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 longitude of the Cape of Good Hope, by his observations of the occultation of Mars by the Moon. Immediately after, he went to St Helena, where he staid till 1678, completing a catalogue of the fixed stars of the southern hemisphere, which was published in the following year, and gained for its author the appellation of the 'Southern Tycho.' In 1679 he was called upon to settle a dispute between the English philosopher Hooke and the celebrated Hevelius, respecting the use of optical instruments in astronomy, and for this purpose went to Dantzic, where with honorable impartiality, he decided against his own countryman. In 1680 he made the tour of Europe, making the acquaintance of Cassini at Paris, and completing his observations from the Royal Observatory of France on the comet which now bears his name. After spending the greater part of 1681 in Italy, he returned to England, and settled at Islington, where he fitted up an observatory for his astronomical researches.

In 1683 he published his Theory of the Variation of the Magnetical Compass, in which he endeavored to account for the phenomenon, by the supposition of the whole globe being one great magnet, having four circulating magnetical poles or points of attraction. His theory, though unsatisfactory, is ingenious. The doctrines of Kepler relative to the motions of the planets next engaged his attention; and finding himself disappointed in his endeavors to obtain information on the subject from Hooke and Sir Christopher Wren, he went to Cambridge, where Newton, then mathematical professor, satisfied his inquiries. In 1691 he was candidate for the Savilian professorship of astronomy at Oxford—a chair which he would have obtained, had he not refused to profess his thorough belief in all the doctrines of the Christian religion, as taught by the church of England. For the purpose of making further observations relative to the variation of the compass, he set sail on a voyage in 1699 (having obtained the command of a vessel from King William, who was anxious to promote the cause of geographical and astronomical science); and after traversing both hemispheres, and making important observations at numerous stations, he returned to England in September 1700. As the result of his researches, he published a general chart, showing at one view the variation of the compass in all those seas where the English navigators were acquainted; and thus laid the foundation of that department of science which has since received the attention of the greatest philosophers. His next employment, under the patronage of the king, was to observe the tides in the English Channel, with the latitudes and longitudes of the principle headlands; observations which were shortly after published in a large map of the Channel. In 1703, he was engaged by the emperor of Germany to survey the coast of Delmatia; and returning in November of that year to England, he was elected Savilian professor of geometry on the death of Dr. Wallis, and was also honored with the diploma of LL.D.; a title somewhat more in consonance with his pursuits than that of 'Captain,' by which he had been styled from the time of his appointment to the command of the surveying vessel furnished him by King William. Dr. Halley now gave his mind more entirely to mathematics, translating into Latin from the Arabic and Greek several treatises, which he afterwards published with supplementary matter, such as those of Appolonius and Serenus.

In 1719 he received the appointment of astronomer-royal at Greenwich, where he afterwards chiefly resided, devoting his time to completing the