The atmosphere is, then, an integral portion of the earth, a body of air revolving with the solid mass upon its axis, the higher strata, of course, increasing in velocity with the distance from the axis of revolution. From hence a conclusion may be drawn respecting its height, for an absolute limit is put to its elevation by this feature of its physical condition. There is a point where the centrifugal force, or the tendency to fly off from the centre, will counterbalance the centripetal, or the gravitation toward the centre, and beyond that point the latter will be vanquished. It is obvious that no portion of the atmosphere can extend beyond the point where the two influences balance, or are in equilibrium, and the projectile force becomes greater than that of gravitation, or its projection into space would follow. At the distance of 6.6 radii from the centre of the earth, or at an elevation of 22,200 miles, about the eleventh part the distance of the moon, this point is fixed, beyond which it is impossible for the atmosphere in the smallest quantity to extend. This consideration is only of importance to show that physical laws rigidly restrict it within finite bounds, for any portion of air at that distance must have a tenuity which is utterly inconceivable. The indications of the height of the atmosphere drawn from its weight, as shown by the barometer, reduce its elevation within a vastly circumscribed limit. A column of the whole circumambient air is nearly equal in weight to a similar column of mercury of thirty inches, or of water of thirty-four feet, which would give it an elevation of but 27,000 feet, or rather under five miles, if its density were uniform. But the elasticity of the air causes it to expand with the diminution of its own pressure, which becomes less at every step from the surface of the earth; and owing to this expansion we must place the limit to its height at a far greater distance than that suggested by the simple barometrical measurement of its weight. A pretty common opinion prevails that its extreme boundary does not exceed forty or fifty miles, and we have sensible evidence on the high lands of the globe, that for all the purposes serviceable to vegetable and animal life, the atmospheric zone is of very contracted elevation. It is a well-known property of the air that the temperature diminishes with its height, a circumstance referable to the general physical law, that as the density of gases decreases they acquire an increased capacity for heat. The higher, therefore, a body ascends in the atmosphere, the greater is the quantity of heat abstracted from it, the surrounding fluid becoming more rare. Hence the perpetual snow, and the piles of glaciers, that crown the summits of mountains, at whose base the orange and the citron bloom, and man pants in the fierce sultriness of a torrid climate.