164 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
is probably not so cold ; and if it be not so cold, its temperature, 
before it comes out again, must be reduced to 28°, or whatever 
be the average temperature of the outer but surface current. 
Moreover, if it be true, as some philosophers have suggested, 
that there is in the depths of the ocean a line from the equator to 
the poles along vi^hich the water is of the same temperature all 
the way, then the question may be asked, Should we not have in 
the depths of the ocean a sort of isothermal floor, as it were, on 
the upper side of which all the changes of temperature are due to 
agents acting from above, and on the lower side of which, the 
changes, if any, are due to agents acting from below ? 
326. This under Polar current water, then, as it rises to the 
top, and is brought to the surface by the agitation of the sea in 
the Arctic regions, gives out its surplus heat and warms the at- 
mosphere there till the temperature of this warm under current 
water is lowered to the requisite degree for going out on the sur- 
face. Hence the water-sky of those regions. 
327. And the heat that it loses in falling from its normal tem- 
perature, be that what it may, till it reaches the temperature of 
28°, is so much caloric set free in the Polar regions, to temper the 
air and mitigate the climate there. Now is not this one of those 
modifications of climate which may be fairly traced back to the 
effect of the saltness of the sea in giving energy to its circula- 
tion? 
Moreover, if there be a deep sea in the Polar basin, which serves 
as a receptacle for the waters brought into it by this under cur- 
rent, which, because it comes from toward the equatorial regions, 
comes from a milder climate, and is therefore warmer, we can 
easily imagine why there might be an open sea in the Polar re- 
gions — why Lieutenant De Haven, in his instructions, was direct- 
ed to look for it ; and why both he and Captain Penny, of one of 
the English searching vessels, found it there. 
328. And in accounting for this polynia, we see that its exist- 
ence is not only consistent with the hypothesis with which we set 
out, touching a perfect system of oceanic circulation, but that it 
may be ascribed, in a great degree at least, if not w^hoUy, to the 
effect produced by the salts of the sea upon the mobility and cir- 
culation of its waters. 
Here, then, is an office which the sea performs in the economy 
