Currents 
The great transportation system of the ocean is its vast 
network of currents. Some of these currents extend for thou- 
sands of miles through the entire length of an ocean. Still 
others are of a more local nature, being confined to a single 
harbor or inlet. The large and small scale currents may be 
studied conveniently if separated. But this separation is more 
than a mere matter of convenience, because there is a real 
difference between the causes of large and small scale cur- 
rents. Therefore we distinguish between: 
1. Ocean currents: large scale currents; caused chiefly by 
differences of density or by prevailing winds. 
2. Tidal currents: small scale currents; caused by the dif- 
ferences of level of tides in small harbors and inlets. 
OCEAN CURRENTS 
There is a general world wide system of ocean currents 
which remains more or less the same throughout the years. It 
includes such great ocean rivers as the Gulf Stream, the 
Kuroshio Current, and the Equatorial Currents. These cur- 
rents are related to differences in density of the water in 
different parts of the ocean. The differences in density are 
due either to temperature (warm water is in general less 
dense than cold water ) or to the salt content (the density is 
greater the more salt the water contains ). The salt content, in 
turn, depends upon the rate of evaporation of sea water at the 
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