25 
CHAPTER II. 
CURRENTS OF THE OCEAN. 
6c 
- 4 + «© « « . 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 
