64 FRAGMENTS OF SCIENCE 



diffused, 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 vary- 

 ing constituent any important influence on terrestrial radi- 

 ation; 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 ex- 

 erts 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 constituents 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. Absolutely 

 considered, however, this substance, notwithstanding 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 life of the world. Imagine the super- 

 ficial molecules of the earth agitated with the motion of 

 heat, and imparting it to the surrounding ether; this mo- 

 tion 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 outward course. But the aqueous 

 vapor takes up the motion, and becomes thereby heated, 



