moon cooled and contracted, leaving other regions elevated, as in the case of the great ocean valleys and continent elevations of our own earth. With further contraction came the formation of great corrugations, the lunar Alps and Apennines and other mountain ranges. But last of all, it may be presumed (if the recent results of Mallet's researches into vulcanology are to be accepted), came the most wonderful of all the stages of disturbances, the great era of crater formation. One would say that the surface of enormous lunar tracts had bub bled over like some seething terrestrial substance, were it not that no materials known to us could form coherent bubbles spanning circular spaces many miles in diameter. Yet no other description gives so just an idea of the actual appearance of extensive tracts of the moon's surface, except one, equally or even perhaps more fanciful :-If the whole of one of these regions, while still plastic from intensity of heat, had been rained upon by liquid meteoric masses many tons or even many hundreds of tons in weight, then something like the observed appearance would probably have resulted. Indeed, it is rather a strange circumstance that a fragment of a slab of green shale, pictured in Lyell's Geology, with casts of rain-prints left by a shower which fell ages on ages since, presents as true a picture of certain lunar tracts, as a model cast expressly to illustrate what is seen in an actual photograph (moon-painted) of one of those regions. Whatever opinion may be formed as to the significance of this fact, it is certain that the present aspect of the crater-covered regions is quite inconsistent with the idea that there was a single continuous era of crater formation. It is manifest that the contour of the whole surface has been changed over and over again by the forces which produced these craters.

Although we find little in the moon't aspect which reminds us of features as present presented by the surface of the earth, we must not too confidently assume that the two globes have been exposed to quite dissimilar processes of change. It is very difficult, indeed, to form clear ideas as to the real conformation of the earth's crust underneath those layers which have been formed, directly or indirectly, by the action of air and water. It requires but a slight study of geology to recognise how importantly such action has affected

our earth. Indeed, there is not a square foot of the earth's surface which does not owe its present configuration either directly to weather changes and the action of water in the form of rain or snow or stream or flood, or else to processes such as vegetation or the succession of various forms of animal life. In the moon, so far as can be judged, we see the natural skeleton, as it were, of a planet, the rock surface precisely as it was left when the internal forces ceased to act with energy. There has been no 66 weathering;" no wearing down of the surface by the action of water; no forests have formed carboniferous layers; no strata like our chalk formations have been deposited; vegetation does not hide any part of the surface; no snows have fallen, and therefore no glaciers grind down the rugged surface of the lunar valleys. With one exception, there is not, so far as can be judged, any process which is at work to disintegrate or modify the sterile face of the moon. The exception is the process of alternate expansion and contraction of the moon's crust, as the lunar day and night pass on in slow succession. Unquestionably, the change from a heat of some five hundred degrees at midday, to a cold far more intense than any with which we are acquainted on earth, must cause a gradual change in portions of the moon's surface.

Yet

But we are thus led to a most interesting question respecting the moon. It is manifest that now, at any rate, there is no water and very little air (if any) on the half of the moon turned towards us. it is argued that those volcanic disturbances which are indicated so strikingly by the moon's aspect, imply the former existence both of water and of air. On our earth water appears absolutely necessary to the occurrence of volcanic eruptions. Our leading seismologist, Mallet, lays down the rule, "without water there can be no eruption," and it was long since pointed out by Humboldt that all the active volcanoes of the earth are close to the sea. Of course the chief evidence in favor of this view consists in the nature of the substances emitted during eruptions; and, in point of fact, the view may be regarded as a demonstrated terrestrial relation. Then it is quite impossible to conceive that so many and such violent eruptions as the lunar volcanoes indicate, can have taken place without the emission of

quantities of vapor so enormous that a discernible atmosphere would from that cause alone, have been formed around our moon. The carbonic acid gas, for example, which would be poured out if the lunar volcanoes in any degree resembled ours, would form a gaseous envelope of no inconsiderable depth. This will be manifest when we recall Galileo's description of the lunar craters as resembling the eyes in a peacock's tail for number. Besides, it is difficult to imagine how any planet could be formed without an atmosphere; and although, no doubt, the moon's small mass would indicate a very inconsiderable aerial envelope, yet it would not explain the complete absence of all traces of air.

The considerations here mentioned have long formed one of the standing mysteries of astronomy. We see in our moon a planet which ought to have oceans and atmosphere, which even would seem once to have had them; and yet she now shows no trace of either.

The efforts made to explain the matter have been sufficiently strenuous.

