Hadley, John

(1682-1744), natural philosopher and mathematician

by Gloria Clifton

© Oxford University Press 2004 All rights reserved

Hadley, John (1682-1744), natural philosopher and mathematician, was born on 16 April 1682 in Bloomsbury, London, the second of six children and eldest son of George Hadley (1649-1729) and his wife, Katherine (1654/5-1712). He was baptized at the parish church of St Giles-in-the-Fields, Bloomsbury, on 21 April by the rector, Dr John Sharp (1645-1714), who later became archbishop of York. Hadley's father was a landed gentleman who became deputy lieutenant and in 1691 high sheriff of Hertfordshire, the family's principal estate being at East Barnet in that county, and his mother was the daughter and coheir of Sir John Fitzjames of Leweston in the county of Dorset. Nothing is known of Hadley's education, but he evidently became proficient in mathematics, mechanics, and optics. He has sometimes been confused with an older John Hadley who patented a device for raising and lowering a water wheel in 1693.

On 21 March 1717 Hadley was elected a fellow of the Royal Society, and he took an active part in the society's proceedings for the rest of his life. In 1718 he was asked to comment on an analysis of the orbits of revolving bodies presented to the society by Colin Maclaurin (1698-1746), which is evidence of his mathematical ability, since the subject was complex and involved an understanding of Newton's theories of gravitation. He was elected to the council of the society in 1720 and, except for the years 1721, 1723, and 1725, was re-elected annually. On 12 February 1728 he was sworn in as vice-president, and he appears to have retained this office until his death, as he is normally referred to as VPRS from that date.

The first achievement that made Hadley's name well known beyond the Royal Society was his improvement of the reflecting telescope in 1719-20. The principle of the instrument had already been demonstrated by James Gregory (1638-1675) and Sir Isaac Newton (1642-1727), but neither had succeeded in producing a telescope with any significant advantage over the older refracting type. Assisted by his brothers George Hadley (1685-1768) and Henry Hadley (1697-1771), Hadley succeeded in making a Newtonian reflector of just over 62 inch focus, with 6 inch mirror and a new kind of stand. It was demonstrated to the Royal Society on 12 January 1721, and on 6 April Hadley reported on the observations he had made with it of Jupiter's satellites and Saturn's ring. Meanwhile Edmond Halley, the astronomer royal, also tried out the reflector and reported in March that it 'Shews the Limbs of the Planets with a greater degree of distinctness than other Sort of Telescopes do, in which particular ... it excells even the great Telescope at Wanstead' (RS, classified papers, 8.1.67). Further trials were carried out by James Bradley, a future astronomer royal, and James Pound (1669-1724), who also compared its performance with the society's aerial 123 foot refractor at Wanstead. They concluded that the definition was about equal, though the image was a little less bright, but the reflector, being only just over 6 feet long, was much easier to manipulate and focus. The other great advantage of the reflector as against an aerial telescope was that the closed tube meant it could be used during twilight. Hadley devised a method of polishing the metal mirror that produced a brighter finish and he attempted to give it a parabolic figure to reduce spherical aberration, hence the performance of his reflector surpassed that of Newton's. Subsequently, Hadley succeeded in producing a successful reflector using the slightly different arrangement of mirrors originated by Gregory. Once Hadley had demonstrated that the reflector could be an effective observing instrument it was further developed with his assistance by a number of opticians, notably Edward Scarlett (d. 1743) and James Short (1710-1768), and became a standard part of their stock by the middle of the century.

Although Hadley's main concerns were his mathematical and scientific studies and the work of the Royal Society in London, he also took some interest in the local community around the family estates, and in 1720 was elected a governor of Barnet grammar school. At his father's death in January 1729 he inherited the landed property, which enabled him to continue his scholarly pursuits.

