482
HISTORY OF PHYSICAL ASTRONOMY.
fleeting quadrant, resembling the sextant of the present day. He com-municated an account of it to Halley, who, however, omitted to imparta knowledge of it to the world; but soon after his death, in 1742, adescription of the instrument in Newton ’s handwriting was found amonghis papers*. In the meantime the instrument had been re-invented bytwo individuals, independently of each other. The reflecting quadrantof Hadley was first described at a meeting of the Royal Society , held onthe 13th of May, 1731 j. It was subsequently found that his inventionof the instrument could be traced back at least as early as the summerof 1730. The other inventor was Thomas Godfrey, of Philadelphia , inAmerica , He appears to have devised an instrument similar to Hadley’stowards the close of the year 1730. The Royal Society , which took intoconsideration the claims of the two individuals, decided that both wereindependent inventors.
The idea of enlarging Hadley’s reflecting quadrant, or rather octant,so as to measure a distance of 120°, was first proposed in 1757, by Gap-tain Campbell, who suggested to the Board of Longitude, that a sextantof large radius, divided by Bird, would serve for making observations atsea, equally well with the reflecting circle as described by Mayer I- Thesextant is the form in which the reflecting instrument has since beengenerally used for nautical purposes.
Accurate celestial observations being now practicable on board a shipby the use of the sextant, it only remained to construct trustworthy lunartables, in order to perfect the method of finding the longitude at sea, bymeans of a comparison between the observed and computed distances ofthe moon from the neighbouring stars. Many years were not allowed toelapse ere this important object was accomplished, as we shall presentlyhave occasion to mention.
James Bradley, the third Astronomer Royal of Greenwich, was bom atSherbourn, in Gloucestershire, in the year 1692. In 1711 he enteredBaliol College, Oxford, where he completed his education. Havingqualified himself for the church, he was presented to a living in the year1719. His predilection for astronomical pursuits, which evinced itself atan early age, was fostered by his uncle, the Rev. James Pound, with whomhe resided for several years at Wanstead, in Essex. In 1721 he wasappointed to the Savilian chair of astronomy in the University of Oxford ,which had then become vacant by the death of Keill. His nomination bythe Government, as successor to Halley in the Observatory of Greenwich,is dated February 2, 1742. His reputation as an astronomer was alreadywell established in Europe by observations of a miscellaneous nature,and more especially by his immortal discovery of the Aberration of Light,and at the time of his appointment he was actually engaged in thoseresearches which resulted in his discovery of the Nutation of the Earth ’saxis.
Bradley ’s first object after his removal to Greenwich was to repair the
* A drawing and description of the instrument, both taken from Newton ’s paper, areinserted in the Phil. Trans, for 1742, p. 155.
■t For a description of Hadley’s reflecting octant see Phil. Trans., 1731, p. 147. Anexamination of the question relative to the invention of this instrument will be found inthe Nautical Magazine, vol. i., p. 351.