376 SPECIAL RESULTS IN THE URANOLOGICAL 



the diameter of the remoter Saturn. The sidereal revolu- 

 tion of Jupiter round the Sun is performed in 11 years, 

 314 days, 20 hours, 2 minutes, and 7 seconds. The ellip- 

 ticity of its figure or compression at the poles, according to 

 the prismatic micrometer measurements of Arago, the 

 result of which was transferred in 1824 into the Exposition 

 du Systeme du Monde (p. 38), is as 167 : 177, or -pf T ; 

 which agrees very nearly with the later investigations (1839) 

 of Beer and Madler ( 612 ), who found it between -^.^ and -^-f .-5- . 

 Hansen and Sir John Herschel prefer -V. The earliest 

 observation of Jupiter's ellipticity by Dominique Cassini was 

 prior to the year 1666, as I have already recalled elsewhere. 

 This circumstance has a peculiar historical interest, from the 

 influence which, as has been acutely remarked by Sir David 

 Brewster, it may have exercised on Newton's ideas respect- 

 ing the figure of 'the Earth. The Principia Philosophise 

 Naturalis seems to afford indications favourable to such a 

 supposition ; but the dates at which the Principia and Cas- 

 sini's observations of the equatorial and polar diameters of 

 Jupiter were respectively published, might occasion some 

 chronological doubts on the subject ( 613 ). 



As the mass of Jupiter is, next to the mass of the Sun, the 

 most important element in the whole of our planetary system, 

 its more exact determination in recent times by Airy (1834), 

 by means of the perturbations of Juno and Vesta, as well 

 as by the elongation of Jupiter's satellites, especially the 

 4th ( 614 ), must be regarded as one of the advances in 

 calculating astronomy most fruitful in consequences. The 

 effect of the corrections obtained is to augment the mass 

 of Jupiter, and to diminish that of Mercury. The mass of 



