(James), a celebrated French astronomer, was born at Paris February 18, 1677; being the younger son of Johannes Dominicus Cassini, of whom some account has been given in the Encyclopaedia.

After his first studies in his father's house, in which it is not to be supposed that mathematics and astronomy would be neglected, he was sent to study philosophy at the Mazarine college, where the celebrated Varignon was then professor of mathematics. From the assistance of this eminent man young Cassini profited so well, that at 15 years of age he supported a mathematical 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 a journey to Italy, where he assisted him in the verification of the meridian at Bologna and other measurements. On his return he performed 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 a member of the Royal Society. In 1712 he succeeded his father as astronomer royal at the observatory of Paris. In 1717 he gave to the Academy his researches on the distance of the fixed stars; in which he showed that the whole annual orbit, of near 300 millions of miles diameter, is but 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 throws sometimes a little towards one side, and sometimes a little on the other, of that half which is commonly beheld 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 its axis in the space of 23 hours; and M. Bianchini had published 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 upon the spots in Venus, he discovered that he had interrupted his observations for the space of three hours, from which cause he had probably mistaken new spots for the old ones, and to had been led into the mistakes. The probability is, that both had fallen into some mistake, or that they had proceeded on very different principles; for otherwise such different results are wholly unaccountable. Dr Herschel seems satisfied that the period of the revolution is less than Bianchini has made; but he does not say what it is, or that it is not much greater than it was supposed by Cassini. Our author, after he had convicted Bianchini, as he thought, of error, 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 1740 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 did not confine himself solely to that branch, but made occasional excursions into 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.

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 the 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 further 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 fort that our author concluded, in the volume published for 1718, 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 axis 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: 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. Though Newton was of all men the most adverse from controversy, the other mathematicians in Britain did not tamely submit to conclusions in direct opposition to the fundamental doctrine of a philosopher of whose talents the nation was justly proud. The consequence was, that the French government sent two different sets of measures, 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, James Cassini died in April 1756, in consequence of a fall, and was succeeded in the Academy and Observatory by the subject of the following article. He published, 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 the year 1699 to 1755.

Cassini de Thury (César-François), a celebrated French astronomer, director of the observatory, pensioner astronomer, and member of most of the learned societies of Europe, was born at Paris June 17, 1714, being the second son of James Cassini, the subject of the preceding memoir, whose occupations and talents he inherited and supported with great honour. He received his first lessons in astronomy and mathematics from MM. Maraldi and Cassini; and made such a rapid progress, that when he was not more than ten years of age he calculated the phases of a total eclipse of the sun. At the age of eighteen he accompanied his father in his two journeys undertaken for drawing the perpendicular to the observatory meridian from Strasbourg to Brest. From that time a general chart of France was devised; for which purpose it was necessary to traverse the country by several lines parallel and perpendicular to the meridian of Paris, and our author was charged with the conduct of this business; in which he was so scrupulous as to measure again what had been measured by his father. This great work was published in 1740, with a chart shewing the new meridian of Paris, by two different series of triangles, passing along the sea coasts to Bayonne, traversing the frontiers of Spain to the Mediterranean and Antibes, and thence along the eastern limits of France to Dunkirk, with parallel and perpendicular lines described at the distance of 6°20' from one another, from side to side of the country.

A tour which, in 1741, our author made in Flanders, in the train of the king, gave rise, at his majesty's instance, to the chart of France; relative to which Cassini published different works, as well as a great number of the sheets of the chart itself. In 1761 he undertook an expedition into Germany, for the purpose of continuing to Vienna the perpendicular of the Paris meridian; to unite the triangles of the chart of France with the points taken in Germany; to prepare the means of extending into that country the same plan as in France; and thus to establish successfully for all Europe a most useful uniformity.

Our author was at Vienna the 6th of June 1761, the day of the transit of the planet Venus over the sun, of which he observed as much as the state of the weather would permit him to do, and published the account of it in his Voyage en Allemagne.

Finally, M. Cassini, always meditating the perfection of his grand design, profited of the peace of 1783 to propose the joining of certain points taken upon the English coast with those which had been determined on the coast of France, and thus to connect the general chart of the latter with that of the British isles, like as he had before united it with those of Flanders and Germany. The proposal was favourably received by the English government, and presently carried into effect under the direction of the Royal Society, the execution being committed to the late General Roy. See the life of that general in this Supplement.

Between the years 1735 and 1770, M. Cassini published, in the volumes of Memoirs of the French Academy, a prodigious number of pieces, consisting chiefly of astronomical observations and questions; among which are observable researches concerning the parallax of the sun, the moon, Mars, and Venus; on astronomical refractions, and the effect caused in their quantity and laws by the weather; numerous observations on the obliquity of the ecliptic, and on the law of its variations. In short, he cultivated astronomy for fifty years, the most important for that science that ever elapsed for the magnitude and variety of objects, in which he commonly sustained a principal share.

M. Cassini was of a very strong and vigorous constitution, which carried him through the many laborious operations... CAT

operations in geography and astronomy which he conducted. An habitual retention of urine, however, rendered the last twelve years of his life very painful and distressing, till it was at length terminated by the smallpox the 4th of September 1784, in the 71st year of his age. He was succeeded in the academy, and as director of the observatory, by his only son John-Dominic Caffini, the fourth in order of direct descent who has filled that honourable station.