CASSINI (John Dominic)

, an eminent astrono- mer, was born of noble parents, at a town in Piedmont in Italy, June 8, 1625. After laying a proper foundation in his studies at home, he was sent to continue them in a college of Jesuits at Genoa. He had an uncommon turn for Latin poetry, which he exercised so very early, that some of his poems were published when he was but 11 years old. At length he met with books of astronomy, which he read, with great eagerness. Pursuing the bent of his inclinations in this way, in a short time he made so amazing a progress, that in 1650 the senate of Bologna invited him to be their public mathematical professor. Cassini was but 25 years of age when he went to Bologna, where he taught mathematics, and made observations upon the heavens, with great care and assiduity. In 1652 a comet appeared, which he observed with great accuracy; and he discovered that comets were not bodies accidentally generated in the atmosphere, as had been supposed, but of the same nature, and probably governed by the same laws, as the planets. The same year he resolved an astronomical problem, which Kepler and Bulliald had given up as insolvable; viz, to determine geometrically the apogee and eccentricity of a planet, from its true and mean place.—In 1653, when a church in Bologna was repaired and enlarged, he obtained leave of the senate to correct and settle a meridian line, which had been drawn by an astronomer in 1575.—In 1657 he attended, as an assistant, a nobleman, who was sent to Rome to compose some differences, which had arisen between Bologna and Ferrara, from the inundations of the Po; and he shewed so much skill and judgment in the management of the affair, that in 1663 the pope's brother appointed him inspector general of the fortifications of the castle of Urbino: and he had afterward committed to him the care of all the rivers in the ecclesiastical state.

Mean while he did not neglect his astronomical studies, but cultivated them with great care. He made several discoveries relating to the planets Mars and Venus, particularly the revolution of Mars upon his own axis: but the point he had chiefly in view, was to settle an accurate theory of Jupiter's satellites; which, after much labour and observation, he happily effected, and published it at Rome, among other astronomical pieces, in 1666.

Picard, the French astronomer, getting Cassini's tables of Jupiter's satellites, found them so very exact, that he conceived the highest opinion of his skill; and from that time his fame increased so fast in France, that the government desired to have him a member of the academy. Cassini however could not leave his station without leave of his superiors; and therefore the king, Lewis the 14th, requested of the pope and the senate of Bologna, that Cassini might be permitted to come into France. Leave was granted for 6 years; and he came to Paris in the beginning of 1669, where he was immediately made the king's astronomer. When this term of 6 years was near expiring, the pope and the senate of Bologna insisted upon his return, on pain of forfeiting his revenues and emoluments, which had hitherto been remitted to him: but the minister Colbert prevailed on him to stay, and he was naturalized in 1673; the same year also in which he was married.

The Royal Observatory of Paris had been finished | some time. The occasion of its being built was this: In 1638, the celebrated Mersenne was the chief institutor and promoter of a society, where several ingenious and learned men met together to talk upon physical and astronomical subjects; among whom were Gassend, Defcartes, Monmort, Thevenot, Bulliald, our countryman Hobbes, &c: and this society was kept up by a succession of learned men for many years. At length the government considering that a number of such men, acting in a body, would succeed much better in the promotion of science, than if they acted separately, each in his particular art or province, established under the direction of Colbert, in 1666, the Royal Academy of Sciences: and for the advancement of astronomy in particular, erected the Royal Observatory at Paris, and furnished it with all kinds of instruments that were necessary to make observations. The foundation of this noble pile was laid in 1667, and the building completed in 1670. Of this observatory, Cassini was appointed to be the first inhabiter; which he took possession of in Sept. 1671, when he set himself with fresh alacrity to attend the duties of his profession. In 1672 he endeavoured to determine the parallax of Mars and the sun: and in 1677 he proved that the diurnal rotation of Jupiter round his axis was performed in 9 hours 58 minutes, from the motion of a spot in one of his larger belts: also in 1684 he discovered four satellites of Saturn, besides that which Huygens had found out. In 1693 he published a new edition of his “Tables of Jupiter's Satellites,” corrected by later observations. In 1695 he took a journey to Bologna, to examine the meridian line, which he had fixed there in 1655; and he shewed, in the presence of eminent mathematicians, that it had not varied in the least, during that 40 years. In 1700 he continued the meridian line through France, which Picard had begun, to the very southern limits of that country.

After our author had resided at the royal observatory for more than 40 years, making many excellent and useful discoveries, which he published from time to time, he died September the 14th, 1712, at 87 years of age; and was succeeded by his only son James Cassini. His publications were very numerous, far too much so, even to be enumerated in this place.