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At length the labours of Picard, continued by La Hire and Cassini, were completed at the commencement of the following century. The astronomical observations, rendered possible by the calculation of the satellites of Jupiter, enabled us to rectify our maps.
J. J. Cassini, however, in 1740, showed that the data collected by both observers were consistent with rotation in twenty-three hours twenty minutes. So the matter rested until Schröter's time.
Cassini had noted an oval form in 1691. This was confirmed by La Hire, Romer, and Picard. W. Herschel supposed the spots to be masses of cloud in the atmosphere an opinion still accepted. Many of them were very permanent. Cassini's great spot vanished and reappeared nine times between 1665 and 1713. It was close to the northern margin of the southern belt.
The great Cassini too, who though of an Italian family, was born at Nice I think, and died at Paris, drew his meridian line through the church of St. Petronius in this city, across the pavement, where it still remains a monument to his memory, who discovered the third and fifth satellites of Jupiter.
Colbert had the true scholar's taste; he had brought Cassini from Italy to take the direction of the new Observatory; he had ordered surveys for a general map of France; he had founded the Journal des Savants; literary men, whether Frenchmen or foreigners, enjoyed the king's bounties. Colbert had even conceived the plan of a Universal Academy, a veritable forerunner of the Institute.
But even if Cassini and the rest had been notoriously untrustworthy persons instead of being some of them distinguished for the care and accuracy with which their observations were made and recorded, these occasional views of a phantom satellite are by no means such observations as they would have invented.
Fontana "discovered" it in 1645; Cassini an adept in the art of seeing recognised it in 1672, and again in 1686; Short watched it for a full hour in 1740 with varied instrumental means; Tobias Mayer in 1759, Montaigne in 1761; several astronomers at Copenhagen in March, 1764, noted what they considered its unmistakable presence; as did Horrebow in 1768.
During the transit of 1769, and again on December 8-9, 1874, Venus certainly had no companion during her transit. What, then, was it that Cassini, Short, Montaigne, and the rest supposed they saw? The idea has been thrown out by Mr. Webb that mirage caused the illusion.
The interference of the German and Russian legations, and of the Clan-na-Gael, with the press and the Senate was innocently undisguised. The charming Russian Minister, Count Cassini, the ideal of diplomatic manners and training, let few days pass without appealing through the press to the public against the government.
These curious peculiarities of the lunar surface had interested the astronomic mind to a very high degree at their first discovery, and have proved to be very perplexing problems ever since. The first observers do not seem to have noticed them. Neither Hevelius, nor Cassini, nor La Hire, nor Herschel, makes a single remark regarding their nature.
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