Pg 112]</a></span> clear-cut
+outlines and black shadows of the Moon have no place here, nor do the
+ruddy plains and blue-grey &#8220;seas&#8221; 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.</p>
+
+<p>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 and mass, and therefore probably in
+atmospheric equipment. If we assume that the atmosphere of any planet is
+in direct proportion to its mass&mdash;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&mdash;the atmosphere of Venus would have a pressure of
+about 11&middot;2 lb. 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.</p>
+
+<p>But Venus is nearer to the Sun than the Earth, and receives nearly double
+the light and heat. Its theoretical equatorial temperature is 368&deg;abs., or
+95&deg;C, and its corresponding mean temperature<span class="pagenum"><a name="Page_113" id="Page_113">[Pg 113]</a></span> is 69&deg; C. But water under a
+pressure of 11&middot;2 lb. will boil at 93&deg; 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.</p>
+
+<p>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 perpetual 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.</p>
+
+<p><span class="pagenum"><a name="Page_114" id="Page_114">[Pg 114]</a></span>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&mdash;water that flows.</p>
+
+<p>But there is still a possible hindrance to life on Venus, a hindrance that
+actually exists in the case of Mercury.</p>
+
+<p>Mercury, the &#8220;Twinkler,&#8221; 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 neighbour, and a certain number of markings have been
+detected upon its disc, markings which, like those of the Moon, appear to
+be permanent.</p>
+
+<p>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<span class="pagenum"><a name="Page_115" id="Page_115">[Pg 115]</a></span> range of temperatures, the low temperature at which water will
+boil.</p>
+
+<p>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 maintained 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.</p>
+
+<p>A reference to Prof. Poynting&#8217;s figures shows that the mean temperature of
+Mercury must approximate to 194&deg; C., while water will boil at 40&deg; 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.</p>
+
+<p>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.</p>
+
+<p><span class="pagenum"><a name="Page_116" id="Page_116">[Pg 116]</a></span>Since Mercury wears no concealing veil of atmosphere, 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 say, 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 development of the
+theory of tidal friction, puts the condition of Mercury in quite a new
+light. No alternation 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&deg; C. above freezing-point, to 270&deg; C. below. It is true
+that between the two hemispheres there is a &#8220;debatable land,&#8221; for, owing
+to the ellipticity of the orbit, the face turned to the Sun is not exactly
+the same at all times, and a region about 47&deg; 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.</p>
+
+<p>The conditions of Mercury are so unfavourable for life that, even if this
+remarkable relation of<span class="pagenum"><a name="Page_117" id="Page_117">[Pg 117]</a></span> 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.</p>
+
+<p>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.</p>
+
+<p>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&deg; 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.</p>
+
+<p>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<span class="pagenum"><a name="Page_118" id="Page_118">[Pg 118]</a></span> 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 habitability
+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 inhabited
+is a matter at present entirely beyond our knowledge.</p>
+
+<p>A page or two back we touched lightly on the eccentricity of the orbit of
+Mercury&mdash;lightly, because it was not the chief factor in disabling the
+planet for habitation. But the condition introduced 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 2&#189; 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 proportional change in this one planet&#8217;s condition than we should
+experience if our Earth were transported from its own orbit to that of
+Mars.</p>
+
+<p>But there are other members of the solar system whose orbits are so
+elongated that that of Mercury<span class="pagenum"><a name="Page_119" id="Page_119">[Pg 119]</a></span> 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&#8217;s is the comet of shortest period, returning
+in about 3&middot;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 <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">17</span>th 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 distance 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 <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">1200</span>th; 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.</p>
+
+<p>Comets cannot be homes of life; they are not<span class="pagenum"><a name="Page_120" id="Page_120">[Pg 120]</a></span> 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.</p>
+
+<p>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&mdash;Ceres&mdash;probably outweighs all the rest, known and unknown, put
+together, though something like 700 have already been detected, and the
+list grows at the rate of about one a week.</p>
+
+<p>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 unfavourable
+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 consequence 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<span class="pagenum"><a name="Page_121" id="Page_121">[Pg 121]</a></span> fall
+far below freezing-point, the minimum on the dark side will approach the
+absolute zero.</p>
+
+<p>With Ceres the whole of the asteroidal 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 supports 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.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_122" id="Page_122">[Pg 122]</a></span></p>
+<h2><a name="CHAPTER_X" id="CHAPTER_X"></a>CHAPTER X</h2>
+<p class="center"><span class="big">THE MAJOR PLANETS</span></p>
+
+<p class="dropcap"><span class="caps">It</span> 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 confronted 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.</p>
+
+<p>And that disc is one that never fails to charm the attentive student, for
+it abounds in colour, movement 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.</p>
+
+<p class="blockquot">&#8220;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 <span class="pagenum"><a name="Page_123" id="Page_123">[Pg 123]</a></span>uniform tint, the surface being mottled with flocculent 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 background. 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, sometimes 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<span class="pagenum"><a name="Page_124" id="Page_124">[Pg 124]</a></span> colour as the belts, but deeper in tint, as if the fluid
+medium could be seen to a greater depth.&#8221;<small><a name="f15.1" id="f15.1" href="#f15">[15]</a></small></p>
+
+<p>In other words, Jupiter is a striped or banded planet, the bands lying
+along the direction of turning. 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&mdash;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&mdash;but the various items that form the bands rotate in
+different times. They may also alter their form and their colour. Jupiter
+seems, then, to be a planet with a great and rapidly changing atmosphere
+that extends above a shoreless sea formed of some liquified substance or
+substances&mdash;the whole in a state of flux.</p>
+
+<p>But if we turn back to the Table, we see that Jupiter at its mean distance
+from the Sun is 5&middot;2 times that of the Earth; that is to say, it receives
+only <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">27</span>th of the light and heat that we receive. But in Chapter VIII, we
+learnt from Mars that as this receives only <span style="font-size: 0.8em;"><sup>3</sup></span>&frasl;<span style="font-size: 0.6em;">7</span>ths of the Earth&#8217;s light
+and heat, its mean temperature would sink to -30&deg;C.; the Earth&#8217;s being
+16&deg;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<span class="pagenum"><a name="Page_125" id="Page_125">[Pg 125]</a></span> 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.</p>
+
+<p>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&middot;4 that of water, Jupiter&#8217;s is 1&middot;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 reconcile these
+inconsistencies?</p>
+
+<p>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 contraction; 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, therefore, all the
+members of the solar system had at<span class="pagenum"><a name="Page_126" id="Page_126">[Pg 126]</a></span> one and the same moment possessed the
+same surface 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.</p>
+
+<p>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.</p>
+
+<p>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,<span class="pagenum"><a name="Page_127" id="Page_127">[Pg 127]</a></span> 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.</p>
+
+<p>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 dealing 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.</p>
+
+<p>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 whole cannot be put
+lower than 0&middot;72, or about that of white paper. But many of the &#8220;belts&#8221; or<span class="pagenum"><a name="Page_128" id="Page_128">[Pg 128]</a></span>
+dark regions are of a dull copper tint, and the polar caps are dusky, so
+that Bond&#8217;s estimate must be realized for the most brilliant &#8220;zones,&#8221; as
+the brighter regions are called; certainly for the whitest of the white
+spots.</p>
+
+<p>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.</p>
+
+<p>Unless there is the counteracting effect of a high temperature, the
+atmosphere of Jupiter would have a pressure at the surface of 104 lb. 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 probably 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
+portholes 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.</p>
+
+<p><span class="pagenum"><a name="Page_129" id="Page_129">[Pg 129]</a></span>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 movements. The clouds and
+storms of our own atmosphere 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 <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">27</span>th 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.</p>
+
+<p>Further, the difference of presentment due to the difference of latitude
+is a fruitful cause of inequalities 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 distance 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.</p>
+
+<p>A notable peculiarity of Jupiter is found in the proper motions of its
+spots. Many of the white<span class="pagenum"><a name="Page_130" id="Page_130">[Pg 130]</a></span> spots are exceedingly swift, giving a rotation
+period of 9h. 50m. while the equatorial belt in general gives a period 5m.
+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.</p>
+
+<p>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
+&#8220;Australia on the loose&#8221; is impossible, even unthinkable here, but the
+Great Red Spot, for all its long duration, is mobile and inconstant, and
+is therefore no portion of a solid permanent crust.</p>
+
+<p>The giant planet Jupiter, therefore, offers us an<span class="pagenum"><a name="Page_131" id="Page_131">[Pg 131]</a></span> example of what we may
+call a &#8220;semi-sun&#8221;; 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&#8217;s &#8220;gloom of iron substance,&#8221; which is necessary, no
+less than its veiling by the plant, as a stage for &#8220;the passion and
+perishing of mankind.&#8221;</p>
+
+<p>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.</p>
+
+<p>Of these eight, only four&mdash;the four discovered by Galileo, in the first
+days of his possession of a telescope&mdash;need be considered; the other four
+are of the same order of size as the asteroids, and are indeed much
+smaller than Ceres.</p>
+
+<p>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; Io, the innermost, is midway between the two in its
+dimensions. But Ganymede, the largest, is almost comparable with Mars, its
+diameter being 0&middot;45 that of the Earth instead of the 0&middot;53 of Mars.</p>
+
+<p><span class="pagenum"><a name="Page_132" id="Page_132">[Pg 132]</a></span>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 <span style="font-size: 0.8em;"><sup>1</sup></span>&frasl;<span style="font-size: 0.6em;">250</span>th as much heat as each unit
+of the Sun&#8217;s surface. In other words, the absolute surface temperature of
+Jupiter should be &#188;th that of the Sun, or about 1550&deg; 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.</p>
+
+<p>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 recipients 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.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_133" id="Page_133">[Pg 133]</a></span></p>
+<h2><a name="CHAPTER_XI" id="CHAPTER_XI"></a>CHAPTER XI</h2>
+<p class="center"><span class="big">WHEN THE MAJOR PLANETS COOL</span></p>
+
+<p class="dropcap"><span class="caps">The</span> question has been asked: &#8220;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 beautiful to our eyes and so vast
+in scale are destined never to be peopled by intelligent beings.&#8221;</p>
+
+<p>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<span class="pagenum"><a name="Page_134" id="Page_134">[Pg 134]</a></span> 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.</p>
+
+<p>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 sufficient. Or we can neglect the question of
+temperature altogether, as we did at first in the case of Mars.</p>
+
+<p>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<span class="pagenum"><a name="Page_135" id="Page_135">[Pg 135]</a></span> 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:</p>
+
+<p class="center"><span class="smcap">Statistics of the Four Outer Planets if with the Same Density as the Earth</span></p>
+
+<table border="0" cellpadding="0" cellspacing="5" summary="table">
+<tr><td colspan="3"><span class="smcap">Proportions of the Planets</span>:&mdash;</td></tr>
+<tr><td>&nbsp;</td>
+    <td>&nbsp;</td><td><span class="spacer">&nbsp;</span></td>
+    <td align="center">Uranus</td><td><span class="spacer">&nbsp;</span></td>
+    <td align="center">Neptune</td><td><span class="spacer">&nbsp;</span></td>
+    <td align="center">Saturn</td><td><span class="spacer">&nbsp;</span></td>
+    <td align="center">Jupiter</td></tr>
+<tr><td><span style="margin-left: 1em;">Diameter in miles</span></td>
+    <td>&nbsp;</td><td>&nbsp;</td>
+    <td align="center">19300</td><td>&nbsp;</td>
+    <td align="center">20400</td><td>&nbsp;</td>
+    <td align="center">36000</td><td>&nbsp;</td>
+    <td align="center">54000</td></tr>
+<tr><td align="center">do</td>
+    <td align="center">&oplus;&nbsp;=&nbsp;1</td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1em;">2&middot;44</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1em;">2&middot;57</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1em;">4&middot;56</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1.5em;">6&middot;82</span></td></tr>
+<tr><td><span style="margin-left: 1em;">Surface,</span></td>
+    <td align="center">&oplus; = 1</td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: .5em;">6&middot;0</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: .5em;">6&middot;6</span></td><td>&nbsp;</td>
+    <td align="center">20&middot;8</td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1em;">46&middot;6</span></td></tr>
+<tr><td><span style="margin-left: 1em;">Mass and Volume,</span></td>
+    <td align="center">&oplus; = 1</td><td>&nbsp;</td>
+    <td align="center">14&middot;6</td><td>&nbsp;</td>
+    <td align="center">17&middot;0</td><td>&nbsp;</td>
+    <td align="center">94&middot;8</td><td>&nbsp;</td>
+    <td align="center">317&middot;7</td></tr>
+<tr><td><span style="margin-left: 1em;">Gravity at surface,</span></td>
+    <td align="center">&oplus; = 1</td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1em;">2&middot;44</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1em;">2&middot;57</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1em;">4&middot;56</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1.5em;">6&middot;82</span></td></tr>
+<tr><td colspan="2">Rate of Fall, Feet in the First Second</td><td>&nbsp;</td>
+    <td align="center">39&middot;2</td><td>&nbsp;</td>
+    <td align="center">41&middot;3</td><td>&nbsp;</td>
+    <td align="center">73&middot;3</td><td>&nbsp;</td>
+    <td align="center">109&middot;7</td></tr>
+<tr><td>&nbsp;</td></tr>
+<tr><td colspan="2"><span class="smcap">Atmosphere</span>, assuming the total mass of<br /><span style="margin-left: 1em;">the atmosphere to be proportional to</span><br />
+<span style="margin-left: 1em;">the mass of the planet:&mdash;</span></td></tr>
+<tr><td colspan="2">Pressure at the surface in lb. per square inch</td><td>&nbsp;</td>
+    <td align="center">88&middot;2</td><td>&nbsp;</td>
+    <td align="center">97&middot;0</td><td>&nbsp;</td>
+    <td align="center">305&middot;8</td><td>&nbsp;</td>
+    <td align="center">685&middot;0</td></tr>
+<tr><td colspan="2">Pressure at the surface in &#8220;atmospheres&#8221;</td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: .5em;">6&middot;0</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: .5em;">6&middot;6</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: .5em;">20&middot;8</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: .5em;">46&middot;6</span></td></tr>
+<tr><td colspan="2">Level of half-pressure in miles</td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1em;">1&middot;37</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1em;">1&middot;30</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1.5em;">0&middot;73</span></td><td>&nbsp;</td>
+    <td align="center"><span style="margin-left: 1.5em;">0&middot;49</span></td></tr>
+<tr><td colspan="2">Boiling point of water at surface</td><td>&nbsp;</td>
+    <td align="center">127&deg;C</td><td>&nbsp;</td>
+    <td align="center">129&deg;C</td><td>&nbsp;</td>
+    <td align="center">148&deg;C</td><td>&nbsp;</td>
+    <td align="center">164&deg;C</td></tr></table>
+
+<p><span class="pagenum"><a name="Page_136" id="Page_136">[Pg 136]</a></span>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 proportion. A man who here weighs 150 lb. will there
+weigh over 1000 lb.; 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 proportion. The atmosphere, supposing it to be in proportion
+to the mass of Jupiter, will exercise a pressure of 46&#189; &#8220;atmospheres,&#8221;
+or more than 680 lb., 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 upward 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 lb. 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 constructed 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<span class="pagenum"><a name="Page_137" id="Page_137">[Pg 137]</a></span> low range of hills that on the Earth would be considered 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 involve.</p>
+
+<p>The sevenfold gravity of Jupiter, taken in connection with this enormous
+atmospheric pressure, would tend to make the meteorological disturbances
+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 suddenness 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 cataclysmic.</p>
+
+<p>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
+atmospheric pressure would be about 21 &#8220;atmospheres,&#8221; or more than 300 lb.
