We may regard the Moon in a twofold aspect, and consider what it is as a mere object to look at, and what it does for us; probably my present readers will prefer that most prominence shall be given to the former aspect. The Moon as seen with the naked eye exhibits a silvery mass of light, which at the epoch of what is called “full Moon” has a seemingly even circular outline. Full or not full, its surface appears to be irregularly shaded or mottled. The immediate cause of this shading is the fact that the surface of the Moon, not being really smooth, reflects irregularly the Sun’s light which falls upon it. The _causa causans_ of this is the existence of numerous mountains and valleys on its surface, and which were first discovered to be such by Galileo. That there are mountains is proved by the shadows cast by their peaks on the surrounding plains, when the Sun illuminates the Moon obliquely—that is, when the Moon is shining either as a crescent or gibbous. Such shadows, however, disappear at the phase of “Full-Moon,” because the Sun’s rays then fall perpendicularly on the Moon’s surface. When the Moon presents either a crescent or a gibbous form (in point of fact when it presents any form except that of “Full-Moon”), the boundary line which separates the illuminated from the unilluminated portion (and which boundary line is generally spoken of as the “terminator”) has a rough, jagged appearance; this is due to the fact that the Sun’s light falls first on the summits of the peaks, and that the adjacent valleys and declivities are in shade. These remain so till by reason of the Moon’s progress in its orbit a sufficient time has elapsed for the Sun to penetrate to the bottom of the valleys. With this explanation the reader will have no difficulty in realising why the terminator always exhibits an irregular or jagged edge. [Illustration: Fig. 11.—Mare Crisium. (Lick Observatory photographs.)] Various mountains on the Moon to the number of more than a thousand have been mapped, and their elevations calculated. Of these fully half have received names, being those of men of various dates and nationalities, who have figured conspicuously in the annals of science, including some, however, who have not done so. Whilst many of these mountains are isolated elevations, not a few form definite chains of mountains, and to certain of these chains definite names, borrowed from the Earth, have been given. Thus we find on maps of the Moon the “Apennines,” the “Alps,” the “Altai Mountains,” the “Dörfel Mountains,” the “Caucasus Mountains,” and so on.