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 remains 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 — water 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; which, 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 where 

 its waters are salt. 



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

 regions is deposited upon those of precipitation, which, for illu!<- 

 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 



