CHAPTEE II. 



CURRENTS OF THE OCEAN. 



" seas that sweep 



The three-decker's oaken mast." TENNYSON. 



THE ocean is a scene of unceasing agitation ; " its vast surface rises and 

 falls," to use the image suggested by Schleiden, " as if it were gifted 

 with a gentle power of respiration ; its movements, gentle or powerful, 

 slow or rapid, are all determined by differences of temperature." 



Heat increases its volume and changes the specific gravity of the 

 water, which is dilated or condensed in proportion to the change of 

 temperature. In proportion as it cools, water increases in density, and 

 descends into the depths until it reaches a constant temperature of 

 four degrees twenty-five minutes Cent, below zero, which it preserves 

 in all latitudes at the depth of a thousand yards, according to 

 M. D'Urville. 



If the water continues to cool, and reaches zero, it becomes lighter 

 than it was at four degrees twenty-five minutes Cent., and ascends in 

 a state of congelation a process which, by an admirable provision of 

 nature, can only take place at the surface. So long as the tempe- 

 rature is above four degrees twenty-five minutes, water is light, and 

 ascends to the surface, while colder water sinks to the bottom. 

 Below four degrees twenty-five minutes the process is reversed ; the 

 first phenomenon is always in force under the Equator, the second 

 near the Poles. The evaporation, which is in continual operation in 

 warm seas, forming vast rain-clouds at the expense of the sea, is com- 

 pensated by unceasing currents of colder water flowing from the Poles. 

 This evaporation has a direct- influence, moreover, on the density of 

 sea water, and is pointed out by Dr. Maury as a remarkable instance 



