170 COSMOS. 



west; apparently enter on the eastern edge of Jupiter, they 

 hide from us, in their passage, successive portions of the disk 

 of their primary, and can be perceived with telescopes of 

 moderate pow^er, since they stand out in lumhious relief 

 from the disk. The visibility of the satellite is attended 

 with more difficulty the nearer it approaches the center of 

 the primary. From this phenomenon, which was early ob- 

 served. Pound, Newton's and Bradley's friend, inferred that 

 the disk was less luminous near the edge than at the center. 

 Arago considers that this assumption, renewed by Messier, 

 involves difficulties which can only be solved by new and 

 more delicate observations. Jupiter was seen without any 

 satellites by Molyneux in November, 1681 ; by Sir William 

 Herschel on the 23d of May, 1802 ; and, lastly, by Griesbach, 

 on the 27th of September, 1843. Such a non- visibility of the 

 satellites has reference, however, to the space without the 

 disk of Jupiter, and is not inconsistent with the theorem that 

 all the four satellites can not be eclipsed at one time. 



Saturn. 



The period of sidereal or true revolution of Saturn is 29y. 

 166d. 23h. 16m. 32s. His mean diameter is 62,028 geo- 

 graphical miles, equal to 9022 terrestrial diameters. The 

 period oi rotation, deduced from the observation of some dark 

 spots (knot-like condensations of the bands) upon the surface,* 

 is lOh. 29m. 17s. Such a great velocity of rotation corre- 

 sponds to the considerable ^«^^em?^g•. William Herschel esti- 

 mated it, in 1776, at -^^.^ ; Bessel, after corresponding observ- 

 ations during a period of more than three years, found that at 



* The earliest and careful observations of William Herschel, in No- 

 vember, 1793, gave for Saturn's period of rotation lOh. 16m. 44s. It 

 has been incorrectly attributed to the great philosopher, Immanuel 

 Kant, that he conjectured the period of Saturn's rotation from theo- 

 retical considerations in his Allgemeincn Naturgeschichte des Himmels, 

 forty years before Herschel. The number that he gives is 6h. 23m. 

 53s. He calls his determination " the mathematical calculation of an 

 unknown motion of a heavenly body, which is, perhaps, the only pre- 

 diction of that kind in pure Natural Philosophy, and awaits confirma- 

 tion at a future period." This confirmation of his conjecture did not 

 take place at all ; observations have shown an error of f of the whole, 

 i. e., of four hours. In the same work it is said respecting the ring of 

 Saturn,. " that in the aggregation of particles which constitute it, those 

 of the inner edge complete their revolution in 10 hours, those of tho 

 external edge in 15 hours. The first of these ring-numbers is the only 

 one which accidentally comes near the planet's observed period of ro- 

 tation (lOd. 29m. 17s.). Compare Kant, Sdmmtlicke Werke, th. vir., 1389 

 p. 135 and 140. 



