Are the Planets Inhabited? Part 2

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 ma.s.s does not exceed that of the Moon in quite the same high ratio; but it is equal in ma.s.s 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 a.s.sure 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 pa.s.sed 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 a.s.sistance 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 p.r.o.nounced 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 Madler, "The full Moon knows no Maginus," has become a proverb amongst selenographers; yet Maginus is a fine walled plain some eighty miles in diameter, and its rampart attains a height in parts of 14,000 feet. Maginus lies near Tycho, which has been well named "the lunar metropolis," for from it radiates the princ.i.p.al 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 antic.i.p.ated. But the question arises whether all the changes that are a.s.sociated 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 surpa.s.ses 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 h.o.a.r frost is formed periodically round the two craters. It is also alleged that whereas Madler described the two craters as being exactly alike eighty years ago, Messier A is now distinctly the larger; but it is very doubtful whether Madler's description can be trusted to this degree of nicety. If it could, this would establish a permanent change in the actual structure of the lunar surface at this point.

There are several other cases of the same order of ambiguity. The most celebrated is Linne, a white spot about six miles in diameter on the Mare Serentatis. This object appears to change in size during the progress of the lunar day, and, as with Messier, some selenographers consider that it has also suffered an actual permanent change in shape within the last sixty or seventy years. Here again the evidence is not decisive; Neison is by no means convinced that a change has taken place, yet does not think it impossible that Linne may once have been a crater with steep walls which have collapsed into its interior through the force of gravity.

Another type of suspected change is a.s.sociated 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 phosph.o.r.escence 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 a.s.sured 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 pa.s.sion 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 pa.s.sage, 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 pa.s.ses 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, pa.s.ses 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 ma.s.s 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 017, 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 050, 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 s.p.a.ce 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 Ca.n.a.lS 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 a.s.serted 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 s.p.a.ce.

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 a.s.sertion 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...o...b..t; 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-gla.s.s 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 Ca.s.sini 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...o...b..t, 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 Madler, 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 cla.s.sic, 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 Ca.s.sini 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 surpa.s.sed since. At this time it was generally a.s.sumed 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...o...b..t 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 "ca.n.a.ls of Mars," and the discussion as to the real nature of these ca.n.a.ls 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 pa.s.s in review the whole of the immense ma.s.s of observations that have been acc.u.mulated 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 pa.s.sed. We now know that the "ca.n.a.ls" vary much in their visibility, and "curiously enough the ca.n.a.ls 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 ca.n.a.ls 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 "ca.n.a.ls" 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 "ca.n.a.ls"; 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 "ca.n.a.ls of Mars" are not mere figments of the imagination, but have a real objective basis.

As this controversy has pa.s.sed 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 "_ca.n.a.li_", 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 "_ca.n.a.li_" was unhappily rendered in English as "ca.n.a.ls," instead of "channels." "Channel" would have left the nature of the marking an open question, but, in English, "ca.n.a.l" 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 "ca.n.a.ls" 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 "ca.n.a.ls"; the discovery of a number of dark, round dots, termed by him "oases," at the junctions of the "ca.n.a.ls"; and the demonstration that the "ca.n.a.ls" 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 "ca.n.a.ls" and "oases" quite precludes the possibility of their being natural formations. Hence there has arisen the second controversy: that on the nature of the "ca.n.a.ls"; for Mr. Lowell considers that their presence proves the existence of inhabitants on Mars, who, by means of a t.i.tanic 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 a.s.sume. 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 ca.n.a.ls 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 s.p.a.ce 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 Ca.n.a.ls_"; and "_Mars as the Abode of Life_." In these he shows that to the a.s.siduity 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 s.p.a.ce it could be recognized only by the imprint it had made on the face of Mars."