THE SALTS OF THE SEA. 161 



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

 bad conductor of heat, this warm water is employed in transport- 

 ing through under currents heat for the mitigation of chmates 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 disturb- 

 ed, for there is no change in the specific gravity of that at the sur- 

 face 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 the top will soon be so changed 

 as to bring that from the very low^est depths of the sea speedily 

 to the top. 



320. If these inferences as to the influence of the salts upon the 

 currents of the sea be correct, the same cause which produces an 

 under current from the Mediterranean, and an under current from 

 the Red Sea into the ocean, should produce an under current 

 from the ocean into the north Polar basin. In each case, the hy- 

 pothesis with regard to the part performed by the salt, in giving 

 vigor to the system of oceanic circulation, requires that, counter 

 to the surface current of water with less salt, there should be an 

 under current of water with more salt in it. 



That such is the case w^ith regard both to the Mediterranean 

 and the Red Sea, has been amply shown in other parts of this 

 work {^ 252 and 239), and abundantly proved. by other observers. 



321. That there is a constant current setting out of the Arctic 

 Ocean through Davis's and other straits thereabout, which con- 

 nect it with the Atlantic Ocean, is generally admitted. Lieuten- 

 ant De Haven, United States Navy, when in command of the 

 American expedition in search of Sir John Franklin, was frozen 

 up with his vessels in the main channel of Welhngton Straits ; 

 and during the nine months that he was so frozen, his vessels, 

 holding their place in the ice, were drifted with it bodily for more 

 than a thousand miles toward the south. 



The ice in which they were bound was of sea water, and the 

 currents by which they were drifted were of sea water — only, it 

 may be supposed, the latter were not quite so salt as the sea wa- 

 ter generally is. The same phenomenon is repeated in the Sound, 

 where (§ 252) an under current of salt water runs in, and an upper 

 current of brackish water (§ 36 and 51) runs out. 



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