CASSINI, JANE.s, a celebrated astronomer, was the youngest son of John Dominique Cassini, and was born at Paris on the 18th of February 1677. After having finished his studies in his father's house, under the care of M. Chazelle of the Academy of Sciences, he was sent to study philosophy at the Mazarin college under the celebrated Varignon, who was then professor of mathe matics ; and so great was his progress, that, at the age of 15, he supported a mathematical thesis, which he dedicated to the Duke of Burgundy. In the year 1694, at the age of 17, he was admitted into the Academy of Sciences ; and, in the year 1695, he accompanied his father to Italy, assisted him in verifying the meridian line at the church of St Petronius, measured the length of the Roman and Greek foot, and determined the latitude of several of the cities through which he passed. Upon his return from Italy he went to Holland, where he de termined the latitude of several places, and discovered some errors which Snellius had committed in his mea surement of the earth. Towards the end of the year 1696, he undertook a journey to England, where he be came acquainted with Newton, Halley, Flamstead, Gre gory and Wallis, and after remaining some time, he re turned to France with an additional ardour for his favour ite science, and adorned with the title of Fellow of the Royal Society.
In the year 1712, James Cassini succeeded his father as Astronomer at the Royal Observatory of Paris; and every volume of the Memoirs of the French Academy contains numerous proofs, both of his assiduity as an observer, and as a physical astronomer. In the year 1705, he presented his paper on I he mining the longitude of places, by the eetitmes of the fixed stars and planets by the moon. His researches on the magnitude of the fixed stars, and their distance from the earth, appeared in 1717; and, in the same year, he published his Theory of the Motion of the Satellites of Saturn. In 1720, he published his work, untitled, De In Grandeur et de la Figure de la Terre. in his paper on
the revolution of Venus !mind her axis, which was print ed in the Memoirs of the Academy for 1732, he eon firmed the results of his father's observations, by point ing out the mistakes committed by Bianchini ; and this determination has been completely established by the very recent obset vations of M. Schroeter. I I is attention was next turned to the curious subject of the accelera tion of the mean motion of Jupiter, and the retardation of that of Saturn; and he shelved that the mean motion of Jupiter should be accelerated half a second every year, while that of Saturn was retarded two minutes in the same fink ; and that these quantities should increase lin. 2000 years, and aft•rw arils diminish.
In the year I71 2, Cassini published his Elemens d' Astronomic, accompanied with Astronomical tables of the sun, moon, planets, stars, and satellites. The Elements of Astronomy were composed a e request of the Duke of Burgundy, who was solicit° is to have in his own language an elementary treatise on that science. The astronomical tables, which were long rec•;_oned the most accurate, were afterwards reprinted by his son, hut with many errors, from which the original edition was free.
Although the attention of Cassini was chiefly direct ed to the study and practice of astronomy, yet the other branches of physical knowledge occasionally attracted his notice. Besides 172 memoirs on astronomical sub jects, which were printed by the Academy of Sciences, he published eight upon the following subjects : 1. Experiences sur le recul des acmes a feu. Mem. :lead. 1703, Hist. 98.
2. Reflexions sur les regles de la condensation de I' air. Id. 1705, p. 61. and p. 272.
3. Observations sur la lurniere des corps frottees. Id. 1707. Hist. 3.
4. Experiences sur les acmes a feu differemment chargees. Id. 1707. Hist. 3.
5. De la necessite de Bien centrer le verre objectif d'une lunette. Id. 1710, p. 223.