Air is chiefly a mixture of two gases, oxygen and nitrogen, in the proportions of one to four by volume. It is believed that the atmosphere at its upper limit presents a definite surface like that of the sea; that disturbing causes produce waves there just as they do on the sea, but that, by reason of greater mobility and other causes, the waves on the surface of this aerial ocean are much larger than any ever seen on the surface of the liquid ocean. The depth of this aerial ocean has been variously estimated at from fifty to two hundred miles.