Cassim, a family of Italian and French astronomers, four members of which were directors of the Paris observatory for the first 122 years of its existence. I. Giovanni Donienico, born at Perinaldo, near Nice, June 8, 1G25, died in Paris, Sept. 14, 1712. While in college at Genoa he gained considerable reputation by his Latin verses; but his attention having been turned to mathematics, he abandoned poetry. He went to Paris as secretary in the suite of Ler-caro, afterward doge of Genoa, who was then the head of an embassy to the court of Louis XIV. After the return of the embassy he accompanied Lercaro to his estates in Lombardy, and while there he devoted some time to the study of astrology, which led him to his lifelong pursuit of astronomy. In 1644, at the invitation of the marquis Malvasia, he went to Bologna, and in 1650 was appointed professor of astronomy in the university there. The marquis had built an observatory at the villa of Pansano near Modena, and here Cassini made observations upon the comet of 1652 from which he published his first work. In the following year the church of St. Petronia in Bologna, where Ignazio Dante in 1575 had established a meridian line and gnomon, was undergoing repairs, and Cassini obtained the privilege of correcting and lengthening this line.
He was very successful, and made observations in regard to the obliquity of the ecliptic, refraction, and parallax, which subsequent observations have shown to be very close approximations to exactness. In 1G5G he published his tables of the sun founded upon these observations. In 1657 he was appointed superintendent of the Po for the city of Bologna, and he was afterward employed in many public duties by different cities and by the pope. He also found time to make a great number of observations upon insects, experiments upon the transfusion of blood, a subject which then attracted great attention, and to continue his astronomical observations. While at Ferrara he conceived the idea of a chart to represent the different appearances of an eclipse of the sun at various places on the surface of the earth; but the inquisitor of that city forbade its publication on account of its novelty. In 1665, at Citta della Pieve in Tuscany, by means of a telescope furnished by Campani, he observed the shadows thrown upon the surface of Jupiter by his satellites when they pass between the planet and the sun, and distinguished them from the fixed spots. Having compared his own observations with those of Galileo, he constructed his first tables of the satellites.
He very nearly approximated the truth in his calculations of the time of rotation of Jupiter, Venus, Mars, and the sun; and these discoveries exalted his reputation above that of any other astronomer then living. Though they really indicated nothing more than the use of good instruments by a careful and accurate observer, yet they were capable of being stated in definite terms which everybody could understand; and while the sublime discoveries of Copernicus and Newton were slowly struggling for recognition, all Europe was filled with the praises of the man who had laid open and stated in numbers the secrets of the heavens. In 1666 Colbert, chief minister of Louis XIV. of France, founded the royal academy of sciences and projected an observatory. He invited Cassini to take up his residence in Paris, offering him a pension equal to the emoluments of all the offices he held in Italy. Cassini arrived in Paris April 4, 1669, and on Sept. 14, 1671, commenced his observations. In this and the following year he discovered the third and fifth satellites of Saturn. In 1673 the Bolognese government requested him to return to that city, but Colbert persuaded him to remain in France. Cassini took out letters of naturalization as a French subject, and the same year married a French lady.
Henceforth his time was mostly occupied in making observations and recording their resuits. His explanation of the lunar libration was more complete and accurate than any previously given, and is considered one of his finest achievements. He was the first who carefully observed the zodiacal light, although he did not, as is frequently asserted, discover it. In 1684 he discovered the first and second satellites of Saturn, and a medal was struck in commemoration of the event, with the legend Satumi Satellites prim urn cogniti. In 1693 he published new tables of the satellites of Jupiter, which were a considerable improvement upon those which he had previously published in 1668. He had long been in possession of all the data necessary to determine the velocity of the transmission of light; but when the announcement of the discovery was made by Olaus Romer, Cassini rejected it. He revisited Italy in 1695, accompanied by his son. The survey of an arc of the meridian of Paris, which had been commenced in 1669 by Picard, and continued to the north of Paris by La Hire in 1683, was completed by Cassini to the south as far as Roussillon in 1700. He continued his observations until a few years before his death, when he became totally blind.
Cassini left a great number of writings, some of which have never been published; but very many appeared in the Journal des Savants and in the Memoires de l'Academic None of them are now consulted except in so far as they may contain records of facts and observations. He nowhere gives evidence of any acquaintance with the writings of Newton, and it has never been ascertained whether he rejected or adopted the Copernican theory of the solar system. He left an autobiography, which was published in 1810 by his great-grandson Cassini de Thury. II. Jacques, son of the preceding, born in Paris, Feb. 18,1677, died on his estate of Thury, April 16, 1756. Some papers on optics by him and an elder brother, who was killed at the battle of La Hogue, were published in 1691. In 1694, when only 17 years of age, he was chosen a member of the academy of sciences, and the following year accompanied his father to Italy. He then travelled in Holland and England, where he made the acquaintance of Newton and many other distinguished men, and in 1696 he was made a member of the royal society, He participated with his father in the survey of the meridian, which he continued to Mt. Canigou in the extreme south of France, and afterward northward to Dunkirk. On the death of his father he succeeded him at the observatory, and was maitre des comptes.
In 1720 he published a work on the magnitude and form of the earth. He determined the times of revolution of the satellites of Saturn then known, first observed the inclination of the orbit of the fifth (now the seventh), and determined very nearly the variation of the obliquity of the ecliptic and the length of the year. In 1740 he published Elements d'astronomic. Like his father, ho was an excellent observer, but little of a philosopher. He seems to have been inclined toward the Copernican system, to have had hardly any acquaintance with the theories of Newton, and to have been ignorant of the discovery of aberration by his contemporary Bradley. III. Cesar Francois (Cassini de Thury), son of the preceding, born in Paris, Jan. 17, 1714, died Sept. 4, 1784. He became a member of the academy of sciences in his 22d year, assisted his father in the survey of the meridian, and succeeded him as director of the observatory and maitre des comptes. His greatest work was the immense topographical map of France, upon which he labored for a large part of his life. It was finished by his son. Like his father and grandfather, he was an accurate and industrious observer, but contributed nothing to the advancement of astronomical science.
IV. Jacqnes Dominique, count de Thury, son of the preceding, born in Paris, June 30, 1748, died there, Oct. 18,1845. In 1770 he published an account of a voyage made by order of the king to test the chronometers of Le Roy, and the same year was admitted a member of the academy of sciences. He succeeded his father as director of the observatory. In 1787 he was associated with Mechain and Legendre in the operations to connect by a series of triangles the observatories of Greenwich and Paris. He completed the great map of France which his father had left unfinished, and on Oct. 13, 1789, he presented it to the national assembly as an aid in the new division of France into departments. In 1793 the national convention decreed that the observatory should no longer be under the control of one person, but of four, who should each serve in rotation for a year. Cassini and three of his pupils were appointed. He refused to submit to this, and on Sept. 6 of the same year he resigned. He was ordered to quit the observatory within 24 hours, and the next year he was imprisoned for seven months. On recovering his liberty he retired to his estate, abandoned astronomy entirely, and refused to take any part in the scientific operations undertaken by the government.
V. Alexandre Henri Gabriel, a French botanist, son of the preceding, born in Paris, May 9, 1781, died of cholera, April 16, 1832. lie commenced astronomical studies at an early age, but soon abandoned them and devoted himself in great measure to the study of botany, and published a large number of papers upon various parts of that science. He also held several judicial offices, being in the latter years of his life a member of the court of cassation. In 1827 he was made a member of the academy of sciences, and in 1830 a peer of Franco.