242 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOIiOLOGY. 



other matter tliat 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, elec- 

 tricit}'', and perhaps magnetism. But with regard to the sea, it 

 is not known what office, if any, is performed by electricity, in 

 crivino: 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 comuience 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 circula- 

 tion; 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- 