+on the square inch. The<span class="pagenum"><a name="Page_138" id="Page_138">[Pg 138]</a></span> level of half-pressure would be reached at about
+three-quarters of a mile, and the force of gravity be nearly 4&#189; times
+that of the Earth.</p>
+
+<p>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.</p>
+
+<p>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&deg; was
+reached. Here the eclipses would begin nearly three terrestrial 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<span class="pagenum"><a name="Page_139" id="Page_139">[Pg 139]</a></span> 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 atmosphere 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.</p>
+
+<p>The case of Uranus introduces us to another class of conditions fatal to
+habitability. The equator of Jupiter is inclined only 3&deg; to the plane of
+its orbit; the difference in its seasons is, therefore, 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 14th 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<span class="pagenum"><a name="Page_140" id="Page_140">[Pg 140]</a></span> 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&deg;. If this were the case for the
+Earth, the noonday sun for London would be, at the spring equinox, 38&#189;&deg;
+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&deg; above the horizon at midnight. At midsummer, indeed,
+it would be only 59&deg; high at noonday, but it would be north of the zenith
+instead of south, and at technical midnight, it would still be 44&deg; in
+altitude, thus moving round in a very small circle, only 15&deg; in diameter.
+From about April 18 to August 25&mdash;that is to say, for 129 days&mdash;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.</p>
+
+<p>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.</p>
+
+<p>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 darkness,
+of unmitigated cold, cannot possibly ever<span class="pagenum"><a name="Page_141" id="Page_141">[Pg 141]</a></span> 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.</p>
+
+<p>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.</p>
+
+<p>It is curious that these four outer planets, that resemble each other so
+strikingly in many of their conditions&mdash;in their vast size, high albedo,
+low density, and vaporous envelopes, that show, in their spectra, not
+merely the lines of reflected sunlight, but also special lines due to
+their own atmospheres (the chief of these being common to all the four
+planets)&mdash;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<span class="pagenum"><a name="Page_142" id="Page_142">[Pg 142]</a></span> 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.</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_143" id="Page_143">[Pg 143]</a></span></p>
+<h2><a name="CHAPTER_XII" id="CHAPTER_XII"></a>CHAPTER XII</h2>
+<p class="center"><span class="big">THE FINAL QUESTION</span></p>
+
+<p class="dropcap"><span class="caps">In</span> 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<span class="pagenum"><a name="Page_144" id="Page_144">[Pg 144]</a></span> 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&#8217;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.</p>
+
+<p>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&mdash;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 &#8220;spectroscopic binaries,&#8221; 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 &#8220;Algol variables.&#8221; The optical double
+stars are cases where the two <span class="pagenum"><a name="Page_145" id="Page_145">[Pg 145]</a></span>components are far distant from each other,
+and hence can be distinguished in our telescopes as separate points of
+light. The &#8220;spectroscopic binaries&#8221; 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 &#8220;Algol variables&#8221; are cases where the two
+components are of very unequal brightness, 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&mdash;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&mdash;and it seems almost certain that simple systems, like that of
+which our Sun is the unchallenged autocrat, must be comparatively rare.</p>
+
+<p>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,<span class="pagenum"><a name="Page_146" id="Page_146">[Pg 146]</a></span> 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 acclimatizing
+themselves to these ever-changing vicissitudes. 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.</p>
+
+<p>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&mdash;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; &#8220;one black ball
+excludes;&#8221; the candidate who fails in a single subject is &#8220;ploughed&#8221;
+without mercy. And in most cases the failure is final; no opportunity is
+given to the candidate to &#8220;sit&#8221; again.</p>
+
+<p>But Space is not the only horizon along which<span class="pagenum"><a name="Page_147" id="Page_147">[Pg 147]</a></span> 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 forbids 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 &#8220;without form and void&#8221;; 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<span class="pagenum"><a name="Page_148" id="Page_148">[Pg 148]</a></span> 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 observers 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 compared with the indefinite duration
+which preceded it, and the indefinite duration which shall follow.</p>
+
+<p>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,<span class="pagenum"><a name="Page_149" id="Page_149">[Pg 149]</a></span> 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.</p>
+
+<p>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
+destroyed. 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 consideration 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,<span class="pagenum"><a name="Page_150" id="Page_150">[Pg 150]</a></span> 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 disqualifications 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 telescopes, 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&mdash;stable and efficient suns&mdash;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<span class="pagenum"><a name="Page_151" id="Page_151">[Pg 151]</a></span> even one in a
+hundred is now at that epoch in its history when it is no longer &#8220;without
+form and void,&#8221; 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 throughout 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 illustrations 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&mdash;indeed of a very few.</p>
+
+<p>And then the objection is raised: &#8220;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?&#8221; 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 &#8220;waste&#8221; in
+the solar system, there is no question. Relatively speaking, this is quite
+insignificant, for we cannot consider that as &#8220;waste material&#8221; which is
+useful and,<span class="pagenum"><a name="Page_152" id="Page_152">[Pg 152]</a></span> 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 <i>upon</i> 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</p>
+
+<p class="center">2,000,000,000,000,000,000,000,000,000 tons</p>
+
+<p>can, in no single unit, be counted &#8220;waste,&#8221; 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&middot;13 per cent of the Sun alone; and a
+wastage, if such it is, like this is insignificant from a material point
+of view.</p>
+
+<p>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<span class="pagenum"><a name="Page_153" id="Page_153">[Pg 153]</a></span> built upon nor cultivated is also essential. Our barren moors and
+bleak hillsides, &#8220;wastelands&#8221; as we call them, are absolutely necessary as
+collectors of the water by which we live. From them our springs take their
+source; and they supply our cities with the first necessity of life.</p>
+
+<p>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, intelligent 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.</p>
+
+<p><span class="pagenum"><a name="Page_154" id="Page_154">[Pg 154]</a></span>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 conditions 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&mdash;the tops of mountains, the polar snows, the waterless deserts,
+the ocean depths&mdash;is only possible because there are more favoured regions
+close at hand, and there are, as it were, &#8220;crumbs that fall from the rich
+man&#8217;s table.&#8221; A well-known litt&eacute;rateur in setting forth &#8220;a hundred ways of
+making money&#8221; 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.</p>
+
+<p>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<span class="pagenum"><a name="Page_155" id="Page_155">[Pg 155]</a></span>
+diverse as they are numerous. And everywhere and always, water is found
+essential to protoplasmic 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.</p>
+
+<p>And neither anthropologist nor biologist would admit that the form of
+intelligent life was an unrelated 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 intelligence
+are associated, and we 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 difference 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 <span class="pagenum"><a name="Page_156" id="Page_156">[Pg 156]</a></span>supposed 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 associated with the development of one type or 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.</p>
+
+<p>Swift, in <i>Gulliver&#8217;s Travels</i>, 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 <i>War of the
+Worlds</i>, attributed intelligence to monsters&mdash;half-cuttlefish and
+half-anemone,&mdash;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 possible for the skeleton of creatures from
+other worlds to be brought to us here, then biologists would as
+confidently pronounce on their intelligence as they do on the extinct
+forms of bygone ages&mdash;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<span class="pagenum"><a name="Page_157" id="Page_157">[Pg 157]</a></span> 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.</p>
+
+<p>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&mdash;they too
+must be filled with Life in like manner. Other believers argue that this
+world was the scene of the<span class="pagenum"><a name="Page_158" id="Page_158">[Pg 158]</a></span> 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 inhabited 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 world of ours. Heavy as the odds are against any
+particular world being an inhabited one, yet when the limitless extent of<span class="pagenum"><a name="Page_159" id="Page_159">[Pg 159]</a></span>
