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.]
HADLEY, JOHN, M.D. (1731–1764), professor of chemistry at Cambridge, eldest son of Henry Hadley (brother of John Hadley, mathematician [q. v.]) and Ann Hoffman (?), was born in London in 1731, and entered Queens' College, Cambridge, in May 1749. He was fifth wrangler, was elected fellow of Queens' in January 1753, and proceeded B.A. in the same year, M.A. in July 1756, and M.D. in 1763. He became professor of chemistry in 1756, and published the 'Plan of a Course of Chemical Lectures,' 1758. He also wrote 'An Introduction to Chemistry, being the Substance of a Course of Lectures read two years successively at the Laboratory in Cambridge,' 1759; the manuscript is in possession of Professor Cumming of Cambridge. In 1758 he became F.R.S., and became, in 1760, assistant physician at St. Thomas's Hospital. In 1763 he was elected physician to the Charterhouse, and also became fellow of the College of Physicians. He died of fever at the Charterhouse 5 Nov. 1764.
The fifty-fourth volume of the 'Philosophical Transactions' contains an account, which Hadley drew up, of 'a mummy inspected in London in 1763,' communicated to Dr. William Heberden. This paper was read 12 Jan. 1764, and on 2 Feb. 'he presented to the society an elegant drawing of