Whiston suggested that a comet had swept away the lunar air and oceans, a view the more remarkable because he held the theory that our Own oceans had been formerly recruited by a comet which produced a universal deluge. Of course what is now known about comets will not permit us for a moment to entertain the supposition that one of these bodies could carry off any portion of the moon's belongings. A comet might rain a shower of meteoric stones upon the moon, and so recruit her mass indeed the idea has been suggested of late that this happened repeatedly in those far-off ages when all the planets were exposed to such influences, their "growing mass," as Wendell Holmes says,

Pelted with star-dust, stoned with meteor-balls.

That the moon should borrow from comets is not unlikely therefore, but that comets should rob the moon is altogether improbable.

There is another theory scarcely less fanciful. It has been suggested that the moon has grown intensely cold. Her small orb, though once instinct with fire, has long since parted, according to this theory, with all its inherent heat. All the forms of life that once existed on the moon,

animal life, vegetable life, and the life which our imagination pictures where great natural changes are in progress, have been, so to speak, frozen out. The moon's oceans have congealed to their utmost depths. The very gases which once formed her atmosphere have frozen, until at last she has become the dead globe we see, never to be warmed again into life, and having no other use in the economy of the universe but to illuminate our earth and regulate her tides.

But while it is quite conceivable that the intensity of cold during the long lunar nights may be amply sufficient to turn every gas we know of into the solid form, it is manifest that the intense heat to which the moon is exposed during her equally long day would produce even more remarkable changes when poured upon such a frozen surface, than it would effect on such a globe as our earth in its present condition. Imagine our oceans frozen, and the air also frozen, so as to lie in great drifts many feet deep* over the whole surface of the globe. Then conceive the sun to pour his rays down upon that frozen surface for a day lasting two of our weeks, his midday place being nearly overhead. Is it not manifest that the frozen air would be melted and vaporised (turned, that is, into our familiar air), and then the ocean melted, and enormous quantities turned into vapor. Such are the actual conditions in those lunar regions which form the middle of the moon's face. Yet at the time of full moon no signs of change can be recognised, at least none which correspond to the vaporisation of a frozen atmosphere, and of frozen oceans. The simple fact, however, that Lord Rosse's experiments prove that the full moon is greatly heated, disposes at once of the fanciful theory we have been considering. For a frozen lunar atmosphere could not be heated beyond the point (corresponding to an exceeding cold) where it becomes gaseous, until the whole of it had assumed this form; and after that, the water under the

* We do not know the actual depth, because we

do not know what is the density of solid oxygen

or solid nitrogen. But we know that if the density of these elements when reduced to the solid state, were equal to that of ice, the atmosphere would be converted into a solid layer, more than thirty feet deep, for the water-barometer stands nitrogen are as dense as mercury, then the layer at more than thirty feet. If frozen oxygen and would be only two and a half feet in depth.

atmosphere could not be heated above. boiling heat without turning altogether into steam. Now of two things one. The boiling heat would be either high or low. If high, that would imply considerable atmospheric pressure, and we could not but recognise an atmosphere producing such pressure; if low, then the degree of heat to which the moon is raised, as Lord Rosse's experiments show,* remains altogether inexplicable.

There is another strange theory in explanation of the absence of water and air in the moon, due to Dr. Frankland. According to this theory, the oceans and atmosphere which once existed on the moon have now withdrawn into the moon's interior. "If water at one time existed on the surface of the moon," says Frankland, "whither has it disappeared? If we assume, in accordance with the nebular hypothesis, that the portions of matter composing respectively the earth and the moon once possessed an equally elevated temperature, it almost necessarily follows that the moon, owing to the comparative smallness of her mass, would cool more rapidly than the earth. This cooling of the moon's mass must, in accordance with all analogy, have been attended with contraction, which can scarcely be conceived as occurring without the development of a cavernous structure in the interior. Much of the cavernous structure would doubtless communicate, by means of fissures, with the surface, and thus there would be provided an internal receptacle for the ocean, from the depths of which even the burning sun of the long lunar day would be totally unable to dislodge more than traces of its vapor. Assuming the solid mass of the moon to contract on cooling at the same rate as granite, its refrigeration through only 180 degrees of the Fahrenheit thermometer (the difference between the boiling heat and the freezing point) would create cellular space equal to nearly 141⁄2 millions of cubic miles, which would be more than sufficient to engulf the whole of the lunar oceans, supposing them to bear the same proportion to the mass of the

Lord Rosse separates the effect of reflected sun heat from that heat which the moon emits as

a warmed body. We do not explain here the principles which render it possible to distinguish between these two forms of heat; but their sufficiency is altogether beyond question.

moon as our own oceans bear to that of the earth."

