In the High Heavens/Chapter 6

by Robert Stawell Ball

CHAPTER VI.

MARS.

 

HE facts with regard to the opposition of Mars in 1892 are sufficiently noteworthy to be stated with some detail, and we may first set them forth even at the risk of repeating a few things that will be familiar to those who have diligently studied the Nautical Almanack. It appears that the orbit of this particular planet is especially remarkable, among planetary orbits generally, for its departure from the circular form so nearly assumed in the movements of most of the other similar bodies. Mars has an orbit of so much eccentricity that its distance from the sun varies very considerably. It is sometimes as much as 153,000,000 miles off. It is sometimes as little as 127,000,000 miles. The orbit in which our earth revolves is much more nearly circular than is the orbit of Mars, but still the variations of the distance between the earth and the sun are too large to be overlooked, even though they may seem relatively unimportant. Under certain circumstances our earth may be as far from the sun as 93,500,000 miles, while the smallest magnitude to which the distance can shrink is 90,500,000 miles. These few facts will enable us to estimate the stretch of space that divides us from the other world in which so much interest is now being taken.

The longest distance that could possibly intervene between the two globes is found when the sun lies between them and when they are each at their greatest possible distance from it. On the other hand the most favourable condition for the observation of Mars will be when the planet is making its nearest approach to the sun, and when the earth happens to be in the same direction as Mars from the sun.

Fig. 13.—Orbits of the Satellites of Mars.

It can be shown that the very lowest value which the planet's distance from the earth can possibly assume would be about 35,000,000 miles. Nor is the condition of things which we have supposed one which will be often realised. No doubt every two years and two months, or more accurately every 780 days, the sun and Mars and the earth come nearly into a straight line, the earth being between the other two bodies; whenever this happens we have what is called the opposition of Mars. If the orbits of both Mars and the earth were circular, then any one opposition would be as good as any other, so far as proximity is concerned: for the distance between the earth and the planet on each such occasion would be simply the difference between their two distances from the sun. But, as we have already seen, the orbits are not circular, and consequently there is very considerable variety in the different oppositions as regards the advantages which they offer to the astronomer. It might, for instance, happen that Mars was at its greatest distance from the sun at the time when the earth crossed between it and the sun. Then the interval between the two bodies would be more than 60,000,000 miles, and the opposition would be as unsuitable as it could possibly be. It thus follows that such a very favourable opposition as that through which Mars passed in 1892 only arises from a particular combination of circumstances which but rarely occur.

It may, however, be of interest to lay down the principles which exhibit the law by which the succession of such oppositions is determined. The opposition of Mars can occur while the earth is at any part of its orbit; that is, the opposition may happen in any month of the year. The part of Mars' path which at present lies nearest the sun runs towards that part of the earth's track through which the earth passes in August. Hence it follows that if an opposition takes places in August it does so at a time when Mars is as near to the sun as is possible. It is true that the earth is not then nearest to the sun, but as the effect contributed by the variation of the earth's distance is of little importance, it follows for all practical purposes that when the opposition takes place in August, it does so under the most desirable circumstances.

On the other hand, if it should happen that the opposition took place about February, then the conditions would be as unfavourable as possible, for though Mars, earth, and sun were in a straight line in the order I have named, yet at this part of its path Mars is at its greatest distance from the sun, and consequently the opposition takes place when the two bodies are separated by the greatest distance that is at present possible on the occasion of an opposition.
Fig. 14.—Apparent dimensions of Mars at its mean and extreme distances. At the greatest distance, Mars, being on the other side of the Sun, is invisible.

It thus happens that in the February oppositions the distance between the two bodies is double as great as it is in the August oppositions. At double the distance the planet only looks one-fourth the size, and hence the appearance of Mars, when the opposition is in February, is widely different from that which it presents in the glories of an August opposition.

We can now understand why such an opportunity as that which we are at present referring to is a rare one. In the first place an opposition of Mars occurs once every 780 days. In the second place the opposition is just as likely in the long run to take place in one month as another. Only, however, when it occurs about August is it a really favourable one. If a friend paid us a visit once every two or three years, and if his visits were impartially distributed over the various seasons, it would not be on many occasions in a lifetime that we should expect to receive him during the grouse shooting. Of somewhat similar infrequency are the favourable visits of Mars, but whenever he does happen to come into opposition about the time when the grouse are being slaughtered, then his ruddy form blazes with an unwonted splendour.

