280 Dr. J. Croll's Remarks on 



It is true that the land is hotter during the day and also 

 during the summer than the ocean, but it is found that the 

 more equable temperature of the ocean gives a higher mean. 

 This is further shown from another consideration. The land 

 is more indebted for heat to the ocean than the ocean is for 

 heat to the land. For example, a very considerable portion 

 of the warmth enjoyed by North-western Europe is derived 

 from the Atlantic. In like manner, Western America is 

 indebted to the Pacific for a large amount of its heat. In 

 addition, an immense quantity of the heat received from the 

 sun by the ocean is consumed in reducing evaporation, and a 

 large portion of this heat latent in the vapour is bestowed on 

 the land during condensation. Yet notwithstanding this 

 transference of heat from the ocean to the land, the mean 

 temperature of the former is greater than that of the latter. 

 Were it not for its store of summer heat, the ocean could not 

 afford to part with so much of its heat to the land during 

 winter and still maintain a higher mean temperature. 



I come now to the consideration of the most singular of 

 all Prof. Newcomb's misapprehensions — that one, namely, 

 which has reference to my third reason. In that reason I 

 stated, what every physicist knows to be perfectly correct, that 

 the aqueous vapour of the air radiates back a portion of its 

 heat ; and the ocean, for reasons which have been already 

 stated, absorbs this radiation more freely than the land. Ra- 

 diation from the air therefore tends more readily to heat 

 the ocean than it does the land. Prof. Newcomb says that 

 this involves the reductio ad absurdum of two bodies heating 

 each other by their mutual radiation. This is not the state 

 of the case at all, for both bodies receive their heat from the 

 sun ; their mutual radiation simply retains them at a higher 

 temperature than they could otherwise have. Here Prof. New- 

 comb appears to get into confusion owing to the meaning 

 which he attributes to the word " heating." The views which 

 I have advocated in reference to this mutual radiation are as 

 follows: — According to the dynamical theory of heat, all bodies 

 above absolute zero radiate heat. If we have two bodies, A 

 at 200° and B at 400°, then, according to Prevost's theory of 

 exchanges, A as truly radiates heat to B as B does to A. The 

 radiation of A, of course, can never raise the temperature of 

 B above 400° ; but nevertheless the tendency of the radia- 

 tion of A, in so far as it goes, is to raise the temperature of B. 

 This is demonstrated by the fact that the temperature of B, 

 in consequence of the radiation of A, is prevented from sink- 

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

 to every student of thermodynamics, that I can hardly think 

 Prof. Newcomb, on reflection, will dispute its accuracy. And 



