174 



COSMOS. 



in their results in favor of fluidity, as well as continuous vari- 

 ability in the figure, and divisibility of the outer ring. The 

 permanence of the whole is considered by Peirce as depend- 

 ent upon the influence and position of the satellites, because 

 without this dependence, even with inequalities in the ring, 

 the equilibrium could not be maintained. 



The Satellites of Saturn. 



The five satellites of Saturn which have been known lon- 

 gest were discovered between the years 1655 and 1684 (Ti- 

 tan, the sixth according to distance, by Huygens ; and four 

 by Cassini, viz., Japetus, the outermost of all, Rhea, Tethys, 

 and Dione). These were followed by the discovery, by Will- 

 iam Herschel, in 1789, of two, Mim^as and Enceladus, situ- 

 ated nearest to the planet. Finally, the seventh satellite, 

 Hyperion, the last but one according to distance, was dis- 

 covered almost simultaneously by Bond, at Cambridge (U. S.), 

 and by Lassell at Liverpool, in September, 1848. The rela- 

 tive magnitudes and relations of distances in this partial sys- 

 tem have been already treated of {Cosmos, vol. i., p. 97 ; 

 vol. iv., p. 105-118.) The periods of revolution and the 

 mean distances, the latter expressed in fractional parts of the 

 equatorial radius of the primary, are, according to the observ- 

 ations instituted by Sir John Herschel at the Cape of Good 

 H.ope,='^ between 1835 and 1837, the following : 



Between the first four satellites nearest to Saturn a re- 

 markable relation oi commensurability in the period of rev- 

 olution presents itself. The period of the third satellite ( Te- 

 thys) is double that of the first {Mimas) ; that of the fourth 

 (Dione) double that of the second {Enceladus). The close- 



* Sir John Herschel, Results of Astron. Observations at the Cape of 

 Good Hope, p. 414-430 ; the same, in the Outlines of Astr., p. 650, and 

 apon the law of distances, $ 550. 



