Among the number of philosophical strangers whom the generosity of Louis XIV. and his minister Colbert had attracted to Paris, were Dominick Cassini and Roe mer. When Cassini was professor of astronomy at Bologna, he constructed an accurate table of refractions, and new tables of the sun's motions. He discovered the diurnal revolutions of Jupiter, Mars, and Venus. By observing the immersions and emersions of Jupiter's satellites, he formed a complete theory of these bodies, and published tables for calculating their eclipses. When be undertook the charge of the royal observatory at Paris, lie commenced a set of regular observations on the equi noxes and solstices, and on the oppositions and conjunc tions of the planets. In the year 1671, 1672, and 1684, he discovered the first, second, third, and fifth satellites of Saturn ; he ascertained the rotation of the fifth satel lite round its axis ; and he observed a belt upon Saturn, and the black line which divides the ring into two equal parts. He discovered also that Jupiter had a spheroidal figure, and that the polar was to the equatorial diameter as 14 to 15. He gave a complete theory of the moon's libration. He observed the belts of Jupiter parallel to his equator ; and in 1683 he discovered the zodiacal light. These brilliant discoveries, the result of forty years labour, have raised their author to a high rank among the astronomers of Europe.
The successive propagation of light, one of the most curious discoveries in astronomy, was about this time ascertained by Roemer, a Danish astronomer. By com paring the eclipses of the first satellite of Jupiter with the times of their immersions and em$rsions given by the tables of Cassini, he found that the error of the ta bles depended on the distance between Jupiter and the earth ; and hence he concluded, that the motion of light was not instantaneous, and that it moved through the diameter of the earth's orbit in the space of eleven minutes.
There flourished about this time a number of minor astronomers, whose labours we have scarcely room to record. Dr Hooke, whom we cannot properly rank under this class, was one of the most ingenious mecha nics and philosophers of his age. His inventive powers have been exhibited in almost every branch of science ; though astronomy is, perhaps, under the least obliga tions to his genius. He invented the zenith sector, in order to determine if the earth's orbit had any sensible parallax ; he gave the first hint of making a quadrant by rellexion ; and he, in some measure, anticipated the discoveries of Newton, when he sheaved that tile motion of the planets resulted from a projectile force combined with the attractive lorce of the sun. John Picard was one of the first who applied telescopes to quadrants; and in 1669, lie determined the earth's diameter, by measur ing a degree of the meridian in France. Richer made a number of astronomical observations at Cayenne ; and he lound that a pendulum which beat seconds at Cay enne, must be about a line and a quarter shorter than one which beat seconds at Paris. This singular obser vat ion being sent to Paris, was ascribed by Huygens to the diminution of gravity at the equator by means of the centrifugal lorce ; and this ingenious philosopher cal culated from this fact the spheroidal form of the earth.
When Charles II. erected the observatory at Green wich, Mr Flamstead was appointed astronomer royal in' 1676, by the recommendation of sir J. More. He was the first who explained the true principles of the equa tion of time ; and during the space of 33 years, he made an immense mass of observations on the planets, comets, and fixed stars, and on the solar spots, and the satel lites of Jupiter. These observations are recorded in his posthumous work, entitled Historia Ctiestis, which was published in 1723. The first volume of this inva luable treasure, contains all the observations which he made at Derby and Greenwich on the fixed stars, pla nets, comets, spots of the sun, and satellites of Jupiter.
The second comprehends, the transits of the planets and fixed stars over the meridian, and the places of the planets, which he deduced from them. The third vo lume contains a description of the instruments used by Tycho and himself ; the catalogues of the fixed stars made by Ptolemy, Ulugh-Beigh, Tycho, the prince of Hesse, and Hevelius ; and likewise the British catalogue of 2884 stars, with their longitudes and latitudes, and their right ascensions and declinations. The tables of Dr Halley, and Newton's theory of the moon, were found ed on the observations contained in that valuable work. The 4tlas Celesfis of Flamstead, another posthumous work, was published in 1753.
The theory of universal gravitation, effected a com plete revolution in physical astronomy. The discoveries of Galileo, Huygens, and Kepler, and the speculations of Dr Hooke concerning gravity, were the first steps in the investigation of this general principle ; and the comprehensive mind of Newton was alone wanting, to combine and generalize them. The reception of a new system of philosophy was rendered less difficult by the revolutions which had already taken place. The hypo thesis of Ptolemy had, in a great measure, yielded to the Copernican system, and Descartes had already sap ped the foundation of the Aristotelian philosophy. The system, indeed, which he himself had reared, presented more allurements to the imagination, and was better calculated to captivate vulgar applause ; but as it had not yet entwined itself with the prejudices of the human mind, the Newtonian philosophy had fewer obstacles to encounter than if it had been proposed at an earlier, or even at a later period. In the first chapter of Physical AsmoNoniy, we have given a full account of the various steps by which Newton established the general law, that all bodies attract each other with a force directly pro portional to their quantity of matter, and inversely as the square of their distance. Under the guidance of this simple principle, which connected together all the phe nomena of the planetary motions, Newton concluded, that the comets, in virtue of the same law, revolved round the sun in very eccentric ellipses; he proved, from the rotatory motion of the earth, that it ought not to be an oblate spheroid ; and, on the supposition of its homoge neity, he fixed the proportion between its polar and equatorial diameters. He demonstrated that the action of the sun and moon upon the equatorial parts of the spheroid ought to produce a motion in its axis and a retrocession in the equinoctial points, and that the tides of the ocean arose from the action of the same lumina ries: he ascribed the lunar irregularities to the combined action of the sun and earth on that satellite ; and he deduced the densities of all the planets that were sur rounded with satellites. These brilliant discoveries, which laid the foundation of those sublime theories, which have been fully developed by the French astrono mers, are fully detaiied in the Principle Philosophix Alituralis, a work which will continue to be one of the most splendid achievements of human genius, and a .durable monument of the patience and penetration of its author. The method which was established by Ba con, of arriving at general laws by a cautious induction and comparison of facts, not only conducted Newton to the principle of universal gravitation, but also to his analytical and optical discoveries. The inutility of hy potheses had been long apparent ; but it was reserved for Newton to banish them for ever from philosophy, to extricate the human mind from the labyrinth in which its energies had been wasted, and to conduct it into a new path of physical research. The trophies which he himself had won by following the method of induction, gave a more powerful recommendation to the Baconian philosophy, than it had ever received from the reasoning and eloquence of its author.