The publication of Newton's _Principia_, which, but for him, might never have seen the light, and most certainly would have been long delayed, is Halley's highest claim to our gratitude. But, apart from this, his record of scientific achievement is indeed a noble one. Always, from boyhood, he had taken a great interest in the behaviour of the magnetic compass, and he now followed up the study of its variations with the greatest energy. For this purpose it was necessary that he should travel, in view of the great importance of the subject to navigation. King William III. gave him a captain's commission in the Royal Navy--a curious and interesting illustration of the close connection between astronomy and the welfare of our navy--and placed him in command of a 'pink,' that is to say, a small vessel with pointed stern, named the Paramour, in which he proceeded to the southern ocean. His first voyage was unfortunate, but the Paramour was recommissioned in 1699, and he sailed in it as far as south latitude 52°. In 1701 and the succeeding year he made further voyages in the Paramour, surveying the tides and coasts of the British Channel and of the Adriatic, and helping in the fortification of Trieste. He became Savilian Professor of Geometry at Oxford in 1703, having failed twelve years previously to secure the Savilian Professorship of Astronomy, mainly through the opposition of Flamsteed, who had already formed a strong prejudice against him, which some writers have traced to Halley's detection of several errors in one of Flamsteed's tide-tables, others to Halley's supposed materialistic views. Probably the difference was innate in the two men. There was likely to be but little sympathy between the strong, masterful man of action and society and the secluded, self-conscious, suffering invalid. At any rate, in the contest between Newton and Flamsteed, which has been already described, Halley took warmly the side of the former, and was appointed to edit the publication of Flamsteed's results, and, on the death of the latter, to succeed him at the Royal Observatory. The condition of things at Greenwich when Halley succeeded to the post of Astronomer Royal in 1720 was most discouraging. The instruments there had all belonged to Flamsteed, and therefore, most naturally, had been removed by his widow. The Observatory had practically to be begun _de novo_, and Halley had now almost attained the age at which in the present day an Astronomer Royal would have to retire. More fortunate, however, than his predecessor, he was able to get a grant for instruments, and he equipped the Observatory as well as the resources of the time permitted, and his transit instrument and great eight-foot quadrant still hang upon the Observatory walls.