32 DISCUSSIONS IN CLIMATOLOGY. 



as it would otherwise do. All this is so well known 

 to every student of thermodynamics, that I can hardly 

 think Professor Newcomb, on reflection, will dispute 

 its accuracy. And if he admits this, then he must 

 also admit the soundness of my third reason, for this 

 is the principle on which it is based. The aqueous 

 vapour of the air absorbs a considerable amount of the 

 heat which is being constantly radiated by the ocean ; 

 a portion of this heat thus absorbed is thrown back 

 upon the ocean, the tendency of which is to keep the 

 surface of the ocean at a much higher temperature than 

 it would otherwise have. Professor Langley has con- 

 cluded, from observations made at Mount Whitney, 

 that were it not for the heat thrown back by the 

 atmosphere, or " trapped," as it is popularly called, 

 mercury would remain solid under a vertical sun. 



He states that reason fourth seems to be little more 

 than a repetition of reason second in a different form. 

 It is, however, much more than that. It is a demon- 

 stration that, were it not for the causes to which I have 

 alluded, the mean temperature of the water hemisphere 

 ought to be higher than that of the land hemisphere ; 

 and for this reason I shall here give the section in full. 



Fourth. — ' The aqueous vapour of the air acts as a 

 screen to prevent the loss by radiation from water, 

 while it allows radiation from the ground to pass more 

 freely into space ; the atmosphere over the ocean 

 consequently throws back a greater amount of heat 

 than is thrown back by the atmosphere over the land. 

 The sea in this case has a much greater difficulty than 

 the land has in getting quit of the heat received from 

 the sun ; in other words, the land tends to lose its heat 

 more rapidly than the sea. The consequence of all these 

 circumstances is that the ocean must stand at a higher 

 mean temperature than the land. A state of equi- 