+space is considered, and the innumerable numbers of stars and systems of
+stars, it seems but reasonable 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.</p>
+
+<p>But there is a religious question connected with this enquiry; one that
+goes down to the very roots of man&#8217;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.</p>
+
+<p class="poem">&#8220;Oh, life as futile then as frail.<br />
+<span style="margin-left: 1em;"><strong><span class="spacer">&#183;</span><span class="spacer">&#183;</span><span class="spacer">&#183;</span><span class="spacer">&#183;</span></strong></span><br />
+What hope of answer or redress?<br />
+Behind the veil, behind the veil.&#8221;</p>
+
+<p>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<span class="pagenum"><a name="Page_160" id="Page_160">[Pg 160]</a></span> 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.</p>
+
+<p>There was a time when on this world there was no life; a time when life
+began. How did it begin? Under what conditions?</p>
+
+<p>Of that great change&mdash;when non-living matter first became endowed with
+life, became so endowed not by the action and intervention of other living
+matter, but without it&mdash;we have no knowledge, no experience. And so long
+as this continues 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 speculate 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.</p>
+
+<p>There is, therefore, a veil behind us as well as<span class="pagenum"><a name="Page_161" id="Page_161">[Pg 161]</a></span> 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.</p>
+
+<hr style="width: 45%;" />
+
+<p>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 foreshadowing of it, still less of all its
+infinities of form, and possibilities of development.</p>
+
+<p>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?</p>
+
+<p>To questions such as these Science has no reply to give; it is even more
+helpless to answer them<span class="pagenum"><a name="Page_162" id="Page_162">[Pg 162]</a></span> than to determine how life first came; its
+experience 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.</p>
+
+<p>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 experience,
+we are not left without an answer. A voice has been heard, the voice of
+the Son of God Himself:</p>
+
+<p>&#8220;I am the Resurrection and the Life. He that believeth on Me, though he
+were dead, yet shall he live.&#8221;</p>
+
+<p>And accepting His word, the Church in all ages, and among all nations,
+peoples, and tongues, has made reply:</p>
+
+<p>&#8220;<span class="smcap">I look for the resurrection of the dead and the life of the world to
+come.</span>&#8221;</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><span class="pagenum"><a name="Page_163" id="Page_163">[Pg 163]</a></span></p>
+<h2>INDEX</h2>
+
+<p class="index">
+Abbot, C. G., <a href="#Page_27">27</a>, <a href="#Page_34">34</a><br />
+<br />
+Albedo of Earth, <a href="#Page_54">54</a>, <a href="#Page_81">81</a><br />
+<span style="margin-left: 1em;">&mdash;&mdash; Jupiter, <a href="#Page_127">127</a></span><br />
+<span style="margin-left: 1em;">&mdash;&mdash; Mars, <a href="#Page_81">81</a></span><br />
+<span style="margin-left: 1em;">&mdash;&mdash; Moon, <a href="#Page_54">54</a></span><br />
+<br />
+<i>Albumin sol</i>, <a href="#Page_15">15</a><br />
+<br />
+Algol-type stars, <a href="#Page_35">35</a>, <a href="#Page_144">144</a>, <a href="#Page_145">145</a><br />
+<br />
+Antares, <a href="#Page_38">38</a><br />
+<br />
+Antoniadi, E. M., <a href="#Page_97">97</a>, <a href="#Page_104">104</a><br />
+<br />
+Archangel, climate of, <a href="#Page_87">87</a>, <a href="#Page_88">88</a><br />
+<br />
+Arcturus, <a href="#Page_35">35</a>, <a href="#Page_37">37</a><br />
+<br />
+Aristarchus, lunar crater, <a href="#Page_48">48</a><br />
+<br />
+&#8220;Astronomical unit,&#8221; <a href="#Page_21">21</a><br />
+<br />
+Atmosphere of, Mars, <a href="#Page_76">76</a><br />
+<span style="margin-left: 1em;">&mdash;&mdash;, Moon, <a href="#Page_53">53</a></span><br />
+<span style="margin-left: 1em;">&mdash;&mdash;, Sun, <a href="#Page_25">25</a></span><br />
+<span style="margin-left: 1em;">&mdash;&mdash;, Venus, <a href="#Page_112">112</a></span><br />
+<br />
+<br />
+Barnard, E. E., <a href="#Page_89">89</a>, <a href="#Page_104">104</a><br />
+<br />
+Beer, <a href="#Page_60">60</a>, <a href="#Page_97">97</a>, <a href="#Page_98">98</a><br />
+<br />
+Bond, G. P., <a href="#Page_127">127</a>, <a href="#Page_128">128</a><br />
+<br />
+Brewster, Sir David, <a href="#Page_4">4</a><br />
+<br />
+<br />
+Calcium, <a href="#Page_12">12</a>, <a href="#Page_36">36</a><br />
+<br />
+Callisto, satellite of Jupiter, <a href="#Page_131">131</a><br />
+<br />
+Calory, <a href="#Page_26">26</a><br />
+<br />
+Campbell, W. W., <a href="#Page_145">145</a><br />
+<br />
+Carbon, <a href="#Page_11">11</a>, <a href="#Page_34">34</a>, <a href="#Page_38">38</a>, <a href="#Page_39">39</a><br />
+<br />
+Carbonic acid, <a href="#Page_11">11</a><br />
+<br />
+Cassini, <a href="#Page_59">59</a>, <a href="#Page_60">60</a>, <a href="#Page_130">130</a><br />
+<br />
+Ceres, minor planet, <a href="#Page_41">41</a>, <a href="#Page_120">120</a>, <a href="#Page_121">121</a>, <a href="#Page_122">122</a>, <a href="#Page_131">131</a><br />
+<br />
+Cerulli, V., <a href="#Page_104">104</a>, <a href="#Page_108">108</a><br />
+<br />
+Chromosphere, <a href="#Page_28">28</a>, <a href="#Page_29">29</a><br />
+<br />
+Cobalt, <a href="#Page_33">33</a><br />
+<br />
+Comet, Encke&#8217;s, <a href="#Page_119">119</a><br />
+<span style="margin-left: 1em;">&mdash;&mdash;, Halley&#8217;s, <a href="#Page_119">119</a></span><br />
+<span style="margin-left: 1em;">&mdash;&mdash;, spectra, <a href="#Page_38">38</a></span><br />
+<br />
+Copernican theory, <a href="#Page_1">1</a><br />
+<br />
+Copper, <a href="#Page_33">33</a><br />
+<br />
+Corona, <a href="#Page_40">40</a><br />
+<br />
+Coxwell, <a href="#Page_25">25</a>, <a href="#Page_75">75</a>, <a href="#Page_79">79</a><br />
+<br />
+Cyanogen, <a href="#Page_37">37</a>, <a href="#Page_38">38</a><br />
+<br />
+Czapek, F., <a href="#Page_11">11</a>, <a href="#Page_13">13</a><br />
+<br />
+<br />
+Darwin, Sir G. H., <a href="#Page_116">116</a><br />
+<br />
+Dawes, W. R., <a href="#Page_60">60</a>, <a href="#Page_63">63</a>, <a href="#Page_97">97</a>, <a href="#Page_99">99</a><br />
+<br />
+Denning, W. F., <a href="#Page_104">104</a><br />
+<br />
+Dispersion, anomalous, <a href="#Page_28">28</a><br />
+<br />
+Doppelmayer, lunar crater, <a href="#Page_52">52</a><br />
+<br />
+Dubois, Eugene, <a href="#Page_155">155</a><br />
+<br />
+<br />
+Eros, minor planet, <a href="#Page_57">57</a><br />
+<br />
+Europa, satellite of Jupiter, <a href="#Page_131">131</a><br />
+<br />
+Evans, J. E., <a href="#Page_107">107</a><br />
+<br />
+<br />
+Faculae, <a href="#Page_29">29</a>, <a href="#Page_30">30</a><br />
+<br />
+Fauth, P., <a href="#Page_50">50</a><br />
+<br />
+Flamsteed, lunar crater, <a href="#Page_52">52</a><br />
+<br />
+Fluorine, <a href="#Page_40">40</a><br />
+<br />
+Fraunhofer, <a href="#Page_33">33</a><br />
+<br />
+<br />
+Galileo, <a href="#Page_51">51</a>, <a href="#Page_59">59</a>, <a href="#Page_131">131</a><br />
+<br />
+Ganymede, satellite of Jupiter, <a href="#Page_131">131</a>, <a href="#Page_132">132</a><br />
+<br />
+Gay-Lussac, <a href="#Page_75">75</a><br />
+<br />
+<span class="pagenum"><a name="Page_164" id="Page_164">[Pg 164]</a></span>Glaisher, J., <a href="#Page_25">25</a>, <a href="#Page_75">75</a>, <a href="#Page_79">79</a><br />
+<br />
+Goodacre, W., <a href="#Page_49">49</a><br />
+<br />
+Green, N. E., <a href="#Page_60">60</a>, <a href="#Page_62">62</a>, <a href="#Page_63">63</a><br />
+<br />
+Greenwich Hospital School, <a href="#Page_107">107</a>, <a href="#Page_108">108</a><br />
+<br />
+&#8220;Gulliver&#8217;s Travels,&#8221; <a href="#Page_82">82</a>, <a href="#Page_156">156</a><br />
+<br />
+<br />
+Haeckel, E., <a href="#Page_12">12</a><br />
+<br />
+Halogens, <a href="#Page_36">36</a>, <a href="#Page_40">40</a><br />
+<br />
+&#8220;Harper&#8217;s Weekly,&#8221; <a href="#Page_93">93</a><br />
+<br />
+Helium, <a href="#Page_37">37</a><br />
+<br />
+Herschel, Sir J., <a href="#Page_54">54</a><br />
+<br />
+&mdash;&mdash; Sir W., <a href="#Page_20">20</a>, <a href="#Page_49">49</a>, <a href="#Page_59">59</a>, <a href="#Page_60">60</a>, <a href="#Page_61">61</a><br />
+<br />
+Hevelius, <a href="#Page_51">51</a><br />
+<br />
+Hippalus, lunar crater, <a href="#Page_52">52</a><br />
+<br />
+Hooke, R., <a href="#Page_59">59</a>, <a href="#Page_60">60</a>, <a href="#Page_130">130</a><br />
+<br />
+Huyghens, <a href="#Page_59">59</a><br />
+<br />
+Hydrocarbons, <a href="#Page_38">38</a><br />
+<br />
+Hydrogen, <a href="#Page_11">11</a>, <a href="#Page_36">36</a>, <a href="#Page_37">37</a>, <a href="#Page_38">38</a>, <a href="#Page_41">41</a>, <a href="#Page_81">81</a><br />
+<br />
+<br />
+&#8220;Inhabitant,&#8221; <a href="#Page_5">5</a><br />
+<br />
+&#8220;Inhabited&#8221; Worlds, <a href="#Page_2">2</a>, <a href="#Page_3">3</a>, <a href="#Page_4">4</a><br />
+<br />
+Io, satellite of Jupiter, <a href="#Page_131">131</a><br />
+<br />
+Iron, <a href="#Page_12">12</a>, <a href="#Page_33">33</a>, <a href="#Page_36">36</a><br />
+<br />
+<br />
+Jupiter, <a href="#Page_122">122-32</a><br />
+<br />
+&mdash;&mdash;, belts, <a href="#Page_127">127</a>, <a href="#Page_129">129</a><br />
+<br />
+&mdash;&mdash;, great red spot, <a href="#Page_130">130</a><br />
+<br />
+&mdash;&mdash;, proper motion of spots, <a href="#Page_129">129</a><br />
+<br />
+&mdash;&mdash;, satellites of, <a href="#Page_128">128</a>, <a href="#Page_131">131</a><br />
+<br />
+&mdash;&mdash;, white spots, <a href="#Page_128">128</a>, <a href="#Page_130">130</a><br />
+<br />
+<br />
+Keeler, J. E., <a href="#Page_122">122</a>, <a href="#Page_125">125</a><br />
+<br />
+Kies, lunar crater, <a href="#Page_52">52</a><br />
+<br />
+Kirchhoff, <a href="#Page_33">33</a><br />
+<br />
+<br />
+Lacus Solis, <a href="#Page_97">97</a>, <a href="#Page_98">98</a>, <a href="#Page_99">99</a><br />
+<br />
+Langley, S. P., <a href="#Page_55">55</a><br />
+<br />
+Lilliputians, <a href="#Page_82">82</a>, <a href="#Page_83">83</a><br />
+<br />
+Linn&eacute;, lunar crater, <a href="#Page_48">48</a><br />
+<br />
+Lockyer, J. N., <a href="#Page_60">60</a><br />
+<br />
+Lowell, P., <a href="#Page_65">65</a>, <a href="#Page_66">66</a>, <a href="#Page_67">67</a>, <a href="#Page_69">69</a>, <a href="#Page_71">71</a>, <a href="#Page_81">81</a>, <a href="#Page_97">97</a>, <a href="#Page_98">98</a>, <a href="#Page_99">99</a>, <a href="#Page_101">101</a>, <a href="#Page_103">103</a>, <a href="#Page_104">104</a>, <a href="#Page_105">105</a>, <a href="#Page_106">106</a>, <a href="#Page_108">108</a>, <a href="#Page_109">109</a>, <a href="#Page_110">110</a><br />
