THE GULF STREA^I. 39 



tlie salt which the trade-wind vapour leaves behind in the tropics 

 has to be conveyed away from the trade-wind region, to be mixed 

 np again in due proportion with the other water of the sea — the 

 Baltic Sea and the Arctic Ocean included — and that these are 

 Gome 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 hy 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 

 djTiamical 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 Strear.i. — As to the tempe- 

 rature 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 

 reasoil of their Avarmth. 



110. Top of Gulf Stream roof-sliaped. — 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 con- 

 tiguous 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 



