Langley, Samuel Pierpont, 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 1896, performed successfully in the air, thus proving to Langley's satisfaction and to the satisfaction of a few of his followers that mechanical flight was possible. Few others were convinced. Langley, assisted by Charles M. Manly, built a machine which in 1903 he twice attempted to launch on the Potomac. His failures brought him a tremendous amount of unmerited ridicule. He maintained that the failures were due to defects in the launching apparatus and not to the machine itself. In 1914, reconstructed and with a higher-powered engine, the machine was actually flown. Most of Langley's many papers are in the publications of the Smithsonian Institution.