RADIATION 65 



thus wrapping the earth like a warm garment, and pro- 

 tecting its surface from the deadly chill which it would 

 otherwise sustain. Various philosophers have speculated 

 on the influence of an atmospheric envelope. De Saus- 

 sure, 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 experi- 

 ments 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 min- 

 imum 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 cal- 

 orific 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 refrigera- 

 tion, 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. 



