REPLY W CRITICS. 29 



this does not prove that the air is not more rapidly 

 heated by radiation from the ocean than from the 

 land. Professor Newcomb says: — "The statement that 

 the aqueous vapour of the air is diathermanous to 

 radiation from land, but not to that from water, is 

 quite new to us, and very surprising." I am surprised 

 that he is not acquainted with the fact, and also with 

 its physical explanation. This will help to account 

 for his inability to perceive how radiation from the 

 ocean may heat the air more rapidly than radiation 

 from the land, even though the surface of the latter 

 may be at a higher temperature than that of the 

 former. 



He says : — " The rapidity with which the heating 

 process goes on depends on the difference of tempera- 

 ture, no matter whether the heat passes by conduction 

 or by radiation." This statement will hardly har- 

 monise with recent researches into radiant heat. It 

 is found that the rapidity with which a body is heated 

 by radiation depends upon the absorbing power of the 

 body ; and the absorbing power again depends upon 

 the quality of the heat-rays. Professor Tyndall, for 

 example, found that in the case of vapours, as a rule, 

 absorption diminishes as the temperature rises. With 

 a platinum spiral heated till it was barely visible, the 

 absorption of the vapour of bisulphide of carbon was 

 6*5, but when the spiral was raised to a white heat the 

 absorption, was reduced to 2"9. A similar result took 

 place in the case of chloroform, formic ether, acetic 

 ether, and other vapours. The physical cause of this 

 is well known. 



If the aqueous vapour of the air, he says, be more 

 diathermanous to radiation from land than from 

 water, as I have stated, then I assigned directly 

 contrary effects to the same cause. For, " reasoning 