+<br />
+Lucifer, <a href="#Page_111">111</a><br />
+<br />
+<br />
+M&auml;dler, <a href="#Page_46">46</a>, <a href="#Page_48">48</a>, <a href="#Page_60">60</a>, <a href="#Page_97">97</a>, <a href="#Page_98">98</a><br />
+<br />
+Maginus, lunar crater, <a href="#Page_46">46</a><br />
+<br />
+Magnesium, <a href="#Page_12">12</a>, <a href="#Page_36">36</a><br />
+<br />
+Manganese, <a href="#Page_33">33</a><br />
+<br />
+Mare Fecunditatis, <a href="#Page_47">47</a><br />
+<br />
+&mdash;&mdash; Humerum, <a href="#Page_52">52</a><br />
+<br />
+&mdash;&mdash; Nubium, <a href="#Page_52">52</a><br />
+<br />
+&mdash;&mdash; Serenitatis, <a href="#Page_48">48</a><br />
+<br />
+Mars, canals of, <a href="#Page_57">57-70</a>, <a href="#Page_78">78</a>, <a href="#Page_101">101</a>, <a href="#Page_102">102</a><br />
+<br />
+&mdash;&mdash;, conditions of, <a href="#Page_71">71-95</a><br />
+<br />
+&mdash;&mdash;, illusions of, <a href="#Page_96">96-110</a><br />
+<br />
+&mdash;&mdash;, meteorology of, <a href="#Page_93">93-4</a><br />
+<br />
+&mdash;&mdash;, oases of, <a href="#Page_65">65</a>, <a href="#Page_98">98</a>, <a href="#Page_99">99</a>, <a href="#Page_101">101</a><br />
+<br />
+&mdash;&mdash;, thermograph of, <a href="#Page_91">91</a>, <a href="#Page_92">92</a><br />
+<br />
+&mdash;&mdash;, winds of, <a href="#Page_77">77</a><br />
+<br />
+Mendel&eacute;eff, <a href="#Page_39">39</a><br />
+<br />
+Mercury, <a href="#Page_114">114-18</a><br />
+<br />
+Messier, lunar crater, <a href="#Page_47">47</a>, <a href="#Page_48">48</a><br />
+<br />
+Metabolism, <a href="#Page_10">10</a>, <a href="#Page_11">11</a>, <a href="#Page_14">14</a>, <a href="#Page_15">15</a>, <a href="#Page_38">38</a><br />
+<br />
+Millechau, <a href="#Page_104">104</a><br />
+<br />
+Milton, <a href="#Page_51">51</a><br />
+<br />
+Mira Ceti, <a href="#Page_150">150</a><br />
+<br />
+Molesworth, P. B., <a href="#Page_49">49</a>, <a href="#Page_104">104</a><br />
+<br />
+Moon, <a href="#Page_43">43-56</a><br />
+<br />
+&mdash;&mdash;, &#8220;terminator&#8221; of, <a href="#Page_51">51</a><br />
+<br />
+Mont Blanc, <a href="#Page_25">25</a>, <a href="#Page_74">74</a>, <a href="#Page_80">80</a><br />
+<br />
+Mount Everest, <a href="#Page_75">75</a>, <a href="#Page_80">80</a><br />
+<br />
+<br />
+Nature of Vision, <a href="#Page_99">99</a><br />
+<br />
+Nebulae, spectrum of, <a href="#Page_38">38</a>, <a href="#Page_40">40</a><br />
+<br />
+Nebulium, <a href="#Page_40">40</a><br />
+<br />
+Negative elements, <a href="#Page_36">36</a><br />
+<br />
+Neison, E., <a href="#Page_48">48</a>, <a href="#Page_53">53</a><br />
+<br />
+Neptune, <a href="#Page_132">132</a>, <a href="#Page_141">141</a><br />
+<br />
+<span class="pagenum"><a name="Page_165" id="Page_165">[Pg 165]</a></span>Newcomb, S., <a href="#Page_93">93</a>, <a href="#Page_109">109</a><br />
+<br />
+Nicholson, J. W., <a href="#Page_40">40</a><br />
+<br />
+Nickel, <a href="#Page_33">33</a><br />
+<br />
+Nilosyrtis, &#8220;canal&#8221; on Mars, <a href="#Page_89">89</a><br />
+<br />
+Nitrogen, <a href="#Page_11">11</a>, <a href="#Page_37">37</a>, <a href="#Page_38">38</a>, <a href="#Page_39">39</a><br />
+<br />
+<br />
+Observatory, Chicago, <a href="#Page_44">44</a><br />
+<br />
+&mdash;&mdash;, Harvard College, <a href="#Page_127">127</a><br />
+<br />
+&mdash;&mdash;, Lick, <a href="#Page_122">122</a><br />
+<br />
+&mdash;&mdash;, Paris, <a href="#Page_44">44</a><br />
+<br />
+Occultation, <a href="#Page_52">52</a>, <a href="#Page_53">53</a><br />
+<br />
+Organic Life, definition of, <a href="#Page_15">15</a><br />
+<br />
+Organism, living, <a href="#Page_6">6-19</a><br />
+<br />
+Organo-genetic elements, <a href="#Page_12">12</a>, <a href="#Page_38">38</a>, <a href="#Page_39">39</a><br />
+<br />
+Osmosis, <a href="#Page_15">15</a><br />
+<br />
+Oxygen, <a href="#Page_11">11</a>, <a href="#Page_36">36</a>, <a href="#Page_37">37</a>, <a href="#Page_38">38</a>, <a href="#Page_41">41</a><br />
+<br />
+<br />
+Periodic Law, Mendel&eacute;eff&#8217;s, <a href="#Page_39">39</a><br />
+<br />
+Phillips, T. E. R., <a href="#Page_104">104</a><br />
+<br />
+Phosphorus, <a href="#Page_12">12</a><br />
+<br />
+Photosphere, <a href="#Page_28">28</a>, <a href="#Page_33">33</a>, <a href="#Page_36">36</a><br />
+<br />
+Pickering, W. H., <a href="#Page_47">47</a>, <a href="#Page_48">48</a>, <a href="#Page_53">53</a>, <a href="#Page_109">109</a><br />
+<br />
+Pithecanthropus, <a href="#Page_155">155</a><br />
+<br />
+Planetary statistics, table of, <a href="#Page_72">72</a>, <a href="#Page_72">73</a>, <a href="#Page_135">135</a><br />
+<br />
+Platinum, <a href="#Page_36">36</a><br />
+<br />
+&#8220;Plurality of Worlds,&#8221; <a href="#Page_2">2</a><br />
+<br />
+Pollock, Master, <a href="#Page_109">109</a><br />
+<br />
+Potassium, <a href="#Page_12">12</a><br />
+<br />
+Poynting, J. H., <a href="#Page_86">86</a>, <a href="#Page_87">87</a>, <a href="#Page_89">89</a>, <a href="#Page_115">115</a><br />
+<br />
+Proctor, R. A., <a href="#Page_34">34</a>, <a href="#Page_77">77</a><br />
+<br />
+Prominences, <a href="#Page_29">29</a>, <a href="#Page_30">30</a>, <a href="#Page_37">37</a><br />
+<br />
+Protofluorine, <a href="#Page_40">40</a><br />
+<br />
+Protonilus, &#8220;canal&#8221; on Mars, <a href="#Page_89">89</a><br />
+<br />
+Protoplasm, <a href="#Page_11">11</a>, <a href="#Page_12">12</a>, <a href="#Page_13">13</a>, <a href="#Page_15">15</a>, <a href="#Page_38">38</a>, <a href="#Page_40">40</a>, <a href="#Page_154">154</a><br />
+<br />
+Pyramid, Great, <a href="#Page_45">45</a><br />
+<br />
+<br />
+Refraction, anomalous, <a href="#Page_28">28</a><br />
+<br />
+Reversing layer, <a href="#Page_36">36</a><br />
+<br />
+&#8220;Rice-grains,&#8221; of Sun&#8217;s surface, <a href="#Page_28">28</a>, <a href="#Page_29">29</a><br />
+<br />
+Ring Nebula in Lyra, <a href="#Page_40">40</a><br />
+<br />
+Rosse, Lord, <a href="#Page_55">55</a><br />
+<br />
+Ruskin, J., <a href="#Page_19">19</a><br />
+<br />
+<br />
+Saturn, <a href="#Page_132">132</a><br />
+<br />
+&mdash;&mdash;, Rings of, <a href="#Page_138">138</a><br />
+<br />
+Schiaparelli, G. V., <a href="#Page_61">61</a>, <a href="#Page_62">62</a>, <a href="#Page_63">63</a>, <a href="#Page_64">64</a>, <a href="#Page_66">66</a>, <a href="#Page_97">97</a>, <a href="#Page_99">99</a>, <a href="#Page_107">107</a>, <a href="#Page_108">108</a>, <a href="#Page_116">116</a>, <a href="#Page_117">117</a><br />
+<br />
+Schooling, T. Holt, <a href="#Page_83">83</a><br />
+<br />
+&#8220;Scientia,&#8221; <a href="#Page_66">66</a><br />
+<br />
+&#8220;Semi-suns,&#8221; <a href="#Page_131">131</a>, <a href="#Page_132">132</a><br />
+<br />
+Serviss, Garrett P., <a href="#Page_17">17</a><br />
+<br />
+Singapore, climate of, <a href="#Page_87">87</a>, <a href="#Page_88">88</a><br />
+<br />
+Sinus Sabaeus, marking on Mars, <a href="#Page_97">97</a>, <a href="#Page_99">99</a><br />
+<br />
+Sirius, <a href="#Page_37">37</a><br />
+<br />
+Sodium, <a href="#Page_33">33</a>, <a href="#Page_36">36</a><br />
+<br />
+&#8220;Solar Constant,&#8221; <a href="#Page_26">26</a><br />
+<br />
+Spectroscopic binaries, <a href="#Page_144">144</a>, <a href="#Page_145">145</a><br />
+<br />
+Spectrum, <a href="#Page_53">53</a><br />
+<br />
+&mdash;&mdash;, heat, <a href="#Page_55">55</a><br />
+<br />
+&#8220;Spurious&#8221; disc, <a href="#Page_103">103</a><br />
+<br />
+Stars, double, <a href="#Page_35">35</a><br />
+<br />
+&mdash;&mdash;, multiple, <a href="#Page_35">35</a><br />
+<br />
+&mdash;&mdash;, red, <a href="#Page_38">38</a><br />
+<br />
+&mdash;&mdash;, spectra of, <a href="#Page_34">34</a>, <a href="#Page_38">38</a>, <a href="#Page_39">39</a><br />
+<br />
+Stefan&#8217;s Law, <a href="#Page_85">85</a><br />
+<br />
+Stoney, G. Johnstone, <a href="#Page_34">34</a><br />
+<br />
+&#8220;Streaming,&#8221; <a href="#Page_15">15</a><br />
+<br />
+Sulphur, <a href="#Page_11">11</a>, <a href="#Page_38">38</a><br />
+<br />
+Sun, <a href="#Page_20">20-32</a><br />
+<br />
+Sunspots, <a href="#Page_29">29</a>, <a href="#Page_30">30</a>, <a href="#Page_31">31</a>, <a href="#Page_38">38</a><br />
+<br />
+&mdash;&mdash;, spectra of, <a href="#Page_37">37</a><br />
+<br />
+Swift, Dean, <a href="#Page_82">82</a>, <a href="#Page_156">156</a><br />
+<br />
+<br />
+Table Mountain, <a href="#Page_54">54</a><br />
+<br />
+Thermograph of Mars, <a href="#Page_91">91</a>, <a href="#Page_92">92</a><br />
+<br />
+<span class="pagenum"><a name="Page_166" id="Page_166">[Pg 166]</a></span>Titanium, <a href="#Page_36">36</a>, <a href="#Page_37">37</a>, <a href="#Page_38">38</a><br />
+<br />
+Tornadoes, <a href="#Page_31">31</a>, <a href="#Page_137">137</a><br />
+<br />
+&#8220;Twinkler,&#8221; <a href="#Page_114">114</a><br />
+<br />
+Tycho, lunar crater, <a href="#Page_46">46</a><br />
+<br />
+<br />
+Uranus, <a href="#Page_132">132</a>, <a href="#Page_140">140</a><br />
+<br />
+<br />
+Venus, <a href="#Page_57">57</a>, <a href="#Page_111">111-18</a><br />
+<br />
+Verworn, Max, <a href="#Page_7">7</a><br />
+<br />
+Very, F. W., <a href="#Page_55">55</a><br />
+<br />
+Vesper, <a href="#Page_111">111</a><br />
+<br />
+&#8220;Victoria,&#8221; hypothetical planet, <a href="#Page_83">83</a><br />
+<br />
+<br />
+Wallace, A. R., <a href="#Page_4">4</a><br />
+<br />
+&#8220;War of the Worlds,&#8221; <a href="#Page_104">104</a>, <a href="#Page_156">156</a><br />
+<br />
+Waste, <a href="#Page_151">151</a>, <a href="#Page_152">152</a><br />
+<br />
+Water, indispensable factor, <a href="#Page_15">15</a>, <a href="#Page_41">41</a><br />
+<br />
+Wells, H. G., <a href="#Page_104">104</a>, <a href="#Page_156">156</a><br />
+<br />
+Whewell, <a href="#Page_4">4</a><br />
+<br />
+Williams, A. Stanley, <a href="#Page_104">104</a><br />
+<br />
+Wolf, Max, <a href="#Page_40">40</a><br />
+<br />
+<br />
+Young, C. A., <a href="#Page_26">26</a>, <a href="#Page_33">33</a><br />
+</p>
+
+<p>&nbsp;</p>
+
+<p class="center">WILLIAM BRENDON AND SON, LTD.<br />
+PRINTERS, PLYMOUTH</p>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<div class="vertsbox">
+<p class="center"><span class="huge">Harper&#8217;s Library of Living Though</span></p>
+
+<p>&nbsp;</p>
+<p class="center">ARTHUR HOLMES<br />
+<span class="huge">THE AGE OF THE EARTH</span><br />
+And Associated Problems. <i>Illustrated</i></p>
+
+<p>Gives us the result of the latest research into this field of enquiry. The
