222 The Smithsonian Institution 



spectrum ; second, in a calculation of the relative intensity 

 of the different rays of the sun before they have entered the 

 earth's atmosphere, which was illustrated by an extra atmo- 

 spheric curve in the spectrum ; third, in the indication that 

 scarcely sixty per cent, of the solar rays penetrate to the 

 earth's surface, the atmosphere as a whole exerting a power- 

 ful selective absorption ; and finally, in a new and important 

 estimate of the "solar constant." The effect of such absorp- 

 tion on the visible rays is to throw out the shorter wave- 

 lengths much more effectively than the longer ones, so that 

 to an eye outside the earth's atmosphere the sun would ap- 

 pear far bluer than to one within, and the estimated amount 

 of heat before absorption is correspondingly measured. 



The total absorption of the heat rays was found to be sur- 

 prisingly great. These experiments then demonstrate that a 

 much greater amount of solar heat reaches the earth than 

 had previously been supposed, sufficient, in fact, to melt each 

 year an ice shell encrusting the earth to the thickness of 1 79 

 feet, instead of no feet, as had before been believed. It was 

 also found that the law of selective absorption modifies pro- 

 foundly the terrestrial manifestations of the heat supplied by 

 the sun, and that were there no such selective absorption, the 

 temperature of the soil in the tropics under a vertical sun 

 would probably not rise above freezing point. 



" The temperature of the earth's surface," he wrote, .... 

 "and with it the existence not only of the human race, but of 

 all organized life on the globe, appears in the light of the con- 

 clusions reached by the Mount Whitney expedition to depend 

 far less on the direct solar heat than on the hitherto little 

 regarded quality of selective absorption in our atmosphere." 



The bearing of these observations on such questions as the 

 temperature of the sun and the radiation from the sky is 



