But we cannot visit the moon because there is no atmospheric air between that planet and the earth. If air existed in this vast space in which a balloon might float, and which a man might breathe, I think that some of us would manage to get to the moon before any one reaches the North Pole. The journey would be longer, but there would be no ice to block up the way. But notwithstanding the fact that we cannot go to the moon, we know a great deal about that planet, especially as it affects the earth. And with the great telescopes that have been constructed, in late years, we can see much of the general configuration of that side of the moon which is turned towards us, and it appears very like the picture at the head of this article. Here we see depressions and elevations, and plain surfaces which may be, and probably are, mountains and beds of dried up oceans and vast plains, which, in all probability, are barren and desolate. For scientific men feel quite certain that the moon has no atmosphere, and of course if there is no air, we have no reason to suppose that there is any life there. But our principal interest in the moon relates to its effects upon ourselves, and our own planet, and therefore we should all understand it as we see and enjoy it from our stand-point. We all know that sometimes the moon is full and bright, flooding the earth with its lovely light, and that, at other times it is quite dim, just a curved strip of light in the sky, and at still other times it seems to be absent altogether. Though we have noticed all this, it is very probable indeed that some of us do not entirely understand these changes, and so I shall briefly explain them. When we cannot see the moon at all, which is the case for two or three days every month, it is because the sun is not shining on that side of the moon which is turned to us. And we might as well remember that although the moon moves around the earth once every four weeks, it always turns the same side to us. We never have seen the other side, with telescopes, or in any other way. When the moon is between us and the sun, the side towards us must of course be dark. Then it is that we do not see the moon at all. But as the moon moves gradually to one side we begin to see a little strip of the bright portion as you notice in Fig. 1. This represents the moon in its first phase. I suppose you have noticed at such times, when the new moon is very small, that we can often see the whole disk of the moon, although the principal portion of it is very dim indeed. Still we can see a faint light shining upon it which makes it comparatively easy for us to discern its outlines. [Illustration: FIG. 1. FIRST PHASE OF MOON.] This pale light is “earth-light.” The earth is then “full” to the moon-folk, (if there be any such creatures who can live without air,) and its brilliancy is partially reflected back to us from the surface of the moon.