150 THE PHYSICAL GEOGRAPHY OF THE SEA. 



rivers of the great hydrographical basins of Arctic Europe, Asia, 

 and America. 



315. 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 extends to no great depth (§ 302), 

 it is only the upper layer of salt water, and that to a moderate 

 depth, which becomes mixed with the fresh. The specific grav> 

 ity 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. 



316. Thus it is to the salts of the sea that we owe that feature 

 in the system of oceanic circulation which causes an under cur- 

 rent to flow from the Mediterranean into the Atlantic (§ 252), and 

 another (^ 245) from the Red Sea into the Indian Ocean. And 

 it is evident, since neither of these seas is salting up, that just as 

 much, or nearly just as much salt as the under current brings out, 

 just so much the upper currents carry in. 



317. We now begin to perceive what a powerful impulse is de- 

 rived from the salts of the sea in giving efTective and active cir- 

 culation to its waters. 



318. Hence w^e infer that the currents of the sea, by reason of 

 its saltness, attain their maximum of volume and velocity. Hence, 

 too, we infer that the transportation of warm water from the equa- 

 tor toward the frozen regions of the poles, and of cold water from 

 the frigid toward the torrid zone, is facilitated ; and consequently 

 here, in the saltness of the sea, have we not an agent by which 

 climates are mitigated — by which they are softened and rendered 

 much more salubrious than it would be possible for them to be 

 were the waters of the ocean deprived of their property of saltness? 



319. This property of saltness imparts to the waters of the 

 ocean another peculiarity, by which the sea is still better adapted 

 for the regulation of climates, and it is this : by evaporating fresh 

 water from the salt in the tropics, the surface water becomes 

 heavier than the average of sea water (§ 127). This heavy wa- 



