We may dismiss, in the first place, without any hesitation, the assertion that regular changes take place in the floor of the great lunar crater Plato. According to statements very confidently advanced a few years ago, this wide circular plain, some sixty miles in diameter, grows darker and darker as the lunar day advances there until the time corresponding to about two o'clock in the afternoon, and then grows gradually lighter again till eventide. The idea seems to have been at first that some sort of vegetation exists on the floor of this mighty ring-shaped mountain, and that, as the sun's heat falls during the long lunar day upon the great plain, the vegetation flourishes, darkening the whole region just as we might imagine that some far-extending forest on the earth would appear darker as seen from the moon when fully clothed with vegetation than when the trees were bare and the lighter tints of the ground could be seen through them. Another idea was that the ground undergoes some change under the sun's heat corresponding to those which are produced in certain substances employed in photography; though it was not explained why the solar rays should produce no permanent change, as in the terrestrial cases adduced in illustration. Yet another and, if possible, an even stranger explanation, suggested that, though the moon has no seas, there may be large quantities of water beneath her crust, which may evaporate when that crust becomes heated, rising in the form of vapour to moisten and so darken the crust. Certainly, the idea of a moistening of the lunar crust, or of portions thereof, as the sun's rays fall more strongly upon it, is so daring that one could almost wish it were admissible, instead of being altogether inconsistent, as unfortunately it is, with physical possibilities.