134 LIGHT SCIENCE FOR LEISURE HOURS. 



must not forget that this action has drying effects also. 

 It evaporates enormous quantities of water, and we 

 have to enquire whence the water comes by which the 

 sea-level is maintained. A surface-flow from the sub- 

 tropical seas would suffice for this purpose, but no such 

 flow is observed. Whence, then, can the water come 

 but from below ? Thus we recognise the fact that a 

 process resembling suction is continually taking place 

 over the whole area of the equatorial Atlantic, the 

 agent being the intense heat of the tropical sun. No 

 one can doubt that this agent is one of adequate power. 

 Indeed, the winds, conceived by Franklin to be the 

 primary cause of the Atlantic currents, are in reality 

 due to the merest fraction of the energy inherent in the 

 sun's heat. 



We have other evidence that the indraught is from 

 below in the comparative coldness of the equatorial 

 current. The Gulf Stream is warm by comparison 

 with the surrounding waters, but the equatorial cur- 

 rent is cooler than the tropical seas. According to 

 Professor Ansted, the southern portion of the equa- 

 torial current, as it flows past Brazil, ' is everywhere a 

 cold current, generally from four to six degrees below 

 the adjacent ocean.' 



Having once detected the mainspring of the Gulf- 

 Stream mechanism, or rather of the whole system of 

 oceanic circulation for the movements observed in the 

 Atlantic have their exact counterpart in the Pacific 

 we have no difficulty in accounting for all the motions 

 which that mechanism exhibits. We need no longer 

 look upon the Gulf Stream as the rebound of the 



