Mars. I. Atmosphere

AMID the seemingly countless stars that on a clear night spangle the vast dome overhead, there appeared last autumn to be a new-comer, a very large and ruddy one, that rose at sunset through the haze about the horizon. That star was the planet Mars, so conspicuous when in such position as often to be taken for a portent. Large as he then looked, however, he is in truth but a secondary planet traveling round a secondary sun; but his interest for us is out of all proportion to his actual size or his relative importance in the cosmos. For that sun is our own; and that planet is, with the exception of the moon, our next to nearest neighbor in space, Venus alone ever approaching us closer. From him, therefore, of all the heavenly bodies, may we expect first to learn something beyond celestial mechanics, beyond even celestial chemistry; something in answer to the mute query that man instinctively makes as he gazes at the stars, whether there be life in worlds other than his own.

Hitherto the question has received no affirmative reply, although the trend of all latter-day investigation has been to such affirmation; for science has been demonstrating more and more clearly the essential oneness of the universe. Matter proves to be common property. We have learnt that the very same substances with which we are familiar on this our earth, iron, magnesium, calcium, and the rest, are present in the far-off stars that strew the depths of space. Nothing new under the sun! Indeed, there is nothing new above it but ever-varying detail. So much for matter. As for mind beyond the confines of our tiny globe, modesty, backed by a probability little short of demonstration, forbids the thought that we are the sole thinkers in this great universe.

That we are the only minds in space it takes indeed a very small mind to fancy. Our relative insignificance commonly escapes us. If we reduce the universe to a scale on which we can conceive it, that on which the earth shall be represented by a good-sized pea, with a grain of mustard seed, the moon, circling about it at a diameter of seven inches, the sun would be a globe two feet in diameter, two hundred and twenty feet away. Mars, a much smaller pea, would circle round the two-foot globe, three hundred and fifty feet from its surface; Jupiter, an orange, at a distance of a quarter of a mile; Saturn, a small orange, at two fifths of a mile; and Uranus and Neptune, good-sized plums, three quarters of a mile and a mile and a quarter away, respectively. The nearest star would lie two hundred and fifty thousand miles off, or at about the actual distance of our own moon, and the other stars at corresponding distances beyond that; that is, on a scale upon which the moon should be but seven inches off, the nearest star would still be as far from us as the moon is now. When we think that each of these stars is probably the centre of a solar system on a grander scale than our own, we cannot seriously take ourselves to be the only minds in the universe.

But improbable as the absence of ultra-terrestrial life in a general way is, up to the present time we have no proof of its particular existence in worlds beyond our own. Whether the observations I am now to describe have revealed something on the point I shall leave the reader to himself to judge, after laying the facts before him; for it is with this in view that the present papers will deal with Mars, since any answer on this point is the most generally interesting outcome of a study of the planet. That the observations also disclose the fact that the hitherto accepted period of its rotation proves to be too small by the hundredth of a second is a matter of far greater moment, of course, but one which leaves the average man comparatively cool. That Mars, however, should be peopled by intelligent beings, although physically they be utterly unlike us, more goblins than men or animals, is a suggestion which appeals romantically, at least, to everybody.

To determine whether a planet be the abode of life, two questions about it must be answered in turn: first, are its physical conditions such as to render it habitable? And secondly, are there any signs of its actual habitation? Unless we can answer the first point satisfactorily, it were futile to seek for evidence of the second.

Of such planets as doubtless circle round other suns we as yet know nothing. Our search is perforce confined at present to the members of our own solar family. Now, when we first scan them for answer to our first query, we find but two that promise even the possibility of an affirmative reply, Mars and Venus. All the others turn out, upon scrutiny, to lie beyond the pale, either because they are too little; for, curiously enough, mere size settles the matter.

The giant Jupiter piques inquiry first by showing us great cloud-belts that recall our own equatorial and temperate cloud-zones. But further study discloses that his clouds are in kind quite unlike those of our earth. Neither the hour of his day nor the season of his year brings change in them. They slowly, very slowly, alter in appearance, indeed, but not in obedience to that central ruler that gathers and dispels our own. In short, the Jovian clouds are not sun-raised, but self-raised ones. It is heat inherent in Jupiter himself, not heat from the sun, that belts him about with his great girdles of cloud. We can even see, in all probability, his glowing inner self; for Jupiter shows brick-red between his belts, like a molten mass.

