520 SAMUEL PIERPONT LANGLEY. 



ermuent in a volunie entitled " Professional Papers of the Signal Service, No. 

 XV. Researches on Solar Heat and its Absorption by the Earth's Atmosphere." 

 Perhaps the most important result of the expedition was the entire change in 

 the hitherto accepted value of the solar constant, while incidentally these 

 and others carried on at Allegheny led to the displacement of the old assump- 

 tion in favor of the present view, namely, that the general absorption is largest 

 as we approach the violet end of the spectrum. 



By 1885 the solar spectrum had been followed by him to wave lengths ten 

 times as great as those of the visible spectrum, and radiations from terrestrial 

 sources even farther, thus overthrowing the ideas previously held of a natural 

 limit to the infra-red wave lengths at about 1 ^. His extended bolometric 

 researches on the heat spectrum of the moon led him to fix the maximum lunar 

 temperature at little above 0° C. In his researches on these long wave-length 

 spectra Mr. Langley developed the optical possibilities and determined the con- 

 stants of rock salt, a substance already employed by Melloni, but whose range 

 of usefulness was now very greatly extended. 



But he did not confine himself during this time either to his Labors 

 in the observatory or to making their resuhs known to scientific men 

 through contributions to^ societies and journals. He had a decided 

 opinion of the right of the world to know what scientific men were 

 doing and a ]-emarkable gift of presenting such knowledge to the 

 man of average intelligence. He occasionally delivered lectures in 

 the city of Pittsburg, which w^ere reported for one or another of the 

 Pittsburg papers, and wrote letters to the Pittsburg Gazette when 

 an}^ unusual astronomical phenomenon which might be of public 

 interest presented itself. By 1875 his reputation had grown to such 

 an extent that he was invited to lecture at Stevens Institute, and his 

 papers, which had heretofore been published only in American jour- 

 nals, commenced to appear abroad in English and Italian periodicals 

 and in the Transactions of the Academy of Sciences of the Institute 

 of France; this, be it noted, within five years from the date of his 

 first publication. 



The trend of his mind toward the popularization of science may be 

 judged from a paper which appeared in the Popular Science Monthly 

 in 1877, entitled " The first popular scientific treatise," in which he 

 declared that " science is not for the professional student only, but 

 that everyone will take an interest in its results if they are only put 

 before the world in the right way." The treatise w^as Fontenelle's 

 " Conversations on the plurality of worlds," and the article, while 

 holding strictly to its subject, showed something of that intimate 

 knowledge of French history to which I shall allude later on. 



The question of the personal error or personal equation, which has 

 attracted so many astronomers, also had his attention, and he de- 

 scribed in a communication to the American Journal of Science in 

 1877 a machine whereby this personal error could be entirely elimi- 

 nated. 



In 1878 he took charge of a party sent out by the United States to 



