REPLY TO CRITICS. 31 



believe in no such thing. But if his meaning be that 

 here we have the air giving back to the ocean a 

 quantity of the heat which it absorbed from it, then 

 he is certainly correct in supposing that this is 

 affirmed by me. But this is a conclusion which no 

 physicist could for a moment doubt. To deny this 

 would be to contradict Prevost's well-known theory 

 of exchanges. Did the air throw back to the ocean 

 none of the heat which it derives from it, the entire 

 waters of the ocean would soon become solid ice. In 

 fact, as we have seen, mercury would not remain fluid, 

 and every living thing on the face of the globe would 

 perish. 



In his "Rejoinder,"* Professor Newcomb still main- 

 tains that this involves the reductio ad absurdum of 

 two bodies heating each other by their mutual radia- 

 tion. 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 Professor 

 Newcomb 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 radiation 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 sinking so low 



* " Amer. Jour, of Science," Jan., 1884; "Phil. Mag.," Feb., 1884 



