Hadley, John (1682-1744) (DNB00)
HADLEY, JOHN (1682–1744), mathematician and scientific mechanist, born on 16 April 1682, was the son of George Hadley, deputy-lieutenant and afterwards, in 1691, high sheriff of Hertfordshire; his mother was Katherine FitzJames. He must be distinguished from an older John Hadley who was also skilled in practical mechanics. Desaguliers, when describing the waterworks put up at London Bridge, near the beginning of the eighteenth century, says that 'the contrivance for raising and falling the waterwheel was the invention of Mr. Hadley, who put up the first of that kind at Worcester, and for which a patent was granted him' in 1693 (Desaguliers, Lectures, ii. 528). On 21 March 1717 the younger Hadley became a fellow of the Royal Society. On 1 May in the following year he drew up a report on an abstruse mathematical question, which had been proposed apparently by Maclaurin, with the conclusion 'that the writer had shown the formation of several trajectories in which bodies might move about a gravitating centre, the gravitating power being as any dignity of the distance, either integer or fracted.' This is evidence of Hadley's knowledge of advanced mathematics, which is confirmed by an analysis which he drew up of Bianchini's work on the planet Venus (Phil. Trans. xxxvi. 158).
In 1719-20 Hadley obtained his first great success by the improvement he effected in the reflecting telescope, which had been left imperfect by both Newton and Gregory, and thus produced the first instrument of that kind which had sufficient size and accuracy to be of service to astronomers. His first large reflector was shown on 12 Jan. 1721 to the Royal Society, who 'ordered their hearty thanks to be recorded,' and state in their journals that 'the force [of the telescope] was such as to enlarge an object near two hundred times, though the length thereof scarcely exceeds six feet.' The reflecting metallic mirror was about six inches in diameter, with a focal length of over five feet two inches. Dr. Bradley reported that with it he had seen 'the transits of Jupiter's satellites and their shadows over the disc, the black list in Saturn's ring, and the edge of the shadow of Saturn cast on the ring … also several times the five satellites of Saturn.' Hadley's new telescope was praised in equally high terms by Dr. Halley, the astronomer royal, who tested it 'on the bodies and satellites of the superior planets,' and on 6 April in the same year Hadley communicated a series of observations which he himself had made on the transit of Jupiter's satellites, &c. (ib. xxxii. 384).
Hadley's success with his first reflector and a second equally large led him to effect great improvements in the Gregorian telescope. His friend Dr. Bradley also acquired a taste for constructing these instruments, and the result of their efforts was that reflecting telescopes speedily came into general use, and have since been supplied regularly by opticians (Brewster, Life of Newton, i. 55).
From 1726 till his death Hadley was annually elected member of the council of the Royal Society, and on 12 Feb. 1728 he was sworn into the office of vice-president. In the summer of 1730 he made his second great success by the invention of the reflecting quadrant, a simple but invaluable improvement of Hooke's instrument. Hooke's octant lacked precisely the quality which makes Hadley's instrument so indispensable at sea, and though Sir Isaac Newton undoubtedly wrote a description to Halley of what was wanting, it is scarcely possible to doubt that Hadley's discovery was reached independently. On 13 May 1731 he read a paper to the Royal Society entitled 'Description of a new Instrument for taking Angles, by John Hadley, Vice-Pres. R.S.' (Phil. Trans. xxxvii. 147-57). This gives a full and exact account of the improved quadrant, the mathematical principles on which it is based, and its special fitness for angle-measurement on board ship. By means of two small mirrors on a portable instrument it was now for the first time possible to easily note the angle subtended by two distant objects independently of small changes of place in the centre of observation. Dr. Whewell, referring to Hadley's 'sextant,' says:' That invaluable instrument in which the distance of two objects is observed by bringing one to coincide apparently with the other' (Ind. Science, ii. 278). The circular arc of the instrument being originally one-eighth of a circumference, it was called 'octant,' and as the double reflection makes one degree on the arc represent two degrees between the objects observed, the octant was therefore a measure of ninety degrees, and thus obtained the name quadrant. In the same way, when Captain Campbell in 1757 first proposed to extend the circular arc to one-sixth of a circumference in order to be able to measure up to 120 degrees, Hadley's instrument then became a sextant (Grant, Phys. Astr. p. 487).
In November 1730 Thomas Godfrey of Philadelphia, Pennsylvania, proposed an improvement of the quadrant similar to that of Hadley, but there is clear evidence that the latter had the priority in point of time (Rigaud, Corresp. of Scientific Men, i. 286, 288).
Soon after the announcement of Hadley's invention, the lords of the admiralty ordered a series of observations to be made 'on board the Chatham yacht' to test the instrument (Phil. Trans. xxxvii. 147). In 1734 Hadley effected a further improvement by fixing a spirit level to his quadrant so as to take a meridian altitude at sea when the horizon is not visible (ib. xxxviii. 167-72). In the following year he wrote his 'proposition relating to the combination of transparent lenses with reflecting planes,' the object being to measure angular distances by the motion of a reflecting plane which transmitted the rays of light without any second reflection in the telescope. We also read (Royal Society Journals, 1734) of a letter 'from M. Godin since his return to Paris, wherein he says he produced Mr. Hadley's instrument for taking angles or distances before a meeting of the Royal Academy of Sciences.'
In 1734 John Hadley married Elizabeth, daughter of Thomas Hodges, F.R.S., who had been attorney-general for Barbadoes. Besides his home at Enfield Chase, near East Barnet, Hadley had a house in Bloomsbury, London, and was a neighbour and intimate friend of Sir Hans Sloane. On a tombstone in East Barnet churchyard is the record, 'John Hadley of East Barnet, Esq., dyed the 14 of February 1743 [i.e. 1743-4], aged 61 years.' His only son John, born in 1738, showed none of the talent of his family, but after inheriting a large fortune in land and houses, died in poverty and obscurity, February 1816.[Biographical Account of John Hadley, esq., V.P.R.S. … and of his brothers George and Henry (anonymous, a copy is in Trinity College Library, Cambridge); Phil. Trans. ut supra (vols. xxxii-xl.), and Dr. Hutton's Abridg. vi. 646; Cass's Hist. East Barnet, pp. 74, 79, 80; Gent. Mag. 1744, p. 108; Scots Mag. vi. 98; Musgrave's Obituary Notices, Addit. MSS. 5727-49; Browne's Translation of Gregory's Optics, App. pp. 252, 285.]