Dictionary of National Biography, 1885-1900/Bradley, James
BRADLEY, JAMES (1693–1762), astronomer-royal, was the third son of William Bradley, a descendant of a family seated at Bradley Castle, county Durham, from the fourteenth century, by his marriage, in 1678, with Jane Pound of Bishop's Canning in Wiltshire. He was born at Sherbourn in Gloucestershire, probably in the end of March 1693, but the date is not precisely ascertainable. He was educated at the Northleach grammar school, and was admitted as a commoner to Balliol College, Oxford, 15 March 1711, when in his eighteenth year, proceeding B.A. 15 Oct. 1714, and M.A. 21 June 1717. His university career had little share in moulding his genius. His uncle, the Rev. James Pound, rector of Wanstead in Essex, was at that time one of the best astronomical observers in England. A warm attachment sprang up between him and his nephew. He nursed him through the small-pox in 1717; he reinforced the scanty supplies drawn from straitened home; above all, he discerned and cultivated his extraordinary talents. Bradley quickly acquired all his instructor's skill and more than his ardour. Every spare moment was devoted to cooperation with him. His handwriting appears in the Wanstead books from 1715, and the journals of the Royal Society notice a communication from him regarding the aurora of 6 March 1716. He was formally introduced to the learned world by Halley, who, in publishing his observation of an appulse of Palilicium to the moon, 5 Dec. 1717, prophetically described him as 'eruditus juvenis, qui simul industria et ingenio pollens his studiis promovendis aptissimus natus est' (Phil. Trans. xxx. 853). The skill with which he and Pound together deduced from the opposition of Mars in 1719 a solar parallax between 9" and 13", was praised by the same authority (ib. xxxi. 114), who again imparted to the Royal Society 'some very curious observations' made by Bradley on Mars in October 1721, implying a parallax for the sun of less than 10" (Journal Books R. Soc. 16 Nov. 1721). The entry of one of these states that 'the 15-feet tube was moved by a machine that made it to keep pace with the stars' (Bradley, Miscellaneous Works, p. 350), a remarkably early attempt at giving automatic movement to a telescope.
Doubtless with the view of investigating annual parallax, Bradley noted the relative positions of the component stars of γ Virginis, 12 March 1718, and of Castor, 30 March 1719 and 1 Oct. 1722. A repetition of this latter observation about 1769 brought the discovery of their orbital revolution almost within his grasp, and, transmitted by Maskelyne to Herschel, served to confirm and correct its theory (Phil. Trans. xciii. 363).
Bradley's first sustained research, however, was concerned with the Jovian system. He early began to calculate the tabular errors of each eclipse observed, and the collation of older observations with his own afforded him the discovery that the irregularities of the three inner satellites (rightly attributed to their mutual attraction) recur in the same order after 437 days. His 'Corrected Tables' were finished in 1718, but, though printed in the following year with Halley's 'Planetary Tables,' remained unpublished until 1749, by which time they had become obsolete. The appended 'Remarks' (Works, p. 81), describing the 437-day cycle, are stated by the minutes to have been read before the Royal Society 2 July 1719. Bradley was then already a fellow; he was elected 6 Nov. 1718, on the motion of Halley, and under the presidential sanction of Newton.
The choice of a profession meantime became 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 (Smith, 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 Newton'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 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 50". The true explanation in a 'nodding' movement of the axis, due to the moon's unequal action upon the equatorial parts of the earth, was more than suspected early in 1732 ; but Bradley did not consider the proof complete until he had tracked each star through an entire revolution of the moon's nodes (18.6 years) back to its mean place (allowance being made for annual precession). In September 1747 he was at length fully satisfied of the correspondence of his hypothesis with facts ; and 14 Feb. 1748 a letter to the Earl of Macclesfield, in which he set forth the upshot of his twenty years' watching and waiting, was read before the Royal Society (Phil. Trans. xlv. 1). The idea of a possible nutation of the earth's axis was not unfamiliar to astronomers ; and Newton had predicted the occurrence of a semi-annual, but scarcely sensible, effect of the kind. A phenomenon such as Bradley detected, however, depending on the position of the lunar orbit, was unthought of until its necessity became evident with the fact of its existence. The complete development of its theory went beyond his mathematical powers, and he invited assistance, promptly rendered by D'Alembert in 1749. Bradley's coefficient of nutation (9") has proved nearly a quarter of a second too small. He might probably have gone even nearer to the truth had he trusted more implicitly to his own observations. His confidence was, however, embarrassed by the proper motions of the stars, the ascertainment of which he, with his usual clear insight into the conditions of exact astronomy, urged upon well-provided observers ; while his sagacious hint that they might be mere optical effects of a real translation of the solar system (Phil. Trans. xlv. 40) gave the first opening for a scientific treatment of that remarkable subject.
