190 THE PHYSICAL GEOGRAPHY OF THE SEA. 



523. 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 (§ 425), and 

 another (§ 413) from the E,ed 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. 



524. We now begin to perceive what a powerful impulse is 

 derived from the salts of the sea in giving effective and active cir- 

 culation to its waters. 



525. 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 

 liere, 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 ? 



526. 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 (§ 181). This heavy water 

 is also warm water ; it sinks, and being a good retainer, but a bad 

 conductor of heat, this warm water is employed in transporting 

 through under currents heat for the mitigation of climates in far- 

 distant regions. Now this also is a property which a sea of fresh 

 water could not have. Let the winds take up their vapor from a 

 sheet of fresh water, and that at the bottom is not disturbed, for 

 there is no change in the specific gravity of that at the surface by 

 which that at the bottom may be brought to the top ; but let 

 evaporation go on, though never so gently, from salt water, and 

 the specific gravity of that at tlie top will soon be so changed as 

 to bring that from the very lowest depths of the sea to the top. 



527. If all tlie salts of the sea were precipitated and spread 

 out equally over the northern half of this continent, it would, it 



