THE SALTS OF THE SEA. I59 



310. Let evaporation now commence in the trade-wind region, 

 as it was supposed to do (§ 304) in the case of the fresh-water 

 seas, and as it actually goes on in nature — and what takes place ? 

 Why, a lowering of the sea level, as before. But as the vapor of 

 salt water is fresh, or nearly so, fresh water only is taken up from 

 the ocean ; that which remams behind is therefore more salt. 

 Thus, while the level is lowered in the salt sea, the equilibrium is 

 destroyed because of the saltness of the water ; for the water that 

 remains after the evaporation takes place is, on account of the 

 solid matter held in solution, specifically heavier than it was be- 

 fore any portion of it was converted into vapor. 



311. The vapor is taken from the surface water; the surface 

 water thereby becomes more salt, and, under certain conditions, 

 heavier ; when it becomes heavier it sinks ; and hence we have, 

 due to the salts of the sea, a vertical circulation, viz., a descent 

 of heavier — because Salter and cooler — vvater from the surface, 

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

 from the depths below. 



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

 regions (^ 126), 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^, &c., than it 

 returns to the atmosphere in the shape of vapor. 



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

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

 regions, by depression ; w^hich, as already stated (^ 305), gives 

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

 the poles toward the equator. 



But we are now considering the effects of evaporation and pre- 

 cipitation in giving impulse to the circulation of the ocean w^here 

 its waters are salt. 



314. The fresh water that has been taken from the evaporating 

 regions is deposited upon those of precipitation, which, for illus- 

 tration merely, we will locate in the north polar basin. Among 

 the sources of supply of fresh water for this basin we must in- 

 clude not only the precipitation which takes place over the basin 

 itself, but also the amount of fresh water discharged into it by the 