The same state of things is yet more strikingly instanced by Saturn; for the tilt of Saturn's pole is not very unlike that of the earth, and in consequence his equatorial regions are at times raised far above the plane of his orbit; at others, dipped far below it. Yet unlike the earth's cloud-belts, his never travel northward when the sun goes north, nor follow the sun when he journeys south again. So far as the sun is concerned, the Saturnian cloud-belts are invariable. Like the Jovian, they owe their formation to the planet's own heat. Like Jupiter, too, Saturn, shows red beneath. From all this it is pretty plain that the giant planets are far from pleasurable abodes, as yet midway in evolution between actual suns and tenantable worlds; too cooled down for the one state, and not yet cooled down enough for the other.

Uranus and Neptune give evidence, also, of being in a chaotic condition, orbs informe, ingens, cui lumen ademptum,—no longer suns, but as yet quite unfit to support beings even distantly analogous to ourselves.

With Mercury littleness is even more fatal to life; for though the giant planets may perhaps, at some future day, grow to be life-supporting, a small one apparently never was, nor ever can be, peopled by beings in the least resembling us. Incapacity to quarter folk is included in the more general incapacity to hold an atmosphere; for absence of atmosphere precludes the possibility of life as we know it. That a planet may be too small to have an atmospheric envelope we shall see more definitely later. That life, however, of a type of which we have no conception may not exist in all these orbs we must be wary of stating, for nothing is more dangerous than a general denial, except a particular statement.

We are limited, therefore, in our present inquiry, to Venus and Mars. But Venus, contrary to her name, proves provokingly modest, the most modest of all the company of heaven, keeping herself so constantly veiled in cloud that we seldom, if ever, are permitted to a peep at her actual surface. In consequence, beyond the fact that she has an atmosphere of considerable though not excessive density, we know little about her.

With Mars, on the other hand, no such false modesty balks us at the outset. The planet named after the old God of War—satirically, it would seem, since he turns out to present characteristics quite the reverse of warlike—lets himself be seen as well as thirty-five millions of miles of separation will allow.

Now, to all forms of life of which we have any conception, two things in nature are vital, air and water. A planet must possess these two things to be able to support any life at all upon its surface. Some articles that we might deem essential to well-being fall cosmically under the head of luxuries; but air and water are necessities of existence. There is no creature which is not in some measure dependent upon both of them. How then is Mars off for air?

Fortunately for an answer to this question, air is as vital to change in the inorganic processes of nature as it is to those other changes which we call peculiarly life. Atmosphere is essential not only to life upon a planet, but to the production of any change whatever upon that planet's surface. Without it, not only development, but decay would come to a standstill, when once all that was friable land crumbled to pieces under the alternate roasting and refrigerating to which the planet's surface would be exposed as it revolved upon its axis toward and away from the sun. Disintegration once effected, the planet would roll, a mummy world, through space. Since atmosphere, therefore, is a sine qua non to any change upon a planet's surface is proof positive of the presence of an atmosphere, however incapable of detection such atmosphere be by direct means.

Now changes take place upon the surface of Mars, changes vast enough to be visible from the earth. When properly observed they turn out to be most marked. We will begin with the look of the planet last June. Its general aspect then was tripartite. Upon the top part of the disc, round what we know to be the planet's pole, appeared a great white cap, the south polar cap. The south lay at the top, because all astronomical views are, for optical reasons, upside down; but inasmuch as we never see the features otherwise, to have them right side up is not vital to the effect. Below the white cap lay a region chiefly bluish-green, interspersed, however, with portions more or less reddish-ochre. Below this, again, came a vast reddish-ochre stretch, the great continental deserts of the planet.

