moved by these causes, before the last stage of all began (at least the last change of a planet's existence as a body undergoing change) is not easily determined. Probably a quarter or a third of the water forming the original oceans of a planet might be withdrawn in one or other of these ways, leaving the rest to be removed during the refrigeration of the nucleus itself — a process requiring many millions, possibly hundreds of millions, of years for its completion.
In whatever way the withdrawal of the lunar seas was accomplished, it is certain that every particle of water has disappeared from the surface of the moon; and as there are clear signs of the former existence of extensive lunar seas, apart from the strong a priori considerations showing that the moon must once have had water on her surface, we have little choice but to admit that the waters of the moon have been withdrawn by such gradual processes as have been described above, and consequently that the era of the moon's existence as a habitable world is really removed from the present epoch by the enormous time-intervals required for the completion of those processes. In fact, we can see clearly pictured on the moon's face the evidence which shows that she has passed through all the stages of planetary life, from the time when her whole frame was glowing with intensity of heat, down to the period when she had reached the condition which our earth in the remote future must attain — that of a cold dead orb, neither living itself (regarding physical changes as corresponding with vitality) nor capable of being the abode of living creatures. Extending the range of our survey, we find in the giant planets, Jupiter and Saturn, the evidence of an earlier stage than any of which the moon's present aspect affords direct evidence. The sun presents a yet earlier stage, while the gaseous nebulæ or masses of luminous star-vapor scattered through the immensity of space illustrate the earliest of all stages of cosmical existence of which we have any direct evidence. On the other hand we see in Mars, with his small ocean-surface and rare atmosphere, the picture of a stage intermediate between that through which the earth is now passing, and the decrepit or death-like condition of the moon. Mercury, if we could examine his condition more satisfactorily than is the case, would probably illustrate a stage somewhat nearer to the moon's present condition. Venus, on the other hand, so far as can be judged, though a somewhat smaller planet than the earth, is in a somewhat earlier stage of planetary existence.
Although the moon may be regarded as to all intents and purposes dead, it must not be supposed that no changes whatever take place upon her surface. On the contrary, some of the peculiarities of the moon s condition must tend to cause even more rapid changes of certain orders than take place in the case of our own earth. Thus the great length of the lunar day, and the moon's waterless condition and rare atmosphere, must help to cause a comparatively rapid crumbling of the moon's surface. During the long and intensely hot lunar day the rock substance of the moon's surface must expand considerably, for it is raised to a degree of heat exceeding that of boiling water. During the long lunar night the surface is exposed to a degree of refrigeration far exceeding that of the bitterest winter in the Arctic regions, and must contract correspondingly. This alternate expansion and contraction must gradually crumble away all the loftiest and steepest portions of the moon's surface, and will doubtless, in the long run — that is, some few hundreds of millions of years hence — destroy all the most marked irregularities of the moon s surface.
The cases of change which have been recognized by telescopists who have carefully studied the moon's surface, may all, without exception, be referred to this process of gradual but steady disintegration. The most remarkable case hitherto known, for example, the disappearance of the lunar crater Linné, is far better explained in this way than as the result of volcanic outburst. This case has recently been described as follows, by the present writer. In the lunar Sea of Serenity there was once a deep crater, nearly seven miles across, a very distinct and obvious feature, even with the small telescope (less than four inches in aperture) used by Beer and Mädler in forming their celebrated chart. But, ten years ago, the astronomer Schmidt, a selenographer of selenographers (who has in fact given the best energies of his life to moon-gazing), found this crater missing. When he announced the fact to the scientific world, other astronomers, armed with very powerful instruments, looked for the crater which had been so clearly seen with Mädler's small telescope; but though they found a crater it was nothing like the crater described by Mädler. The present crater is scarcely two miles in diameter, and only just visible