The energy phenomena illustrated by the experimental devices above are to be observed, in their aspects of greatest perfection, in the natural world. In the earth, united to its encircling atmosphere by the invisible bond of gravitation, we find the prototype of the hypothetical system just described. Its uniformity of rotation is an established fact of centuries, and over its spheroidal surface we have, corresponding to the motion of our illustrative spherical masses, the movement of enormous quantities of atmospheric air in the general directions from equatorial to polar regions and vice versa. This circulatory movement, and the internal energy reactions which it involves, have been already fully dealt with (§ 88); we have now to consider it in a somewhat more comprehensive fashion, in the light of the pendulum systems described above. As already explained (§ 13), the operation of mechanical energy is not confined to solid and liquid masses only, but may likewise be manifested by the movements of gaseous masses. The terrestrial atmospheric machine provides an outstanding example. In its working conditions, and in the general nature of the energy operations involved, the terrestrial atmospheric machine is very clearly represented by the rotating pendulum system (§ 42). The analogy is still closer in the case of the hypothetical system just described. The actual terrestrial energy machine differs from both only in that the energy processes, which they illustrate by the movements of solid material, are carried out in the course of its working by the motion of gaseous masses. It is obvious, however, that this in no way affects the inherent nature of the energy processes themselves. They are carried out quite as completely and efficiently--in fact, more completely and more efficiently--by the motions of gaseous as by the motions of solid material.