AIRY, SIR GEORGE BIDDELL (1801-1892), British astronomer royal, was born at Alnwick on July 27, 1801. He was educated at Colchester grammar school and at Trinity college, Cambridge, where he graduated as senior wrangler in 1823. He became a fellow of his college (1824), Lucasian professor of math ematics (1826), and then (1828) Plumian professor of astronomy and director of the new Cambridge observatory. Before long a mural circle was installed, and regular observations were insti tuted with it in 1833. In the same year the duke of Northumber land presented the Cambridge observatory with a fine object-glass of 12in. aperture, which was mounted according to Airy's designs and under his superintendence, although the erection was not com pleted until after his removal to Greenwich in 1835. Airy's writ ings during this time are divided between mathematical and astronomy. The former were for the most part concerned with questions relating to the theory of light, arising out of his professorial lectures, among which may be specially mentioned his paper "On the Diffraction of an Object-Glass with Circular Aperture." Of his astronomical writings during this period the most important are his investigation of the mass of Jupiter, his report to the British Association on the progress of astronomy dur ing the i9th century, and his memoir On an Inequality of Long Period in the Motions of the Earth and V enus.
Airy's discovery of a new inequality in the motions of Venus and the earth was a remarkable achievement. In correcting the elements of Delambre's solar tables he discovered an inequality overlooked by their constructor. Eight times the mean motion of Venus is so nearly equal to 13 times that of the earth that the difference amounts to only the -2-hth of the earth's mean motion, and from the fact that the term depending on this difference, al though very small in itself, receives in the integration of the dif ferential equations a multiplier of about 2,2oo,000, Airy was led to infer the existence of a sensible inequality extending over 24o years (Phil. Trans., cxxii. 67). The investigation leading to this result was probably the most laborious that had been made up to Airy's time in planetary theory, and represented the first specific improvement in the solar tables effected in England since the establishment of the theory of gravitation.
In June, 1835, Airy was appointed astronomer royal in succes sion to John Pond. Under Airy's administration modem ap paratus was installed, and the whcle organization placed on a scientific footing. No fewer than 8,000 lunar observations were rescued from oblivion, and were, in 1846, placed at the disposal of astronomers in such a form that they could be used directly f or comparison with the theory and for the improvement of the tables of the moon's motion. (See HANSEN, PETER ANDREAS.) One of the most remarkable of Airy's researches was his deter mination of the mean density of the earth. In 1826 the idea occurred to him of attacking this problem by means of pendulum experiments at the top and bottom of a deep mine. After some failures, successful experiments were made at the Harton pit, near South Shields, in 1854. Their immediate result was to show that gravity at the bottom of the mine exceeded that at the top by i-T,JT--"th of its amount, the depth being 1,256ft. From this Airy was led to the final value of 6.566 for the mean density of the earth as compared with that of water (Phil. Trans. cxlvi. 342). At the age of 71 Airy embarked on a new method of treating lunar theory. A general description of his method will be found in the Monthly Notices of the Royal Astronomical Society, Vol. xxxiv. No. 3. It consisted essentially in the adoption of Delauny's final numer ical expressions for longitude, latitude and parallax, with a sym bolic term attached to each number, the value of which was to be determined by substitution in the equations of motion. The work was published in 1886, when its author was 85 years of age, In 1881 Sir George Airy resigned the office of astronomer royal and resided at the White house, Greenwich, not far from the Royal observatory, until his death, which took place on Jan.,2, 1892. The learned societies of his own and other countries conferred many honours on him.
BIBLIOGRAPHY.-A list of Airy's papers will be found in his AutobiBibliography.-A list of Airy's papers will be found in his Autobi- ography, edited in 1896 by his son, Wilfrid Airy, B.A. M. Inst. C.E. See also his Mathetnatical Tracts (1826) on the Lunar'Theory, Figure of the Earth, Precession and Nutation, and Cakulus of Variations, to which, in the second edition (of 1828), were added tracts on the Planetary Theory and the Undulatory Theory of Light; Experiments on Iron-built Ships, instituted for the purpose of discovering a cor rection for the deviation of the Compass produced by the Iron of the Ships 0839); On the Theoretical Explanation of an apparent new Polarity in Light (184o) ; Tides and Waves (1842).