40 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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. 



iVs 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 New- 

 foundland in mid ocean, a difference between its waters and those 

 of the ocean near by of 20°, and even 30°. Water, we know, ex- 

 pands 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 lighter by reason of their warmth. 



39. 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 allow- 

 ing 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 

 trom 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 shal- 

 lower 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 to show, but observations to prove. 



40. Navigators, while drifting along with the Gulf Stream, have 

 lowered a boat to try the surface current. In such cases, the boat 

 would drift either to the east or to the west, as it happened to be 

 on one side or the other of the axis of the stream, while the ves- 

 sel herself would drift along with the stream in the direction of 

 its course ; thus showing the existence of a shallow roof-current 

 from the middle toward either edge, which would carry the boat 

 along, but which, being superficial, does not extend deep enough 

 to affect the drift of the vessel. 



