To be sure the earth does not loom very large in the eye of the sun. It receives a positively trifling fraction of the total output of sunheat. So negligible is this amount that it would not be worth our mentioning if we did not owe our existence to it. It is thanks to the atmosphere, however, that the earth attains this (borrowed) importance. It is thanks to this thin layer of gases that we are protected from that fraction of sunheat which, however trifling when compared with the whole, would otherwise be sufficient to fry us all in a second. Without this gas wrapping we would all freeze (if still unfried) immediately after sunset. The atmosphere keeps us in a sort of thermos globe, unmindful of the burning power of the great star, and of the uncalculated cold of outer space. Yet, limitless as it seems to us and inexhaustible, our invaluable atmosphere is a small thing after all. Half of its total bulk is compressed into the first three and a half miles upward. Only one sixty-fourth of it lies above the twenty-one mile limit. Compared with the thickness of the earth this makes a very thin envelope. Light as air, we say, forgetting that this stuff that looks so thin and inconsequential weighs fifteen pounds to the square inch. We walk around carrying our fourteen tons gaily enough. The only reason that we don't grumble is because the gases press evenly in all directions permeating our tissues and thereby supporting this crushing burden. A layer of water thirty-four feet thick weighs just about as much as this air-pack under which we feel so buoyant. But if these gases get in motion we feel their pressure. We say the wind is strong to-day. As it blows along the surface of the earth this wind is mostly nitrogen, oxygen, moisture, and dust. The nitrogen occupies nearly eight-tenths of a given bulk of air, the oxygen two-tenths, and the moisture anything up to one-twentieth. Five other gases are present in small quantities. The dust and the water vapor occupy space independently of the rest. As one goes up mountains the water vapor increases for a couple of thousand feet and then decreases to the seven mile limit after which it has almost completely vanished. The lightest gases have been detected as high up as two hundred miles and scientists think that hydrogen, the lightest of all, may escape altogether from the restraint of gravity. One strange fact about all of these gases is that they do not form a separate chemical combination, although they are thoroughly mixed.