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 success the ‘abstruse mystery’ of secular magnetic changes. It was revived by Hansteen in 1819. Desirous of investigating thoroughly phenomena which he hoped might prove regular enough to serve for the determination of longitudes, Halley obtained from William III in 1698 the command of a war-sloop, the Paramour Pink, with orders to study the variation of the compass, and ‘attempt the discovery of what land lies to the south of the western ocean.’ He sailed from Portsmouth at the end of November 1698, but was compelled by the refractory conduct of his crew to return from Barbadoes in the following June. Having got his lieutenant cashiered, he started again in September, and penetrated to 52° south latitude, where he ‘fell in with great islands of ice, of so incredible a height and magnitude that I scarce dare write my thoughts of it.’ After a narrow escape from destruction he steered north, explored the Atlantic from shore to shore, and cast anchor in the Thames on 7 Sept. 1700, his ship's company diminished only by the loss of one boy swept overboard. Of this incident he could never afterwards speak without tears. His ‘General Chart’ of the variation of the compass appeared in 1701. It set the example of a method, since extensively employed, of representing to the eye a mass of complex facts, and gave the first general view of the distribution of terrestrial magnetism by means of lines of equal declination, long called ‘Halleyan lines.’

Resuming the command of the Paramour Pink, Halley made in 1701, by the king's orders, a thorough survey of the tides and coasts of the British Channel, of which he published a map in 1702. He was next sent by Queen Anne, at the Emperor Leopold's request, to inspect the harbours of the Adriatic, and, on a second journey thither, aided the imperial engineers to fortify Trieste. In passing through Hanover he supped with the elector (afterwards George I) and his sister, the queen of Prussia, and at Vienna was presented by the emperor with a diamond ring from his own finger. Dr. Wallis [q. v.] having died just before his arrival in England, in November 1703, he was appointed in his room Savilian professor of geometry at Oxford, where he was created D.C.L. on 16 Oct. 1710. He was no sooner installed in the Savilian chair than Dr. Aldrich engaged him to complete a translation from Arabic into Latin, begun by Dr. Bernard, of Apollonius's ‘De Sectione Rationis,’ till then unknown to European scholars. His success, and the useful emendations of the original manuscript which, notwithstanding his previous ignorance of Arabic, he suggested, were extremely surprising to Dr. Sykes, the greatest orientalist of his time. He added a restoration, from the description of Pappus, of ‘De Sectione Spatii,’ by the same author, and the whole was published from the university press in 1706. The first complete edition of the ‘Conics’ of Apollonius, including a masterly restoration of the lost eighth book, was issued by him, with Serenus's ‘De Sectione Cylindri et Coni,’ in 1710. His edition of Ptolemy's ‘Catalogue’ formed part of the third volume of Hudson's ‘Geographiæ Veteris Scriptores Græci’ (Oxford, 1712), and his edition of the ‘Spherics’ of Menelaus was published by his friend Dr. Costard in 1758.

Halley was a leading member of the committee entrusted by Prince George of Denmark with preparing Flamsteed's observations for the press, and edited the first or ‘spurious’ version of the ‘Historia Cœlestis’ in 1712. His accurate prediction of the circumstances of the total solar eclipse of 2 May 1715 added greatly to his reputation. He observed the event, in company with the Earl of Abingdon and Chief-justice Parker (afterwards Earl of Macclesfield), from the roof of the Royal Society's house in Crane Court; and minutely described the corona, without venturing to decide whether it belonged to the sun or to the moon (Phil. Trans. xxix. 245). The great aurora of 16 March 1715, the first he had seen, was observed by him at London. He explained the auroral crown as an optical effect due to the ‘concourse’ of many streamers, and suggested a mode of determining the height of such phenomena (ib. p. 407). The hypothesis of their magnetic origin was a development of his views on terrestrial magnetism. He supposed auroræ to be occasioned by the escape of a ‘luminous medium,’ by which a subterranean globe was rendered habitable.

Halley became secretary to the Royal Society on Sir Hans Sloane's resignation, 13 Nov. 1713, and on 9 Feb. 1721 was appointed, through Lord-chancellor Parker's interest, astronomer-royal in succession to Flamsteed. He took possession of the house on 7 March, but on 6 May had not ‘yet got into the observatory,’ which he found ‘wholly unprovided with instruments, and, indeed, of everything else that was moveable.’ Five hundred pounds were allotted by the board of ordnance for supplying the needful apparatus, and in 1721 the first transit-instrument erected at Greenwich—one 5½ feet in length, constructed twenty years earlier by Hooke—was in its place. Halley's observations with it, however, begun on 1 Oct. 1721, were rendered useless by the absence of any means of taking zenith distances. After October 1725 his main depen-