A knowledge of these facts points out that the opposition of Mars in 1892 was the best that has offered itself since 1877, and the best that will offer itself for many years to come. Hence it is that so much interest has been manifested in this phenomenon, for though it would not be true to say that Mars is our nearest neighbour in the heavenly host, yet there are circumstances which render his globe much more instructive to us than any of the other heavenly bodies.

Of course, the moon is always much closer to the earth than is Mars. Even when the moon is at its greatest Fig. 15.—The Seasons in Mars. May 27, 1890.

distance from us it is still not one-hundredth part of the distance by which we are divided from Mars when that planet is at its nearest. Yet we can never look on the moon as a neighbouring world in the same sense in which we look at Mars. The moon is a globe of quite a different order from the earth. Its want of air and water in any measure comparable with the abundance of such elements on the earth at once establishes so profound a difference between it and the earth, that we naturally relinquish the supposition that our satellite can have any resemblance whatever to the earth when considered as the abode of organized life.

But there is another planet with which, in all probability, we have much closer affinities than we have even with Mars. Fig. 16.—The Seasons in Mars. August 4, 1892.The planet Venus happens to be almost exactly of the same size as the earth. If models of the two globes were inspected, it would require careful measurement to say which of the two was the greater, though, as a matter of fact, to some insignificant extent, we may remark that both in volume and in mass the earth exceeds the sister planet. Venus is also, in a strict sense, a closer neighbour to us than Mars. At no time can it wander so far from us as Mars is accustomed to do, while at its closest approach the distance from Yen us to the earth is less than two-thirds of that by which Mars when nearest still remains separated from us. Nor are other points of resemblance between the earth and Venus wanting. Especially may we notice that, like its companion globe, Venus is encompassed with a copious atmosphere. Everything, therefore, so far as we can judge, points to the conclusion that Venus is a world resembling our own in important features of physical constitution, so that quite possibly it is adapted to be a residence for organized beings.

Here, unfortunately, telescopic examination gives us but little aid. Notwithstanding the considerable size of Venus, and the closeness with which she makes her approach, we are unable to scrutinize her surface with the success that we desire. That very splendour which makes the evening star so lustrous an object decks the planet in such a shining robe that we are unable to make out the details on its surface. We can, no doubt, sometimes see that her form is an exquisite crescent which passes through a succession of phases. We can occasionally detect, under rare favourable circumstances of climate and instrumental equipment, slight indications or marks on the surface of the planet which, with some help from the imagination, we can suppose to be indications of continents. Then, again, some observers have noticed that in the "cusps" at the ends of the crescent occasional interruptions and irregularities are presented which have been interpreted as implying the existence of great mountains on Venus. But when this is admitted we have said almost all that has ever been alleged to be discernible by us of the topography of that globe which is really our nearest planetary neighbour. The little that we have seen merely suggests what a wonderful spectacle might be disclosed could we put Venus into a more favourable position. If Venus were placed where Mars is, then the greater size of the former planet would make it a far more striking spectacle than Mars ever can be. Mars happens to be the more interesting globe to us simply because it is better placed for observing. Everybody knows that you can read your book comfortably if you sit with the light so nearly behind you that it may fall on the page at which you are looking. This is the aspect in which Mars is presented at opposition. The sun, which illuminates Mars, is then at midnight, behind the observer, but its beams are directed full on the planet, and exhibit it under the most favourable conditions. But Venus is presented to us in quite a different manner. It is not pleasant to try to read with the lamp in front of you, and your book held up between you and the lamp. Yet this is the way we have to look at Venus when it makes its closest approach. The consequence is that, while astronomers have abundance to tell us about the appearance of Mars, they have but little to say about the features of that other globe which is both larger and nearer to us than Mars, and with which, in all probability, we have closer affinities than we have with any other body in the universe.

