The largest volcano-crater known on the earth is perhaps not more than eight or ten miles across. On the moon there are numbers and numbers of little craters about that size, too many to count. But there are also many huge craters, far bigger than anything of the kind ever seen here. Some of the Moon mountains are very high, a good deal higher than Mont Blanc in Switzerland; and they often lie in vast rocky heights around some enormous crater. Such craters are to be seen fifty miles across from edge to edge, and some a hundred miles across, and even more. These monster craters make our little Earth craters seem very small; do they not? In a photograph of the Moon’s surface, taken when she is at her First or Last Quarter, the steep mountains and great craters often stand out very clearly. Of course they do not _look_ large to us at this distance, but quite small. If there were air on the moon her sharp outline and the mountain edges would be much softer than they are now. The Moon must be a very, very cold world. True, she is no farther off from the Sun than we are, so his bright rays have the same strength there as here. But she has no thick coverlet of air wrapped round her, to act as a blanket and to keep in the heat which the Sun gives. That is what our air does for us: and that is what the Moon lacks. Through the long fortnight of darkness, on the side of the Moon which is turned away from the Sun, the cold must be perfectly awful. But even during the long fortnight of day-time following, when that part of the Moon which has been in darkness gets round into sunlight, things are not much better. The Sun indeed beats down upon the Moon with frightful power, and with a desperate glare such as we never know on the Earth. For the same air which keeps prisoner the warmth of the sunbeams for our use also softens their glare. But in spite of all this it is likely that the Moon’s surface never gets warm--that in the noon of her long day the ground is far more than ice-cold. On a high mountain-top of the Earth, where the air is thin, although the glare of the Sun becomes fierce yet the ice and snow are not melted. A little thawing of the outermost snow takes place, but often no more than this. When the sun shines through air which is too thin to capture and store up his heat, then he is quite overmatched by the grip of King Frost. If this is so on the Earth how much more is it likely to be so on the Moon. At the top of the highest Earth mountain there is still a good deal of air, enough for a man to breathe. But on the Moon there is no air at all worth speaking of; not enough to keep alive any creature of which we know. So, though the Sun does his best, though he floods the Moon with his warmth, all the heat is poured out again just as fast as he pours it in. For want of a sheltering air-coverlet the ground there may remain, and doubtless does remain, ice-cold through all the long Moon-day.