The atmosphere is not of uniform density throughout. At high altitudes it is extremely rare. That is, there is little of it in a given volume. Close to the earth's surface, however, it is comparatively dense. Half of all the atmosphere is within three and one-half miles of the surface and half of the remainder lies within the next three and a half miles. We may consider it as made up, on the whole, of layers of different densities, strongly compressed near the surface.