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 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 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 lowest 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 with 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 Wellington 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. 
L 
