260 DISCUSSIONS IN CLIMATOLOGY. 



was written more than forty years ago, when the 

 data available for elucidating the subject were far 

 more imperfect than now, especially as regards the 

 influence of the atmosphere on radiant heat. For 

 example, Pouillet comes to the conclusion that, owing 

 to the fact of our atmosphere being less diathermanous 

 to radiation from the earth than to radiation from the 

 sun and the stars, were the sun extinguished the radia- 

 tion of the stars would still maintain the surface of our 

 globe at - 89° C, or about 53° C. above that of space. 

 The experiments of Tyndall, however, show that the 

 absorbing power of the atmosphere for heat-rays is due 

 almost exclusively to the small quantity of aqueous 

 vapour which it contains. It is evident, therefore, that 

 but for the sun there would probably be no aqueous 

 vapour, and consequently nothing to protect the earth 

 from losing its heat by radiation. Deprived of solar 

 heat, the surface of the ground would sink to about as 

 low a temperature as that of stellar space, whatever 

 that temperature may actually be. 



But before we are able to answer the foregoing 

 questions, and tell, for example, how much a given 

 increase or decrease in the quantity of sun's heat will 

 raise or lower the temperature, there is another physical 

 point to be determined, on which a considerable amount 

 of uncertainty still exists. We must know in what 

 way the temperature varies with the amount of heat 

 received. In computing, say, the rise of temperature 

 resulting from a great increase in the quantity of heat 

 received, should we assume with Newton that it is 

 proportional to the increase in the quantity of heat 

 received, or should we adopt Dulong's and Petit's 

 formula ? 



In estimating the extent to which the temperature 

 of the air would be affected by a change in the sun's 