+radioactive minerals are shown to be recording their own age with the
+exquisite accuracy of a chronometer&mdash;their records checking physical,
+astronomical, and geological methods of computation.</p>
+
+
+<p>&nbsp;</p>
+<p class="center">PROF. A. W. BICKERTON<br />
+<span class="huge">THE BIRTH OF WORLDS AND SYSTEMS</span><br />
+<i>Illustrated</i><br />
+<i>Preface by Prof. Ernest Rutherford, F.R.S.</i></p>
+
+<p>A graphic account of the formation of new stars from the collision of dead
+suns or other celestial bodies. The theory throws light on many
+astronomical problems, and with its conception of an immortal cosmos, is
+of great philosophical importance.</p>
+
+
+<p>&nbsp;</p>
+<p class="center">PROF. SVANTE ARRHENIUS<br />
+<span class="huge">THE LIFE OF THE UNIVERSE</span><br />
+<i>2 Vols. Illustrated</i></p>
+
+<p>&#8220;We can thoroughly recommend these volumes. The information is accurate,
+useful, and most suggestive. There are many for whom the first chapters of
+Genesis are a subtle allegory covering the profoundest truths, and we are
+grateful to the author for having set out this mass of facts.&#8221;&mdash;<i>The
+Globe.</i></p>
+
+
+<p>&nbsp;</p>
+<p class="center">SIR OLIVER LODGE, F.R.S.<br />
+<span class="huge">THE ETHER OF SPACE</span><br />
+<i>Illustrated</i></p>
+
+<p>&#8220;This work by the great physicist will be found to possess an abiding
+charm and an intellectual stimulation.&#8221;-<i>Observer.</i></p>
+
+<p>&#8220;Opens up new views into the nature of the universe. Precise and lucid, it
+summarises our knowledge of the substance which fills all space and
+penetrates all matter&mdash;the substratum of matter itself.&#8221;&mdash;<i>Birmingham
+Post.</i></p>
+
+<p><br /><i>Please write for announcements and descriptive list:</i></p>
+
+<p class="center"><span class="smcap">Harper &amp; Brothers</span>, 45 Albemarle Street, London, W.</p></div>
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<div class="vertsbox">
+<p class="center"><span class="huge">Harper&#8217;s Library of Living Thought</span></p>
+
+<p class="center"><i>Foolscap 8vo, gilt tops, decorative covers, richly gilt backs<br />
+Per Volume: Cloth 2s. 6d. net. Leather 3s. 6d. net.</i></p>
+
+<p>&nbsp;</p>
+<p class="center">PROF. ARTHUR KEITH, M.D.<br />
+(Hunterian Professor Royal College of Surgeons)<br />
+<span class="huge">ANCIENT TYPES OF MAN</span><br />
+<i>Illustrated</i></p>
+
+<p>&#8220;The kind of book that only a master of his subject could write. It must
+interest every thinking person.&#8221;<i>&mdash;British Medical Journal.</i></p>
+
+
+<p>&nbsp;</p>
+<p class="center">PROF. FREDERICK CZAPEK<br />
+<span class="huge">CHEMICAL PHENOMENA IN LIFE</span></p>
+
+<p>Discusses in clear, concise terms the great question&mdash;&#8220;Can life be
+explained by physics and chemistry?&#8221; It deals with the life-processes of
+plants, the molecular structure of protoplasm, organic synthesis in the
+cell, the nature of ferments, and the subject of inheritance.</p>
+
+
+<p>&nbsp;</p>
+<p class="center">SIR A. TILDEN, F.R.S.<br />
+<span class="huge">THE ELEMENTS</span><br />
+Speculations as to their Nature and Origin<br />
+<i>Diagrams, &amp;c.</i></p>
+
+<p>Points to the conclusion that the elements resulted from a change in some
+primal essence, and discusses &#8220;whether all may not be suffering a slow
+waste, which, in the long run, must lead back to the primal chaos.&#8221;</p>
+
+
+<p>&nbsp;</p>
+<p class="center">SIR WILLIAM RAMSAY, F.R.S.<br />
+<span class="huge">ELEMENTS AND ELECTRONS</span><br />
+<i>Diagrams</i></p>
+
+<p>The electron&mdash;&#8220;the atom of electricity&#8221;&mdash;is shown to be separable from
+matter, and to be capable under certain circumstances of independent
+existence. The book shows that the electron must be regarded as a kind of
+&#8220;element&#8221; itself, with much stronger claims to &#8220;elementary&#8221; or
+undecomposable characters than the bodies hitherto ranked as elements.</p></div>
+
+
+<p>&nbsp;</p><p>&nbsp;</p>
+<hr style="width: 50%;" />
+<p><strong>Footnotes:</strong></p>
+
+<p><a name="f1" id="f1" href="#f1.1">[1]</a> <i>Chemical Phenomena in Life</i>, pp. 62-3, by Dr. Frederick Czapek
+(Harper&#8217;s Library of Living Thought). The reader is strongly recommended
+to study this work in the present connection.</p>
+
+<p><a name="f2" id="f2" href="#f2.1">[2]</a> <i>Wonders of Life</i>, by Ernst Haeckel, Professor at Jena University, p.
+130.</p>
+
+<p><a name="f3" id="f3" href="#f3.1">[3]</a> <i>Wonders of Life</i>, pp. 127-8.</p>
+
+<p><a name="f4" id="f4" href="#f4.1">[4]</a> <i>Chemical Phenomena in Life</i>, p. 58.</p>
+
+<p><a name="f5" id="f5" href="#f5.1">[5]</a> <i>Ibid.</i>, p. 22.</p>
+
+<p><a name="f6" id="f6" href="#f6.1">[6]</a> <i>Other Worlds</i>, by Garrett P. Serviss, pp. 63-4.</p>
+
+<p><a name="f7" id="f7" href="#f7.1">[7]</a> <i>Modern Painters</i>, by John Ruskin.</p>
+
+<p><a name="f8" id="f8" href="#f8.1">[8]</a> If this experiment could be carried out, it would be necessary 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.</p>
+
+<p><a name="f9" id="f9" href="#f9.1">[9]</a> The movements of not a few double stars point to perturbations caused
+by the attraction of unseen bodies. There are also a number of instances
+known of &#8220;Eclipse&#8221; or &#8220;Algol-type&#8221; variable stars, in which the presence
+of a dark companion is indicated by the diminution of the light of the
+star at regular intervals.</p>
+
+<p><a name="f10" id="f10" href="#f10.1">[10]</a> <i>Proc. R. Soc.</i>, LXXX, 50, 1907.</p>
+
+<p><a name="f11" id="f11" href="#f11.1">[11]</a> <i>Nature</i>, LXXX, 158 (April 8th, 1909).</p>
+
+<p><a name="f12" id="f12" href="#f12.1">[12]</a> &#8220;Periodic Changes upon the Moon,&#8221; <i>Memoirs</i>, British Astronomical
+Association, Vol. XIII, p. 88.</p>
+
+<p><a name="f13" id="f13" href="#f13.1">[13]</a> <i>The Moon</i>, by Philip Fauth, p. 156.</p>
+
+<p><a name="f14" id="f14" href="#f14.1">[14]</a> <i>Radiation in the Solar System: Its Effects on Temperature, and its
+Pressure on Small Bodies</i>, by Dr. J. H. Poynting (<i>Phil. Trans. of the
+Royal Society</i>, Vol. 202 A).</p>
+
+<p><a name="f15" id="f15" href="#f15.1">[15]</a> <i>Publ. of the Astron. Soc. of the Pacific</i>, Vol. II, pp. 286-8.</p>
+
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
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+Project Gutenberg's Are the Planets Inhabited?, by E. Walter Maunder
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
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+
+Title: Are the Planets Inhabited?
+
+Author: E. Walter Maunder
+
+Release Date: April 23, 2011 [EBook #35937]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK ARE THE PLANETS INHABITED? ***
+
+
+
+
+Produced by Jonathan Ingram and the Online Distributed
+Proofreading Team at https://www.pgdp.net (This file was
+produced from images generously made available by The
+Internet Archive/American Libraries.)
+
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+
+
+
+
+HARPER'S LIBRARY of LIVING THOUGHT
+
+
+
+
+  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 & BROTHERS
+  LONDON AND NEW YORK
+
+  45 ALBEMARLE STREET, W.
+  1913
+
+
+
+
+  _Published March, 1913_
+
+
+
+
+CONTENTS
+
+
+  CHAPTER                                           PAGE
+
+     I. THE QUESTION STATED                            1
+
+    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             111
+
+     X. THE MAJOR PLANETS                            122
+
+    XI. WHEN THE MAJOR PLANETS COOL                  133
+
+   XII. THE FINAL QUESTION                           143
+
+        INDEX                                        163
+
+
+
+
+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 difference 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 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 ourselves, 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
+richness of vegetation and no fulness and complexity of animal life would
+justify an explorer in describing some land that he had discovered as
+being "inhabited" if no men were there, so we cannot rightly speak of any
+other world as being "inhabited" 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 hereafter into manifestations with
+which we could claim a nearer kinship.
+
+On the other hand, of necessity we are precluded from extending our
+enquiry to the case of disembodied intelligences, if such be conceived
+possible. All created existences must be conditioned, 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 subsist. Gnomes, sylphs, elves, and
+fairies, and all similar conceptions, escape the possibility of discussion
+by our ignorance of their properties. As nothing can be asserted of them
+they remain beyond investigation, as they are beyond sight and touch.
+
+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 world 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.
+
+For myself, if there be any theological significance 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 theological 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 concerned with the
+highest form of organism, the one furthest from these two dividing lines.
+
+It suffices that living organisms do exist here, and exist under
+well-defined conditions. Wanting these conditions, they perish. We can, to
+a varying degree, determine the physical conditions prevailing 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
+essentially 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
+particularly 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; surrounding this, a bluish
+zone that is faintly luminous; 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 continuously 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 restricted, 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 which have been produced by the chemical reactions 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 more than twenty to thirty years.
+These experiences 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 production 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 temporarily, really suspended.
+
+"Life is, therefore, quite inseparable from chemical 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."[1]
+
+The essential quality, therefore, of life is continual 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, nitrogen, hydrogen, and sulphur. The
+compounds which these five elements form with each other are most complex
+and varied, and they also admit to combination--but in smaller
+proportions--some of the other elements, of which phosphorus, potassium,
+calcium, magnesium, and iron are the most important.