From one cause or another, it happens that Mars is the most world-like of all the other globes which come within the range of effective observation. It would, indeed, be very rash to assert that other bodies may not have a closer resemblance to our earth than Mars has, but of them we have either little knowledge, as in the case of Venus, or no knowledge at all. No doubt both Jupiter and Saturn can vie with Mars in the copiousness of detail with which they delight the astronomers who study them. These grand planets are deserving of every attention, but then the interest they excite is of a wholly different kind from that which makes a view of Mars so attractive. Jupiter offers us a meteorological study of the most astounding cloud-system in creation. Saturn gives an illustration of a marvellous dynamical-system the like of which would never have been thought possible had it not actually presented itself to our notice. But the significance of Mars is essentially derived from those points of resemblance to the earth which are now engrossing attention. Mars is clearly a possible world, presenting both remarkable analogies and remarkable contrasts to our own world, and inducing us to put forth our utmost endeavours to utilise so exceptional an occasion as that presented in the close approach which it made in 1892. Let us see what we have learned about this globe.

In the first place, it should be noticed that Mars must be a small world in comparison with our own. The width of this globe is only 4,200 miles, so that its volume is but the seventh part of that of the earth. The weight of Mars is even less than what might have been expected from his bulk. It would take nearly ten globes, each as heavy as Mars, to form a weight equal to that of the earth. This fundamental difference in dimensions between Mars and our globe is intimately connected with certain points of contrast which it offers to the earth.

Of these the most important is that which concerns the atmosphere. When we consider what the qualifications for a globe as a possible abode for organic beings should be, it is natural to inquire first into the presence or the absence of an atmosphere. Seeing that our earth is enveloped by a copious covering of air, it follows that the beings which dwell upon its surface must be specially adapted to the conditions which the atmosphere imposes.

Fig. 17.—Mars and one of its Satellites.

Most, if not all, animals utilise this circumstance by obtaining a proximate source of energy in the union of oxygen from the atmosphere with oxidizable materials within their bodies. In this respect the atmosphere is of such fundamental importance that it is difficult for us to imagine what that type of life best adapted for existence on an airless globe would be. In other respects which are hardly less important, the conditions of life are also dependent on the fact that we live at the bottom of an ocean of air. It is the atmosphere which, to a large extent, mitigates the fierceness with which the sun's rays would beat down on the globe if it were devoid of such protection. Again, at night, the atmospheric covering serves to screen us from the cold that would otherwise be the consequence of unrestricted radiation from the earth into space. It is, therefore, obvious that the absence of a copious atmosphere, though perhaps not so absolutely incompatible with life of some kind, must still necessitate types of life of a wholly different character from those with which we are familiar. In attempting, therefore, to form an estimate of the probability of life on another world, it is of essential importance to consider whether it possesses an atmosphere.

We may here lay down a canon which appears to be pretty general among the celestial bodies accessible to our observations. It may be thus stated. The larger the body the more abundant the atmosphere by which that body is surrounded. Of course this rule has to be understood with certain qualifications, and perhaps some exceptions to it might be suggested, but as a broad general fact it will hardly be questioned. Thus, to take at once the largest body of our system and one of the smallest—the sun and the moon—they both provide striking exemplifications of the principle in question. It is well known that the sun is enveloped by an atmosphere remarkable for the prodigious extent that it occupies. On the other hand the moon, which is by far the smallest of the bodies readily accessible to our observations, is, if not entirely devoid of gaseous investment, at all events only provided with the scantiest covering of this nature. But the chief interest that the principle we have laid down possesses, is found in the explanation which has been given of it. That explanation is both so recent and so remarkable that I am glad here to have the opportunity of setting it forth, as it has an important application in the case of Mars.

Modern research has demonstrated that what we call a gas is in truth a mighty host of molecules far too small to be perceptible by the most powerful microscope. Each of these molecules is animated by a rapid movement, which is only pursued for a short distance in one direction before an encounter takes place with some other molecule, in consequence of which the directions and velocities of the individual molecules are continually changing. For each gas the molecules have, however, a certain average pace, which is appropriate to that gas for that temperature, and when two or more gases are blended, as in our atmosphere, then each molecule of the constituent gases continues to move with its own particular speed. Thus, in the case of the air, the molecules of oxygen as well as the molecules of nitrogen, are each animated by their characteristic velocity, and the same may be said of the molecules of carbonic acid or of any other gas which, in more or less abundance, may happen to be diffused through our air. For two of the chief gases the average velocities of the molecules are as follows: oxygen, a quarter of a mile per second; hydrogen, one mile per second; in each case the temperature is taken to be 64°C. below zero, being presumably that at the confines of the atmosphere. It will be noticed that there is a remarkable difference between the speeds of the two molecules here mentioned. That of hydrogen is by far the of any gas.