The first sign of change occurred in the polar cap. It proceeded slowly to dwindle in size. Such obliteration it has, with praiseworthy regularity, undergone once every two years for the last two hundred. Since the polar cap was first seen it has waxed and waned with clock-like precision, a precision timed to the change of season in the planet's year. During the spring, these snow-fields, as analogy at once guesses them to be, and as beyond doubt they really are, stretch into the southern hemisphere, the one presented to us at this last opposition, down to latitude seventy, and even sixty-five south; covering thus more than the whole of the planet's south frigid zone. As summer comes on they dwindle gradually away, till be early autumn they present but tiny patches, a few hundred miles across. This year, for the first time in human experience, they melted, apparently, completely. This unprecedented event happened on October 13, or forty-three days after the summer solstice of the southern hemisphere, a date corresponding to about the middle of July on Earth. Evidently, it was a phenomenally hot season on Mars, for the minimum of the polar patch is reached usually about three months after Martian midsummer. It will be noticed how nearly such melting parallels what takes place with our arctic ice-cap on earth.

But the appearance of the polar snows is by no means the only change discernable upon the surface of the planet. Several years ago Shiaparelli noticed differences in tint at successive oppositions, both in the dark areas and in the bright ones. These, he suggested, might be due to the seasons. This year it has been possible to watch the change take place. From the Martian middle of August, the bluish-green areas have been steadily undergoing a most marked transformation. These proves, in fact, to be a wave of seasonal change that sweeps over the face of the planet from pole to pole. We will examine this more in detail when we take up the question of water. For the present point it suffices that it takes place; for it constitutes proof positive of the presence of an atmosphere.

A moment's consideration will show how absolutely positive this proof is; for it is the inevitable deduction from the simplest of observed facts. Its cogency consists in its simplicity. It is independent of difficult detail or of doubtful interpretation. It is not concerned with that may be the constitution of the polar caps, nor with the character of the transformation that sweeps, wavelike, over the rest of the planet. It merely states that changes occurs, and that statement is conclusive.

Having thus seen with the brain as much as with the eye, and in the simplest possible manner, that a Martian atmosphere exists, we will go on to consider what it is like.

The first and most conspicuous of its characteristics is its cloudlessness. A cloud is an event on Mars, a rare and unusual phenomenon, which should make it more fittingly appreciated there than Ruskin lamented was the case on earth. For it is almost perpetually fine weather on our neighbor in space. From the days beginning to its close, and from one end of a year to the other, nothing appears to veil the greater part of the planet's surface.

This is more completely the case than has hitherto been supposed. We read sometimes in astronomical books and articles picturesque accounts of clouds and mist gathering over certain regions of the disc, hiding the coast lines and continents from view, and then, some hours later, clearing off again. No instance of such blotting out of detail has been seen this year at Flagstaff. Though the planets face been scanned there almost every night, from the last day of May to the end of November, not a case of obscuration of any part of the central portions of the planet, from any Martian cause, has been detected by any one of three observers. Certain peculiar brightish patches have from time to time been noted, but, with a courtesy uncommon in clouds, they have carefully refrained from obscuring in the slightest degree any detail the observer might be engaged in looking at.

The only dimming of detail upon the Martian disc has been along its bright edge, what is technically called its limb. Fringing this is a permanent lune of light that swamps all except the very darkest markings in its glare. This limb-light has commonly been taken as evidence of sunrise or sunset mists on Mars. But observations of mine during last June show that such cannot be the case. In June Mars was gibbous,—that is, he showed a face like the moon between the quarter and the full,—and along his limb, then upon his own western side, lay the bright-limb-light, stretching inward about thirty degrees. Since the face turned toward us was the only in part illumined by the sun, the centre of it did not stand at noon, but some hours later, and the middle of the limb consequently not at sunrise, but at about nine o'clock of a Martian morning. As the limb-light extended in from this thirty degrees, or two hours in time, the mist, if mist it was, must have lasted till eleven o'clock in the day. Furthermore, it must have been mist of a singularly mathematical turn of mind, for it made a perfect semi-ellipse from one pole to the other, quite oblivious of the fact that every hour from sunrise to sunset lay represented along its edge, including high noon. What is more, as the disc passed, in course of time, from gibbous form to the full, and then to the gibbous form on the other side, the limb-light obligingly clung to the limb, regardless of everything except its geometric curve. But as it did so, the eleven o'clock meridian swung from one side of the centre of the disc to the other. As it crossed the centre its regions showed perfectly clear; not a trace of obscuration as it passed directly under the eye. It was evident, therefore, that Martian morning mists were not responsible for the phenomenon.

