$ 159-161] Measurements of the Earth : Cassini 205 



of Jupiter (1665), of Mars (1666), and possibly of Venus 

 (1667), and also by his tables of the motions of Jupiter's 

 moons (1668). The last caused Picard to procure for him 

 an invitation from Louis XIV. (1669) to come to Paris 

 and to exercise a general superintendence over the Obser- 

 vatory, which was then being built and was substantially 

 completed in 1671. Cassini was an industrious observer 

 and a voluminous writer, with a remarkable talent for 

 impressing the scientific public as well as the Court. He 

 possessed a strong sense of the importance both of himself 

 and of his work, but it is more than doubtful if he had as 

 clear ideas as Picard of the really important pieces of work 

 which ought to be done at a public observatory, and of 

 the way to set about them. But, notwithstanding these 

 defects, he rendered valuable services to various departments 

 of astronomy. He discovered four new satellites of Saturn : 

 Japetus in 1671, Rhea in the following year, Dione and 

 Thetis in 1684; and also noticed in 1675 a dark marking 

 in Saturn's ring, which has subsequently been more dis- 

 tinctly recognised as a division of the ring into two, an 

 inner and an outer, and is known as Cassini's division 

 (see fig. 95 facing p. 384). He also improved to some 

 extent the theory of the sun, calculated a fresh table of 

 atmospheric refraction which was an improvement on 

 Kepler's (chapter vn., 138), and issued in 1693 a fr esn set 

 of tables of Jupiter's moons, which were much more accurate 

 than those which he had published in 1668, and much the 

 best existing. 



1 6 1. It was probably at the suggestion of Picard or Cassini 

 that one of their fellow astronomers, John Richer (?-i696), 

 otherwise almost unknown, undertook (1671-3) a scientific 

 expedition to Cayenne (in latitude 5 N.). Two important 

 results were obtained. It was found that a pendulum of 

 given length beat more slowly at Cayenne than at Paris, 

 thus shewing that the intensity of gravity was less near the 

 equator than in higher latitudes. This fact suggested that the 

 earth was not a perfect sphere, and was afterwards used in 

 connection with theoretical investigations of the problem of 

 the earth's shape (cf. chapter ix., 187). Again, Richer's 

 observations of the position of Mars in the sky, combined 

 with observations taken at the same time by Cassini, Picard, 



