But an atmosphere 2,000 times rarer than our air can scarcely be regarded as an atmosphere at all. The contents of an air-pump receiver can seldom be rarefied to a greater extent than to about 1/1000 of the density of air at the earth’s surface, with the best of pneumatic machines; and the lunar atmosphere, if it exist at all, is thus proved to be twice as attenuated as what we are accustomed to recognise as a vacuum. In discussing the physical phenomena of the lunar surface, we are, therefore, perfectly justified in omitting all considerations of an atmosphere, and adapting our arguments to the non-existence of such an appendage.
And if there be no air upon the moon, we are almost forced to conclude that there can be no water; for if water covered any part of the lunar globe it must be vapourised under the influence of the long period of uninterrupted sunshine (upwards of 300 hours) that constitutes the lunar day, and would manifest itself in the form of clouds or mists obscuring certain parts of the surface. But, as we have already said, no such obliteration of details ever takes place; and, as we have further seen, no evidence of aqueous vapour is manifested upon the occasion of spectrum observations. Since, then, the effects of watery vapour are absent, we are forced to conclude that the cause is absent also.
Those parts of the moon which the ancient astronomers assumed, from their comparatively smooth and dusky appearance, to be seas, have long since been discovered to be merely extensive regions of less reflective surface material; for the telescope reveals to us irregularities and asperities covering well nigh the whole of them, which asperities could not be seen if they were covered with water; unless, indeed, we admit the possibility of seeing to the bottom of the water, not only perpendicularly, but obliquely. Some observers have noticed features that have led them to suppose that water was at one time present upon the moon, and has left its traces in the form of appearances of erosive action in some parts. But if water ever existed, where is it now? One writer, it is true, has suggested as possible, that whatever air, and we presume he would include whatever water also, the moon may possess, is hidden away in sublunarean caves and hollows; but even if water existed in these places it must sometimes assume the vapoury form, and thus make its presence known.
Fig. 13.
Fig. 14.
Sir John Herschel pointed out that if any moisture exists upon the moon, it must be in a continual state of migration from the illuminated or hot, to the unilluminated or cold side of the lunar globe. The alternations of temperature, from the heat produced by the unmitigated sunshine of 14 days’ duration, to the intensity of cold resulting from the absence of any sunshine whatever for an equal period, must, he argued, produce an action similar to that of the cryophorus in transporting the lunar moisture from one hemisphere to the other. The cryophorus is a little instrument invented by the late Dr. Wollaston; it consists of two bulbs of glass connected by a bent tube, in the manner shown in the annexed illustration, [fig. 13]. One of the bulbs, A, is half-filled with water, and, all air being exhausted, the instrument is hermetically sealed, leaving nothing within but the water and the aqueous vapour which rises therefrom in the absence of atmospheric pressure. When the empty bulb, B, is placed in a freezing mixture, a rapid condensation of this vapour takes place within it, and as a consequence the water in the bulb A gives off more vapour. The abstraction of heat from the water, which is a natural consequence of this evaporation, causes it to freeze into a solid mass of ice. Now upon the moon the same phenomenon would occur did the material exist there to supply it. In the accompanying diagram let A represent the illuminated or heated hemisphere of the moon, and B the dark or cold hemisphere; the former being probably at a temperature of 300° above, and the latter 200° below Fahrenheit’s zero. Upon the above principle, if moisture existed upon A it would become vapourised, and the vapour would migrate over to B, and deposit itself there as hoarfrost; it would, therefore, manifest itself to us while in the act of migrating by clouding or dimming the details about the boundary of the illuminated hemisphere. The sun, rising upon any point upon the margin of the dark hemisphere, would have to shine through a bed of moisture, and we may justly suppose, if this were the case, that the tops of mountains catching the first beams of sunlight would be tinged with colour, or be lit up at first with but a faint illumination, just as we see in the case of terrestrial mountains whose summits catch the first, or receive the last beams of the rising or setting sun. Nothing of this kind is, however, perceptible: when the solar rays tip the lofty peaks of lunar mountains, these shine at once with brilliant light, quite as vivid as any of those parts that receive less horizontal illumination, or upon which the sun is almost perpendicularly shining.
All the evidence, then, that we have the means of obtaining, goes to prove that neither air nor water exist upon the moon. Two complicating elements affecting all questions relating to the geology of the terraqueous globe we inhabit may thus be dismissed from our minds while considering the physical features of the lunar surface. Fire on the one hand and water or the other, are the agents to which the configurations of the earth’s surface are referrable: the first of these produced the igneous rocks that form the veritable foundations of the earth, the second has given rise to the superstructure of deposits that constitute the secondary and tertiary formations: were these last removed from the surface of our planet, so as to lay bare its original igneous crust, that crust, so far as reasoning can picture it to us, would probably not differ essentially from the visible surface of the moon. In considering the causes that have given birth to the diversified features of that surface, we may, therefore, ignore the influence of air and water action and confine our reasoning to igneous phenomena alone: our task in this matter, it is hardly necessary to remark, is materially simplified thereby.