1834–1906, American scientist, b. Roxbury, Mass., received only a high school education but continued his studies in science in Boston libraries. He became, in 1866, professor of physics at the Western Univ. of Pennsylvania (now the Univ. of Pittsburgh) and director of the Allegheny Observatory there. He did much to popularize astronomy; his book The New Astronomy (1888) was widely read. He invented the bolometer, a highly sensitive instrument for recording variations in heat radiation, and with it measured the distribution of heat in the solar and lunar spectra. In 1887, Langley became secretary of the Smithsonian Institution and established the Astrophysical Observatory and the National Zoological Park there. He continued his study of the solar spectrum and made new determinations of the solar constant of radiation and, in 1904, announced his conclusion that this solar constant was a variable. He constructed power-driven model aircraft with specially designed light engines, which, in…