32 SATELLITES OF SATURN AND UEANUS. SECT. IV. 



and prove connections which we might otherwise be unable to 

 trace. The identity of the velocity of light, at the distance of 

 Jupiter, and on the earth's surface, shows that its velocity is 

 uniform ; and as light consists in the vibrations of an elastic 

 medium or ether filling space, the uniformity of its velocity 

 shows that the density of the medium throughout the whole 

 extent of the solar system must be proportional to its elasticity 

 (N". 99). Among the fortunate conjectures which have been 

 confirmed by subsequent experience, that of Bacon is not the 

 least remarkable, " It produces in me," says the restorer of true 

 philosophy, " a doubt whether the face of the serene and starry 

 heavens be seen at the instant it really exists, or not till some 

 time later : and whether there be not, with respect to the 

 heavenly bodies, a true time and an apparent time, no less than 

 a true place and an apparent place, as astronomers say, on 

 account of parallax. For it seems incredible that the species or 

 rays of the celestial bodies can pass through the immense interval 

 between them and us in an instant, or that they do not even 

 require some considerable portion of time." 



Great discoveries generally lead to a variety of conclusions : 

 the aberration of light affords a direct proof of the motion of the 

 earth in its orbit ; and its rotation is proved by the theory of 

 falling bodies, since the centrifugal force it induces retards the 

 oscillations of the pendulum (N. 100) in going from the pole to 

 the equator. Thus a high degree of scientific knowledge has 

 been requisite to dispel the errors of the senses (N. 237). 



The little that is known of the theories of the satellites of 

 Saturn and Uranus is, in all respects, similar to that of Jupiter. 

 Saturn is accompanied by eight satellites. The seventh is about 

 the size of Mars, and the eighth was simultaneously discovered 

 by Mr. Bond in America, and that distinguished astronomer Mr. 

 Lassell, of Liverpool. The orbits of the two last have a sensible 

 inclination to the plane of the ring ; but the great compression 

 of Saturn occasions the other satellites to move nearly in the 

 plane of his equator. So many circumstances must concur to 

 render the two interior satellites visible, that they have very 

 rarely been seen. They move exactly at the edge of the ring, 

 and their orbits never deviate from its plane. In 1789 Sir 

 William Herschel saw them like beads, threading the slender 

 line of light which the ring is reduced to when seen edgewise 



