25 



CHAPTER 11. 



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 4° 25' Centigrade below zero, which it preserves in all latitudes at 

 the depth of 1,000 yards, according to M. D'Urville. 



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

 lighter than it was at 4° 25' Centigrade, 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 temperature is 

 above 4° 25', water is light, and ascends to the surface, while colder 

 water sinks to the bottom. Below 4" 25' 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 

 compensated 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 of the compensations by which the oceanic 

 waters are governed. " According to Rodgers' observations," he 

 says, " the average specific gravity of the sea water on the parallels 

 of 34° north and south, at a mean temperature of 64", is just what 

 it ought to be, according to saline and thermal laws ; but its specific 

 gravity, when taken from the Equator at a mean temperature of 