+
+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 composition 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."[2]
+
+Haeckel, the writer just quoted, describes the plasm, the universal basis
+of all the vital phenomena, 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 distinguishes
+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 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 substance 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 distributed 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."[3] 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 are carried on contemporaneously,
+without one disturbing the other in the slightest degree."[4]
+
+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."[5]
+
+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 undergoing 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.
+
+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 for its
+continuance the presence of one indispensable factor--WATER.
+
+Protoplasm itself, as Czapek 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 protoplasm, its continual movement in the cell, can
+only be carried on in water.
+
+WATER is the compound of oxygen and hydrogen 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 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 0 deg. Centigrade, it boils at 100 deg. Centigrade.
+Necessarily, our experiences are mostly confined within this range, and
+therefore 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
+destroyed, 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 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 dissolved; 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 outside the atmosphere. As the Earth
+condensed from the original nebula, and cooled and solidified, a 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 ceaseless 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."[6]
+
+"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 intermediate 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 spread of intermediate
+being;--which should appease 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."[7]
+
+The leaf and the cloud are the signs of a habitable 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."
+
+
+
+
+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 points 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 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 perpetual
+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
+years 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 introduction to our subject. Further, those conditions
+themselves will bring out certain principles that are of necessary
+application when we come to consider 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 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 deduce 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 describing 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 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.[8]
+
+This relation is one of great importance when we realize that the pressure
+of the Earth's atmosphere is 14.7 lb. 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
+atmosphere 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
+lb.
+
+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 atmosphere, 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 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 remaining
+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, 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
+transcends 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
+surface 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
+temperature 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 pressures, on the other hand, apart from the high
+temperatures, would probably solidify every element, 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 appearance 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 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 impressed with the
+difficulty of conceiving that a gaseous body like the Sun could, under the
+conditions 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 incompatible 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 brightness, and
+irregularly scattered. This mottling is sometimes coarsely, sometimes
+finely textured; in some regions it is sharp and well defined, in others
+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 _sunspots_, 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
+_faculae_, 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 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 completely disappeared within
+less than half an hour. The velocity of the gas streams in such eruptions
+often exceeds one hundred miles a second; sometimes, 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 thousand 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 constitution,
+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, 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 negative 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 condition, enclosed by the
+luminous shell which we term the photosphere. This shell Prof. C. A. Young
+and the majority of astronomers regard as consisting of a relatively thin
+layer of glowing clouds, justifying the quaint conceit of R. A. Proctor,
+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 substance 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, refuses
+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 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 of life. 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.[9] 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 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 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 essentially 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
+temperatures 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 flutings 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.[10]
+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 metabolism of nitrogenous carbon
+compounds in association 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 compounds 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 Mendeleeff's 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 algebraically) 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 acquainted. New elements, no doubt,
+will be discovered in the future, as in the past; and indeed we may be
+able to discover them and learn their atomic weights and properties
+without 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 _Nebulium_,
+will have an atomic weight of 2.1. Prof. Max Wolf, of Heidelberg, has
+recently pointed out[11] 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 abundant 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 discovered between the orbits of Mars and
+Jupiter, 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 planetoidal 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 0 deg. Centigrade, and
+boils at 100 deg. 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 elements 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 0 deg. to 100 deg. Centigrade, have we any reason to
+suppose that protoplasm itself would be able to endure these outlying
+temperatures? Looking through the 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?
+
+
+
+
+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 looking 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 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 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 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 suspected,
+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
+recognized with difficulty. Thus the terse remark of Maedler, "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 metropolis," 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.
+
+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
+triangular 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."[12]
+
+Some observers explain this cycle of changes as due merely to the peculiar
+contour of the two 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
+probably 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 Maedler described the two craters as being
+exactly alike eighty years ago, Messier A is now distinctly the larger;
+but it is very doubtful whether Maedler'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 brightest formation on the Moon, so bright indeed that
+Sir William Herschel, observing it when illuminated 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 unusual 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 Molesworth 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.
+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."[13]
+
+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 manifestly 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 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 observations. 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 contained 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 because 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 "terminator," 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 perfectly smooth. The grey plains are therefore not expanses of
+water now, nor were they in time past. 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 formations 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
+destroyed 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 reappearance 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 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
+disconcerting; it is hardly less disconcerting to observe 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 spectroscope, 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 1/300th of
+that of the Earth by Neison, and as 1/10000th by W. H. Pickering. If the
+Moon possessed an atmosphere bearing the same proportion 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 Sun as the Earth, and
+therefore, surface for surface, 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 reflection, and is only
+bright by contrast with the darkness 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 Mountain, 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 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 accomplish, 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 wave
+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
+immediately. In 1898, Prof. Very, following up Langley's line of work,
+concluded that the temperature of the lunar soil must range through about
+350 deg. Centigrade, considerably exceeding 100 deg. at the height of the lunar
+day, and falling to about the temperature of liquid air during the 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, therefore, that the
+time of rotation of a planet is an important 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
+inhabited, 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
+tremendous 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
+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 superhuman 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, however, 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 1/70th of the diameter of the Moon to 1/130th. 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 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 telescopes.
+
+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 unmistakable. 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 oppositions 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 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 knowledge of Mars must be
+ascribed to the two German astronomers, Beer and Maedler, who made a series
+of drawings in the years 1830, 1832 and 1837, by means of a telescope of 4
+inches aperture, from which they were able to construct a chart of the
+entire globe. This chart may be 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 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 satisfaction 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 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 astronomers of skill and experience had observed the
+planet in 1877 without detecting the network of 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 observatories, 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 confirmation of not a few of Schiaparelli's "canals"; even
+in 1877 a few of the easiest and most conspicuous 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
+discoveries increased. It has now long been known 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
+objective basis.
+
+As this controversy has passed away, another and a very different one has
+arisen out of an unfortunate 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
+descriptions." 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
+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 believing that the "canals" are
+artificial constructions, or may they be merely natural formations?
+
+In 1894, Mr. Percival Lowell founded at Flagstaff, 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 irrigation, are fighting a losing battle against the gradual
+desiccation of their planet.
+
+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 unlocking 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 curiously 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 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 character: "_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. 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 regularly 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,
+    unconscious 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).
+
+    "That intelligence should thus mutely communicate 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
+achievement 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 existence 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 the evolutionary
+development necessary to its attaining its highest expression,--that where
+the material living organism has become the tabernacle and instrument of
+the conscious intelligent spirit.
+
+
+
+
+CHAPTER VII
+
+THE CONDITION OF MARS
+
+
+The planet Mars is the debatable ground between 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
+living 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
+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.
+
+PLANETARY STATISTICS
+
+  +--------------------------------------------++--------++----------------
+  |                                            || Minor  ||          Inner
+  |                                            ||Planets.||
+  +--------------------------------------------++--------++-------+-------+
+  |                                            || Ceres  || Moon  |Mercury|
+  +--------------------------------------------++--------++-------+-------+
+  |PROPORTIONS OF THE PLANETS:--               ||        ||       |       |
+  | Diameter in miles                          ||   477  || 2163  | 3030  |
+  |     "    [Symbol] = 1                      ||  0.06  || 0.273 | 0.383 |
+  | Surface, [Symbol] = 1                      ||  0.004 || 0.075 | 0.147 |
+  | Volume,  [Symbol] = 1                      ||  0.0002|| 0.02  | 0.06  |
+  | Density, Water = 1                         ||  2.8 ? || 3.39  | 4.72  |
+  |     "    [Symbol] = 1                      ||  0.5 ? || 0.61  | 0.85  |
+  | Mass,    [Symbol] = 1                      ||  0.0001|| 0.012 | 0.048 |
+  | Gravity at surface, [Symbol] = 1           ||  0.028 || 0.17  | 0.33  |
+  | Rate of Fall, Feet in the First Second     ||  0.45  || 2.73  | 5.30  |
+  | Albedo                                     ||  0.14  || 0.17  | 0.14  |
+  |                                            ||        ||       |       |
+  |DETAILS OF ORBIT:--                         ||        ||       |       |
+  | Mean Distance from Sun in millions of miles||257.1   ||92.9   |36.0   |
+  |      "          "   Earth's distance = 1   ||  2.767 || 1.000 | 0.387 |
+  | Period of Revolution, in years             ||  4.60  || 1.00  | 0.24  |
+  | Velocity, in miles per second              || 11.1   ||18.5   | 9.7   |
+  | Eccentricity                               ||  0.0763|| 0.0168| 0.2056|
+  | Aphelion Distance, Perihelion = 1          ||  1.157 || 1.034 | 1.517 |
+  | Inclination of Equator to Orbit            ||  (?)   || 1 deg..32'|  (?)  |
+  |                                            ||        || d h m |   d   |
+  | Rotation period                            ||  (?)   ||27.7.43| 88(?) |
+  |                                            ||        ||       |       |
+  |ATMOSPHERE, assuming the total mass of the  ||        ||       |       |
+  |  atmosphere to be proportional to the mass ||        ||       |       |
+  |  of the planet:--                          ||        ||       |       |
+  | Pressure at the surface in lb. per sq. in. ||  0.014 || 0.40  | 1.6   |
+  |     "       "      "    in "atmospheres"   ||  0.0009|| 0.027 | 0.108 |
+  | Level of half surface pressure in miles    ||119.0   ||19.6   |10.1   |
+  | Boiling point of water at the surface      ||        || 22 deg.C  | 53 deg.C  |
+  |                                            ||        ||       |       |
+  |TEMPERATURE:--                              ||        ||       |       |
+  | Light and heat received from Sun,          ||        ||       |       |
+  |                               [Symbol] = 1 ||  0.13  || 1.00  | 6.67  |
+  | Reciprocal of square-root of distance,     ||        ||       |       |
+  |                               [Symbol] = 1 ||  0.60  || 1.00  | 1.61  |
+  | Equatorial temp. of ideal planet, Absolute ||   188  ||  312 deg. |  502 deg. |
+  |     "       "       "       "    Centigrade||   -65  ||  +39  | +229  |
+  | Average temp. of ideal planet, Absolute    ||   174  ||  290  |  467  |
+  |     "       "       "       "  Centigrade  ||   -99  ||  +17  | +194  |
+  | Upper limit under zenith sun, Absolute     ||   248  ||  412  |  664  |
+  |     "       "       "       " Centigrade   ||   -25  || +139  | +391  |
+  | Average temp. of equivalent disc, Absolute ||   223  ||  371  |  598  |
+  |     "       "       "       "    Centigrade||   -50  ||  +98  | +325  |
+  |                                            ||        ||       |       |
+  +--------------------------------------------++--------++-------+-------+
+
+  ------------------++--------++--------------------------------------+
+   Planets.         ||        ||            Outer Planets.            |
+                    ||        ||                                      |
+  +--------+--------++--------++---------+---------+--------+---------+
+  |  Mars  | Venus  || Earth  || Uranus  | Neptune | Saturn | Jupiter |
+  +--------+--------++--------++---------+---------+--------+---------+
+  |        |        ||        ||         |         |        |         |
+  |  4230  |  7700  ||  7918  ||   31900 |   34800 |  73000 |   86500 |
+  |  0.534 |  0.972 ||  1.000 ||   4.029 |   4.395 |  9.219 |  10.924 |
+  |  0.285 |  0.945 ||  1.000 ||  16.2   |  19.3   | 85.0   | 119.3   |
+  |  0.15  |  0.92  ||  1.00  ||  65.    |  85.    |760.    |1304.    |
+  |  3.92  |  4.94  ||  5.55  ||   1.22  |   1.11  |  0.72  |   1.32  |
+  |  0.71  |  0.89  ||  1.00  ||   0.22  |   0.20  |  0.13  |   0.24  |
+  |  0.107 |  0.820 ||  1.000 ||  14.6   |  17.0   | 94.8   | 317.7   |
+  |  0.38  |  0.87  ||  1.00  ||   0.90  |   0.89  |  1.18  |   2.65  |
+  |  6.11  | 13.99  || 16.08  ||  14.47  |  14.31  | 18.97  |  42.61  |
+  |  0.22  |  0.76  ||  0.50? ||   0.60  |   0.52  |  0.72  |   0.62  |
+  |        |        ||        ||         |         |        |         |
+  |        |        ||        ||         |         |        |         |
+  |141.5   | 67.2   || 92.9   ||1781.9   |2791.6   |886.0   | 483.3   |
+  |  1.524 |  0.723 ||  1.000 ||  19.183 |  30.055 |  9.539 |   5.203 |
+  |  1.88  |  0.62  ||  1.00  ||  84.02  | 164.78  | 29.46  |  11.86  |
+  | 15.0   | 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 |   1.101 |
+  |24 deg..0'  |  (?)   || 23 deg..27'||   (?)   |   (?)   | 26 deg..49'|  3 deg..5'  |
+  |h  m  s |        || h  m  s|| h m     |         | h  m   | h  m    |
+  |24.37.23|  (?)   || 23.56.4|| 9.30(?) |   (?)   |  10.14+-|  9.55+-  |
+  |        |        ||        ||         |         |        |         |
+  |        |        ||        ||         |         |        |         |
+  |        |        ||        ||         |         |        |         |
+  |        |        ||        ||         |         |        |         |
+  |  2.1   | 11.1   || 14.7   ||  11.9   |  11.5   | 19.4   | 103.8   |
+  |  0.143 |  0.754 ||  1.000 ||   0.81  |   0.78  |  1.32  |   7.06  |
+  |  8.8   |  3.8   ||  3.3   ||   3.7   |   3.8   |  2.8   |   1.3   |
+  |  53 deg.C  |  92 deg.C  || 100 deg.C  ||  94 deg.C   |  93 deg.C   | 108 deg.C  |  166 deg.C  |
+  |        |        ||        ||         |         |        |         |
+  |        |        ||        ||         |         |        |         |
+  |  0.43  |  1.91  ||  1.00  ||   0.003 |   0.001 |  0.011 |   0.037 |
+  |  0.81  |  1.18  ||  1.00  ||   0.23  |   0.18  |  0.32  |   0.44  |
+  |   253 deg. |   368 deg. ||   312 deg. ||    71 deg.  |    56 deg.  |   101 deg. |    137 deg. |
+  |   -20  |   +95  ||   +39  ||   -202  |   -217  |  -172  |   -136  |
+  |   235  |   342  ||   290  ||    66   |    52   |   94   |    127  |
+  |   -38  |   +69  ||   +17  ||   -207  |   -221  |  -179  |   -146  |
+  |   337  |   486  ||   412  ||    94   |    74   |   133  |    180  |
+  |   +64  |  +213  ||  +139  ||   -179  |   -199  |  -140  |    -93  |
+  |   300  |   438  ||   371  ||    84   |    67   |   120  |    162  |
+  |   +27  |  +165  ||   +98  ||   -189  |   -206  |  -153  |   -111  |
+  |        |        ||        ||         |         |        |         |
+  +--------+--------++--------++---------+---------+--------+---------+
+
+For in regard to this particular premiss we can do away with hypothesis,
+and deal only with certain 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 dispute 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 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 deg. C., sank to 9 deg. 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 120 pulsations a minute instead of his normal 66. When Mr.
