RADIATION. 59 



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 superficial molecules of the earth agitated 

 with the motion of heat, and imparting it to the sur- 

 rounding ether; this motion would be carried rapidly 

 away, and lost for ever to our planet, if the waves of 

 ether had nothing but the air to contend with in their 

 outward course. But the aqueous vapour takes up the 

 motion, and becomes thereby heated, thus wrapping 

 the earth like a warm garment, and protecting its 

 surface from the deadly chill which it would other- 

 wise sustain. Various philosophers 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 contribu- 

 tions 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 vapour. 



The observations of meteorologists furnish impor- 

 tant, though hitherto unconscious evidence of the in- 

 fluence 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 maximum 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 forci- 

 bly contrasted with the chill of night. In the Sahara 

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



