44 FRA OMENTS OF SCIENCE. 



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 philoso- 

 phers have speculated on the influence of an atmospheric 

 envelope. De Saussure, Fourier, M. Pouillet, and Mr. 

 Hopkins, have, one and all, enriched scientific literature 

 with contributions on this subject, but the considerations 

 which these eminent men have applied to atmospheric air, 

 have, if my experiments be correct, to be transferred 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 

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

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

 in Australia wherever drought reigns, we have the heat of 

 day forcibly contrasted with the chill of night. In the 

 Sahara itself, when the sun's rays cease to impinge on the 

 burning soil, the temperature runs rapidly down to freez- 

 ing, because there is no vapor overhead to check the calo- 

 rific drain. And here another instance might be added to 

 the numbers already known, in which nature tends, as it 

 were, to check her own excess. By nocturnal refrigeration, 

 the aqueous vapor of the air is condensed to water on the 

 surface of the earth; and, as only the superficial portions 

 radiate, the act of condensation makes water the radiating 

 body. Now experiment proves that to the rays emitted by 

 water, aqueous vapor is especially opaque. Hence the very 

 act of condensation, consequent on terrestrial cooling, 

 becomes a safeguard to the earth, imparting to its radia- 

 tion that particular character which renders it most liable 

 to be prevented from escaping into space. 



It might, however, be urged that, inasmuch as we derive 

 all our heat from the sun, the selfsame covering which 

 protects the earth from chill must also shut out the solar 

 radiation. This is partially true, but only partially; the 

 sun's rays are different in quality from the earth's rays, 

 and it does not at all follow that the substance which 