As regards nutation, the novelty of his announcement had been somewhat taken off by previous disclosures. On his return from Lapland, Maupertuis consulted him as to the reduction of his observations, when Bradley imparted to him, 27 Oct. 1737, his incipient discovery. Maupertuis was not bound to secrecy, nor did he observe it. He transmitted the information to the Paris Academy (Mem. de l'Ac. 1737, p. 411), while Lalande published in 1745 (ib. 1745, p. 512) the confirmatory results of observations undertaken at Bradley's suggestion.
The discovery of aberration earned for its author, 14 Dec. 1730, exemption on the part of the Royal Society from all future payments ; that of nutation was honoured in 1748 with the Copley medal. His heightened reputation further enabled him to ask and obtain a new instrumental outfit for the Royal Observatory. He took advantage of the annual visitation by members of the Royal Society to represent its absolute necessity ; and a petition drawn up by him and signed by the president and members of council in August 1748 produced an order for 1,000l. under the sign-manual, paid, as a note in Bradley's handwriting informs us, by the treasurer of the navy out of the proceeds of the sale of old stores. The wise expenditure of this paltry sum laid the firm foundation of modern practical astronomy. Bradley was fortunate in the co-operation of John Bird. The eight-foot mural quadrant, for which he paid him 300l., was an instrument not unworthy the eye and hand that were to use it. He had also from him a movable quadrant forty inches in radius, and a transit-instrument of eight-feet focal length. From Short a six-foot reflector was ordered, but not delivered until much later ; and 20l. was paid for a magnetic apparatus, changes in dip and variation having been objects of attention to Bradley as early as 1729. For the Wanstead sector, removed to Greenwich in July 1749, 45l. was allowed to him.
The first employment of Bird's quadrant was in a series of observations, 10 Aug. 1750 to 31 July 1753, for the purpose of determining the latitude of the observatory and the laws of refraction. Simultaneously with Lacaille and Mayer, Bradley introduced the improvement of correcting these for barometrical and thermometrical fluctuations. His formula for computing mean refraction at any altitude closely represented the actual amounts down to within 10 of the horizon (Grant, Hist. Phys. Astr. pp. 329-30). After its publication by Maskelyne in 1763, it was generally adopted in England, and was in use at Greenwich down to 1833.
In 1751 Bradley made observations for determining the distances of the sun and moon in concert with those of Lacaille at the Cape of Good Hope (Mém. de l'Ac. 1752, p. 424). From the combined results for Mars, Delisle deduced a solar parallax of 10.3" (Bradley, Misc. Works, p. 481). A series of 230 comparisons with the heavens of Tobias Mayer's 'Lunar Tables,' between December 1755 and February 1756, enabled Bradley to report them to the admiralty as accurate generally within 1'. His hopes of bringing the lunar method of longitudes into actual use were thus revived ; and he undertook, aided by Mason, a laborious correction of the remaining errors founded on 1,220 observations. The particulars of these were inserted in the 'Nautical Almanac' for 1774 but the amended tables, completed from them in 1760, never saw the light, and were superseded by Mayer's own improvements in 1770. The regular work of the observatory, consisting in meridian observations of the sun, moon, planets, and stars, was meanwhile carried on with unremitting diligence and unrivalled skill.
