other matter that shall very much affect its character, or alter the proportion of its ingredients, and affect the health of its inhabitants; but in the open sea or open air we can do no such thing.
468. Dynamical agents.—The principal agents that are supposed to be concerned in giving circulation to the atmosphere, and in preserving the ratio among its components, are light, heat, electricity, and perhaps magnetism. But with regard to the sea, it is not known what office, if any, is performed by electricity, in giving dynamical force to its system of circulation. The chief motive power from which marine currents derive their velocity has been ascribed to heat; but a close study of the agents concerned has suggested that an important—nay, a powerful and active agency in the system of oceanic circulation is derived from the salts of the sea water, through the instrumentality of the winds, of marine plants, and animals. These give the ocean great dynamical force. Let us, for the sake of illustrating and explaining the nature of this force, suppose the sea in all its parts—in its depths and at the surface, at the equator and about the poles—to be of one uniform temperature, and to be all of fresh water; and, moreover, that there be neither wind to disturb its surface, nor tides nor rains to raise the level in this part, or to depress it in that. In this case there would be nothing of heat to disturb its equilibrium, and there would be no motive power (§ 461) to beget currents, or to set the water in motion by reason of the difference of level or of specific gravity due to water at different densities and temperatures. Now let us suppose the winds, for the first time since the creation, to commence to blow upon this quiescent sea, and to ruffle its surface; they, by their force, would create partial surface currents, and thus agitating the waters, as they do, but only for a little way below the surface, would give rise to a feeble circulation in the supposed sea of fresh water. The surface drift thus created—currents they would hardly be,—would set with the wind, giving rise to counter movements in the shape of under-tows and eddies. This, then, is one of the sources whence power is given to the system of oceanic circulation; but, though a feeble one, it is one which exists in reality, and, therefore, need not be regarded as hypothetical. Some (§ 79) think it the "sole cause!" Let us next call in evaporation and precipitation, with heat and cold—more powerful agents still. Suppose the evaporation to commence from this imaginary fresh-