Room might certainly be found in this way for all the lunar oceans, because the moon's surface amounts only to 14,600,000 square miles, and therefore the cellular space deduced above amounts to the volume of an ocean competent to cover the whole surface of the moon to the depth of a mile. But then, where has the lunar atmosphere gone to? It would require much more room than the oceans, if originally comparable to our own atmosphere in density. For even at a height of 22 miles from the moon's surface the density of the air would only be reduced one-half, so that half the lunar air would occupy a shell of space covering the whole moon to a depth of 22 miles. It would thus require 22 times as much space as Frankland's theory gives, and still the other half would be left outside the moon. But even the oceans are not very easily accounted for on this theory. We must assume that when they existed on the moon's surface they were not quite so hot as boiling water on the earth. In fact Frankland's theory depends in great part on the probable existence of glaciers on the moon, and it need hardly be said that there would be no glaciers while the oceans, and therefore the solid moon, were at the temperature of boiling water. How then is the refrigeration through 180 degrees to take place without passing far below the freezing point? But frozen oceans would assuredly not find their way into the moon's interior through the fissures of Frankland's theory. Apart from this it must be remembered that if the moon had a very rare atmosphere, the boiling point would be very much lower than on the earth; while, if she had an atmosphere as dense as ours, it remains impossible to understand where that atmosphere can have gone to.

I have said that the theory requires that formerly glaciers should have existed on the moon. It is manifest that, apart from the theory, the question whether there were ever any glaciers on the moon is full of interest. For if there were glaciers there must have been snow and rain, as well as wind currents to bear the moisture laden air against the slopes of the lunar mountain ranges. It will be well, therefore, to indicate the evidence which Frankland finds for the lunar glaciers of his theory. "What may we expect to

see?" he says. "Under favorable circumstances the terminal moraine of a glacier attains enormous dimensions; and consequently of all the marks of a glacier valley, this would be the one most likely to be first perceived. Two such terminal moraines, one of them a double one, have appeared to observers to be traceable upon the moon's surface." His description of the position of these would not be intelligible without a lunar chart; but students of the moon will understand where to look for them when we mention simply that one lies near the end of the remarkable streak from Tycho to Bullialdus, crossing this streak exactly opposite Lubiniezky, while the other lies at the northern extremity of the lunar valley which runs past the eastern edge of Rheita.

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Describing the first, Frankland says, there are two ridges forming the arcs of eccentric circles. Beyond the second ridge a talus slopes down gradually northward to the general level of the lunar surface, the whole presenting an appearance reminding the observer of the concentric moraines of the Rhone glacier. These ridges are visible for the whole period during which that portion of the moon's surface is illuminated; but it is only about the third day after the first quarter, and at the corresponding phase of the waning moon, when the sun's rays falling nearly horizontally, throw the details of this part of the moon's surface into strong relief, and the appearances suggest this explanation of them.” It will be manifest that the evidence for glaciers on the moon is not altogether irresistible. On the whole face of that hemisphere, seven mil

* Tycho is that spot where the full moon shows a gathering together of streaks, somewhat as at either core-end of a peeled orange. Indeed, small photographs of the full moon look so much like photographs of a peeled orange that, as Wendell Holmes notes, many persons suppose astronomers have substituted the orange for the moon, so as to save themselves trouble. Imagine how pleasing such an idea must be to our De la Rues, Rutherfurds, and others, who have exhausted the contrivances of mechanical ingenuity to make their great telescopes truly follow the moon, and have devised at infinite labor the best photographic appliances to secure good results. It is only right to say, however, that no one would for a moment mistake the masterpieces of these astronomers for photographs of a peeled orange, since they are equal in distinctness to views of the moon with excellent telescopes.

lions of square miles in extent, which the moon turns earthwards, there are but two spots where appearances are recognized which suggest the idea of glacial moraines. raines. This is not convincing, especially when we remember that under the best telescopic scrutiny yet applied to the moon we see her surface only as we should see a mountain region on the earth from a distance of more than one hundred miles, and through a dense and perturbed atmosphere. For all the atmospheric effects are multiplied precisely in proportion to the power of the telescope employed, so that even when we use so high a power as 2,400, which would theoretically reduce the moon's distance to 100 miles, the atmosphere between us and the moon is, as it were, multiplied 2,400 times.

