SAMUEL PIERPONT LANGLEY. 
7 
devise something more satisfactory, and in 1879 and 1880 
was successful in the invention of the bolometer. This in- 
strument has found high favor for a wide range of experi- 
mental work, but in his hands it has been used from 1880 to 
the present time to open up a great new field of investigation 
in connection with the invisible long wave-length rays pro- 
ceeding from all heated bodies, and to change many of the 
older ideas concerning them. 
“The more important of his many researches published 
during this period were upon the energy spectrum of the 
sun, the transmission of the earth’s atmosphere and the solar 
constant, the behavior of prisms toward long wave-length 
radiators, the energy-spectra of heated terrestrial bodies, and 
the energy spectrum of the moon, the moon’s heat hitherto 
having been recognized with difficulty even in gross by the 
thermopile, but now, by the bolometer, being analyzed in 
minute detail in a lunar heat spectrum. More recently, a 
comparison of the proportions of luminous and non-lumi- 
nous heat in the spectra of the sun and artificial light sources 
with the corresponding proportions of the light and heat in 
the radiations emitted by the glow-worm, gave important 
economical results. 
“In 1881, previous observations at Allegheny having led 
him to believe that there was a great, and then unappreciated 
selective absorption both in the sun’s and in the earth’s 
atmosphere, which rendered in the latter case Pouillet’s 
methods inapplicable, and which, when recognized, tended 
to give a far larger value to the solar constant, he, with 
the aid of the Government, organized an expedition to the 
top of Mount Whitney, the loftiest mountain in southwestern 
California, whose abrupt precipices permitted observations to 
be made from two neighboring stations, yet with a distance 
of more than two miles of altitude between them. These 
observations were published by the United States Govern- 
ment in a volume entitled 'Professional Papers of the Signal 
Service, No. XV. Researches on Solar Heat, and its Ab- 
sorption by the Earth’s Atmosphere.’ Perhaps the most im- 
portant result of the expedition was the entire change in the 
hitherto accepted value of the solar constant, while incident- 
ally these and others carried on at Allegheny led to the dis- 
placement of the old assumption in favor of the present view, 
namely, that the general absorption is largest as we approach 
the violet end of the spectrum. 
