34 DISCUSSIONS IN CLIMATOLOGY. 



getting quit of the heat which it is receiving, then, i'or 

 the reason just assigned, A will necessarily stand at 

 a higher temperature than B. Let us now suppose 

 the southern, or water hemisphere, to be A, and the 

 northern, or land hemisphere, to be B. I have 

 endeavoured to show (' Climate and Time,' and else- 

 where) that A, the water hemisphere, ought to have 

 a higher mean temperature than B, the land hemi- 

 sphere, because the former has a greater difficulty in 

 getting quit of the heat which it is receiving from 

 the sun than the latter. The question then arises, 

 How is it that the water hemisphere has a greater 

 difficulty than the land hemisphere in getting rid of 

 its heat ? It is mainly due to that cause which Pro- 

 fessor Newcomb says is quite new to him, viz. the 

 fact that the aqueous vapour of the air ' is far less 

 diatherwianous to radiation from water than from 

 land. It is a curious fact that Prof. Newcomb, in his 

 "Rejoinder," entirely overlooks this cause assigned 

 by me, although I have stated it fully in my fourth 

 reason. The period of the heat-vibrations of the 

 aqueous vapour of the air is the same as that of the 

 ocean, and consequently the aqueous vapour will 

 absorb radiation from the ocean more readily than 

 from the land. A considerable portion of the heat 

 absorbed by the aqueous vapour of the air is thrown 

 back upon the ocean, and in this way the aqueous 

 vapour acts as a screen, or like the glass of a green- 

 house, in preventing the ocean from getting quit of 

 its heat so rapidly as the land. The result is that the 

 temperature of equilibrium of the ocean must be higher 

 than that of the land. In other words, before the 

 ocean can manage to throw off its heat into space 

 as rapidly as it is receiving it, its temperature must 

 be higher than that of the land. 



