the entire horizon. An attempt was made at that time to fix the meridian line, but this was done more accurately by James Cassini, son of J. D. Cassini, in 1733. Imperfect as were their instruments, large sums of money were expended on them, and excellent work was done. The principal marvels of the heavens were discovered, the sun's rotation on itself, the revolution of Mars, Venus and Jupiter, the nebulæ, variable stars, four of Saturn's satellites and the division of his ring. Cassini's atlas of the heavens of sixty-two pages or plates was graciously accepted by the king as a gift from the astronomer. Huyghens of Holland succeeded Cassini as director of the observatory and he was followed by Roemer, the Dane, who discovered the velocity of light. Although after the death of Colbert in 1683 the resources of the observatory were diminished, the astronomers continued their work and by their discoveries brought fame to the nation. After 1689 and for many years, it is said, their salaries were only one third of what they had been in the time of Colbert.
The work of the astronomers of the seventeenth and eighteenth centuries has sometimes been treated as of small importance. But it was as good as the means for the study of the heavens permitted. The French astronomers, according to the testimony of Airy of England, did the best work in the world. Their efforts to measure accurately a degree of the meridian attracted universal notice. In this work the Abbé Picard was prominent. J. D. Cassini, his son James, Philippe de la Hire and a few others visited India and America to secure favorable observations. In 1755 Godin, Bougner and la Condaminer went to Peru, Maupertuis and Clairvant to Laponie, le Maurier and the Abbé de la Caillet to the Cape of Good Hope. The transit of Venus in 1761 was observed with great care. The abbé Chappe d'Auteroche went to Tobolsk, Francis Cassini de Maury to Venice, and Pingré to the island of Rodriguez. Observations were also made at San José, California. In 1769 other observations of the transit were made, but war with the English prevented some that might have been very valuable.
In 1770 the condition of the observatory was discouraging. Reports were to the effect that it was dangerous to occupy the buildings. The ministers of Louis XVI. aided J. D. Cassini de Thury in his work. But perfect instruments and promised repairs of the buildings were insufficient to persuade Cassini to remain in Paris after the Revolution and the decree of 1791. In June of this year the convention turned over all the observatories of France to the Bureau of Longitude. Under the influence and direction of such men as Laplace, Delambre, Legendre, Lagrange, Michain, Arago, Bouvard and his son, Mathieu and Mauduis the observatory regained its former reputation and even added to it. Still, as late as 1832, the astronomers found it neither altogether safe nor comfortable to live in the quarters provided for them.