296 LIGHT SCIENCE FOR LEISURE HOURS. 



undoubtedly great, but it cannot be more than the 

 equivalent of the amount of heat rendered latent as the 

 vapours are formed, and therefore the expansive effects 

 due to the liberation of heat cannot be greater than the 

 contrary effects due to the prior imprisonment of heat. 

 It is quite true, and has been accepted as the undoubted 

 explanation of many climatic effects, that if vapour be 

 raised in one place and condensed over another, then 

 the temperature of the air over the latter place is raised. 

 But when we have to consider a phenomenon extending 

 over a zone twenty or thirty degrees in width, we can- 

 not argue in this manner. Nay, it is necessary to the 

 force of Maury's second cause that the condensation of 

 vapour should take place over the very zone in which 

 the vaporisation is proceeding. To assign similar effects 

 to both processes, is to require that the winding up and 

 the loosening of the spring should take place in the 

 same direction. 



Whatever effects, then, are due to the constant 

 evaporation going on in the southern hemisphere, must 

 not be derived from changes of temperature. So far 

 as these are effective at all, they must depend on the 

 excess of evaporation over condensation (since the 

 excess cannot possibly lie the other way), and therefore 

 represent diminution of heat or increase of pressure, the 

 contrary effect to that we have to account for. We 

 have, therefore, only to consider the first cause men- 

 tioned by Maury ; that is the expulsive effects due to 

 the formation of aqueous vapour. , 



At first sight, this process of expulsion appears simple 



