CAUSES OF OCEAN CURRENTS. 55 
of inaccessible mountains, shame the flight of the condor as he 
towers over the summit of the Andes, and penetrate deeper into 
the bowels of the earth than the miner has ever sunk his shaft. 
Leaving their wanderings through the regions of air to the 
next chapter, I shall now describe the principal ocean currents, 
the simple, but powerful agencies by which they are set in 
motion, their importance in the economy of nature, and their 
influence on the climate of different countries. 
Even in the torrid zone, the waters of the ocean, like a false 
friend, are warm merely cn the surface, and of an almost icy cold- 
ness at a considerable depth. This low temperature cannot be 
owing to any refrigerating influence at the bottom of the sea, 
as the internal warmth of the earth increases in proportion to its 
depth, and the waters of protuund lakes, in a southern climate, 
never show the same degree of cold as those of the vast ocean. 
The phenomenon can thus only arise from a constant sub- 
marine current of cold water from the poles to the line, and 
strange as it may seem, its primary cause is to be sought for in 
the warming rays of the sun, which, as we all know, distributes 
heat in a very unequal manner over the surface of the globe. 
Heat expands all liquid bodies, and renders them lighter ; 
cold increases their weight by condensation. In consequence of 
this physical law, the waters of the tropical seas, rendered 
buoyant by the heat of a vertical sun, must necessarily rise and 
spread over the surface of the ocean to the north and south, 
whilst colder and heavier streams from the higher latitudes 
flow towards the equator along the bottom of the ocean, to re- 
place them as they ascend. 
In this manner, the unequal action of the sun calls forth a 
general and constant movement of the waters from the poles to 
the equator, and from the equator to the poles; and this per- 
petual migration is one of the chief causes by which their purity 
is maintained. These opposite currents would necessarily flow 
direct to the north or south, were they not deflected from their 
course by the rotation of the earth, which gradually gives them 
a westerly or easterly direction, 
The unequal influence of the sun in different parts of the 
globe, and the rotation of the earth, are, however, not the only 
causes by which the course of ocean-currents is determined. 
Violent storms move the waters to a considerable depth, and 
