Born: 16 April 1682 in Bloomsbury, London, England
Died: 14 February 1744 in East Barnet, Hertfordshire, England

John Hadley's mother was Katherine Fitzjames and his father was George Hadley. Both parents were from leading families, with his father having an estate at Enfield Chase near East Barnet, Hertfordshire which is now in London. George Hadley became high sheriff of Hertfordshire when John was nine years old. John was the second of his parents' six children having an older sister and two younger brothers George and Henry. John was five years older than Henry and three years older than George. No records survive to show how and where he was educated but certainly he must have acquired a high level of expertise in mechanics, and optics as well as mathematics.

Hadley was elected a fellow of the Royal Society on 21 March 1717 and he attained high office in the Society when he was elected Vice President on 12 February 1728. Although he had no need to earn a living, he did devote considerable time to looking after the family estates and also served the local communityas governor of Barnet grammar school from 1720. His father died in 1729 and, as the eldest son, Hadley inherited the estate near East Barnet as well as other land owned by the family. This certainly put in in an exceptionally good financial position and enabled him to pursue his scientific interests. On 6 June 1734 he married Elizabeth Hodges who herself was wealthy. They had one child, a son John born in 1738.

We have indicated above that Hadley had a financial position which enabled him to devote both time and money to science. We must now look at his achievements in this area. He built, in collaboration with his brothers George and Henry, the first effective Newtonian reflecting telescope during the years 1719 to 1720. It had a 6 inch mirror and proved a very useful instrument. It was tested by Hadley and also by Halley, the astronomer royal, in 1721 and Halley reported to the Royal Society (see for example [2]) that the telescope:-

... shows the limbs of the planets with a greater degree of distinctness than other sort of telescopes do, in which particular ... it excels even the great telescope at Wanstead.

Bradley also tested the telescope, comparing it with the refractor at Wanstead, and found it an excellent instrument which produced as good results as much larger refracting telescopes which, because of their size, were much more unwieldy to use. He built a Gregorian reflector in 1726. It was due to him thatreflecting telescopes of sufficient accuracy and power to be useful in astronomy were developed.

At this time one of the major problems which many scientists attacked was that of determining longitude at sea. The shipwreck of the fleet under the command of Sir Cloudesley Shovell on the Scilly Islands in 1707 led to Parliament putting up a large amount of money for any method to find longitude at sea to within one degree. This motivated Hadley to tackle the problem and in 1730 he invented the reflecting octant which measured the altitude of the sun or of a star. The construction of the octant was, as his telescope had been, undertaken jointly by Hadley together with his brothers George and Henry. In 1731 Hadley showed his new quadrant to theRoyal Society and published a description in his paper Description of a new instrument for taking angles which appeared in the Philosophical Transactions of the Royal Society. Hadley wrote in the paper:-

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.

In 1734 he showed a new version of the instrument, with an added bubble-level, to the Royal Society. It was used to determine position at sea. Edmund Stone wrote:-

Mr Hadley tells us, that upon trial of one of these instruments, three observations made at sea of the distance between two stars with a brass octant of this kind differed from Mr Flamsteed's at land, only about a minute.

1. Biography in Encyclopaedia Britannica. 
   http://www.britannica.com/eb/article-9038742/John-Hadley Biography (Oxford, 2004).

Articles:

1. E G Forbes, The scientific and technical bases for longitude determination at sea, NTM Schr. Geschichte Natur. Tech. Medizin 16 (1) (1979), 113-118.
2. E G Forbes, Die Entwicklung der Navigationswissenschaft im 18. Jahrhundert, RETE (4) 2 (1975), 307-321.
3. P Müürsepp and R Preem, Die ersten Versuche zur Deutung der Naturerscheinungen durch die Erdrotation, in Theoria cum praxi: on the relationship of theory and praxis in the seventeenth and eighteenth centuries, Vol. IV, Hannover, 1977 (Wiesbaden, 1982), 37-39.
4. S P Rigaud, Biographical account of John Hadley, Esq. V.P.R.S., the inventor of the quadrant, and of his brothers George and Henry, Nautical Magazine 4 (1835), 12-22; 137-146.
5. A Stimson, The influence of the Royal Observatory at Greenwich upon the design of 17th and 18th century angle-measuring instruments at sea, Vistas Astronom. 20 (1-2) (1976), 123-130.
6. E G R Taylor, The mathematical practitioners of Hanoverian England 1714-1840 (Cambridge, 1966), 123-124.