Cassini, James

, a celebrated French astronomer, and member of the several academies of sciences of France, England, Prussia, and Bologna, was born at Paris Feb. 18, 1677, being the younger son of the preceding, whom he succeeded as astronomer at the royal observatory, the elder son having lost his life at the battle of La Hogue.

After some education in his father’s house he was sent to study philosophy at the Mazarine college, where the celebrated Varignon was then professor of mathematics; from whose assistance young Cassini profited so well, that at fifteen years of age he supported a mathematical thesis with | great honour. At the age of seventeen 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 measurement* On his return he made other similar operations in a journey into Holland, where he discovered some errors in the measure of the earth by Snell, the result of which was communicated to the academy in 1702. He made also a visit to England in 1696, where he was made r a member of the royal society. In 1712 he succeeded his father as astronomer royal at the observatory. In 17 17 he gave to the academy his researches on the distance of the fixed stars, in which he shewed that the whole annual orbit, of near 200 million of miles diameter, is but as a point in comparison of that distance. The same year he communicated also his discoveries concerning the inclination of the orbits of the satellites in general, and especially of those of Saturn’s satellites and ring. In 1723 he undertook to determine the cause of the moon’s libration, by which she shews 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 astronomy exercised the ingenuity of our author. His father had determined, by his observations, that the planet Venus revolved about her axis in the space of twenty-three hours: and M. Bianchini had published a work in 1729, in which he settled the period of the same revolution at twenty-four days eight hours. From an examination of Bianchini’s observations, which were upon 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 determined the nature and quantity of the acceleration 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 174O he published his Astronomical Tables, and his Elements of Astronomy; very extensive and accurate works.

Although astronomy was the principal object of our author’s consideration, he 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 fire-arms; 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. The French academy had properly judged that one of its most important objects was the measurement of the earth. In 1669, Picard measured a little more than a degree of latitude to the north of Paris; but as that extent appeared too small from which to conclude the whole circumference with sufficient accuracy, it was resolved to continue that measurement on the meridian of Paris to the north and south, through the whole extent of the country. Accordingly, in 1683, the late M. de la Hire continued that on the north side of Paris, and the older Cassini that on the south side. The latter was assisted in 1700 in the continuation of this operation by his son, our author. The same work was farther continued by the same academicians; and finally the part left unfinished by de la Hire in the north, was finished in 1718 by our author, with the late Maraldi, and de la Hire the younger.

These operations produced a considerable degree of precision. It appeared also, from this measured extent of six degrees, that the degrees were of different lengths in different parts of the meridian; and in such sort that our author concluded, in the volume published for 17 18^ that they decreased more and more towards the pole, and that therefore the figure of the earth was that of an oblong spheroid, or having its axe longer than the equatorial diameter. He also measured the perpendicular to the same meridian, and compared the measured distance with the differences of longitude as before determined by the eclipses of Jupiter’s satellites; from whence he concluded that the length of the degrees of longitude was smaller than it would be on a sphere, and that therefore again the figure of the earth was an oblong spheroid; contrary to the determination of Newton by the theory of gravity. In consequence of these assertions of our author, the French government sent two different sets of measurers, the one to measure a degree at the equator, the other at the polar circle; and the comparison of the whole determined the figure to be an oblate spheroid, contrary to Cassini’s determination.

After a long and laborious life, our author, James Cassini, lost his life by a fall, in April 1756, in the eightieth | year of his age, and was succeeded in the academy and observatory, by his second son, Cæsar- Francois de Thury. He published '5 A Treatise on the Magnitude and Figure of the Earth“as also” The Elements or Theory of “the Planets, with Tables” beside an infinite number of papers in the Memoirs of the Academy, from 1699 to 1755. ¹