160 
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 effective and active cir- 
culation to its waters. 
318. Hence we 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 
w^ere 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- 
