CURRENTS. 65 



of the poles, and that the warmer water makes its way thence, 

 to be cooled down ready for its return journey. 



Sir Wyville Thomson believes that the cold water comes 

 mainly from the Southern or Antarctic Sea, and assigns as a 

 reason the fact that the several series of temperature soundings, 

 which were taken in the North Atlantic, seemed to show that 

 the lower stratum of water at or near the equator was colder 

 than any between it and the Arctic Sea. 



The heat of the sun's rays not only induces this circulation 

 of surface and under-currents by its direct action, but it also 

 plays a similar part through the medium of evaporation and 

 precipitation. 



The amount of evaporation from the surface of the ocean 

 within the tropics has been estimated, from experiments made 

 at St. Helena by Dr. Halley, to be about 8600 cubic yards per 

 square statute mile per diem. 



We will assume, for the purpose of obtaining some idea of 

 the volume of water thus raised, that the average rate of evapo- 

 ration from the whole water-surface of the globe is equal to 

 one-half of this. We shall thus have 146,000,000 square statute 

 miles the computed approximate 1 water-area of the globe 

 multiplied by ^- 6 2 OJI cubic yards per square mile per diem, which 

 gives about 115 cubic miles, or say 42,000 cubic miles per 

 annum, as the quantity of water taken from the ocean by 

 evaporation. This corresponds to an evaporation of only 18 

 inches per annum, which is a small allowance. 



Maury estimates the evaporation from the ocean at a very 

 much higher figure. 



In the Persian Gulf, it is said to be upwards of 15 feet. 



However, whatever the correct amount may be, the principle 

 involved, viz. that ocean currents are affected by evaporation, 

 remains the same. 



The capacity of the North Sea basin, assuming an average 

 depth of 35 fathoms (which is approximately correct), is roughly 

 6300 cubic miles. 



A mass of water nearly seven times greater than that which 

 the North Sea contains, is therefore at my low estimate 

 abstracted fron* the ocean annually by evaporation, and is 



1 The water-area of the globe is variously estimated, the differences in the 

 several calculations being mainly due to the manner in which the unexplored 

 regions of the poles are dealt with. 



F 



