In speaking of the surface of the moon, we are able only to speak about one side. The other is entirely hidden from us. This is caused by the curious fact that the moon turns on her axis and travels round the earth in exactly the same length of time. One-half of the moon is thus always turned towards us, though of that half we can only see so much as is receiving the light of the sun. But the half turned in our direction is always the same half. One part of the moon--not quite so much as half, though always the same portion--is turned away from us. A small border on each side of that part becomes now and then visible to us, owing to certain movements of the earth and the moon. What sort of a landscape may lie in the unknown district, it is idle to imagine. Many guesses have been made. Some have supposed it possible that air _might_ be found there; that water _might_ exist there; that something like earthly animals _might_ live there. It is difficult to say what may not be, in a place about which we know nothing whatever. But judging from our earthly experience, nothing seems more unlikely than that air, water, clouds, should be entirely banished from over one-half of a globe, and collected together in the space remaining. We are on safer ground when we speak about that part of the moon which is turned towards us. For we can say with confidence that if any atmosphere exist there, it must be in thickness less than the two-thousandth part of our earthly atmosphere. It seems equally clear that water also must be entirely wanting. The tremendous heat of the long lunar day would raise clouds of vapor, which could not fail to be visible. But no such mistiness ever disturbs the sharply-defined outline of the moon, and no signs of water action are seen in the craggy mountains and deep craters. [Illustration: THE LUNAR CRATER COPERNICUS.] The craters which honeycomb the surface of the moon are various in size. Many of the larger ones are from fifty to a hundred miles in diameter. These huge craters--or, as we may call them, deep circular plains--are surrounded by mighty mountain ramparts, rising to the height of thousands of feet. Usually they have in their center a sugar-loaf or cone-shape mountain, or even two or more such mountains, somewhat lower in height than the surrounding range. The sunset-lights upon certain of these distant mountain-peaks were first watched by Galileo through his telescope, and have since been seen by many an observer--intense brightness contrasting with intense blackness of shadow.