196 FRAGMENTS OF SCIENCE. 



But it must be borne in mind that these large numbers 

 depend in part upon 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 con- 

 sidered, 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 feet of the earth's surface. This 

 must evidently be of the utmost consequence to the life of 

 the world. Imagine the superficial molecules of the earth 

 trembling with the motion of heat, and imparting 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 outward 

 course. But the aqueous vapor takes up the motion of the 

 ethereal waves, and becomes thereby heated, thus wrapping 

 the earth like a warm garment, and protecting its surface 

 from the deadly chill which it would otherwise sustain. 

 Various philosophers have speculated on the influence of 

 an atmospheric envelope. De Saussure, Fourier, M. Pouil- 

 let and Mr. Hopkins have, one and all, enriched scientific 

 literature with contributions on this subject, but the con- 

 siderations which these eminent men have applied to atmos- 

 pheric air, have, if my experiments be correct, to be trans- 

 ferred to the aqueous vapor. 



The observations of meteorologists furnish important, 

 though hitherto unconscious evidence of the influence of 

 this agent. Wherever the air is dry we are liable to daily 

 extremes of temperature. By day, in such places, the sun's 

 heat reaches the earth unimpeded, and renders the maxi- 

 mum high ; by night, on the other hand, the earth's heat 

 escapes unhindered into space, and renders the minimum 

 low. Hence the difference between the maximum and min- 

 imum is greatest where the air is driest. In the plains 

 of India, on the heights of the Himalaya, in central Asia, in 



