The hot and salt waters of the tropics are carried north by the Gulf Stream. There are here two counteracting agents at work. Heat inclines the Gulf Stream to rise; saltness inclines it to sink. During the first part of its journey, as we know, its great heat prevails over the other influence, and it flows as a surface-current. But, at a certain point in its northward route, it meets with the cold, brackish, ice-bearing currents that flow out of the arctic basin. Having lost much of its heat (though still possessing a great deal more than the arctic currents), the saltness of the Gulf Stream prevails; it dips below the polar waters, and thenceforth continues its course as an under-current, salt, and comparatively warm. To state the matter briefly: The hot water, which _ought_ to keep on the surface because of its heat, is sunk by its superabundant salt; and the cold water, which _ought_ to sink because of its cold, is buoyed on the surface because of its want of salt. Now arises the question--what becomes of the great quantity of salt that is thus being carried perpetually into the polar basin? Manifestly it must be carried out again by the surface-current, otherwise the polar basin would of necessity become a basin of salt. The under-current _must_, therefore, rise to the surface somewhere near the pole, with its temperature necessarily only a little, if at all, below the freezing-point--which, be it observed, is a _warm_ temperature for such regions. Here, then, where the warm waters from the south rise to the surface, it is supposed this open Arctic Ocean must exist. So much for theory. Now for facts that have been observed, and that tend, more or less, to corroborate this proposition of an open polar sea. The habits of the whale have gone far to prove it. The log-books of whalers have for many years been carefully examined and compared by scientific men. These investigations have led to the discovery "that the tropical regions of the ocean are to the `right' whale as a sea of fire, through which he cannot pass, and into which he never enters." It has also been ascertained that the same kind of whale which is found off the shores of Greenland, in Baffin's Bay, etcetera, is found in the North Pacific, and about Behring's Straits; and that the `right' whale of the southern hemisphere is a different animal from that of the northern. How, then, came the Greenland whales to pass from the Greenland seas to the Pacific? Not by the Capes Horn or Good Hope; the "sea of fire" precluded that. Clearly there was ground here for concluding that they did so through the (supposed) open sea lying beyond, or rather within, the frozen ocean. It is true the objection might be made, that the same kind of whale which exists in the North Pacific exists also in the North Atlantic, although they never cross over to see each other. But another discovery has met this objection.