166 COSMOS. 



^^ho found the flattening to "be between yi if and ^| g. Han- 

 sen and Sir John Herschel give the preference to Jj. The 

 earliest observation of the flattening, by Dominique Cassini, 

 is older than the year 1666, as I have already pointed out 

 elsewhere. This circumstance has an especial historical im« 

 portance, on account of the influence which, according to Sir 

 David Brewster's acute remark, the discovery of this flatten- 

 ing by Cassini exercised upon Newton's ideas as to the figure 

 of the Earth. The Frincipia PhilosophicB Naturalis bears 

 witness to this, but the epochs at which the Principia and 

 Cassini's observation of equatorial and polar diameters of 

 Jupiter appeared, might excite chronological doubts.* 



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,! must be considered as 

 one of the most productive improvements of calculating astron- 

 omy. The valua 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 to4 7^T9' 

 while Laplace gave it as yoeVog-^ 



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. 56in., by means of a 

 spot which was visible^ for many years, even indeed to 1691, 

 and was always of the same color and outline. Tho greater 

 part of these spots are of greater blackness than the streaks 

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



Laplace {Syst. du Monde, p. 266) found it theoretically between -^^ 

 and -^^j with increasing density of the strata. 



* Newton's immortal work, Philosophice Naturalis Principia Mathe 

 matica, appeared as early as May, 1687, and the papers of the Paris 

 Academy did not contain the notice of Cassini's determination of the 

 flattening (y^^) until the year 1691 ; so that Newton, who might cer- 

 tainly have known of Richer's pendulum-experiment at Cayenne, fronr 

 the account of the journey printed in 1679, must have become acquaint- 

 ed with the configuration of Jupiter by verbal intercourse and the act- 

 ive correspondence of that time. With regard to this subject, and tho 

 only apparent early acquaintance of Huygens with the pendulum-ex 

 periment of Richer, compare Cosmos, vol. i., p. 165, note, and vol. ii., 

 p. 146, note. 



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

 p. 43. 



t As early as the year 1824. (Laplace, op. cit., p. 207.) 



$ Delambre, /fW. de t Astron. Mod., torn, ii., p. 754. 



