Page:Dictionary of National Biography volume 06.djvu/180

 explanation offered of a highly complex phenomenon. It was never disputed, and has scarcely been corrected. Bradley found the 'constant' of aberration to be 20-25" (reducing it, however, in 1748 to 20"). Struve fixed it at 20.445". Bradley concluded, from the amount of aberration, the velocity of light to be such as to bring it from the sun to the earth in 8m 13s, although Roemer had, from actual observation, estimated the interval at llm . The best recent determination (Glasenapp's) of the 'light equation' is 8m 21s . Bradley's demonstration of his rules for aberration remained unpublished till 1832 (Works, p. 287). He observed only the effects in declination ; but his theory was verified as regards right ascension also, by Eustachio Manfredi at Bologna in 1729. The subject was fully investigated by Clairaut in 1737 (Mém. de l'Ac. 1737, p. 205). An important secondary inference from the Wanstead observations was that of the vast distances of even the brighter stars. Bradley stated decisively that the parallax neither of γ Draconis nor of η Ursæ Majoris reached 1", and believed that he should have detected half that quantity (''Phil. Trans.'' xxxv. 660. Double parallaxes are there spoken of). This well-grounded assurance shows an extraordinary advance in exactness of observation.

Bradley succeeded Whiteside as lecturer on experimental philosophy at Oxford in 1729, and resigned the post in 1760, after the close of his seventy-ninth course. There was no endowment, Lord Crewe's benefaction of 30l. per annum becoming payable only in 1749 ; but fees of three guineas a course, with an average attendance of fifty-seven, produced emoluments sufficient for his wants. His lectures were delivered in the Ashmolean Museum, of which he vainly sought the keepership in 1731. In 1732 he took a share in a trial at sea of Hadley's sextants, and wrote a letter warmly commendatory of the invention (Works, p. 505). His removal to Oxford occurred in May of the same year, when he occupied a house in New College Lane attached to his professorship. His aunt, Mrs. Pound, accompanied him, with two of her nephews, and lived with him there five years. He transported thither most of his instruments, but left Graham's sector undisturbed. An important investigation was in progress by its means, for the purposes of which he made during the next fifteen years periodical visits to Wanstead.

It is certain that Halley desired to have Bradley for his successor, and it is even said that he offered to resign in his favour. But death anticipated his project, 14 Jan. 1742. Through the urgent representations of George, earl of Macclesfield, who quoted to Lord-chancellor Hardwicke Newton's dictum that he was 'the best astronomer in Europe,' Bradley was appointed astronomer-royal 3 Feb. 1742. The honour of a degree of D.D. was conferred upon him by diploma at Oxford 22 Feb., and in June he went to live at Greenwich. His first care was to remedy, so far as possible, the miserable state of the instruments, and to procure an assistant in the person of John Bradley, son of his eldest brother, who, at a stipend of 26l., diligently carried out his instructions during fourteen years, and was replaced successively by Mason and Green.

With untiring and well-directed zeal Bradley laboured at the duties of his new office. He took his first transit at Greenwich 25 July 1742, and by the end of the year 1500 had been entered. The work done in 1743 was enormous. The records of observations with the transit instrument fill 177, with the quadrant 148 folio pages. On 8 Aug. 255 determinations of the former, 181 of the latter kind were made. His efforts towards a higher degree of accuracy were unceasing and successful ; yet he never possessed an achromatic telescope. He recognised it as the first duty of an astronomer to make himself acquainted with the peculiar defects of his instruments, and was indefatigable in testing and improving them. By the addition of a finer micrometer screw, 18 July 1745, he succeeded in measuring intervals of half a second with the eight-foot quadrant erected by Graham for Halley, but was deterred from attempting further refinements by discovering it a year later to be sensibly eccentric. At various times between 1743 and 1749 he made experiments on the length of the seconds pendulum, giving the most accurate result previous to Kater's in 1818. The great comet of 1743 was first seen at Greenwich 26 Dec., and was observed there until 17 Feb. 1744. Bradley roughly computed its trajectory, but went no further, it is conjectured, out of kindness towards young Betts, who had the ambition to try his hand on it. He also observed the first comet of 1748, and calculated that of 1707. His observations of Halley's comet in 1759 have for the most part perished. The time was now ripe for the publication of his second great discovery. From the first the Wanstead observations had shown the displacements due to aberration to be attended by a 'residual phenomenon.' A slight progressive inequality was detected, occasioning in stars near the equinoctial colures an excess, in those near the solstitial colures a defect of movement in declination, as compared with that required by a precession of