OCEANIC CIRCULATION. 255 



into clouds, much heat being given out in the process, 

 causing the air to expand in the neighbourhood of the 

 clouds so formed, and thus giving to these clouds their 

 peculiar rounded tops. (At least this feature seems 

 better explained thus than by De Saussure's theory.) 

 Now suppose the conditions changed to those existing 

 at the equator. The supply of vapour is very much 

 greater, the saturation point is very much higher near 

 the sea-surface, and the contrast between the conditions 

 prevailing there and in the region where condensation 

 begins is very much more marked. The air above the 

 equatorial and tropical seas contains, in the form of 

 invisible aqueous vapour, an enormous quantity of water ; 

 this vapour rises and extends itself, its place being con- 

 tinually supplied by fresh evaporation. What must 

 happen when the process has continued for several 

 hours, but precisely what is observed to happen ? There 

 is an overflow, so to speak, resembling, only much more 

 marked, that which causes the formation of our summer 

 clouds. Enormous cloud-masses are formed, which 

 cannot be carried away by the atmospheric circulation 

 (very high above the calm zone), so fast as they are 

 formed. Hence follows excessive accumulation, pre- 

 sently resulting in precipitation, accompanied by re- 

 markable electrical phenomena. 



But to suppose that the whole quantity of water 

 evaporated at the equator and in tropical regions, is 

 precipitated there in the form of rain, corresponds to 

 such a supposition as that the water overflowing a dam 

 includes all that has risen to the level of the dam. 



