Page:Dictionary of National Biography volume 06.djvu/179

 imperative. He had been brought up to the church, and in 1719 Hoadly, bishop of Hereford, presented him to the vicarage of Bridstow. On this title, accordingly, he was ordained deacon at St. Paul's, 24 May, and priest, 25 July, 1719. Early in 1720 the sinecure rectory of Llandewi-Velfry in Pembrokeshire was procured for him by his friend Samuel Molyneux, secretary to the Prince of Wales, and he also became chaplain to the bishop of Hereford. His prospects of promotion were thus considerable, but he continued to frequent Wanetead, and took an early opportunity of extricating himself from a position in which his duties were at variance with inclinations. The Savilian chair of astronomy at Oxford became vacant by the death of Keill in August 1721. Bradley was elected to fill it 31 Oct., and, immediately resigning his preferments, found himself free to follow his bent on an income which amounted in 1724 to 138l. 5s. 9d. He read his inaugural lecture 26 April 1722.

In 1723 we find him assisting his uncle in experiments upon Hadley's new reflector (Phil. Trans. xxxii. 382); and Hadley's example and instructions encouraged him, about the same time, to attempt the grinding of specula (, A Compleat System of Opticks, ii. 302). In this be was only partially successful, though his mechanical skill sufficed, at all times for the repair and adjustment of his instruments. His observations and elements of a comet discovered by Halley 9 Oct. 1723 formed the subject of his first paper in 'Philosophical Transactions' (xxxiii, 41 ; see 's Principia, 3rd edit. lib. iii. prop. 42, p. 523, 1726). Bradley was the first succeseor of Halley in the then laborious task of computing the orbits of comets. He published parabolic elements for those of 1737 and 1757 (Phil. Trans. xl. iii, 1, 408), and by his communication to Lemonnier of the orbit of, and process of calculation applied to, the comet of 1742, knowledge of his method became diffused abroad.

By the death of Pound, which took place 16 Nov. 1724, he lost 'a relation to whom he was dear, even more than by the ties of blood.' He continued, however, to observe with his instruments, and to reside with his widow (visiting Oxford only for the delivery of his lectures) in a small house in the town of Wanstead memorable as the scene of his chief discoveries. On 26 Nov. 1725, a 24¼-foot telescope by Graham was fixed in the direction of the zenith at the house of Mr.Samuel Molyneux on Kew Green. It had been resolved by him and Bradley to subject Hooke's supposed detection of a large parallax for γ Draconis to a searching inquiry, and the first observation for the purpose was made by Molyneux at noon 3 Dec. 1725. It was repeated by Bradley, 'chiefly through curiosity,' 17 Dec., when, to his surprise, he found the star pass a little more to the southward. This unexpected change, which was in the opposite direction to what could have been produced by parallax, continued, in spite of every precaution against error, at the rate of about 1" in three days; and at the end of a year's observation the star had completed an oscillation 39" in extent.

Meanwhile an explanation was vainly sought of this enigmatical movement, perceived to be shared, in degrees varying with their latitude, by other stars. A nutation of the earth's axis was first thought of, and a test star, or 'anti-Draco,' on the opposite side of the pole (35 Camelopardi) was watched from 7 Jan. 1726; but the quantity of its motion was insufficient to support that hypothesis. The friends next considered 'what refraction might do,' on the supposition of an annual change of figure in the earth's atmosphere through the action of a resisting medium ; this too was discarded on closer examination. Bradley now resolved to procure an instrument of his own, and, 19 Aug. 1727. a zenith-sector of 12½ feet radius, and 12½° range, was mounted for him by Graham in the upper part of his aunt's house. Thenceforth he trusted entirely to the Wanstead results. A year's assiduous use of this instrument gave him a set of empirical rules for the annual apparent motions of stars in various parts of the sky ; but he had almost despaired of being able to account for them, when an unexpected illumination fell upon him. Accompanying a pleasure party in a sail on the Thames one day about September 1728, be noticed that the wind seemed to shift each time that the boat put about, and a question put to the boat man brought the (to him) significant reply that the changes in direction of the vane at the top of the mast were merely due to changes in the boat's course, the wind remaining steady throughout. This was the clue he needed. He divined at once that the progressive tranamission of light, combined with the advance of the earth in its orbit, must cause an annual shifting of the direction in which the heavenly bodies are seen, by an amount depending upon the ratio of the two velocities. Working out the problem in detail, he found that the consequences agreed perfectly with the rules already deduced from observation, and announced his memorable discovery of the 'aberration of light' in the form of a letter to Halley, read before the Royal Society 9 and 16 Jan. 1729 (Phil. Trans. xxxv. 637).

Never was a more minutely satisfactory