We may now recall a fundamental fact in connection with any celestial body large or small. It is well known that, with the most powerful pieces of artillery that can be forged, a projectile can be launched with a speed of about half a mile per second. If the cannon were pointed vertically upwards the projectile would soar to a great elevation, but its speed would gradually abate, the summit of its journey would be duly reached, after which it would fall back again on the earth. Such would undoubtedly be the case if the experiment were made on a globe resembling our own in size and mass. But on a globe much smaller than the earth, not larger, for instance, than are some of the minor planets, it is certain that a projectile shot aloft from a great Armstrong gun would go up and up and would never return. The lessening gravitation of the planet would fail to recall it. Of course we are here reminded of Jules Verne's famous projectile. According to that philosopher, if a cannon was pointed vertically, and the projectile was discharged with a speed of seven miles a second it would soar aloft, and whether it went to the moon or not, it would at all events not return to the earth except by such a marvellous series of coincidences as those which he has described. But the story will at all events serve to illustrate the fact that for each particular globe there is a certain speed with which if a body leaves the globe it will not return.

It is a singular fact that hydrogen in the free state is absent from our atmosphere. Doubtless many explanations of a chemical nature might be offered, but the argument employed by Dr. G. J. Stoney is most interesting, inasmuch as it shows that the continued existence of hydrogen in our atmosphere would seem to be impossible. No doubt the average speed at which the molecules of this gas are hurrying about is only one mile a second,

Fig. 18.—Comparative sizes of Earth, Mars, Mercury, and the Moon.

and, therefore, only a seventh of the critical velocity required to project a missile from the earth so as not to return. But the molecules are continually changing their velocity and may sometimes attain a speed which is seven times as great as the average. Suppose, therefore, that a certain quantity of hydrogen were diffused through our air, every now and then a molecule of hydrogen in its wanderings would attain the upper limit of our atmosphere, and then it would occasionally happen that with its proper speed it would cross out into space beyond the region in which its movements would be interfered with by the collisions between other atmospheric molecules. If the attraction of the earth was sufficient to recall it, then, of course, it would duly fall back, and in the case of the more sluggishly moving atmospheric gases the velocity seems always small enough to permit the recall to be made. But it happens in the case of hydrogen that the velocity with which its molecules are occasionally animated rises beyond the speed which can be controlled by terrestrial gravity. The consequence is that every now and then a molecule of hydrogen succeeds in bolting away from the earth altogether, and escaping into open space. Thus it appears that every molecule of free hydrogen which happened to be present in an atmosphere like ours, would have an unstable connection with the earth, for wherever in the vicissitudes of things it happened to reach the very uppermost strata it would be liable to escape altogether. In the course of countless ages it would thus come to pass that the particles of hydrogen would all effect their departure, and thus the fact that there is at present no free hydrogen in the air over our heads may be accounted for.

If the mass of the earth were very much larger than it is, then the velocities with which the molecules of hydrogen wend their way would never be sufficiently high to enable them to quit the earth altogether, and consequently we might in such a case expect to find our atmosphere largely charged with hydrogen. Considering the vast abundance of hydrogen in the universe, it seems highly probable that its absence from our air is simply due to the circumstances we have mentioned. In the case of a globe so mighty as the sun, the attraction which it exercises, even at the uppermost layers of its atmosphere, is so intense that the molecules of hydrogen never attain pace enough to enable them to escape. Their velocity would have to be much greater than it ever can be if they could dart away from the sun as they have done from the earth. It is not, therefore, surprising to find hydro* gen in the solar atmosphere. In a similar manner we can explain the abundance of hydrogen with which the atmospheres of other massive suns like Sirius or Vega seem to be charged. The attraction of these vast globes is sufficiently potent to retain even an atmosphere solely composed of this subtle element.

