§ 468. THE SALTS OF THE SEA. 241 



US, for tlie 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 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 tempera- 

 tares. Now let us suppose the winds, for the first time since the 

 creation, to commence to blow upon this quiescent sea, and to 

 rufSe 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 currents thus 

 created would set with the wind, giving rise to counter currents 

 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 T Let us next call in evaporation and 

 precipitation, with heat and cold — more powerful agents still. 

 Suppose the evaporation to commence fr9m this imaginary fresh- 

 water ocean, and to go on as it does from the seas as they are. In 

 those regions, as in the trade- wind regions, where evaporation is 

 ill excess of precipitation (§ 545), the general level of this sup- 

 posed sea Would be altered, and immediately as much water as is 

 carried off by evaporation would commence to flow in from north 

 and south toward the trade- wind or evaporating region to restore 

 the level. On the other hand, the winds would have taken this 

 vapor, borne it off to the extra-tropical regions, and precipitated 

 it, we >vill suppose, where precipitation is in excess of evaporation. 

 Here is another alteration of sea level by elevation instead of by 

 depression ; and hence we have the motive power for a surface 

 current from each pole toward the equator, the object of which is 

 only to supply the demand for evaporation in the trade-wind re- 

 gions — demand for evaporation being taken here to mean the dif- 

 ference between evaporation and precipitation for any part of the 



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