30 THE WORLD'S WONDERS. 



All the water poured by the trade currents from the tropical 

 ocean, and all raised from it by evaporation and transported 

 through aerial channels to feed the rivers of the temperate and 

 polar regions, must find its way back by counter currents. Heat , 

 according to the dictum of modern Science, may be reduced to 

 force. The force of the sun's rays poured upon the tropical 

 ocean is sufficient to raise thousands of yards into the air five 

 hundred cubic miles of water every day, and to put and keep in 

 motion the mighty currents which sweep back and forth from the 

 equator to the poles. The study of the course, direction and 

 elevation of these currents has as yet only begun. We know 

 that sometimes, as on the coast of America, the currents of cold 

 and warm water run side by side in opposite directions ; sometimes 

 a warm current is on the surface and sometimes below it. In the 

 Gulf Stream the warm current is above, the cold below ; while 

 on the coast of Japan a cold current from the Okotsk Sea runs 

 on the surface, giving rise to a fishery not inferior in magnitude 

 to that caused on the banks of Newfoundland by the cold cur- 

 rents from Baflins Bay. Enough, however, is now known of 

 ocean currents to warrant the assumption that they are mainly 

 governed by the great law of gravitation. The lighter water 

 flows on the surface, the heavier underneath. But the specific 

 gravity of ocean water depends upon two things, the temperature 

 and amount of salt contained. The heated water of the tropics 

 is rendered lighter than that which surrounds it of the same 

 saltness and so floats on the surface ; but the cold currents from 

 the poles are less saline, and consequently lighter than the 

 tropical waters of the same temperature. When these two 

 opposing currents meet there is a struggle ; but at length the one 

 which is specifically heavier sinks, while the lighter rises. So 

 facile is the movement of fluids among each other, that a 

 difference in gravity which we can scarcely detect with our nicest 

 instruments may be abundantly sufficient to decide which of two 

 opposing currents shall run above and which below. 



The air has currents as well as the ocean, and these have 

 very much to (Jo in modifying the climate of the tropical 