But we have not even yet exhausted all the ingenious theories which have been devised by those who insist on endowing the moon of former ages with oceans and an atmosphere. We have seen a comet called in to carry away the lunar air and water, next we have had them frozen up, and thirdly the moon's interior has opened to remove them from our sight. But a fourth theory remains, which, though not less startling than the others, has found singular favor even among astronomers of repute. According to this fourth theory, the lunar oceans and atmosphere have withdrawn, not into the inside of the moon, but to her other or unseen side. The farther half of the moon is never seen by us, and being unknown has appeared to afford a favorable opportunity of applying the principle "omne ignotum pro mirifico." Accordingly, it has been supplied with oceans and an atmosphere, in fact with a double of course, not wanting where circumstances quantity of air and water; inhabitants are, are so suitable for their subsistence; and in fine, another world exists on the unseen half of the moon.

It would be unfair, however, to describe this theory as though it were merely based on our ignorance of the state of things on the farther side of the moon,—as though in fact it resemble done of the peut-êtres of Fontenelle (who was an ardent believer, by the way, in the habitability of our satellite). The theory was originally suggested by a mathematical inquiry of singular profundity. The skilful German mathematician, Hansen, found reason to believe that

if the moon's centre of gravity is not exactly at the middle point of that diameter of hers which is directed earthwards, her movements must give evidence of the fact. If the centre of gravity were farther away than the middle point she would show a slight peculiarity of motion in one direction, while if the centre of gravity were nearer than the middle point she would show a peculiarity of the opposite kind. On examining the moon's actually recorded motions, Hansen considered that he had evidence sufficing to prove that the centre of gravity is more than thirty miles farther away than the middle point just mentioned. Now clearly, if the moon's shape is very nearly globular, but she is like a loaded die, heavier on one side than the other, her oceans and atmosphere must pass over to the loaded side. To use the emphatic mode of describing matters employed by Sir John Herschel in a letter to the present writer, the farther side of the moon, according to Hansen's view, is "like a great lake basin, nearly forty miles deep." Of course, Herschel did not mean that there is a great concavity on that side, any more than a geographer would mean that the ocean bottom is concave, if he spoke of the ocean basin. But the state of the farther side of the moon, according to the theory we are considering, is precisely as though matter were excavated away to a depth of nearly forty miles, leaving, of course, ample room for every drop of water to flow to that unseen half. The air would also flow to that side. It is not, however, altogether so clear that the air would be concealed in the same way that the water would be. The fact is, one half of the moon is not wholly hidden from our view. There is a "balancing motion" (technically called the "libration") of the moon, by which she now tilts one part of the farther hemisphere towards the earth, and then another part, with a singular alternation which brings the balancing round so as to affect in turn every part of the moon's edge. And owing to this peculiarity, instead of one half of the moon remaining concealed from us, about forty-two parts out of 100 only are altogether and at all times unseen. It is difficult to believe that an atmosphere coerced so much less than our own (since the moon's attractive power at her surface is but one-sixth of the earth's at hers)

would confine itself strictly within limits so narrow.

But in reality, evidence has been obtained in favor of Hansen's fundamental theory which, if admitted, disposes altogether of the conclusions based upon that theory. The continental astronomer, Gussew of Wilna, has very carefully examined some of De la Rue's lunar photographs, taken when the moon was at opposite stages of her balancing motion, and by noting how much the several craters, &c, are displaced, he has found the means of determining the shape of the moon's surface. According to his measurements the greater part of the visible surface of the moon must be regarded as an enormous elevation, rising in the middle fully seventy miles above the mean level. In fact, the moon, according to these measurements, would come to be regarded as egg-shaped, the smaller end of the egg being turned earthwards,-only it will of course be understood that, regarded as a whole, the moon's body would not differ very markedly from the globular form. It would be shaped, to speak plainly, like a nearly round egg.

Of course, this way of throwing the centre of gravity farther away than the middle of the lunar diameter directed towards the earth, leads to results quite different from those which would follow if the moon were a globe in shape but loaded like a die internally. That great hill of matter on the earthward side of the moon would draw the oceans and air away from the farther side-not, indeed, to its own summit, that is, not to the middle of the disc we see, but to its base. In fact, there would be a gathering of the waters in a zone all round the edge of the moon's visible disc, and over this zone the atmospheric pressure would also be greatest. Since, as a matter of fact, there is no sign either of water or air on this zone of the moon's surface, we must perforce abandon the theory that lunar oceans and air still lie anywhere on the surface of the moon.

The reader will probably conclude, as the evidence seems to require, that all ideas to the contrary notwithstanding, the moon has never had either a watery envelope or an aerial one in the slightest degree comparable in relative extent with those on our earth.

But before we pass to the curious ques