THE SALTS OF THE SEA. 
159 
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 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 
