the salt which the trade-wind vapour leaves behind in the tropics has to be conveyed away from the trade-wind region, to be mixed up again in due proportion with the other water of the sea—the Baltic Sea and the Arctic Ocean included—and that these are some of the waters, at least, which we see running off through the Gulf Stream. To convey them away is doubtless one of the offices which, in the economy of the ocean, has been assigned to it. But as for the seat of the forces which put and keep the Gulf Stream in motion, theorists may place them exclusively on one side of the ocean with as much philosophical propriety as on the other. Its waters find their way into the North Sea and the Arctic Ocean by virtue of their specific gravity, while water thence, to take their place, is, by virtue of its specific gravity and by counter currents, carried back into the Gulf. The dynamical force which causes the Gulf Stream may therefore be said to reside both in the polar and in the intertropical waters of the Atlantic.
109. Winter temperature of the Gulf Stream.—As to the temperature of the Gulf Stream, there is, in a winter's day, off Hatteras, and even as high up as the Grand Banks of Newfoundland in mid-ocean, a difference between its waters and those of the ocean near by of 20° and even 30°. Water, we know, expands by heat, and here the difference of temperature may more than compensate for the difference in saltness, and leave, therefore, the waters of the Gulf Stream, though salter, yet lighter by reason of their warmth.
110. Top of Gulf Stream roof-shaped.—If they be lighter, they should therefore occupy a higher level than those through which they flow. Assuming the depth off Hatteras to be one hundred and fourteen fathoms, and allowing the usual rates of expansion for sea-water, figures show that the middle or axis of the Gulf Stream there should be nearly two feet higher than the contiguous waters of the Atlantic. Hence the surface of the stream should present a double inclined plane, from which the water would be running down on either side as from the roof of a house. As this runs off at the top, the same weight of colder water runs in at the bottom, and so raises up the cold-water bed of the Gulf Stream, and causes it to become shallower and shallower as it goes north. That the Gulf Stream is therefore roof-shaped, causing the waters on its surface to flow off to either side from the middle, we have not only circumstantial evidence
