SKETCH OF PROFESSOR S. P. LANGLEY. 407 



resembles the pure original sunlight less than the electric beam 

 which has come to us through reddish-colored glasses resembles the 

 original brightness. With this visible heat was included the large 

 amount of invisible heat, and, if there was any law observable in 

 this 'capricious' action of the atmosphere, it was found to be this, 

 that, throughout the whole range of the known heat-spectrum the 

 large wave-lengths passed with greater facility than the shorter ones." 



The effect of this selective absorption on the visible rays is to cut 

 out the shorter wave-lengths proportionally more ; so that to an eye 

 outside of the earth's atmosphere the sun would be far bluer than to us. 

 On the heat-rays taken together, the total amount of the absorption is 

 very great, far greater than had been previously supposed. Professor 

 Langley's experiments give a very great increase in the amount of 

 solar heat reaching the earth over previous determinations, so that for 

 example, according to him, the solar radiation is sufficient to annually 

 melt an ice-shell one hundred and seventy-nine feet thick all round 

 the earth. According to previous determinations, one hundred and ten 

 feet in thickness could be melted. But while Professor Langley finds 

 a vastly greater amount of heat supplied by the sun, his law of the 

 selective absorption comes in to profoundly modify its terrestrial 

 manifestations. Were there no such selective absorption, the temper- 

 ature of the soil in the tropics, under a vertical sun, would probably 

 not rise to that of the freezing-point of mercury. "The temperature 

 of this planet, and with it the existence, not only of the human 

 race but of all organic life on the globe, appears, from the results 

 of the Mount Whitney expedition, to depend far less on the direct 

 solar heat" than on the hitherto neglected quality of selective ab- 

 sorption. 



The bearing of the observations at Mount Whitney on a great 

 number of important questions, the temperature of the sun, the radia- 

 tion from the sky, etc., etc., can not be here considered for want of 

 space. The solar spectrum previously known was but half of that 

 mapped out by the expedition, and there is good reason to believe that 

 Professor Langley's observations have now revealed the whole of it 

 to us. 



The partial results of these investigations, published from time to 

 time in foreign periodicals, have done much to make Professor Lang- 

 ley honored in other countries than his own. In 1882 he was invited 

 to address the British Association for the Advancement of Science at 

 Southampton, and did so. His paper on that occasion reminds one 

 of that of February, 1874, in the astonishing fullness of experiment, 

 thought, and judgment which seems to lie just back of the sentences. 

 It comes from a full mind. In the spring of 1885 Professor Langley 

 goes to England at the invitation of the Royal Institution to lecture 

 before it. 



There are many other most interesting researches of Professor Lang- 