The salary of astronomer-royal was then, as in Flamsteed's time, 100l. a year, reduced to 90l. by fees at public offices. This pittance was designed to be supplemented by Mr. Pelham's offer to Bradley, in the king's name, of the vicarage of Greenwich ; which was, however, refused on the honourable ground of incompatibility of clerical with official obligations. His disinterestedness was compensated by a crown pension of 250l. per annum, granted under the privy seal 15 Feb. 1752, and continued to his successors. Honours now fell thickly upon him. From 1725 he had frequently been chosen a member of the council of the Royal Society, and he occupied that position uninterruptedly from 1752 until his death. In July 1746 Euler wrote to announce his admission to the Berlin Academy of Sciences ; he was associated to those of Paris and St. Petersburg respectively in 1748 and 1750, and, probably in acknowledgment of his services in superintending the construction of a quadrant by Bird for the latter body, complimented with its full membership in 1754 ; while the institute of Bologna enrolled his name 16 June 1757. Scarcely an astronomer in Europe but sought a correspondence with him, which he usually declined, being averse to writing, and leaving many letters unanswered.
No direct descendant of Bradley survives. He married, 25 June 1744, Susannah,daughter of Mr. Samuel Peach of Chalford in Gloucestershire. She died in 1757, leaving a daughter, Susannah, born at Greenwich in 1745, who married in 1771 her first cousin, the Rev. Samuel Peach, and had in turn an only daughter, who died childless in 1806. Bradley's intimacy with the Earl of Macclesfield grew closer after his removal to Oxford in 1732. He co-operated with him in the establishment (about 1739) of an observatory at Shirburn Castle, and in the reform of the calendar, calculating the tables appended to the bill for that purpose. Until near the close of his life he continued to reside about three months of each year at Oxford, but resigned his readership through ill- health in 1760. For several years he had felt the approach of an obscure malady in occasional attacks of severe pain. His labours in correcting the lunar tables overtasked his hitherto robust strength, and from 1760 a heavy cloud of depression settled over his spirits, inducing the grievous apprehension of surviving his mental faculties, which remained nevertheless clear to the end. He attended, for the last time, a meeting of the Royal Society 31 Jan. 1761, and drew up a paper of instructions for Mason, on his departure to observe the transit of Venus, the latest astronomical event in which he took an active interest. But already in May he was obliged to ask Bliss to replace him, and when the day of the transit, 6 June 1761, arrived, he was unable to use the telescope. He, however, took a final observation with the transit-instrument in September, after which his handwriting disappears from the Greenwich registers. The few months that remained he spent at Chalford, being much attached to his wife's relations, and there died, in the house of his father-in-law, after a fortnight's acute suffering, 13 July 1762, in his seventieth year, and was buried with his wife and mother at Minchinhampton. His disease proved on examination to be a chronic inflammation of the abdominal viscera. The case was described by Daniel Lysons, M.D., in the 'Philosophical Transactions' (lii. 635).
In character Bradley is described as 'humane, benevolent, and kind ; a dutiful son, an indulgent husband, a tender father, and a steady friend'(Suppl. to New Biog. Dict., 1767, p. 58). Many of his poorer relatives experienced his generosity. His life was blameless, his habits abstemious, his temper mild and placid. He was habitually taciturn, but was clear, ready, and open in explaining his opinions to others. No homage could overthrow his modesty or disturb his caution. He was always more apprehensive of injuring his reputation than sanguine of enhancing it, and thus shrank from publicity; polished composition, moreover, was irksome to him. His only elaborate pieces were the accounts of his two leading discoveries ; and the preservation of several unfinished drafts of that on aberration affords evidence of toil unrewarded by felicity of expression. Nor had he any taste for abstract mathematics. His great powers were those of sagacity and persistence. He possessed 'a most extraordinary clearness of perception, both mental and organic ; great accuracy in the combination of his ideas ; and an inexhaustible fund of that "industry and patient thought" to which Newton ascribed his own discoveries ' (Rigaud, Memoirs of Bradley, p. cv). Less inventive than Kepler, he surpassed him in sobriety and precision. No discrepancy was too minute for his