To what, then, was the limb-light due? At first sight, it would seem as if the moon might help us; for the moon's limb is similarly ringed by a lune of light. In her case the effect has been attributed to mountain slopes catching the sun's light at angles beyond the possibilities of plains. But Mars has few mountains worthy the name. His terminator—that is, the part of the disc which is just passing in or out of sunlight, and discloses mountains by the way in which they catch the coming light before the plains at their feet are illuminated—shows irregularities quite inferior to the lunar ones, proving that his elevations and depressions are relatively insignificant.

On the whole, the best explanation of the phenomenon seems to be that the Martian atmosphere itself is somewhat of a veil, and this veiling effect, though practically imperceptible in the centre of the disc, becomes noticeable as we go from the centre to the edge, owing to the greater thickness of the stratum through which we look. At thirty degrees in from the limb the observer would look through twice as much of it as when he looked plumb down upon the centre of the disc; in consequence, what would be diaphanous at the centre might well seem opaque toward the edge. The effect we are familiar with on earth in the haze that always borders the horizon,—a haze most noticeable in places where there is much water in the air. Here, then, we have a hint of what is the matter on Mars. Were his atmosphere charged with water-vapor, just such an effect as is observed should take place.

This first hint receives an independent support from another Martian phenomenon. Contrary to what the distance of the planet from the sun and the thinness of its atmospheric envelope would lead us to expect, the climate of Mars proves astonishingly mild. Whereas calculation from distance and atmospheric density puts its average temperature below freezing, thus relegating it to perpetual ice, the planetÕs surface features show that the temperature is relatively high. Observation reveals the fact that the mean temperature must actually be above that of the earth; for not only is there practically no snow or ice outside the frigid zone at any time, but the polar snow-caps melt to a minimum quite beyond that of our own, affording the Martians rare chance for quixotic polar expeditions. Such pleasing amelioration of the climate must be accounted for, and the aqueous vapor is quite specific as a planetary comforter, being the very best of blankets. It acts, indeed, like the glass of a conservatory, letting the light rays in, and opposing the passage of the heat rays out.

The state of things thus disclosed by observation, the cloudlessness and the rim of limb-light, turns out to agree in a most happy manner with what probability would lead us to expect: for the most natural supposition to make a priori about the Martian atmosphere is the following. When each planet was produced by fission from the parent nebula, we may suppose that it took with it as its birthright its proportion of chemical constituents; that is, that its amount of oxygen, nitrogen, and so forth was proportional to its mass. Doubtless its place in the primal nebula would to a certain extent modify the ratio, just as the size of the planet would to a certain extent modify the relative amount of these elements that would thereupon enter into combination. Supposing, however, that the ratio of free oxygen and so forth to the other elements remained substantially the same, we should have in the case of any two planets the same relative quantity of atmosphere. But the size of the planet would entirely alter the distribution of this air.

Three causes would all combine to rob the smaller planet of efficient covering, on the general principle that he that hath little shall have less.

In the first place, the smaller the planet, the greater would be its volume in proportion to its mass, because the materials of which it was composed, being subjected to less pressure owing to a lesser pull, would not be crowded so closely together. This is one reason why Mars should have a thinner atmosphere than is the case with our earth.

Secondly, of two similar bodies, spheres or others, the smaller has the greater surface for its volume, since the one quantity is of two dimensions only, the other of three. An onion will give us a good instance of this. By stripping off layer after layer we reach eventually a last layer which is all surface, inclosing nothing. We may, if we please, observe something analogous in men, among whom the most superficial have the least in them. In consequence of this principle, the atmosphere of the smaller body finds itself obliged to cover relatively more surface, which still further thins it out.