+Glaisher and Mr. Coxwell made their celebrated ascent between 1 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 deg.; at 26,000 feet to -15.2 deg.;
+and at 39,000 feet the temperature was down to -16.0 deg. 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 useless
+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
+attained 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 with moisture, 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 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
+ascending current on Mars would therefore take 1/0.38 x 1/0.38 = 1/0.145,
+or seven times as long to 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 circulation, 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 immense thickness of an obscuring medium. And yet the details of the
+surface are seen with remarkable 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 1/16th,
+and 1/12th, 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 transparent.
+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 practically 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 atmosphere. 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 1/9th of that of the Earth; exactly 0.107. It is
+distributed over a smaller surface, 0.285. Consequently 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 their great ascent; it is about
+one-half the pressure that is experienced on the top of the very highest
+terrestrial mountains. But the habitable regions of the Earth do not
+extend even so far upward as to the level of a pressure of 7.3 lb. 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 atmosphere 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 0 deg. C. and boils at 100 deg. C.; so that for one hundred degrees it
+remains in a liquid condition. On Mars, under the assumed conditions,
+water would boil at 53 deg. 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 atmosphere 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 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 determining 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 read 2.5
+inches; and water would boil at 44 deg. 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 diminished.
+
+These statistics may seem rather dull and tiresome, 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 unchanged. 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 visiting 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 Lilliputians take a truer view of the effect of the
+alteration 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 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 anywhere; 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 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 deg.F., or 16 deg.C.
+Three-sevenths of this would give us 7 deg.C. as the mean temperature of Mars,
+which would signify a planet not impossible 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 deg. on the
+Centigrade scale; the temperature of the Earth on the absolute scale
+therefore should be taken as 289 deg., and three-sevenths of this would give
+124 deg. of absolute temperature. But this is 149 deg. below freezing-point, and
+no life could exist on a planet under such conditions.
+
+But the mean temperature of Mars cannot be computed 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 Law, the radiation varies for
+a perfect radiator with the 4th power of the absolute temperature; so that
+if Mars were at 124 deg. abs., while the Earth were at 289 deg. 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 expenditure 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 reflecting one-eighth would
+be, as shown in the Table, 64 deg.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 resembling our Moon, and take the Moon's effective average
+temperature as 297 deg. abs., the corresponding temperature for Mars is 240 deg.
+abs., and the highest temperature is four-fifths of 337 deg. = 270 deg. 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 deg. 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
+temperature of his surface is everywhere below the freezing-point of
+water."[14] As the atmospheric circulation on Mars must be languid, and
+the atmosphere itself is very rare, the general condition 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 64-1/2 deg.--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, latitude 64-1/2 deg. 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.
+
+Now the mean temperature of latitude 64-1/2 deg., say the latitude of
+Archangel, is just about freezing-point (0 deg.C.), while that of the equator
+is about 28 deg.C. We should therefore expect from this a difference between
+the mean temperatures of the Earth and Mars of 28 deg.; that is to say, as the
+Earth stands at 16 deg.C, Mars would be at -12 deg.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 moving 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 deg. which we
+find on the Earth between 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 deg. as the true equivalent difference between the mean
+temperatures of Singapore and Archangel; i.e. of the Earth and Mars; and
+Mars would stand at -40 deg.C. The 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 condition 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 telescope 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--presumed 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 Protonilus is seen as a
+dark canal. Now the Protonilus is in North Lat. 42 deg., 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 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 temperatures has been laid down
+for every 10 deg. of latitude on the Earth, derived as far as possible from
+accepted isothermals in continental countries in the northern hemisphere.
+From this curve ordinates have been drawn at each 10 deg., upward to show
+average deviation from the mean temperature for the hottest part of the
+day in summer, downward 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 temperature
+of the Earth has been taken as 16 deg.C, and as representing that prevailing
+in about 42 deg. 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 state.
+
+In constructing a similar diagram for Mars, three modifications have to be
+made. First of all, the mean temperature of the planet must be
+considerably 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 atmosphere, and partly on account of the length of the Martian year.
+
+
+[Illustration: THERMOGRAPHS OF THE EARTH AND MARS]
+
+
+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 temperature 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 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 confined 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 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 substitute 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 warmest 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 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.
+
+
+
+
+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
+improvement 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 objects appear repeatedly on the drawings
+made by Beer and Maedler 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 Maedler undoubtedly portrayed 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 growth 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 Maedler, 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 attention to these two objects,
+and a mass of complicated structure has been brought out in the regions
+which were so simple in the sight of Beer and Maedler, 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
+Maedler, 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 themselves as uniform spots,
+precisely circular in outline? It is clear that Beer and Maedler 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 Maedler
+was that they were too small to be defined in their instruments; their
+minor irregularities were therefore invisible, and their apparent
+circularity covered detail of an altogether different form.
+
+Beer and Maedler only drew two such spots; Lowell shows about two hundred.
+Beer and Maedler's two spots seemed to them exactly alike; these two spots
+as we see them to-day have no resemblance to each other. Prof. Lowell's
+two hundred oases, with few exceptions, seem all of the same character; is
+it possible to suppose, if telescopes 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 defined vision, as would
+be 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 defined; that is to say for the
+spots to be seen as two separate objects.
+
+It will be seen that there is a wide range between objects that are large
+enough to be quite unmistakably perceived, and objects which are large
+enough to have their true outline really defined. 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 defined, 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;" 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 defined
+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 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 telescope;
+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 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 appearance 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 geometrical 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 described by H. G. Wells in the
+_War of the Worlds_.
+
+There are too many oversights in the canal theory.
+
+Thus no source is indicated for the moisture supposed 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 polar 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 conveyance 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
+conveyed 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 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 significance 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 continent, and its breadth would not be
+uniform from end to end, but the region fertilized would grow narrower
+with increase of distance from the fountain head of the canal.
+
+Under what conditions can we see straight lines, perfectly uniform from
+end to end, spring into existence, 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 defined. 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 experiments; they were keen-sighted,
+well drilled; accustomed to do what they were told without asking
+questions; and they knew nothing whatsoever of astronomy, certainly
+nothing about Mars.
+
+A diagram was hung up, based upon some drawing or other of the planet made
+by Schiaparelli, 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 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 markings 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 marvellous 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. Newcomb 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 Remembrancer,
+sent to me, in his 91st 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 supporting 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 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 disappear. In this, and not in any gigantic engineering 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.
+
+
+
+
+CHAPTER IX
+
+VENUS, MERCURY AND THE ASTEROIDS
+
+
+Of all the planets, Venus appears, to the unassisted 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 satisfying. 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 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 and 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 lb. 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 deg.abs., or
+95 deg.C, and its corresponding mean temperature is 69 deg. C. But water under a
+pressure of 11.2 lb. will boil at 93 deg. 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 perpetual 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.
+
+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 neighbour, 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 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 maintained 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 deg. C., while water will boil at 40 deg. 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.
+
+Since Mercury wears no concealing veil of atmosphere, 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 say, 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 development of the
+theory of tidal friction, puts the condition of Mercury in quite a new
+light. No alternation 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 deg. C. above freezing-point, to 270 deg. C. below. It is true
+that between the two hemispheres there is a "debatable land," for, owing
+to the ellipticity of the orbit, the face turned to the Sun is not exactly
+the same at all times, and a region about 47 deg. 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 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 deg. 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 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 habitability
+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 inhabited
+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 introduced 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 2-1/2 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 proportional change in this one planet's condition than we should
+experience if our Earth were transported 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 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 1/17th 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 distance 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 1/1200th; 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 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 all the rest, known and unknown, put together,
+though something 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 unfavourable
+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 consequence 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 fall
+far below freezing-point, the minimum on the dark side will approach the
+absolute zero.
+
+With Ceres the whole of the asteroidal 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 supports 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.
+
+
+
+
+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 confronted 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, movement 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 uniform tint, the surface being mottled with flocculent 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 background. 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, sometimes 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 colour as the belts, but deeper in tint, as if the fluid
+    medium could be seen to a greater depth."[15]
+
+In other words, Jupiter is a striped or banded planet, the bands lying
+along the direction of turning. 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 in
+different times. They may also alter their form and their colour. Jupiter
+seems, then, to be a planet with a great and rapidly changing atmosphere
+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 1/27th of the light and heat that we receive. But in Chapter VIII, we
+learnt from Mars that as this receives only 3/7ths of the Earth's light
+and heat, its mean temperature would sink to -30 deg.C.; the Earth's being
+16 deg.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 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 reconcile 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 contraction; 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, therefore, all the
+members of the solar system had at one and the same moment possessed the
+same surface 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, 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 dealing 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 whole cannot be put
+lower than 0.72, or about that of white paper. But many of the "belts" or
+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 lb. 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 probably 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
+portholes 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.
+
+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 movements. The clouds and
+storms of our own atmosphere 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 1/27th 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 inequalities 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 distance 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 spots are exceedingly swift, giving a rotation
+period of 9h. 50m. while the equatorial belt in general gives a period 5m.
+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 impossible, even unthinkable here, but the
+Great Red Spot, for all its long duration, is mobile and inconstant, and
+is therefore no portion of a solid permanent crust.
