CASSINI (JAMES,) a celebrated French astronomer, and member of the several Academies of Sciences of France, Eng land, Prussia, and Bologna, was born at Paris, Feburary 18,1677, being the young er son of John Dominic Cassini, above mentioned, whom he succeeded as as tronomer at the Royal Observatory, the elder son having lost his life at the battle of La Hogue.
After his first studies in his father's house, in which it is not to be supposed that mathematics and astronomy were neglected, he was sent to study philoso phy at the Mazarine college, where the celebrated Varignon was then professor of mathematics ; from whose assistance young Cassini profited so well, that at 15 years of age he supported a mathemati cal thesis with great honour. At the age of 17 he was admitted a member of the Academy of Sciences ; and the same year he accompanied his father in his journey to Italy, where he assisted him in the verification of the meridian at Bologna, and other measurements.
In 1712 he succeeded his father as as tronomer royal at the Observatory. In 1717 he gave to the academy his re searches on the distance of the fixed stars, in which he showed that the whole annu lar orbit of near 200 millions of miles dia meter is but as a point in comparison of that distance. The same year he com municated also his discoveries concerning the inclination of the orbits of the satel lites in general, and especially of those of Saturn's satellites and ring. In 1725 he undertook to determine the cause of the moon's libration, by which she shows sometimes a little towards one side, and Sometimes a little on the other, of that half which is commonly behind or hid from our view.
In 1732 an important question in astro nomy exercised the ingenuity of our au thor. His father had determined, by his observations, that the planet Venus re volved about her axis in the space of 23 hours : and M. Bianchini had publish ed a work in 1729, in which he settled the period of the same revolution at 24 days 8 hours. From an examination of
Bianchini's observations, which were up on the spots in Venus, he discovered that he had intermitted his observations for the space of three hours, from which cause he had probably mistaken new spots for the old ones, and so had been led into the mistake. He soon afterwards deter mined the nature and quantity of the ac celeration of the motion of Jupiter at half a second per year, and of that of the retardation of Saturn at two minutes per year ; that these quantities would go on increasing for 2000 years, and then would decrease again. In 1740 he published his "Astronomical Tables," and his " Ele ments of Astronomy ;" which were very extensive and accurate works.
Although astronomy was the principal object of our author's consideration, he did not confine himself absolutely to that branch, but made occasional excursions into other fields. We owe also to him, for example, experiments on electricity, or the light produced by bodies by friction; experiments on the recoil of firearms; researches on the rise of the mercury in the barometer at different heights above the level of the sea; reflections on the perfecting of burning-glasses, and other memoirs.
After a long and laborious life our au thor, James Cassini, lost his life by a fall, in April 1756, in the 80th year of his age, and was succeeded in the Academy and Observatory by his second son, Caesar Francois de Thury ; who also distinguish ed himself in the sciences connected with astronomy ; and, as well as his father and grandfather, published many valuable works. He died in 1784, of the small pox, and was succeeded by his only son count John Dominic Cassini.