JOHANNES DOMINICUS, an excellent astronomer, was born at Piedmont in 1635. His early proficiency in astronomy procured him an invitation to become mathematical professor at Bologna when he was only fifteen years of age; and a comet having appeared in 1652, he discovered that bodies of this kind are not accidental meteors, but of the same nature, and probably governed by the same laws, as the planets. In the same year he solved a problem abandoned by Kepler and Bullialdus as insoluble; which was to determine geometrically the apoee and eccentricity of a planet from its true and mean place. In 1663 he was appointed inspector-general of the fortifications of the castle of Urbino, and had afterwards the care of all the rivers in the ecclesiastical states. He still, however, prosecuted his astronomical studies, in the course of which he discovered the revolution of Mars round his own axis; and in 1666 he published his theory of Jupiter's satellites. Cassini was invited by Louis XIV. in 1669 to visit France, where he settled as the first professor in the royal observatory. In 1677 he demonstrated the line of Jupiter's diurnal rotation; and in 1684 discovered four more satellites belonging to Saturn, Huygens having observed one. Cassini was principally instrumental in causing the French government undertake the expedition to Cayenne, which served to correct many errors relative to the figure of the earth, at the same time that it demonstrated the decrease of the intensity of terrestrial gravity from the equator to the poles; a phenomenon which exhibits a striking confirmation of the theory of gravitation. In 1693 he published new tables of the satellites of Jupiter, more exact than those which he had given to the world in 1668. In 1693 he went to revisit his meridian of St Pétron, which he regarded with honest pride; but he was now occupied with another and much longer meridian, commenced in 1669 by Picard, continued to the north of Paris in 1683 by De Lahire, and lastly, in 1700, carried by Cassini himself to the extremity of Roussillon; being the same line which, forty years after, was measured anew by François Cassini and La Caille, and, at the same distance of a century, measured a third time by Méchain and Delambre, with a precision which left little more to be desired. In the last years of his life he lost his sight, a misfortune which befell him in common with Galileo, and which probably proceeded from the same cause, namely, excessive application to the delicate observations of astronomy. Cassini inhabited the royal observatory at Paris more than forty years, and died on the 14th September 1712, "sans maladie, sans douleur, uniquement par la nécessité de mourir." In Lalande's Bibliographie Astronomique will be found a complete enumeration of the various works of Cassini; we need only cite here, 1. Observationes Comete Ann. 1655 et 1653, Modena, 1653, fol.; 2. Opera Astronomica, Rome, 1666; 3. Nuntii Syderei Interpretis; 4. Cosmographie.

James, another celebrated astronomer, the only son of the former. He was born at Paris on the 18th February 1677. His early studies were conducted in his father's house, where, from the pursuits and studies of his father, mathematics, and their application to astronomy, were probably not neglected. He afterwards became a student at the Mazarin College, at the time that the celebrated Varignon was professor of mathematics. With the assistance of this eminent man, young Cassini made such progress, that at fifteen years of age he supported a mathematical thesis with great honour. At the age of seventeen he was admitted as 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 in other measurements. After his return he performed similar operations in Holland, and discovered some errors in the measurement of the earth by Snell, the result of which was communicated to the academy in 1702. In 1696 he also made a visit to England, where he was elected 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 two hundred millions of miles diameter, is but as a point in comparison of their distance. The same year he also communicated 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 shows sometimes a little on the 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 engaged 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 in 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 also 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; also that these quantities would go on increasing for two thousand years, and then would decrease again. In 1740 he published his Astronomical Tables, and his Elements of Astronomy; both very extensive and accurate works.

Although astronomy was the principal object of our author's pursuit, he did not confine himself absolutely to that science, but made occasional excursions into other fields. We owe to him Experiments on Electricity; Experiments on the Recoil of Fire-arms; Researches on the Rise of the Mercury in the Barometer at different Heights; Reflections on the perfecting of Burning-glasses; and other interesting memoirs.

One of the most important objects of the French academy 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 deduce the whole circumference with sufficient accuracy, it was resolved to continue the measurement on the meridian of Paris to the north and the south, throughout the whole extent of the country. Accord- ingly, in 1683, M. de Lahire continued that on the north side of Paris, and the older Cassini that on the south side; while, in the continuation of this operation, the latter was, in 1700, assisted by his son. The same work was further continued by the same academicians; and finally, the part left unfinished by De Lahire in the north was finished in 1718 by our author, Maraldi, and De Lahire the younger.

