RADIATION. 59 



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 superficial molecules 

 of the earth agitated with the motion of heat, and 

 imparting it to the surrounding 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 otherwise sustain. Various philo- 

 sophers have speculated on the influence of an atmo- 

 spheric 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 vapour. 



The observations of meteorologists furnish impor- 

 tant, 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 unim- 

 peded, 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 forcibly contrasted with the chill of night. In 

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



