# 1911 Encyclopædia Britannica/Langley, Samuel Pierpont

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​LANGLEY, SAMUEL PIERPONT (1834–1906), American physicist and astronomer, was born at Roxbury, Boston, Massachusetts, on the 22nd of August 1834. After acting for a short time as assistant in Harvard College Observatory, he was appointed assistant professor of mathematics in the U.S. Naval Academy in 1866, and in the following year became director of the Allegheny Observatory at Pittsburg, a position which he held until his selection in 1887 as secretary of the Smithsonian Institution at Washington. His name is especially associated with two main branches of investigation—aeronautics, and the exploration of the infra-red portions of the solar spectrum. The study of the latter he took up as a result of the publication in 1871 of an energy-curve of the spectrum by S. I. Lamansky. The imperfections of the thermopile, with which he began his work, led him, about 1880, to the invention of the bolometer, an instrument of extraordinary delicacy, which in its most refined form is believed to be capable of detecting a change of temperature amounting to less than one-hundred-millionth of a degree Centigrade. Depending on the fact that the electrical conductivity of a metallic conductor is decreased by heat, it consists of two strips of platinum, arranged to form the two arms of a Wheatstone bridge; one strip being exposed to a source of radiation from which the other is shielded, the heat causes a change in the resistance of one arm, the balance of the bridge is destroyed, and a deflection is marked on the galvanometer. The platinum strips are exceedingly minute, being in some cases only 1/250 in. in width, and less than one-tenth of that amount in thickness. By the aid of this instrument, Langley, working on Mount Whitney, 12,000 ft. above sea-level, discovered in 1881 an entirely unsuspected extension of the invisible infra-red rays, which he called the “new spectrum.” The importance of his achievement may be judged from the fact that, while the visible spectrum includes rays having wave-lengths of from about 0.4 μ to 0.76 μ, and no invisible heat-rays were known before 1881 having a wave-length greater than 1.8 μ, he detected rays having a wave-length of 5.3 μ. In addition, taking advantage of the accuracy with which the bolometer can determine the position of a source of heat by which it is affected, he mapped out in this infra-red spectrum over 700 dark lines or bands resembling the Fraunhofer lines of the visible spectrum, with a probable accuracy equal to that of refined astronomical observations. In aeronautics he succeeded in demonstrating the practicability of mechanical flight. He first undertook a preliminary inquiry into the principles upon which flight depends, and established at Allegheny a huge “whirling table,” the revolving arm of which could be driven by a steam-engine at any circumferential speed up to 70 m. an hour. The construction of a flying machine was next attempted. The first difficulty was to make it sufficiently light in relation to the power its machinery could develop; and several machines were built in which trials were made of steam, and of compressed air and carbonic acid gas as motive agents. About 1893 a ​satisfactory machine was ready, and a new series of troubles had to be faced, for it had to be launched at a certain initial speed, and in the face of any wind that might be blowing. To enable these conditions to be fulfilled, as well as to ensure that the machine, when it fell, should fall on water, the experiments were carried out on the Potomac river, some 30 m. below Washington. It was not till the autumn of 1894 that an efficient launching apparatus was devised, and then the wings were found not to be strong enough to bear the pressures to which they were subjected. Various other delays and mishaps followed, but ultimately, on the 6th of May 1896, a successful flight was made. On that day an aerodrome, weighing about 30 ℔ and about 16 ft. in length, with wings measuring between 12 and 13 ft. from tip to tip, twice sustained itself in the air for 11/2 minutes (the full time for which it was supplied with fuel and water), and traversed on each occasion a distance of over half a mile, falling gently into the water when the engines stopped. Later in the same year, on the 28th of November, a similar aerodrome flew about three-quarters of a mile, attaining a speed of 30 m. an hour. In 1903 he experimented with an aerodrome capable of carrying a man, but repeated accidents prevented it from being launched, and finally through lack of funds the experiments had to be abandoned without the machine ever having been free in the air (see also Flight and Flying). Langley died on the 27th of February 1906.