42 THE PHYSICAL GEOGRAPHY OF THE SEA. 



in tlie ocean on the left of tlie stream. It is tlie height of sum- 

 mer : 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 as- 

 sumed a normal condition between the two divisions, adjusting it- 

 self 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 likewise by a change 

 in volume of those waters, amounting, no doubt, in the aggregate, 

 to many hundred millions of tons, over the whole ocean ; for sea 

 water, unlike fresh (§ 103), contracts to freezing, and below. 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 level, than Na- 

 ture is sure with her efforts to restore equilibrium in both sea and 

 air whenever, wherever, and by whatever it be disturbed. There- 

 tore, though the waters of the Gulf Stream do not extend to the 

 bottom, and though they be not impenetrable to the waters on 

 either hand, yet, seeing that they have a waste of waters on the 

 right and a waste of waters on the left, to which (§ 70) they offer 

 a sort of resisting permeability, we are enabled to comprehend 

 how the waters on either hand, as their specific gravity is in- 

 creased or diminished, will impart to the trough of this stream a 

 vibratory motion, pressing it now to the right, now to the left, 

 according to the seasons and the consequent changes of tempera- 

 ture in the sea. 



126. Plate VI. shows the limits of the Gulf Stream for March 

 Limits of the Gulf and September. The reason for this change of po- 

 and September. ' sitiou is obvious. The bauks of the Gulf Stream 

 (§ 70) are cold water. In winter the volume of cold water on the 

 American, or left side of the stream, is greatly increased. It must 

 have room, and gains it by pressing the warmer waters of the 

 stream farther to the south, or right. In September, the tempera- 

 ture of these cold waters is modified ; there is not such an extent 

 of them, and then the warmer waters, in turn, press them back, 

 and so the pendulum-like motion is preserved. 