These operations produced a very considerable degree of precision. From this measured extent of six degrees, it appeared also that the degrees were of different lengths in different parts of the meridian; and 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 a 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 which 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 that of an oblong spheroid, contrary to the determination of Newton by the theory of gravity. Newton was indeed of all men the most averse from controversy; but the other mathematicians in Britain did not tamely submit to conclusions in direct opposition to the fundamental doctrine of this great philosopher. The consequence was, that the French government sent two different sets of mathematicians, 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 that of an oblate spheroid, contrary to the conclusion of Cassini.

After a long and laborious life, James Cassini died in April 1756, and was succeeded in the academy and observatory by his second son. He published, 1. Réponse à la Dissertation de M. Celsius sur les Observations faites pour pouvoir determiner la figure de la Terre, 1738, 8vo; 2. Éléments d'Astronomie, Paris, 1740, 4to; 3. Tables Astronomiques du Soleil, de la Lune, des Planètes, des Étoiles, et des Satellites, Paris, 1740, 4to.

Cassini de Thury, Caspar François, a celebrated French astronomer, director of the observatory, and member of most of the learned societies of Europe, was born at Paris on the 17th June 1714. He was the second son of James Cassini, whose occupations and talents he inherited and supported with great honour. He received his first lessons in astronomy and mathematics from MM. Maraldi and Camus; and made such 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 to draw a perpendicular to the meridian of the observatory, from Strasbourg to Brest. A general chart of France was then projected; and for this purpose it was necessary to traverse the country by several lines parallel and perpendicular to the meridian of Paris. Our author was charged with the conduct of this work, in the execution of which he was so scrupulous as to measure again what had been previously measured by his father. This great work was published in 1740, with a chart showing 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 passing along the eastern limits of France to Dunkirk, with parallel and perpendicular lines drawn at the distances respectively of 6000 toises from side to side of the country.

In 1741 he made a tour in Flanders, in the train of the king; and this gave rise, at his majesty's instance, to the chart of France; in relation 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 Parisian meridian, in order 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 plan adopted in France, and thus to establish successively for all Europe a most useful uniformity. He was at Vienna the 6th of June 1761, the day of the transit of the planet Venus over the sun's disc, a phenomenon which he observed as accurately as the state of the weather would permit him to do, and of which he published an account in his Voyage en Allemagne.

Cassini, always meditating the perfection of his grand design, profited by 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 connecting the general chart of the latter with that of the British isles, as he had before united it with those of Flanders and Germany. The proposal was favourably received by the English government, and carried into effect by General Roy, under the direction of the Royal Society.

Between the years 1735 and 1770, M. Cassini published, in the memoirs of the French Academy, a great number of pieces, consisting chiefly of astronomical observations and questions, among which may be mentioned researches concerning the parallax of the sun, the moon, Mars, and Venus; remarks 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. He cultivated astronomy during half a century, the most important for that science that ever elapsed, on account of the magnitude and variety of the objects that had been investigated, in all which Cassini 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 had conducted. An habitual retention of urine, however, rendered the last twelve years of his life very painful and distressing, till at length it was terminated by the small-pox, on the 4th of September 1784, in the seventy-first year of his age. He was succeeded in the academy, and as director of the observatory, by his only son John Dominic Cassini, the fourth in order of direct descent who filled that honourable station. The following is a pretty correct list of his works, viz. 1. La Méridienne de l'Observatoire Royal de Paris, vérifiée dans toute l'étendue du Royaume, avec des Observations d'Histoire Naturelle par Lemonnier, 1744, 4to; 2. Cartes des Triangles de la France (avec Maraldi), 1744, 4to; 3. Additions aux Tables Astronomiques de Cassini, 1755, 4to; 4. Relation de deux Voyages faits en 1761 et 1762 en Allemagne, pour determiner la grandeur des degrés de longitude, par rapport à la géographie et à l'astronomie, 1763, 4to; 5. Opuscules Divers, 1771, 8vo; 6. Description d'un instrument pour prendre hauteur, et pour trouver l'heure vraie sans aucun calcul, 1770, 4to; 7. Relation d'un Voyage en Allemagne, qui comprend les opérations relatives à la figure de la terre et à la géographie particulière du Palatinat, etc., 1775, 4to; 8. Description Géométrique de la Terre, 1775, 4to; 9. Description Géométrique de la France, 1784, 4to. He also edited Observations sur la Comète de 1551, pendant le temps de son retour en 1652, faites par J. D. Cassini, 1759, 12mo.