The invention for which Hadley was best remembered is the navigational instrument known as the octant, or Hadley's quadrant. With the help of his brothers he made the first octant in 1730, and reported it to the Royal Society on 13 May 1731 in a paper entitled 'Description of a new instrument for taking angles' (PTRS, 37, 1731-2, 147-57). He explained that 'The Instrument is designed to be of use where the Motion of the Objects, or any Circumstance Occasioning an unsteadiness in the common Instruments renders the Observations difficult or uncertain' (ibid., 147). By means of two mirrors the images of two objects could be made to appear to coincide and the angle between them read from the scale. The arc of the instrument was one-eighth of a circle, hence the name octant, but, because of the double reflection, 1° on the arc represented 2° between the objects observed, so that the instrument could measure 90° or a quarter of a circle and be called a quadrant. Hadley gave a full mathematical explanation to the society, showing that the invention depended on his appreciation of the optical laws involved. The idea was not entirely original, as Hooke had made a quadrant that incorporated a single mirror to provide a reflected image and Newton had written to Halley with ideas for improving it by using a double reflection, though he seems not to have attempted to put them into practice. There is no evidence, however, that Hadley knew of Newton's proposals, and he was certainly the first to produce a practical instrument. Thomas Godfrey of Philadelphia later claimed the invention, but news of his design did not reach London until 1732, and it is clear that the two men independently arrived at a similar device. The octant was tested on board the yacht Chatham in 1732, the observers including James Bradley and Hadley's brother Henry, and was found to be superior to other nautical instruments for measuring angles. Hadley added a refinement in 1734 in the form of a spirit level to make altitude measurements possible when the horizon was obscured. He took out a patent (no. 550) for his instrument in November of that year. A sign that his octant had superseded earlier navigational instruments for measuring the altitudes of the sun and stars for finding latitude at sea came in 1754, when Christ's Hospital ordered that the boys in the mathematical school should be issued with Hadley quadrants rather than the outdated Davis quadrant and forestaff. In 1757 Captain John Campbell enlarged Hadley's octant to a sixth of a circle, so that it could measure up to 120°, thereby creating the sextant.

On 6 June 1734 Hadley married Elizabeth Hodges (d. 1752) at the parish church of St George's, Bloomsbury. She was the daughter of Thomas Hodges FRS, attorney-general for Barbados under Queen Anne, and had a personal fortune of £5000. The trustees of the marriage settlement were her younger brother, Colonel John Hodges, and Sir Hans Sloane, who had a town house near Hadley's in Bloomsbury; a letter in the British Library indicates that Hadley's friendship with Sloane went back to at least his earliest days as a fellow of the Royal Society. The Hadleys' only child, John, was born on 21 February 1738. Hadley died at East Barnet on 14 February 1744, and was buried there in the churchyard of St Mary the Virgin on 22 February. His wife was amply provided for in his will, being given the rent from his estates at East Barnet, Edmonton, and Enfield for her lifetime, as well as £100 from annuities. On her death in 1752 all the property passed to their son, who sold off the estates and spent all the proceeds.

GLORIA CLIFTON

Sources  
S. P. Rigaud, 'Biographical account of John Hadley, esq., VPRS, the inventor of the quadrant, and of his brothers George and Henry', Nautical Magazine, 4 (1835), 12-22, 137-46
F. C. Cass, East Barnet, 2 vols. (1885-92), 1.74-80
PRO, PROB 11/731/41
J. Hadley, 'An account of a catadioptrick telescope', PTRS, 32 (1722-3), 303-12
parish register (baptism), 21 April 1682, Bloomsbury, St Giles-in-the-Fields
parish register, Bloomsbury, St George, 6 June 1734, LMA, P82/GEOI/015 [marriage]
monumental inscriptions, East Barnet, St Mary the Virgin, Society of Genealogists, London [microfilm]
index to parish register, St George, Bloomsbury, London, 1737-8, LMA, P82/GEOI/14 [baptism]
W. Bulloch, 'Roll of the fellows of the Royal Society', index, RS
RS, classified papers, 2.16.21; 8.1.67; 8.2.44

Archives  
RS, classified papers
Sci. Mus., reflecting telescope, inv. 1937/601
Sci. Mus., speculum, inv. 1932/459 |  BL, letters, Sloane MSS

Likenesses  
B. Dandridge, oils, NMM
engraving, repro. in Rigaud, 'Biographical account'
lithograph, Sci. Mus.

Wealth at death  
landed estates sold after death for almost £16,000: Rigaud, 'Biographical account'


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