Langley, Samuel Pierpont

(b. Roxbury [now part of Boston], Massachusetts, 22 August 1834; d. Aiken, Soth Carolina, 22 February 1906),

astrophysics.

Langley’s parents, Mary Sumner Williams and Samuel Langley, a wholesale merchant of broad interests, traced their ancestry to old and prominent New England families. Langley attended private schools, Boston Latin School, and completed his formal education upon graduation from Boston High School in 1851. From his youth he read extensively in science, literature, and history. Langley had a Puritan morality but was an ardent seeker for religious truth and the nature of the soul, including study of metaphysics, psychology, and psychic phenomena. A bachelor, he was a large man with a florid countenance and robust health but a tendency to be melancholy. Because of a deep-seated shyness, he maintained a front of severe and haughty dignity. He was irascible and often gave offense, but to his intimates he was warmhearted, displaying great charm. Highly respected in scientific circles, he received several scientific honors and honorary degrees. He was a member of the National Academy of Sciences, on whose council he served; a foreign member of the Royal Society of London, the Royal Society of Edinburgh, and the Accademia dei Lincei of Rome; a correspondent of the Institut de France; and a president of the American Association for the Advancement of Science.

Langley worked as a civil engineer and architect from 1851 to 1864, ranging west to Chicago and St. Louis. After a tour of Europe with his brother John Williams Langley, a chemist, he was an assistant at the Harvard observatory for a year. In 1866 he was appointed assistant professor of mathematics at the Naval Academy and placed in charge of restoring its neglected observatory.

In 1867 Langley was appointed director of the Allegheny Observatory and professor of physics and astronomy at the Western University of Pennsylvania (later University of Pittsburgh). He pioneered in providing standard time signals to railroads; the income from this service helped support the observatory. Langley did his most original work in his twenty years at Pittsburgh.

In January 1887 Langley was appointed assistant secretary, and in November of that year secretary, of the Smithsonian Institution. He did not cease work at Allegheny or take up full-time residence in Washington until 1889, after the death of his Pittsburgh benefactor, william Thaw. Langley then fulfilled the long-held goal of a Smithsonian observatory by founding the Smithsonian Astrophysical Observatory, using private bequests and government support (1890). Besides giving new life to the physical sciences at the Smithsonian, Langley broadened the scope of the institution by beginning what became the National Zoological Park and the National Gallery of Art.

Langley began research on aerodynamics at Pittsburgh in 1887, investigating the pressure on a plane surface inclined to its direction of travel. The results were of minor importance. In 1891 Langley announced that “Mechanical Flight is possible with engines we now possess,” on the basis of his prediction that a twenty-pound, one-horsepower steam engine could sustain the flight of a 200–pound airplane at a speed of forty-five miles per hour. In 1896 Langley successfully flew models over the Potomac.

Langley’s models were developed in slow stages from bench tests and were designed to be inherently stable. When in 1898, at the request of the War Department, Langley began work on a man-carrying machine, he continued this evolutionary process. His one-quarter (of man-carrying size) model flew very successfully in 1903. The man-carrying airplane did not fly; on two attempts in 1906 the launching apparatus failed. The ridicule of the press, offended by Langley’s aloofness, and the mismanaged combination of official secrecy but public testing ended Langley’s expensive program.

It has been said that the work of the scientifically respected Langley brought experiments on flight out of the stage of ridicule and into a stage of science. His failure was the occasion for sharp ridicule, however; and the procedures of the Wright brothers were unlike those of Langley because they first mastered the art of flying inherently unstable gliders before adding power. The main line of development of flight followed the work of the Wright brothers and not that of Langley.

It is difficult to agree on what constitutes successful flight of powered, man-carrying, heavier-than-air craft; and there are many motivated by a desire to improve Langley’s popular image or to discount the contributions of the Wright brothers in favor of supposed precursors. In 1914, with the support of Smithsonian officials, Glenn Curtiss, a patent opponent of the Wright brothers, barely flew Langley’s airplane after significant but unreported modifications. The resulting controversy caused bitter feelings between the Wright brothers and the Smithsonian and did no service to Langley.

Langley’s work in astrophysics and infrared spectroscopy was very much in the mainstream of those branches of physics. Although a keen visual observer, Langley made his contribution in the development and application of apparatus and techniques for the measurement of radiation.

Correctly recognizing the need for measurements of the energy of radiation as a function of wavelenght, Langley developed a new instrument—the bolometer—between 1879 and 1881 to do this. The bolometer was an ingenious application of the principle—the temperature coefficient of the resistance of metals—being used by William Siemens for the measurement of temperature. It was a Wheatstone bridge with similar narrow platinum strips in opposite arms, one of which was exposed to radiation. With this Langley could measure a temperature difference of 10^-5° C. in a one-second exposure with an error of less than 1 percent. The bolometer allowed continuous and direct measurement of E(?) versus ? for the full known spectrum of radiation, through what had been delineated (because of the different techniques necessary for measurement before the bolometer) as actinic, visual, and thermal regions.

