58 FRAGMENTS OF SCIENCE. 



12. Aqueous Vapour in relation to the Terrestrial 

 Temperatures. 



"VVe are now fully prepared for a result which, with- 

 out such preparation, might appear incredible. Water 

 is, to some extent, a volatile body, and our atmosphere, 

 resting as it does upon the surface of the ocean, receives 

 from it a continual supply of aqueous vapour. It 

 would be an error to confound clouds or fog or any 

 visible mist with the vapour of water, which is a per- 

 fectly impalpable gas, diffused, even on the clearest 

 days, throughout the atmosphere. Compared with the 

 great body of the air, the aqueous vapour it contains 

 is of almost infinitesimal amount, 99^ out of every 100 

 parts of the atmosphere being composed of oxygen and 

 nitrogen. In the absence of experiment, we should 

 never think of ascribing to this scant and varying con- 

 stituent any important influence on terrestrial radia- 

 tion; and yet its influence is far more potent than that 

 of the great body of the air. To say that on a day of 

 average humidity in England, the atmospheric vapour 

 exerts 100 times the action of the air itself, would 

 certainly be an understatement of the fact. Compar- 

 ing a single molecule of aqueous vapour with an 

 atom of either of the main constituents of our atmos- 

 phere, I am not prepared to say how many thousand 

 times the action of the former exceeds that of the 

 latter. 



But it must be borne in mind that these large num- 

 bers depend, in part, on the extreme feebleness of the 

 air; the power of aqueous vapour seems vast, because 

 that of the air with which it is compared is infinites- 

 imal. Absolutely considered, however, this substance, 

 notwithstanding its small specific gravity, exercises a 



