THE GULF STREAM. 43 



United States and the Shoals of Nantucket to turn the Gulf Stream 

 toward the east. 



53. But there are other forces operating upon the Gulf Stream. 

 They are derived from the effect of changes in the waters of the 

 whole ocean, as produced by changes in their temperature from 

 time to time. As the Gulf Stream leaves the coasts of the United 

 States, it begins to vary its position according to the seasons ; the 

 limit of its northern edge, as it passes the meridian of Cape Race 

 (Plate VI.), being in winter about latitude 40O-41°, and in Septem- 

 'ber, when the sea is hottest, about latitude 45*^-46°. The trouo-h 

 of the Gulf Stream, therefore, may be supposed to waver about in 

 the ocean not unlike a pennon in the breeze. Its head is confined 

 between the shoals of the Bahamas and the Carolinas, but that 

 part of it w^hich stretches over toward the Grand Banks of New- 

 foundland is, as the temperature of the waters of the ocean changes, 

 first pressed down toward the south, and then again up toward 

 the north, according to the season of the year. 



To appreciate the extent of the force by wdiich it is so pressed, 

 let us imagine the waters of the Gulf Stream to extend all the way 

 to the bottom of the sea, so as completely to separate, by an im- 

 penetrable liquid wall, if you please, the waters of the ocean on 

 the right from the waters in the ocean on the left of the stream. 

 It is the height of summer : the waters of the sea on either hand 

 are for the most part in a liquid state, and the Gulf Stream, let it 

 be supposed, has assumed a normal condition between the two 

 divisions, adjusting itself to the pressure on either side so as to 

 balance them exactly and be in equilibrium. Now, again, it is the 

 dead of winter, and the temperature of the waters over an area of 

 millions of square miles in the North Atlantic has been changed 

 many degrees, and this change of temperature has been followed 

 by a change in the specific gravity of those waters, amounting, no 

 doubt, in the aggregate, to many hundred millions of tons, over 

 the whole ocean ; for sea w^ater, unlike fresh (§ 31), contracts to 

 freezing. Now is it probable that, in passing from their summer 

 to their winter temperature, the sea waters to the right of the Gulf 

 Stream should change their specific gravity exactly as much in 

 the aggregate as do the waters in the whole ocean to the left of 

 it ? If not, the difference must be compensated by some means. 

 Sparks are not more prone to fly upward, nor water to seek its 