Lastly, gravity being less on the surface of the smaller body, the atmosphere is less compressed, and, being a gas, seizes that opportunity to spread out to a greater height, which renders it still less dense at the planet's surface.

Thus for three reasons Mars should have a thinner air at his surface than is found on the surface of the earth.

Calculating the effect of the above causes numerically, we find that on this a priori supposition; for the cloudless character of the Martian skies is precisely what we should look for in a rare air. Clouds are congeries of globules of water or particles of ice buoyed up by the air about them. The smaller these are, the more easily are they buoyed up, because gravity, which tends to pull them down, acts upon their mass, while the resistance they oppose to it varies as their surface, and this, as we saw just now, is relatively greater in the smaller particles. The result is that the smaller particles can float in thinner air. We see the principle exemplified in our terrestrial clouds; the low nimbus being formed of comparatively large globules, while the high cirrus is made up of very minute particles. If we go yet higher, we reach a region incapable of supporting clouds of any kind, so rarefied is its air. This occurs about five miles above the earth's surface; and yet even at this height the density of our air is greater than is the probably density of the air at the surface of Mars. We see, therefore, that the Martian atmosphere should from its rarity prove cloudless, just as we observe it to be.

So far in this our investigation of the Martian atmosphere we have been indebted solely to the principles of mathematics and molar physics for help, and these have told us something about the probable quantity of that atmosphere, though silent as to its possible quality. On this latter point, however, molecular physics turns out to have something to say; for an Irish gentleman, Dr. G. Johnstone Stoney, has recently made an ingenious deduction from the kinetic theory of gases bearing upon the atmospheric envelope which any planet can retain. His deduction is as acute as it appears from observation to be in keeping with the facts. It is this:—

The molecular theory of gases supposes them to be made up of myriads of molecules in incessant motion. What a molecule may be nobody knows; some scientists supposing it to be a vortex ring in miniature,—something like the swirl produced by a teaspoon when drawn through a cup of tea. But whatever it be, the idea of it accounts for the facts. The motion of the molecules is almost inconceivably swift as they dart hither and thither throughout the space occupied by the gas, and their speed differs for different gases. It is calculated that the molecules of oxygen travel, on the average, at the rate of fifteen miles a minute, and those of hydrogen, which are the fastest known, at the enormous speed of a mile a second. But this average velocity may, in any particular case, be increased by collisions of the molecules among themselves something like sevenfold. What is more, each molecule of the gas is bound, sooner or later, to attain this maximum velocity of its kind merely on the doctrine of chances. When it is attained, the molecule of water vapor at the rate of two and one third miles a second, and the molecule of hydrogen actually at seven miles at second, six hundred times as fast as our fastest express train.

Now, if a body, whether it be a molecule or a cannon-ball, be projected away from the earth's surface, the earth will at once try to pull it down again: this instinctive holding on of Mother Earth to what she has we call gravity. In the cases with which we are personally familiar, her endeavor is eminently successful; what goes up usually coming down again, either on the thrower or on some other person. But even the earth is not omnipotent. As the velocity with which the body is projected increases, it takes the earth longer and longer to overcome it and compel the body's return. Finally there comes a speed which the earth is just able to overcome, if she take an infinite time about it. In that case, the body would continue to travel away from her, at a constantly diminishing rate, but still at some rate, on and on into the depths of space, till it attained infinity, at which point the truant would stop, and reluctantly begin to return again. This velocity we may call the critical velocity. It is the velocity which the earth would cause in a body falling to it from an infinite distance, since gravity is able to destroy on the way up just the speed it is able to create on the way down. But now, if the body's departure were even hastier than this, the earth would never be able wholly to annihilate its speed, and the body would travel forever away out and out, till it fell, probably, under the sway of some distant star. In any case, the earth would know the vagabond no more.

As gravity, depends upon mass, the larger the attracting planet, the greater is its critical velocity, the velocity it can just control; and, reversely, the smaller the planet, the less its restraining power. With the earth the critical velocity is between six and seven miles a second. If any of us, therefore, could manage to become faster than this, socially or otherwise, we could bid defiance to the whole earth, and begin to voyage on our own account through space.

