186 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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 (§ 490) to beget currents, or to set 

 the water in motion by reason of the difference of level or of spe- 

 cific gravity due to water at different densities and temperatures. 



507. 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 and partial 

 aqueous circulation in the supposed sea of fresh water. 



508. 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. 



509. Let us next call in evaporation and precipitation, with 

 heat and cold — more powerful agents. Suppose the evaporation 

 to commence from 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 in excess of precipitation 

 (§ 178), 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 toward the trade- 

 wind or evaporating region, to restore the level. 



510. On the other hand, the winds have taken this vapor, borne 

 it off to the extra-tropical regions, and precipitated it, we will sup- 

 pose, where precipitation is in excess of evaporation. Here is 

 another alteration of sea level by elevation instead of by depres- 

 sion ; 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 regions — 

 demand for evaporation being taken here to mean the difference 

 between evaporation and precipitation for any part of the sea. 



511. Now imagine this sea of uniform temperature (§ 50G) to 