It is now easy to account for the absence of atmosphere from the moon. We may feel confident from the line of reasoning here followed that neither of the gases, oxygen or nitrogen, to say nothing of hydrogen, could possibly exist in the free state on a globe of the mass and dimensions of our satellite. The pace with which the molecules of oxygen and nitrogen speed on their way would be quite sufficient to render their abode unstable if it should ever have appeared that circumstances placed such gases on the moon. We need, therefore, feel no surprise at the absence of any atmosphere from the neighbouring globe. The explanation is given by the laws of dynamics. We are placed at too great a distance from the small planets or asteroids, as they are called, to be able to see whether or not they have any gaseous surroundings. But it is possible, from the ingenious argument we are discussing, to assure ourselves that such small bodies must be quite as devoid of air as the moon. There are, we know, globes in our system only a few miles in diameter, and so small in mass, that a cricket-ball there, receiving the velocity it would get from the bat of a W. G. Grace, would go off into space never to return. It is quite obvious that the molecules of any gases we know would be far too nimble in their movements to remain prisoners at the surface of little globes of this description, to which their only bond was the feeble attraction of gravitation. It is, therefore, in the highest degree improbable we might, indeed, almost say impossible for gaseous surroundings to be preserved by any globe where the mass is not considerably greater than that of the moon.

In applying these considerations to Mars we have first to note that its mass and size are intermediate between those of the earth and the moon. It is much more capable of retaining an atmosphere than the moon, though its capability in this respect falls short of that possessed by the earth. But in such a case it is essential to depend not on mere generalities but on the actual numerical facts of the case. Without going too deeply into detail it is sufficient to observe that there must be for each globe a certain critical velocity represented by the least pace at which a missile projected from it will succeed in escaping altogether. In discussing this we may leave out of view the question of the resistance which the air opposes to the passage of the projectile. This is, no doubt, of vital importance in cases where actual artillery practice is concerned, yet it is not material to our present inquiry. The problem which we are considering depends on the movements of the molecules of air at the summit of the atmosphere, and the question is simply whether after they have made an incursion into free space there is sufficient efficiency in the attraction of the globe to effect their recall.

At the surface of Mars the speed which would carry a body away from its surface altogether is about three miles per second. It seems certain that the velocity of the molecules of hydrogen is often far in excess of this, and consequently free hydrogen is impossible as a permanent ingredient of the Martian atmosphere. Oxygen, however, has a molecular velocity only about one-fourth of that of hydrogen, and it seems unlikely that the oxygen molecules can ever have sufficient velocity to permit their escape from an atmosphere surrounding Mars. There is nothing, therefore, to prevent this element from being now present.

But the case of the vapour of water is especially instructive and interesting. Its molecules have a speed which averages about one-third of that attained by the molecules of hydrogen. It would seem that the utmost pace that the molecules of water could attain (being perhaps seven times the average velocity) would be about 2 miles per second. Now this would not be enough for escape from Mars, for we have seen that a speed of three miles per second would be required for this purpose. This argument suggests that the globe of Mars happens to approach very closely the dimensions and mass of the smallest world on which the continued existence of water would be possible. It would perhaps be going rather too far to say that a world almost the size of Mars must therefore be the smallest on which life could possibly be supported, but it is plain that our argument tends to support such a proposition.

Fig. 19.—Mars through the telescope.

The discussion we have just given will prepare us to believe that a planet with the size and mass of Mars may be expected to be encompassed with an atmosphere. Our telescopic observations completely bear this out. It is perfectly certain that there is a certain shell of gaseous material investing Mars. This is shown in various ways. We note the gradual obscuration of objects on the planet as they approach the edge of the disc, where they are necessarily viewed through a greatly increased thickness of Martian atmosphere. We also observe the clearness with which objects are exhibited at the centre of the disc of Mars, and though this may be in some measure due to the absence of distortion from the effects of foreshortening, it undoubtedly arises to some extent from the fact that objects in this position are viewed through a comparatively small thickness of the atmosphere enveloping the planet.

Fig. 20.—Mars through the telescope.