+
+The giant planet Jupiter, therefore, offers us an 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; Io, the innermost, is midway between the two in its
+dimensions. But Ganymede, the largest, is almost comparable with Mars, its
+diameter being 0.45 that of the Earth instead of the 0.53 of Mars.
+
+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 1/250th as much heat as each unit
+of the Sun's surface. In other words, the absolute surface temperature of
+Jupiter should be 1/4th that of the Sun, or about 1550 deg. 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 recipients 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.
+
+
+
+
+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 beautiful 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 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 sufficient. Or we can neglect the question of
+temperature 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 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                [Symbol] = 1    2.44     2.57     4.56    6.82
+    Surface,            [Symbol] = 1    6.0      6.6     20.8    46.6
+    Mass and Volume,    [Symbol] = 1   14.6     17.0     94.8   317.7
+    Gravity at surface, [Symbol] = 1    2.44     2.57     4.56    6.82
+  Rate of Fall, Feet in
+    the First Second                   39.2     41.3     73.3   109.7
+
+  ATMOSPHERE, assuming the
+  total mass of the atmosphere
+  to be proportional to
+  the mass of the planet:--
+
+  Pressure at the surface in lb.
+    per square inch                    88.2     97.0    305.8   685.0
+  Pressure at the surface in
+    "atmospheres"                       6.0      6.6     20.8    46.6
+  Level of half-pressure in miles       1.37     1.30     0.73    0.49
+  Boiling point of water at
+    surface                            127 deg.C    129 deg.C    148 deg.C   164 deg.C
+
+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 proportion. A man who here weighs 150 lb. will there
+weigh over 1000 lb.; 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 proportion. The atmosphere, supposing it to be in proportion
+to the mass of Jupiter, will exercise a pressure of 46-1/2 "atmospheres,"
+or more than 680 lb., 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 upward 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 lb. 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 constructed 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 low range of hills that on the Earth would be considered 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 involve.
+
+The sevenfold gravity of Jupiter, taken in connection with this enormous
+atmospheric pressure, would tend to make the meteorological disturbances
+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 suddenness 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 cataclysmic.
+
+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
+atmospheric pressure would be about 21 "atmospheres," or more than 300 lb.
+on the square inch. The level of half-pressure would be reached at about
+three-quarters of a mile, and the force of gravity be nearly 4-1/2 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 deg. was
+reached. Here the eclipses would begin nearly three terrestrial 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 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 atmosphere 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 deg. to the plane of
+its orbit; the difference in its seasons is, therefore, 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 14th 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 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 deg. If this were the case for the
+Earth, the noonday sun for London would be, at the spring equinox, 38-1/2 deg.
+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 deg. above the horizon at midnight. At midsummer, indeed,
+it would be only 59 deg. high at noonday, but it would be north of the zenith
+instead of south, and at technical midnight, it would still be 44 deg. in
+altitude, thus moving round in a very small circle, only 15 deg. 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 darkness,
+of unmitigated cold, cannot possibly ever 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 sunlight, but also special lines due to
+their own atmospheres (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 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.
+
+
+
+
+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 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 components 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 brightness, 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, 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 acclimatizing
+themselves to these ever-changing vicissitudes. 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 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 forbids 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 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 observers 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 compared 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, 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
+destroyed. 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 consideration 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, 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 disqualifications 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 telescopes, 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 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 throughout 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 illustrations 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, 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
+
+  2,000,000,000,000,000,000,000,000,000 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 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 we 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, intelligent 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.
+
+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 conditions 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 waterless 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 litterateur 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
+diverse as they are numerous. And everywhere and always, water is found
+essential to protoplasmic 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 unrelated 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 intelligence
+are associated, and we 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 difference 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 supposed 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 associated with the development of one type or 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 possible for the skeleton of creatures from
+other worlds to be brought to us here, then biologists would as
+confidently pronounce on their intelligence 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 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 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 inhabited 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 world of ours. Heavy as the odds are against any
+particular world being an inhabited one, yet when the limitless extent of
+space is considered, and the innumerable numbers of stars and systems of
+stars, it seems but reasonable 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 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 endowed not by the action and intervention of other living
+matter, but without it--we have no knowledge, no experience. And so long
+as this continues 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 speculate 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 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 foreshadowing 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 than to determine how life first came; its
+experience 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 experience,
+we are not left without an answer. A voice has been heard, the voice of
+the Son of God Himself:
+
+"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."
+
+
+
+
+INDEX
+
+
+  Abbot, C. G., 27, 34
+
+  Albedo of Earth, 54, 81
+    ---- Jupiter, 127
+    ---- Mars, 81
+    ---- 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, 11, 34, 38, 39
+
+  Carbonic acid, 11
+
+  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, 1
+
+  Copper, 33
+
+  Corona, 40
+
+  Coxwell, 25, 75, 79
+
+  Cyanogen, 37, 38
+
+  Czapek, F., 11, 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
+
+  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, 81
+
+
+  "Inhabitant," 5
+
+  "Inhabited" Worlds, 2, 3, 4
+
+  Io, satellite of Jupiter, 131
+
+  Iron, 12, 33, 36
+
+
+  Jupiter, 122-32
+
+  ----, belts, 127, 129
+
+  ----, great red spot, 130
+
+  ----, proper motion of spots, 129
+
+  ----, 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
+
+  Lowell, P., 65, 66, 67, 69, 71, 81, 97, 98, 99, 101, 103, 104, 105, 106,
+  108, 109, 110
+
+  Lucifer, 111
+
+
+  Maedler, 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, 57-70, 78, 101, 102
+
+  ----, conditions of, 71-95
+
+  ----, 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, 11, 14, 15, 38
+
+  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
+
+  Nicholson, J. W., 40
+
+  Nickel, 33
+
+  Nilosyrtis, "canal" on Mars, 89
+
+  Nitrogen, 11, 37, 38, 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, 11, 36, 37, 38, 41
+
+
+  Periodic Law, Mendeleeff's, 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, 115
+
+  Proctor, R. A., 34, 77
+
+  Prominences, 29, 30, 37
+
+  Protofluorine, 40
+
+  Protonilus, "canal" on Mars, 89
+
+  Protoplasm, 11, 12, 13, 15, 38, 40, 154
+
+  Pyramid, Great, 45
+
+
+  Refraction, anomalous, 28
+
+  Reversing layer, 36
+
+  "Rice-grains," of Sun's surface, 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, T. 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, 11, 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
+
+  Tornadoes, 31, 137
+
+  "Twinkler," 114
+
+  Tycho, lunar crater, 46
+
+
+  Uranus, 132, 140
+
+
+  Venus, 57, 111-18
+
+  Verworn, Max, 7
+
+  Very, F. W., 55
+
+  Vesper, 111
+
+  "Victoria," hypothetical planet, 83
+
+
+  Wallace, A. R., 4
+
+  "War of the Worlds," 104, 156
+
+  Waste, 151, 152
+
+  Water, indispensable factor, 15, 41
+
+  Wells, H. G., 104, 156
+
+  Whewell, 4
+
+  Williams, A. Stanley, 104
+
+  Wolf, Max, 40
+
+
+  Young, C. A., 26, 33
+
+
+WILLIAM BRENDON AND SON, LTD.
+
+PRINTERS, PLYMOUTH
+
+
+
+
+Footnotes:
+
+[1] _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.
+
+[2] _Wonders of Life_, by Ernst Haeckel, Professor at Jena University, p.
+130.
+
+[3] _Wonders of Life_, pp. 127-8.
+
+[4] _Chemical Phenomena in Life_, p. 58.
+
+[5] _Ibid._, p. 22.
+
+[6] _Other Worlds_, by Garrett P. Serviss, pp. 63-4.
+
+[7] _Modern Painters_, by John Ruskin.
+
+[8] If this experiment could be carried out, it would be necessary 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.
+
+[9] The movements of not a few double stars point to perturbations 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.
+
+[10] _Proc. R. Soc._, LXXX, 50, 1907.
+
+[11] _Nature_, LXXX, 158 (April 8th, 1909).
+
+[12] "Periodic Changes upon the Moon," _Memoirs_, British Astronomical
+Association, Vol. XIII, p. 88.
+
+[13] _The Moon_, by Philip Fauth, p. 156.
+
+[14] _Radiation in the Solar System: Its Effects on Temperature, and its
+Pressure on Small Bodies_, by Dr. J. H. Poynting (_Phil. Trans. of the
+Royal Society_, Vol. 202 A).
+
+[15] _Publ. of the Astron. Soc. of the Pacific_, Vol. II, pp. 286-8.
+
+
+
+
+Harper's Library of Living Though
+
+
+ARTHUR HOLMES
+
+THE AGE OF THE EARTH
+
+And Associated Problems. _Illustrated_
+
+Gives us the result of the latest research into this field of enquiry. The
+radioactive minerals are shown to be recording their own age with the
+exquisite accuracy of a chronometer--their records checking physical,
+astronomical, and geological methods of computation.
+
+
+PROF. A. W. BICKERTON
+
+THE BIRTH OF WORLDS AND SYSTEMS
+
+_Illustrated_
+
+_Preface by Prof. Ernest Rutherford, F.R.S._
+
+A graphic account of the formation of new stars from the collision of dead
+suns or other celestial bodies. The theory throws light on many
+astronomical problems, and with its conception of an immortal cosmos, is
+of great philosophical importance.
+
+
+PROF. SVANTE ARRHENIUS
+
+THE LIFE OF THE UNIVERSE
+
+_2 Vols. Illustrated_
+
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+useful, and most suggestive. There are many for whom the first chapters of
+Genesis are a subtle allegory covering the profoundest truths, and we are
+grateful to the author for having set out this mass of facts."--_The
+Globe._
+
+
+SIR OLIVER LODGE, F.R.S.
+
+THE ETHER OF SPACE
+
+_Illustrated_
+
+"This work by the great physicist will be found to possess an abiding
+charm and an intellectual stimulation."-_Observer._
+
+"Opens up new views into the nature of the universe. Precise and lucid, it
+summarises our knowledge of the substance which fills all space and
+penetrates all matter--the substratum of matter itself."--_Birmingham
+Post._
+
+_Please write for announcements and descriptive list:_
+
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+Harper's Library of Living Thought
+
+_Foolscap 8vo, gilt tops, decorative covers, richly gilt backs Per Volume:
+Cloth 2s. 6d. net. Leather 3s. 6d. net._
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+
+PROF. ARTHUR KEITH, M.D.
+
+(Hunterian Professor Royal College of Surgeons)
+
+ANCIENT TYPES OF MAN
+
+_Illustrated_
+
+"The kind of book that only a master of his subject could write. It must
+interest every thinking person."_--British Medical Journal._
+
+
+PROF. FREDERICK CZAPEK
+
+CHEMICAL PHENOMENA IN LIFE
+
+Discusses in clear, concise terms the great question--"Can life be
+explained by physics and chemistry?" It deals with the life-processes of
+plants, the molecular structure of protoplasm, organic synthesis in the
+cell, the nature of ferments, and the subject of inheritance.
+
+
+SIR A. TILDEN, F.R.S.
+
+THE ELEMENTS
+
+Speculations as to their Nature and Origin
+
+_Diagrams, &c._
+
+Points to the conclusion that the elements resulted from a change in some
+primal essence, and discusses "whether all may not be suffering a slow
+waste, which, in the long run, must lead back to the primal chaos."
+
+
+SIR WILLIAM RAMSAY, F.R.S.
+
+ELEMENTS AND ELECTRONS
+
+_Diagrams_
+
+The electron--"the atom of electricity"--is shown to be separable from
+matter, and to be capable under certain circumstances of independent
+existence. The book shows that the electron must be regarded as a kind of
+"element" itself, with much stronger claims to "elementary" or
+undecomposable characters than the bodies hitherto ranked as elements.
+
+
+
+
+Transcriber's Notes:
+
+Passages in italics are indicated by _italics_.
+
+Superscripted characters are indicated by {superscript}.
+
+The original text includes symbols which are represented by [Symbol] in
+this text version.
+
+
+
+
+
+
+End of Project Gutenberg's Are the Planets Inhabited?, by E. Walter Maunder
+
+*** END OF THIS PROJECT GUTENBERG EBOOK ARE THE PLANETS INHABITED? ***
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