ON THE SUN'S HEAT. 389 



what could be added to the past must be taken 

 from the future. 



In our calculations we have taken Pouillet's 

 number for the total activity of solar radiation, 

 which practically agrees with Herschel's. Forbes 1 

 showed the necessity for correcting the mode 

 of allowing for atmospheric absorption used by 

 his two predecessors in estimating the total 

 amount of solar radiation, and he was thus led 

 to a number r6 times theirs. Forty years later 

 Langley, 2 in an excellently worked out con- 

 sideration of the whole question of absorption by 

 our atmosphere, of radiant heat of all wave- 

 lengths, accepts and confirms Forbes's reasoning, 

 and by fresh observations in very favourable 

 circumstances on Mount Whitney, 15,000 feet 

 above the sea-level, finds a number a little 

 greater still than Forbes (17, instead of Forbes's 

 i -6, times Pouillet's number). Thus Langley's 

 measurement of solar radiation corresponds to 

 !33>ooo horse-power per square metre, instead 



1 Edin. New Phil. Journal, vol. xxxvi. 1844. 



2 American Journal of Science, vol. xxvi. March, 1883. 