This is actually what happens, as we have seen, to the molecules of hydrogen. If, therefore, free hydrogen were present at the surface of the earth, and met with no other gas attractive enough to tie it down by uniting with it, the rover would, in course of time, attain a speed sufficient to allow it to bid good-by to earth, and start on interspacial travels of its own. That it should reach its maximum speed is all that is essential to liberty, the direction of its motion being immaterial. To each molecule in turn would come this happy dispatch, till the earth stood deprived of every atom of free hydrogen she possessed.

It is a highly significant fact that there is no free hydrogen found in the earth's atmosphere. With oxygen and water vapor, and indeed all the other gases we know, the case if different; for their maximum speed falls far short of the possibility of escape. So they have stayed with us solely because they must. And, as a matter of fact, the earth's atmosphere contains plenty of free oxygen, nitrogen, and the like. The actions of the heavenly bodies confirm this conclusion. The moon, for example, possesses no atmosphere, and calculation shows that the velocity it can control falls short of the maximum of any of these gases. All were, therefore, at liberty to leave it, and all have promptly done so. Whatever the moonÕs attraction for lovers, no gas was sufficiently attracted by it to stay. On the other hand, the giant planets give evidence of very dense atmospheres. They have kept all that they ever had.

But the most striking confirmation of the theory comes from the cusps of Venus and Mercury; for an atmosphere would prolong, by its refraction, the cusps of a crescent beyond their true limits. Length of cusp becomes, consequently, a criterion of the presence of an atmosphere. Now, in the appearance of their cusps there is a notable difference between Venus and Mercury. The cusps of Venus extend beyond the semi-circle; Mercury do not. We see, therefore, that Mercury has no appreciable atmospheric envelope.

Turning to the case of Mars, we find with him the critical velocity to be about three miles a second. This is, like the earth's, below the maximum for the molecules of hydrogen, but also, like the earth's, above that of any other gas; from which we have reason to suppose that, except for possible chemical combinations, his atmosphere is in quality not unlike our own.

Having seen what the atmosphere of Mars is probably like, we may draw certain interesting inferences from it as to its capabilities for making life comfortable. The first consequence is that Mars is blissfully destitute of weather. Unlike New England, which has more than it can accommodate, Mars has none of the article. What takes its place as the staple topic of conversation for empty-headed folk remains one of the Martian mysteries yet to be solved. What takes its place in fact is a perpetual serenity, such as we can scarcely conceive of. Although over what we shall later see to be the great continental deserts the air must at midday be highly rarefied, and cause vacuums into which the surrounding air must rush, the actual difference gradient owing to the initial thinness of the air must be very slight. With a normal barometer of four and a half inches, a very great relative fall is a very slight actual one. In consequence, storms would such mild-mannered things that, for objectionable purposes, they might as well not be. In the first place, if we are right, there can be no rain, nor hail, nor snow in them, for the particles would be deposited before they gained the dignity of such separate existence. Dew or frost would be the maximum of precipitation that Mars could support. The polar snow-cap or ice-cap, therefore, is doubtless formed, not by the falling of snow, but by successive depositions of dew. Secondly, there would be about the Martian storms no very palpable wind. Though the gale might blow at fairly respectable rates, so flimsy is the substance moved that it might buffet a man unmercifully without reproach.

Another interesting result of the rarity of the air would be its effect upon the boiling-point of water. Reynault's experiments have shown that, in air at a density 14/100 of our own, water would boil at about 127° Fahrenheit. This, then, would be the temperature at which water would be converted into steam on Mars. So low a boiling-point would make it impossible to cook anything in the open air. Boiled eggs could be prepared only under cover, and such people as liked their meat boiled would probably find it convenient to prefer it done differently. Fortunately, roasts would still remain possible. The lowering of the boiling-point would raise the relative amount of aqueous vapor held in suspension by the air at any temperature. At about 127o the air would be saturated, and even at lower temperatures much more of it would evaporate and load the surrounding air than happens at similar temperatures on earth. Thus at the heels of similarity treads contrast. We may now go on to such phenomena bearing on the Martian atmosphere as show it to differ from ours. Some of them we were able more or less imperfectly to explain; some we are not.

