612 COSMOS. 



As the mass of Jupiter after that of the Sun is the most 

 important element of the whole planetary system, its accurate 

 determination, which has recently been effected through the 

 disturbances of Juno and Vesta, as well as by the elongation 

 of his satellites, especially the fourth, 70 must be considered 

 as one of the most productive improvements of calculating 

 astronomy. The value of the mass of Jupiter is greater now 

 than formerly ; that of Mercury, on the contrary, smaller. 

 The former, together with that of the four satellites, is 

 rcTT-iTfT' while Laplace gave it as T^.^ n . 



Jupiter's period of rotation is, according to Airy, 9h. 55' 

 21"-3 mean solar time. Dominique Cassini first found it 

 (1665) to be between 9h. 55m. and 9h. 56m., by means of a 

 spot which was visible 78 for many years, even indeed to 

 1691, and was always of the same colour and outline. The 

 greater part of these spots are of greater blackness than the 

 streaks upon Jupiter. They do not, however, appear to 

 belong to the surface of the planet itself, as they sometimes 

 have a different velocity from that of the equatorial regions. 

 According to a very experienced observer, Heinrich Schwabe 

 of Dessau, the dark, more sharply bounded spots have been 



papers of the Paris Academy did not contain the notice of 

 Cnssiui's determination of the flattening (y^) until the year 

 1691 : so that Newton, who might certainly have known of 

 Rieher's pendulum-experiment at Cayenne, from the account 

 of the journey printed in 1679. must have become acquainted 

 with the configuration of Jupiter by verbal intercourse and 

 the active correspondence of that time. With regard to this 

 subject, and the only apparent early acquaintance of Huygens 

 with the pendulum-experiment of Richer, compare Cosmos, 

 vol. i. p. 156, note; and vol. ii. p. 136, note. 



10 Airy, in the Mem. of the Royal Astron. Soc. vol. ix. p. 7 ; 

 vol. x. p. 43. 



n As early as the year 1824. (Lnplace, op. cit. p. 207 v 

 w Delambn. Hist, de VAstroi\ mod. toni. ii. p. 754. 



