148 LIGHT SCIENCE FOR LEISURE HOURS. 



current westward, let us consider whether Ilerschel's 

 arguments against the " heat-theory " may not suggest 

 a hint for our guidance. He points out that an over- 

 flow from the equator poleward would result in an 

 eastward, and not in a westward current. This is 

 true. It is equally true that a flow of water toward 

 the equator would result in a westward current. But 

 no such flow is observed. Is it possible that there may 

 be such a flow, but that it takes place in a hidden 

 manner? Clearly there may be. Sub-marine currents 

 toward the equator would have precisely the kind 

 of motion we require, and, if any cause drew them to 

 the surface near the equator, they would account in 

 full for the great equatorial westward current. 



At this point we begin to see that an important 

 circumstance has been lost sight of in dealing with the 

 heat-theory. The action of the sun on the surface- 

 water of the equatorial Atlantic has only been con- 

 sidered with reference to its warming effects. But we 

 must not forget that this action has drying effects also. 

 It evaporates enormous quantities of water, and we 

 have to inquire 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 recognize the fact that a 

 process resembling suction is continually taking place 

 over the wl\ple area of the equatorial Atlantic, the 



