32 SATELLITES OF JUPITER AND URANUS. SKCT. IV. 



dulum (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. 



The little that is known of the theories of the satel- 

 lites of Saturn and Uranus, is, in all respects, similar to 

 that of Jupiter. Saturn is accompanied by seven satel- 

 lites, the most distant of which is about the size of the 

 planet Mars. Its orbit has 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 re- 

 duced to, when seen edgewise from the earth. And 

 for a short time he perceived them advancing off it at 

 each end, when turning round in their orbits. The 

 eclipses of the exterior satellites only take place when 

 the ring is in this position. Of the situation of the equa- 

 tor of Uranus we know nothing, nor of his compression ; 

 but the orbits of his satellites are nearly perpendicular 

 to the plane of the ecliptic ; and, by analogy, they ought 

 to be in the plane of his equator. Uranus is so remote 

 that he has more the appearance of a planetary nebula 

 than a planet, which renders it extremely difficult to 

 distinguish the satellites at all ; and quite hopeless with- 

 out such a telescope as is rarely to be met with even in 

 observatories. Sir William Herschel discovered six, 

 and determined the motions of two of them ; but from 

 that time the position of the planet has been such as to 

 render farther observations impossible. The subject 

 has recently occupied the attention of his son, who has 

 found evidence of the general correctness of his father's 

 views, and has been enabled to determine the elements 

 of the motions of these minute objects with more accu- 

 racy. The first satellite performs its revolution about 

 Uranus in 8 d 16 h 56 ra 28 s -6 ; and the second satellite ac- 

 complishes its period in 13 d ll h 7 m 12 B 6. The orbits of 

 both seem to have an inclination of about 101 -2 to the 

 plane of the ecliptic ; and their motions offer the singu- 