Langley developed the bolometer to study the solar constant by integrating the energy versus wavelength curves it gave; to study the selective absorption of the earth’s atmosphere by repeating measurements for different thicknesses of the atmosphere; and to study the selective absorption of the sun’s atmosphere by repeating measurements on various parts of the sun. On his expedition to Mt. Whitney to accomplish this in 1881, Langley discovered significant radiation beyond a wavelength of one micron, which had been thought to be the limit of solar radiation. The superior measurements by means of the bolometer, the newly discovered extent of the solar spectrum, and the new results for selective absorption of the earth’s atmosphere were significant contributions to the study of the sun and its effects on the earth.

The dispersion and absorption of prisms depend on wavelength. Grating spectra, although without this disadvantage, are weak and overlap at large wavelengths. Langley’s technique was to determine the dispersion and absorption of prisms, rock salt in particular, as a function of wavelength by explicitly using the overlap in grating spectra and the bolometer. The prism was then used in an automatically recording spectrobolometer, which allowed convenient measurement of infrared spectra well beyond previous capability.

The investigations of long wavelengths in the late 1890’s which prompted Planck’s interpolated radiation law were based, in part, on bolometer measurements. Langley extended measurement of spectra to a fivemicron wavelength. The failure of Wein’s law was seen beyond a ten-micron wavelenth. Langley did some work measuring the spectrum of Leslie cubes—showing, for example, migration of the maximum of radiation intensity as a function of the temperature of the source—but did not study blackbody radiation as such.

In a paper titled “Laws of Nature,” Langley expressed his empiricist creedcolon “[The] practical rule of life [is] that we must act with the majority where our faith does not compel us to do otherwise, [remembering that] we know nothing absolutely or in its essence [and science has authority in matters] only as far as they are settled by evidence and observation alone’ (“The Laws on Nature,” in Report of the Board of Regents of the Smithsonian Institution for 1901 [1902], p. 551).

BIBLIOGRAPHY

I. Original Works. There are bibliographies of Langley’s published works appended to Walcott’s memoir of Langley and to the report of the Langley Memorial Meeting (see below). Three major publications–“Researches on Solar Heat and Its Absorption by the Earth’s Atmosphere, A Report of the Mount Whitney Expedition,” Professional Papers of the Signal Service, no. 15 (1884); Annals of the Astrophysical Observatory of the Smithsonian Institution,1 (1900); and “Langley Memoir on Flight,” in Smithsonian Contributions to Knowledge, 27 (1911)—report most of his work.

II. Secondary Literature. Langley prepared some biographical fragments which were used by his very close friend Cyrus Adler for his memoir, “Samuel Pierpont Langley,” in ulletin of the Philosophical Society of Washington,15 (1907), 1-26. Adler contemplated a full biography of Langley, and there may be valuable materials among Adler’s papers at the American Jewish Historical Society in New York.

Other notices of Langley are “Samuel Pierpont Langley Memorial Meeting,” in Smithsonian Miscellaneous Collections,49 (1907); C. G. Abbot, “Samuel Pierpont Langley,” in Astrophysical Journal,23 (1906), 271-283; George Brown Goode, “Samuel Pierpont Langley,” in The Smithsonian Institution: 1846 to 1896, George Brown Goode, ed. (Washington, D.C., 1896); and C. D. Walcott, “Biographical Memoir of Samuel Pierpont Langley,” in Biographical Memoirs. National Academy of Sciences,7 (1917), 247-268.

Autobiographies of Langley’s associates containing significant mention of Langley are Charles G. Abbot, Adventures in the World of Science (Washington, D.C., 1958); Cyrus Adler, I Have Remembered the Days (Philadelphia, 1941); and John A. Brashear, The Autobiography of a Man Who Loved the Stars, W. Lucien Scaife, ed. (Boston, 1925).

Among works on the Smithsonian the following, along with the Goode volume liusted above, are valuable: Bessie Zaban Jones, Lighthouse of the Skies, the Smithsonian Astrophysical Observatory: Background and History 1846-1955 (Washington, 1965); and Paul H. Oehser, Sons of Science: The Story of the Smithsonian Institution and Its Leaders (New York, 1949).

For Langley’s place in the history of flight, see C. G. Abbot, “The 1914 Tests of the Langley ‘Aerodrome,’” in Smithsonian Miscellaneous Collections,103, no. 8 (1942); and Charles H. Gibbs-Smith, The Aeroplane: An Historical Survey of Its Origins and Development (London, 1960).

Langley’s bolometer work, concerning as it does instrumentation and technique, has not been itself the subject of historical scholarship, although there is perfunctory mention of Langley’s contribution in works on astrophysics, infrared spectroscopy, and the prehistory of quantum mechanics.

Don F. Moyer