322 Samuel Pierpont Langley. 



The last published memoirs by Langley and his assistant, Mr. 

 C. G. Abbot, demonstrate beyond all perad venture the steady 

 decrease of the absorption of the earth's atmosphere as the 

 wave lengths increase. 



From the measurement of the distribution of heat in the 

 solar spectrum Langley then passed to the distribution of 

 energy, and from this to the sum-total of energy in the spec- 

 trum. Now the sum total of energy was also supposed to be 

 given by Pouillet's or some other form of pyrhelio meter, and 

 Langley at once began to experiment with every form or modi- 

 fication of this apparatus. He traveled extensively in this 

 country and Europe in order to consult with all those who 

 were working on these problems, and his progress, up to the 

 end of 1883, was presented in his report on the Mount Whit- 

 ney expedition, published as Professional Paper Number xv 

 of the Signal Service, under the title, " Researches in Solar 

 Heat," He had already observed from the summit of Mount 

 Etna, and in 1878 from the summit of Pike's Peak, but these 

 new observations were made from the still higher summit of 

 Mount "Whitney. In fact the study of the solar atmosphere 

 forced him, first of all modern physicists, to undertake the 

 greatest expense and labor in order to realize even a few days 

 of work at the highest possible altitude. The general result 

 of the expedition to Mount Whitney was to demonstrate that, 

 in all probability, the universal opinion of physicists was in 

 error in believing that the infra-red rays are more absorbed 

 than the luminous and ultra-violet : as Langley himself states, 

 considering the weight of authority against him, he felt bound 

 to repeat his experiments in every manner, and with every pre- 

 caution. It also followed that the accepted value of the heat 

 radiated from the sun, as received at the outer surface of the 

 earth's atmosphere, must be very greatly increased, and that its 

 value could not be less than 2"6 calories, while it might be as 

 high as 3*5, the most probable figure being 3*0 calories.* We 

 believe that up to the last Professor Langley saw no reason to 

 depart greatly from this result. He made this experience 

 on Mount Whitney a strong argument as to the importance to 

 meteorology of similar observations at other great elevations. 



In 1887 Professor Langley was appointed Secretary of the 

 Smithsonian Institution, succeeding Professor S. F. Baird. 

 This necessitated his removal to Washington, where he was 

 able to established the Astro-physical Observatory, and to con- 

 tinue the researches begun in Allegheny. Washington is 

 unfortunate as regards the steadiness of the atmosphere, but its 

 selection was forced upon Langley by circumstances over which 



* Namely small calories, or a gram of water heated from 0° Centigrade to 

 1° Centigrade per square minute per centimeter. 



