“Before quitting this photograph permit me to recall you to the neighborhood of Tycho and Clavius. To the left of a line joining them you will perceive a flat, oval plain with a much broken mountain ring. This is Maginus. Last evening while we were looking at one of the smaller photographs I pointed it out under a more favorable illumination, telling you at the same time that it possessed the peculiarity of almost completely disappearing at Full Moon. Already, although day has not advanced very far upon it, you observe that it has become relatively inconspicuous. This is a lesson in the curious effects of light and shadow in alternately revealing and concealing vast objects on the moon. You will notice that in many particulars Maginus resembles a reduced copy of Clavius. But the walls of Clavius are in a comparatively perfect condition while those of Maginus have apparently crumbled and fallen, destroyed by forces of whose nature we can only form guesses. Evidently the destruction has not been wrought, like that of some of the rings in the _Mare Nubium_, by an inundation of liquid rock from beneath the crust. It resembles the effects of the ‘weathering’ which gradually brings down the mountains of the earth, but if such agencies ever acted upon the moon, then it must have had an atmosphere and an abundance of water. In any event, here before us is another page of lunar chronology. Maginus is evidently far older than Clavius; Clavius is older than the craters standing on its own walls.” We now took up the third of the large photographs representing a part of the southwestern quarter of the moon, more extraordinary for its mountains, plateaus, and extinct volcanoes than the famous southwestern region of the United States. “Here is something that you will surely recognize without any assistance,” I said. “In the lower left-hand corner of the picture is the great three-link chain of crater rings, of which Theophilus is the principal and most perfect member.” “Oh, I recall them well,” replied my friend. “And yet they do not appear to me exactly the same as when I saw them before.” [Illustration: THE GREAT SOUTHWEST ON THE MOON. ]