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 VL), being in winter about latitude 40°-41°, and in Septem- 
ber, when the sea is hottest, about latitude 45°-46'^. The trough 
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 which stretches over toward the Grand Banks of New- 
foundland is, as the temperature of the waters of the ocean changes, 
first pressed dow^n 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 which 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 water, 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 w^hole ocean to the left of 
it ? If not, the difl^erence must be compensated by some means. 
Sparks are not more prone to fly upward, nor water to seek its 
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