THE SALTS OF THE SEA. 189 



an ascent of water that is lighter — because it is not so salt — from 

 the depths below. 



519. This vapor, then, which is taken up from the evaporating 

 regions (§ 179), is carried by the winds through their channels of 

 circulation, and poured back into the ocean where the regions of 

 precipitation are ; and by the regions of precipitation I mean those 

 parts of the ocean, as in the polar basins, where the ocean receives 

 more fresh water in the shape of rain, snow, etc., than it returns 

 to the atmosphere in the shape of vapor. 



520. In the precipitating regions, therefore, the level is de- 

 stroyed, as before explained, by elevation ; and in the evaporating 

 regions, by depression ; which, as already stated (§ 509), gives rise 

 to a system of surface currents, moved by gravity alone, from the 

 poles toward the equator. 



521. But we are now considering the effects of evaporation and 

 precipitation in giving impulse to the circulation of the ocean where 

 its waters are salt. The fresh water that has been taken from the 

 evaporating regions is deposited upon those of precipitation, which, 

 for illustration merely, we will locate in the north Polar basin. 

 Among the sources of supply of fresh water for this basin, we 

 must include not only the precipitation which takes place over 

 the basin itself, but also the amount of fresh water discharged into 

 it by the rivers of the great hydrographical basins of Arctic Eu- 

 rope, Asia, and America. 



522. This fresh water, being emptied into the Polar Sea and 

 agitated by the winds, becomes mixed with the salt ; but as the 

 agitation of the sea by the winds is supposed to extend to no great 

 depth (§ 507), it is only the upper layer of salt water, and that to 

 a moderate depth, which becomes mixed with the fresh. The 

 specific gravity of this upper layer, therefore, is diminished just as 

 much as the specific gravity of the sea water in the evaporating 

 regions was increased. And thus we have a surface current of 

 saltish water from the poles toward the equator, and an under 

 current of water Salter and heavier from the equator to the poles. 

 This under current supplies, in a great measure, the salt which the 

 upper current, freighted with fresh water from the clouds and riv- 

 ers, carries back. 



