Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/822

756 the erection of the superstructure. II e showed that the motions of all the bodies of the planetary system are regu lated by its influence ; he determined the figure of the earth on the supposition of its homogeneity ; he gave a theory of the tides, discovered the cause of the precession of the equinoxes, and determined some of the principal lunar inequalities and planetary perturbations. Many of his theories were left in an imperfect state j for it is not in matters of science that it is given to the same individual to invent and bring to perfection : their complete develop ment required that several subsidiary sciences should be farther advanced ; but it has been the triumph of his system, that every subsequent discovery has only tended to strengthen and confirm it.

While physical astronomy was undergoing a complete revolution in the hands of Newton, the practical part was receiving great improvement from, the first astronomer royal, who conducted the Greenwich Observatory. This celebrated institution, from which so many important discoveries have emanated, was erected under the reign of Charles II., in 1675. Flamsteed was appointed to it in 1676, and continued with indefatigable zeal to discharge the duties of the office during the long period of 33 years. In the course of this time he made an immense number of excellent observations, the results of which are given in the Historia Ccelestis, the first edition of which was pub lished in 1712, at the expense of Prince George of Denmark, the husband of Queen Anne. The second appeared in 1723, some time after the death of the author, in three volumes folio. The first volume contains the observations which he made, first at Derby, and afterwards at Greenwich, of the fixed stars, planets, comets, spots of the sun, and Jupiter s satellites. The second volume contains the tran sits of the planets and stars over the meridian, and the places of the planets deduced from these observations. The third contains a historical notice, in which he gives a description of the instruments used by Tycho and him self ; catalogues of fixed stars by Ptolemy, Ulugh Begh, Tycho, the landgrave of Hesse, and Hevelius ; together with the British Catalogue, containing the places of 2884 stars. The labours of Flamsteed were, however, confined entirely to the practical part of astronomy. He made no improvements in theory ; but he is entitled to the merit of ha r ing been the first who brought into common use the method of simultaneously observing the right ascension of the sun and a star, a method by means of which the determination of the positions of the stars is reduced to the observation of meridional transits and altitudes. He was likewise the first who explained the true principles of the equation of time ; and he improved the lunar tables by introducing into them the annual equation which had been suggested by Horrox.

Flamsteed was succeeded in the observatory by, who held a prominent place among English astronomers. From early youth he applied himself with ardour to the study of mathematics and astronomy ; and having pro cured a few instruments, he began to make observations, by which he was led to remark the inaccuracy of the tables of Jupiter and Saturn. In his 19th year he published a direct and geometrical method of finding the eccentricities and aphelia of the orbits of the planets ; and in the year following he undertook a voyage to St Helena, with a view to form a catalogue of the stars in the southern hemi sphere. The station was unfortunately chosen, for, owing to the incessant rains and foggy atmosphere of that island, he was able to determine the places of only 360 stars in the course of a whole year. He had, however, the satis faction of observing a transit of Mercury over the sun s disk, a phenomenon which suggested to him the important idea, that the transits of the inferior planets might be advantageously employed in determining one of the most essential elements of the planetary system, viz., the parallax of the sun, and consequently the diameters of the orbits. The method has since been successfully employed in the case of Venus : the transits of Mercury, though much more frequent, are not so well adapted to the purpose. The other additions which he made to astronomical knowledge are chiefly recorded in his Synopsis Astronomiae Cometicce, a work abounding in profound and original views, and which, in respect of theory, formed perhaps the most remarkable accession to the science that had been made since the time of Kepler. In this work he revived an ancient opinion, that the comets belong to the solar system, and move in very eccentric orbits round the sun, returning after stated but long intervals. He also ventured to pre dict that the comet of 1681 would again return to its perihelion in 1759, the first prediction of the kind ever verified. In 1720 Halley was appointed to succeed Flamsteed in the Royal Observatory ; and though then in the 64th year of his age, he undertook, with a view to improve the lunar theory, to observe the moon through a whole revolution of her nodes, erroneously supposing that after such a revolution the errors of the tables would again appear in the same order. He was the first who, by a comparison of ancient and modern observations, remarked the acceleration of the mean motion of the moon, and thus called the attention of mathematicians to an important and curious phenomenon, the physical cause of which was at length detected by the powerful analysis of Laplace. He was also the first who pointed out the secular inequalities of Jupiter and Saturn, occasioned by their mutual pertur bations, a theory that formed the subject of several pro found memoirs of Euler and Lagrange, and for the complete development of which astronomy is likewise indebted to Laplace. (See .)

The discoveries of, who succeeded Halley as astronomer royal, form a memorable epoch in the history of the science. It was reserved to him to give the theoretical explanation of a singular motion of the polar star which had been first observed by Picard, who had remarked that the inequality was annual, and amounted to about 40 seconds, but had been unable to refer it to any law. Hooke, in 1674, a few years after the observations of Picard, imagined that he had discovered a parallax in some of the stars ; and Flamsteed, following the ideas of Hooke, explained, by means of parallax, the minute changes of position which he had observed in Polaris and some cir- cumpolar stars. Manfredi and Cassini demonstrated the error of Flamsteed, but were not more successful in their attempts to explain the motion in question. Samuel Molyneux conceived the idea of verifying all that had been said respecting the supposed parallaxes, and for this pur pose commenced a series of observations at Kew, with an excellent 24-feet sector constructed by Graham. Bradley, who happened at that time to reside at Kew, took part in these observations, the result of which was, that the remarks of Picard were confirmed beyond the possibility of doubt. It was, however, abundantly evident that the apparent motions observed were not connected in any manner with parallax ; it therefore became an object of the greatest interest to determine their physical cause, and assign their law and period. The first idea that occurred was to inquire whether they arose from a change of position in the earth s axis ; but this supposition was found to be inadequate to the explanation of the phenomena. Moly neux having been in the meantime appointed a lord of the Admiralty, the observations were discontinued at Kew ; they were, however, shortly after resumed by Bradley at Wanstead, with a smaller but more convenient instrument ; and after they had been continued several years, it was 