Cassini De Thury, Cæsar-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 the preceding, whose occupations and talents our author inherited and supported with great honour. He received his first lessons in astronomy and mathematics from Messieurs Maraldi and Camus. He was hardly ten years of age when he calculated the phases of the total eclipse of the sun of 1727. 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. He did not content himself with the measure of a degree by Picard; suspecting even that the measures which had been taken by his father and grandfather were not exempt from some errors, which the imperfections of their instruments at least would be liable to, he again undertook to measure the meridian of Paris, by means of a new series of triangles, of a smaller number, and more advantageously disposed. 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 6000 toises from one another, from side to side of the country. Jn 1735 he had been received into the academy as adjoint supernumerary, at twenty-one years of age.

A tour which our author made in Flanders, in company with the king, about 1741, gave rise to the particular chart of France, at the instance of the king. Cassini | published different works relative to these charts, and a great number of the sheets of the charts themselves. In 1761, Cassini 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 this country the same plan as in France; and thus to establish successively 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.” M. Cassini, always meditating the perfection of his grand design, profited of the peace of 1762, 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 after whose death the business was for some time suspended but it was afterwards revived under the auspices of the duke of Richmond, master-general of the ordnance, and the execution committed to the care of col. Edward Williams and capt. William Mudge, both respectable officers of the artillery, and Mr. Isaac Dalby, who had before accompanied and assisted general Roy.

M. Cassini published in the volumes of Memoirs of the French academy a prodigious number of pieces, chiefly astronomical, too numerous to particularize in this place, between the years 1735 and 1770; consisting 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, of 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 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 small-pox, the 4th of September, 178*, in the seventy-first year of his age; being succeeded in the academy, and as director of the observatory, by his only son, the present count John Dominic Cassini who is the fourth in order by direct descent in that honourable station. ¹