THE SALTS OF THE SEA. 233 



perhaps magnetism. But with regard to the sea, it is not kiio^\Ti 

 what office, if any, is performed by electricity, in giving d^qiami- 

 cal force to its system of circulation. The chief motive power 

 from which marine cm-rents 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 sys- 

 tem 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 sur- 

 face, 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 distm^b 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 difierent densities and temperatures. Now 

 let us suppose the winds, for the first time since the creation, ta 

 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 httle way below the 

 sm-face, 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 vdth. the wind, giving rise to counter 

 movements in the shape of under-tows and eddies. This, then, is 

 one of the som'ces whence power is given to the system of oceanic 

 circulation; but, though a feeble one, it is one which exists in 

 reahty, and, therefore, need not be regarded as hypotheticaL 

 Some (§ 79) think it the " sole cause r Let us next call in eva- 

 poration and precipitation, with heat and cold — more powerful 

 agents still. Suppose the evaporation to commence from this- 

 imaginary fresh-water ocean, and to go on as it does from the sea& 

 as they are. In those regions, as in the trade-wind regions, where 

 evaporation is in excess of precipitation (§ 545), the general level 

 of this supposed sea would be altered, and immediately as much 

 water as is carried off by evaporation would commence to flow in 

 from north and south towards the trade-wind or evaporating region 

 to restore the level. On the other hand, the winds would have 

 taken this vapour, borne it ofi' to the extra-tropical regions, and 