Clouds are also sometimes seen apparently floating in the upper region of Mars. This, of course, is only possible on the supposition that there must be an atmosphere which formed the vehicle by which clouds were borne along. It is, however, quite obvious that the extent of the Martian atmosphere must be quite insignificant when compared with that by which our earth is enveloped. It is a rare circumstance for any of the main topographical features, such as the outlines of its so-called continents or coasts of its so-called seas, to be obscured by clouds to an extent which is appreciable except by very refined observations.

Quite otherwise would be the appearance which our globe would present to any observer who should view it say from Mars, or from some other external world at the same distance. The greater part of our globe would seem swathed with vast clouds through which only occasional peeps could be had at the actual configuration of its surface. I dare say a Martian astronomer who had an observatory with sufficiently good optical appliances, and who possessed sufficient patience, might in the course of time, and by availing himself of every opportunity, gradually limn out a chart of the earth which would in some degree represent that with which we are familiar in our atlases. It would, however, be a very tedious matter owing to the interruptions to the survey caused by the obscurities in our atmosphere. The distant astronomer would never be able to comprehend the whole of our earth's features in a bird's-eye glance as we are able to do those features on that hemisphere of Mars which happens to be turned towards us on a clear night.

As to what the composition of the atmosphere on Mars may be we can say but little. In so far as the sustenance of life is concerned, the main question, of course, turns on the presence or the absence of oxygen. It may be pertinent to this inquiry to remark here that a globe surrounded by air may at one epoch of its career have free oxygen as an ingredient in its atmosphere, while at other epochs free oxygen may be absent. This may arise from another cause besides the possible loss of the gas by diffusion into space from small globes in the manner already explained. Indeed, it seems quite probable that the oxygen in our own air is not destined for ever to remain there. It passes through various vicissitudes by being absorbed by animals and then restored again in a free state under the influence of vegetation. But there is an appetite for oxygen among the inorganic materials of our globe which seems capable of using up all the oxygen on the globe and still remaining unsatiated. We have excellent grounds for believing that there is in the interior of the earth a quantity of metallic iron quite sufficient to unite with all the free oxygen of the air so as to form iron oxide. In view of the eagerness with which oxygen and iron unite, and the permanence of the compound which they form, it is impossible for us to regard the presence of oxygen in the air as representing a stable condition of things. It follows that, even though there may now be no free oxygen in the atmosphere of Mars, it is by no means certain that this element has always been absent. It is, however, not at all beyond the reach of scientific resources to determine what the actual composition and extent of the atmosphere of Mars may be, though it can hardly be said that as yet we are in full possession of the truth.

An almost equally important question is as to the telescopic evidence of the presence of water on Mars. Here, again, we have to be reminded of the fact that even when the planet makes its closest approach it is still actually a very long way off. It would be impossible for us to say with certainty that a region which by its colour and general appearance looked like an ocean of water was really water or was even a fluid at all. It is so easy to exaggerate the capabilities of our great telescopes that it may be well to recount what is the very utmost that could be expected from even our greatest instrument when applied to the study of Mars. Let us consider, for example, the capabilities of the Lick telescope in aiding such an inquiry as that before us. This instrument, both from its position and its optical excellence, offers a better view of Mars at the present time than can be obtained elsewhere. But the utmost that this telescope can perform in the way of rendering remote objects visible is to reduce the apparent distance of the object to about one-thousandth part of its actual amount. Some, indeed, might consider that even the Lick instrument would not be capable of giving so great an accession to our powers as this statement expresses. However, I am willing to leave the figure at this amount, only remembering that if I estimated the powers of the telescope less highly than these facts convey, the argument on which I am entering would be correspondingly strengthened.

As we have already said, Mars is at a distance of 35,000,000 miles during a favourable opposition, and if we look at it through a telescope of such a power as we have described the apparent distance is reduced to one-thousandth part. In other words, all that the best telescope can possibly do is to exhibit the planet to us as it would be seen by the unaided eye if it were brought within a distance of 35,000 miles. This will demonstrate that even our greatest telescopes cannot be expected to enable us to answer the questions that are so often asked about our neighbouring globe. What could we learn of Europe if we had only a bird's-eye view of it from a height of
Fig. 21.—Part of a Map of Mars.

