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 John-Dominic Cassini, above mentioned, whom he succeeded as astronomer 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 philosophy 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 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 his journey to Italy, where he assisted him in the verification of the meridian at Bologna, and other measurements. 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 a member of the Royal Society.—In 1712 he succeeded his father as astronomer royal at the observatory.—In 1717 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 1725 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 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 intermitted his observations for the space of 3 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 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 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 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 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 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 6 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 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 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 80th year of his age, and was succeeded in the Academy and Observatory by his second son Cesar-François de Thury. 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 (Cesar-Francois), 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, whose occupations and talents our author inherited and supported, with great honour. He received his first lessons in astronomy and mathematics from MM. Maraldi and Camus. He was hardly 10 years of age when he calculated the phases of the total eclipse of the sun of 1727. At the age of 18 he accompanied his father in his two journies 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.—In 1735, he had been received into the academy as adjoint supernumerary at 21 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 Allemague.
Finally, M. Cassini, always meditating the perfection of his grand design, prosited of the late peace 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 has lately been 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; from whose united skill and zeal the happiest prosecution of this business may be expected.
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 50 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 12 years of his life very painful and distressing, till it was at length terminated by the small-pox the 4th of September 1784, in the 71st year of his age; being succeeded in the academy, and as director of the observatory, by his only son the present count JohnDominic Cassini; who is the 4th in order by direct descent in that honourable station. |
