1678 at the age of 22, and was, six months later, sent by that body to Danzig as arbiter of a dispute between Hooke and Hevelius on the respective advantages of telescopic and plain sights. He shared the observations of Hevelius from 26 May to 18 July 1679, and testified to their accuracy in a letter printed by Hevelius in his ‘Annus Climactericus’ (1685, p. 101).
Towards the close of 1680 he started on a continental tour with his school-friend, Robert Nelson, and caught sight near Calais of the great comet of that year, upon which he made, with Cassini, at Paris, observations of great service to Newton in fixing its orbit. He spent most of 1681 in Italy, and married in England in 1682 Mary, daughter of Mr. Tooke, auditor of the exchequer, an amiable and attractive woman. His first house was at Islington, where his instruments excited much curiosity; but he removed later to Golden Lion Court, Aldersgate Street. He lost no time in entering upon his favourite project of perfecting the lunar theory by means of observations continued through a ‘sarotic’ period of 223 lunations, or a little more than eighteen years, and secured at Islington in 1683–4 nearly two hundred observations, by which his expectation of the regular recurrence of errors was confirmed. These results were published by him in 1710 as an appendix to the second edition of Street's ‘Caroline Tables.’ He was, however, interrupted by the death of his father in 1684 in unexpectedly bad circumstances, and was obliged to postpone everything to the defence of the little that was left of his patrimony.
In an address delivered at Cambridge on 19 April 1888 Dr. Glaisher expressed the conviction that ‘but for Halley the “Principia” would not have existed.’ His suggestions originated it; he averted the threatened suppression of the third book. ‘He paid all the expenses, he corrected the proofs, he laid aside all his own work in order to press forward to the utmost the printing. All his letters show the most intense devotion to the work.’ Keenly alive to the importance of the problem of gravity, Halley obtained from Kepler's third law in January 1684 the law of inverse squares, but failed to deduce from it the planetary motions. Having fruitlessly applied to Wren and Hooke, he in August 1684 paid a visit to Newton at Cambridge, and ‘learned from him the good news that he had brought this demonstration to perfection.’ The first eleven propositions of the ‘Principia’ were communicated three months later to Halley, who again repaired to Cambridge to confer with their author, and on 10 Dec. gave an account of them to the Royal Society. Although now a poor man, he undertook on 2 June 1686 to print Newton's work at his own charge, and in a letter to him of 5 July 1687 was able to announce its completion. His outlay was eventually reimbursed by the sale of copies. A ‘Discourse concerning Gravity’ was read by Halley before the Royal Society on 21 April 1686, by way of preparation for the ‘incomparable treatise of motion almost ready for the press’ (Phil. Trans. xvi. 3). He prefixed to the first edition a set of Latin verses ending with the line
Nec fas est propius mortali attingere Divos,
and presented to James II a copy of the ‘Principia’ with a discourse ‘On the true Theory of the Tides’ (ib. xix. 445).
Halley was refused the Savilian professorship of astronomy at Oxford in 1691, owing to a suspicion, which he vainly tried to combat, of his holding materialistic views. Flamsteed, lately become his enemy, did his utmost to hinder his election. Halley acted as assistant secretary to the Royal Society and editor of the ‘Philosophical Transactions’ from 1685 to 1 Jan. 1693. Among his numerous contributions to them about this time were an ‘Historical Account of the Trade Winds and Monsoons’ (ib. xvi. 153), giving the first detailed description and a sketch of a circulatory theory of these winds; ‘An Account of the Circulation of the Watery Vapours of the Sea, and of the Cause of Springs’ (ib. xvii. 468), establishing an equilibrium between expenditure by evaporation and supply by condensation in the waters of the globe; a ‘Discourse tending to prove at what Time and Place Julius Cæsar made his first Descent upon Britain’ (ib. p. 495); and a ‘New and General Method of finding the Roots of Equations’ (ib. xviii. 136). Appointed by Newton's influence deputy-controller of the mint at Chester in 1696, he held the post, in spite of ‘intolerable’ annoyances from his fellow-officials, until its abolition two years later. He corresponded meantime actively with the Royal Society through Sir Hans Sloane, observed at Chester the partial lunar eclipse of 19 Oct. 1697 (ib. xix. 784), and ascended Snowdon for the purpose of testing his method of determining heights by the barometer. His theory of the variation of the compass was proposed in 1683, and further developed in 1692 (ib. xiii. 208, xvii. 563). It assumed the direction of the needle to be governed by the influence of four magnetic poles, two fixed in the outer shell of the earth, two revolving with an inner nucleus in a period roughly estimated at seven hundred years. This hypothesis explained with surprising
