RADIATION. 43 



amounts to 372. It would be idle to speculate on the 

 quantities of matter concerned in these actions. 



12. Aqueous Vapor in Relation to the Terrestrial Tem- 

 peratures. 



We are now fully prepared for a result which, without 

 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 vapor. It would be an error 

 to confound clouds or fog or any visible mist with the 

 vapor of water, which is a perfectly impalpable gas, dif- 

 fused, even on the clearest days, throughout the atmos- 

 phere. Compared with the great body of the air, the aque- 

 ous vapor 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 

 constituent 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 vapor exerts 100 

 times the action of the air itself, would certainly be an 

 understatement of the fact. Comparing a single molecule 

 of aqueous vapor with an atom of either of the main con- 

 stituents of our atmosphere, 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 numbers 

 depend, in part, on the extreme feebleness of the air; the 

 power of aqueous vapor seems vast, because that of the 

 air with which it is compared is infinitesimal. Abso- 

 lutely considered, however, this substance, notwithstand- 

 ing its small specific gravity, exercises a very potent action. 

 Probably from 10 to 15 per cent, of the heat radiated from 

 the earth is absorbed within 10 or 20 feet of the earth's 

 surface. This must evidently be of the utmost consequence 

 to the Jife of the world. Imagine the superficial molecules 

 of the earth agitated with the motion of heat, and impart- 

 ing it to the surrounding ether; this motion would be carried 

 rapidly away, and lost forever to our planet, if the waves of 

 ether had nothing but the air to contend with in their out- 

 ward course. But the aqueous vapor takes up the motion, 