Although no case of obscuration has been seen at Flagstaff this summer, certain bright patches have been observed on special portions of the planet's disc. That they are not storm-clouds, like those which, by a wavelike process of generation, travel across the American continent, for example, is shown by the fact that they do not travel, but are local fixtures. Commonly, they appear day after day, and even year after year, in the same spots; for identical patches have been observed by different astronomers at successive oppositions. To this category belong the regions known as Elysium, Ophir, Memnonia, Eridania, and Tempe. Still smaller patches, apparently, more fugitive in character, have been seen this year by Professor W. H. Pickering. But the most marked instance of variability was detected in September last by Mr. Douglass, in the western part of Elysium. On September 22 and 23 he found this blissfully named region, as usual, equally bright throughout. But on September 24 he noticed that the western half of it had suddenly increased in brightness and far outshone the eastern half, being almost as brilliant as the polar cap. When he looked at it again the next night, September 25, the effect of the night before had vanished, the western half being now actually the darker of the two. So fugitive an effect suggests cloud, forming presumably over high ground, and subsequently dissipating; it also suggests a deposition of frost that melted on the next day. It is specially noteworthy that the canals inclosing the region, Galaxias and Hyblaeus, were not in any way obscured by the bright apparition. On the contrary, Mr. Douglass found them perceptibly darker than they had been, an effect attributable perhaps to contrast.

Although not storm-clouds, it is possible that these appearances may have been due to cloud capping high land. There are objections, however, to this view, as in the first place, there is evidence that the Martian mountains are low; in the second place that they would have to be phenomenally high to produce a change in temperature sufficient to condense the air about them and so cap them with cloud; and in the third place that the air is not dense enough to support clouds, anyway. Nevertheless a most singular phenomenon was seen by Mr. Douglass on November 24, a bright detached projection, for which from measurement he deduced a height of thirty miles. This would seem to have been a cloud. With regard to its enormous height, it is not to be forgotten that a few years ago, on the earth, phenomenal dust-clouds were observed as high as one hundred miles.

Something more in the line of the explicable was a phenomenon observed in 1879 and in 1881 by Schiaparelli. From October, 1879, to January, 1880, he noticed certain bright patches which appeared to surround the north pole in a sort of crown, the pole itself being invisible. In 1881 he saw the same ramifications again, in apparently the same place. At this latter opposition the north pole was much better placed for observation, and he was able to mark a curious subsequent action in these spots; for as time went on they gradually contracted toward the pole, till finally they consolidated into the north polar patch, which up to that time had been absent. The polar patch proper did not thus appear till more than a month till more than a month after the vernal equinox of the northern hemisphere.

Here, then, we have a very curious phenomenon, a phenomenon which seems to indicate that the seasonal wave of change acts as a unit across the planet's face; that instead of a more or less continuous deposit of moisture at the pole, such as occurs on earth, Martian atmospheric conditions oblige such deposit to creep gradually with the season up into polar latitudes, where it appears first as a crown of frost, and does not envelop the pole and become a polar cap till it has got higher. No sooner has this happened than the advance of following warmer isotherms causes it to begin to melt. One deduction from this thin air we must, however, be careful not to make: that because it is thin it is incapable of supporting intelligent life. That beings constituted physically as we are would find it a most uncomfortable habitat is pretty certain. But lungs are not wedded to logic, and there is nothing in the world or beyond it to prevent, so far as we know, a being with gills, for example, from being a most superior person. A fish doubtless imagines life out of water to be impossible; and similarly, to argue that life of an order as high as our own, or higher, is impossible, because of less air to breathe than that to which we are locally accustomed, is, as Flammarion happily expresses it, to argue, not as a philosopher, but as a fish.

To sum up, now, what we know about the atmosphere of Mars: we have proof positive that Mars has an atmosphere; we have reason to believe that this atmosphere is very thing,—thinner at least by half than the air upon the summit of the Himalayas,—that in constitution it does not differ greatly from our own, and that it is relatively heavily charged with water vapor.

In the next paper I shall take up the question of water upon the planet.