The Three Secretaries 223 



manifestly very important. The extent of the solar spectrum 

 previously known was but a fraction of that discovered by 

 this expedition.^ 



Mr. Langley's determination of the power of the sun's light 

 and heat, as made at Pittsburg in 1878, is one based upon 

 definite standards of comparison. He then demonstrated that 

 the sun's disk radiates fifty-three hundred times as much 

 light, and eighty-seven times as much heat, as would an equal 

 area of metal in the converter of a Bessemer furnace in full 

 blast. 



Of Mr. Langley's numerous subsequent investigations with 

 the bolometer, there can only be mentioned his researches on 

 the temperature of the moon, which entirely changed the 

 conclusions previously held from the statements of Sir John 

 Herschel and the experiments of Lord Rosse, and his meas- 

 ures of the amount of energy realized in the form of light by 

 different natural and artificial methods of producing it. 



Extremely significant in this latter respect were his ob- 

 servations made in Washington upon the spectrum of the 

 firefly, PyropJiorus noctiluciis. He showed that its radiation 

 consists wholly of visible radiations, or, in other words, that 

 there exists in use a natural process by which all the heat 

 generated is converted into light, a process probably imitable, 

 and which if successfully imitated would be of immense indus- 

 trial importance. In the gas flame only two per cent, of the 

 heat is utilized in visible radiation and ninety-eight per cent, 

 is wasted. 



Within comparatively few years Mr. Langley has taken up 

 the study of the physics of the atmosphere and the conditions 

 of artificial flight. This is a subject in which he has been in- 

 terested from boyhood, though it was not until 1889 that he 



1 In this connection reference should be Allegheny Observatory, upon the influence of 

 made to the work of J. E. Keeler, one of his absorption of certain rays in the visible spec- 

 students, and his successor as director of the trum by the carbon dioxide of the atmosphere. 



