CURRENTS. 67 



currents carry off the warm surface water before it has time to 

 acquire enough additional density, by evaporation, to outweigh, 

 or even balance, the reduction in its specific gravity resulting 

 from increase of temperature. 



In such seas, however, as the Mediterranean and Red Sea, no 

 circulation corresponding to that of ocean currents exists. The 

 surface water is therefore not carried off, as in the open ocean, 

 but it subsides when its salinity reaches a point which renders 

 flotation no longer possible. Hence the salinity, and therefore 

 specific gravity of the waters, increases with depth, being much 

 greater at the bottom than at the surface. 



The quantity of water taken from the surface of the Mediter- 

 ranean by evaporation considerably exceeds that poured into it 

 by rivers and rainfall combined, the deficiency being made up 

 by a constant indraught from the Atlantic. 



The amount of salt thus carried into the Mediterranean is 

 very great, and is the means provided by Nature of setting up 

 a circulation, without which, there can be little doubt, the waters 

 of that sea would soon become like those of the Dead Sea. This 

 salt, by descending, as before described, so increases the density 

 of the water in the lower compartment of the sea that it enables, 

 or indeed compels, it to force its way out against the lighter 

 water of the Atlantic as an under-current, just in the same 

 manner as so much quicksilver would do. 



The waters of the Mediterranean and similar seas are thus 

 made to circulate in quite a different manner to those of the 

 ocean, notwithstanding that in each case the heat of the sun's 

 rays is the prime mover. 



We have now to consider the effect of wind upon ocean 

 currents. 



Winds, especially if blowing steadily in one direction for a 

 considerable time, will set up surface currents; and constant 

 winds, like the " Trades," undoubtedly influence ocean currents 

 to a very great extent. 



It seems pretty certain that the great equatorial westerly 

 going current is much accelerated, if not chiefly caused, by the 

 trade winds. 



After leaving the region of the " Trades," this current very 

 soon comes within the influence of the prevailing south-west 

 winds. Its course is diverted by the eastern cape of Brazil 

 (Cape San Roque), by which it is split into two streams of about 