35,000 miles, that is to say, from a height which was a dozen times as far as from the shores of Europe to America? The broad outlines of the coast might, of course, be seen by the contrast between the colour of a continent and the colour of the ocean. Possibly a great mountain mass like the Alps would be sufficiently noticeable to permit some conjectures as to its character to be formed. But it is obvious that it would be hopeless to expect to see details. The smallest object that would be discernible on Mars must be as large as London. It would not be possible to see a point so small as either Liverpool or Manchester would be if they were on that planet. There is no doubt a remarkable contrast between the dark colours of certain parts of Mars and the ruddy colours of other parts. We would, however, be unwarranted in asserting that the former must be oceans of water, and the latter continents of land. This may indeed be the case, and most astronomers, I believe, think that it is the case, but it certainly has not yet been proved to be so.

Undoubtedly the most striking piece of evidence that can be adduced in favour of the supposition that there is water on Mars is derived from the "snowy" poles on the planet. The appearance of the poles on Mars with their white caps is one of the most curious features of the solar system. The resemblance to the structure of our own polar regions is extremely instructive. It is evident that there must be some white material which from time to time gathers in mighty volume round the north and south poles of the planet. It is also to be noticed that this accumulation is not permanent. The amount of it waxes and wanes in correspondence with the variations of the seasons on Mars. It increases at either pole of Mars during the winter of that pole. In this respect the white regions, whatever they may be composed of, present a noteworthy contrast to the majority of the other features on the planet. The latter offer no periodic changes to our notice; they are evidently comparatively permanent marks, not to any appreciable extent subject to seasonal variations. When we reflect that this white material is something which grows and then disappears according to a regular period, it is impossible to resist the supposition that it must be snow, or possibly the congealed form of some liquid other than water, which during Mars' summer is restored to a fluid state. There can hardly be a doubt that if we were ever able to take a bird's-eye view of our own earth its poles would exhibit white masses like those which are exhibited by Mars, and the periodic fluctuations at different seasons would produce changes just like those which are actually seen on Mars. It seems only reasonable to infer that we have in Mars a repetition of the terrestrial phenomenon of arctic regions on a somewhat reduced scale.

Among the features presented by Mars there are others, in addition to the polar caps, which seem to suggest the existence of water. It was in September, 1877, when Mars was placed in the same advantageous position for observation that it subsequently occupied in 1892, that a remarkable discovery was made by Professor Schiaparelli, the director of the Milan Observatory. In the clear atmosphere and the convenient latitude of the locality of his observatory, he was so fortunate as to observe marks not readily discernible under the less advantageous conditions in which our observatories are placed. Up to this time it was no doubt well known that the surface of Mars could be mapped out into districts marked with more or less distinctness, so much so that charts of the planet had been carefully drawn and names had been assigned to the various regions which could be detected with sufficient certainty. But at the memorable opposition to which we have referred, the distinguished Italian astronomer discovered that the tracts generally described as "continents" on Mars were traversed by long, dark "canals," as he called them. They must have been each at least sixty miles wide, and in some cases they were thousands of miles in length. Notwithstanding the dimensions to which these figures correspond, the detection of the Martian canals indicates one of the utmost refinements of astronomical observation. The fact that they are so difficult to see may be taken as an illustration of what I have already said as to the hopelessness of discerning any object on this planet unless it be of colossal dimensions.

It is impossible to doubt that considerable changes must be in progress on the surface of Mars. It is true that, viewed from the distance at which we are placed, the extent of the changes, though intrinsically vast, seem relatively insignificant. There is, however, too much testimony as to the changes to allow of hesitation. As an illustration of what is meant, we may refer to the subsequent observations of the canals made by Schiaparelli, their discoverer. During the opposition of 1881 and 1882, he again recognised the presence of these curious objects, but it would seem that a very extraordinary transformation had taken place in some of them. They had become doubled. In certain cases a pair of canals could be detected, separated by an interval of two hundred miles or more, and running parallel to each other throughout their whole length. Again, in the opposition of 1888, other astronomers, notably Dr. Terby and M. Perrotin, have also made observations confirming the
Fig. 22.—Canals on Mars observed by MM. Perrotin and Thollon, 1886.

remarkable phenomenon of the duplicity in the canals. Professor Schiaparelli has, on the same occasion, confirmed his previous observations, and, notwithstanding that the opposition of 1888 was not really an advantageous one, yet under exceptionally favourable circumstances, he declares that he saw the hemisphere of Mars so exquisitely delineated that the canals had all the distinctness of an engraving on steel, with the magical beauty of a coloured painting.