consideration ; his scrutiny of possible causes and their consequences was keen, dispassionate, and complete; his mental grasp was close and unrelaxing. He ranks as the founder of modern observational astronomy; nor by the example of his 'solicitous accuracy' alone or chiefly, though this was much. But his discoveries of aberration and nutation first rendered possible exact knowledge of the places of the fixed stars, and thereby of the movements of the other celestial bodies. Moreover, he bequeathed to posterity, in his diligent and faithful record of the state of the heavens in his time, a mass of documentary evidence invaluable for the testing of theory, or the elucidation of change. The publication, for the benefit of his daughter, of his observations, contained in thirteen folio and two quarto volumes, was interrupted by official demands for their possession, followed up by a lawsuit commenced by the crown in 1767, but abandoned in 1776. The Rev. Mr. Peach, Bradley's son-in-law, thereupon offered them to Lord North, to be printed by the Clarendon Press, and after many delays the first of two volumes appeared in 1798, under the editorship of Dr. Hornsby, with the title 'Astronomical Observations made at the Royal Observatory at Greenwich, from the year 1750 to the year 1762;' the second, edited by Dr. Abram Robertson, in 1805. They number about 60,000, and fill close upon 1,000 large folio pages. A sequel to Bradley's work, in the observations of Bliss and Green down to 15 March 1765, was included in the second volume. A catalogue of 387 stars, computed by Mason from Bradley's original manuscripts, and appended to the 'Nautical Almanac' for 1773, formed the basis of a similar work inserted by Hornsby in vol. i. (p. xxxviii); and 1,041 of Bradley's stars, reduced by Pilati, were added to Piazzi's second catalogue (1814). In the hands of Bessel, however, his observations assumed a new value. With extraordinary skill and labour he deduced from them in 1818 a catalogue of 3,222 stars for the epoch 1755, so authentically determined as to afford, by comparison with their later places, a sure criterion of their proper motions. The title of 'Fundamenta Astronomiæ' fitly expressed the importance of this work. More accurate values for precession and refraction were similarly obtained. Bradley's observations of the moon and planets, when reduced by Airy, supplied valuable data for the correction of the theories of those bodies.
Portraits of him are preserved at Oxford (by Hudson), at Shirburn Castle, at Greenwich, and in the rooms of the Royal Society. A dial, erected in 1831 by command of William IV, marks the spot at Kew where he began the observations which led to the discoveries of aberration and nutation. His communications to the Royal Society, besides those already adverted to, were on 'The Longitude of Lisbon and the Fort of New York, from Wanstead and London, determined by Eclipses of the First Satellite of Jupiter' (Phil. Trans. xxxiv. 85); and 'An Account of some Observations made in London by Mr. George Graham, and at Black River in Jamaica by Colin Campbell, Esq., concerning the going of a Clock; in order to determine the Difference between the Lengths of Isochronal Pendulums in those Places' (ib. xxxviii. 302). His 'Directions for using the Common Micrometer' were published by Maskelyne in 1772 (ib. lxii. 46). The originals of Bradley's Greenwich observations having been deposited in the Bodleian, the confused mass of his remaining papers, disinterred by Professor S. P. Rigaud, afforded materials for a large quarto volume, published by him in 1832 at Oxford, with the title 'Miscellaneous Works and Correspondence of James Bradley, D.D., Astronomer-Royal.' It includes, besides the Kew and Wanstead journals, every record of the slightest value in his handwriting, not omitting papers already printed in the 'Philosophical Transactions,' with many letters addressed to him by persons of eminence in England and abroad, and in some cases his replies. The prefixed memoir embodies all that the closest inquiry could gather concerning him. The investigation of his early observations, thus brought to light after nearly a century's oblivion, was made the subject of a prize by the Royal Society of Copenhagen in 1832; whence the publication by Dr. Busch of Königsberg of 'Reduction of the Observations made by Bradley at Kew and Wanstead to determine the Quantities of Aberration and Nutation' (Oxford, 1838).
[Rigaud's Memoirs of Bradley; New and Gen. Biog. Dict. xii. 54, 1767; Biog. Brit. (Kippis); Fouchy's Eloge, Mém. de l'Ac. des Sciences, 1762, p. 231 (Hist.); same trans. in Annual Reg. 1765, p. 23, and Gent. Mag. xxxv. 361; Delambre's Hist, de 1'Astronomie au xviiie siècle, p. 413; Thomson's Hist. of R. Soc. p. 344; Watt's Bibl. Brit.]