Speculations have naturally been made as to the explanation of these wonderful canals. It has been suggested that they may indeed be rivers; but it hardly seems likely that the drainage of continents on so small a globe as Mars would require an elaborate system of rivers each sixty miles wide and thousands of miles in length. There is, however, a more fatal objection to the river theory, in the fact that the marks we are trying to interpret sometimes cross a Martian continent from ocean to ocean, while on other occasions they seem to intersect each other. Such phenomena are, of course, well-nigh impossible if these so-called canals were in any respect analogous to the rivers which we know on our own globe. It can, however, hardly be doubted that if we assume the dark regions to be oceans the canals do really represent some extension of the waters of these oceans into the continental masses. Other facts which are known about the planet suggest that what seem to be vast inundations of its continents must occasionally take place. Nor is it surprising that such vicissitudes should occur on a globe circumstanced like Mars. Here again it is well to remember the small size of the planet, from which we may infer that it has progressed through its physical evolution at a rate more rapid than would be possible with a larger globe like the earth. The sea is constantly wearing down the land, but by upheavals arising from the intensely heated condition of the interior of our globe the land is still able to maintain itself above water. It can, however, hardly be doubted, that if our earth had so far cooled that the upheavals had either ceased or were greatly reduced, the water would greatly encroach on the land. On a small globe like Mars the cooling of the interior has so far advanced that, in all probability, the internal heat is no longer an effective agent for indirectly resisting the advance of the water, and, consequently, the observed submergence is quite to be expected.

That there may be types of life on Mars of some kind or other is, I should think, very likely. Two of the elements, carbon and hydrogen, which are most intimately associated with the phenomena of life here, appear to be among the most widely distributed elements throughout the universe, and their presence on Mars is in the highest degree probable. But what course the progress of evolution may have taken on such a globe as Mars, it seems totally impossible to conjecture. It has been sometimes thought that the ruddy colour of the planet may be due to vegetation of some peculiar hue, and there is certainly no impossibility in the conception that vast forests of some such trees as copper-beeches might impart to continental masses hues not unlike those which come from Mars. Speculations have also been made as to the possibility of there being intelligent inhabitants on this planet, and I do not see how anyone can deny the possibility at all events of such a fact. I would suggest, however, that as our earth has only been tenanted by intelligent beings for an extremely brief part of its entire history, say, for example, for about one-thousandth-part of the entire number of years during which our globe has had an independent existence, so we may fairly conjecture that the occupancy of any other world by intelligent beings might be only a very minute fraction in the span of the planet's history. It would, therefore, be highly improbable, to say the least of it, that in two worlds, so profoundly different in many respects as are this earth and Mars, the periods of occupancy by intelligent beings should happen to be contemporaneous. I should therefore judge that, though there may once have been, or though there may yet be, intelligent life on Mars, the laws of probability pronounce against the supposition that there is such life there at this moment.

We have also heard surmises as to the possibility of the communication of inter-planetary signals between the earth and Mars, but the suggestion is a preposterous one. Seeing that a canal, sixty miles wide and a thousand miles long, is an object only to be discerned on exceptional occasions, and under most favourable circumstances, what possibility could there be that, even if there were inhabitants on Mars who desired to signal to this earth, they could ever succeed in doing so? We are accustomed to see ships signalling by flags, but what would have to be the size of the flags by which the earth could signal to Mars, or Mars signal to the earth? To be effective for such purpose each of the flags should be, at least, as big as Ireland. It is true, no doubt, that small planets would be fitted for the residence of large beings, and large planets would be proper for small beings. The Liliputians might be sought for on a globe like Jupiter, and the Brobdingnagians on a globe like Mars, and not vice versa as might be hastily supposed. But no Brobdingnagian's arms would be mighty enough to wave the flag on Mars which we should be able to see here. No building that we could raise, even were it a hundred times more massive than the Great Pyramid, would be discernible by the Martian astronomer, even had he the keenest eyes and the most potent telescopes of which our experience has given us any conception.