SECT. IV. SATELLITES OF SATURN AND U1UNUS. 33 



from the earth. And for a short time he perceived them advanc- 

 ing 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. Mr. Lassell, with a powerful telescope, made 

 by himself, has seen lapetus, the nearest of the two, on several 

 occasions, even when the opening of the ring was very wide, 

 which made it extremely difficult to see so minute an object. Of 

 the situation of the equator of Uranus we know nothing, nor of 

 his compression ; but the orbits of his satellites are nearly per- 

 pendicular 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 without such a telescope as 

 is rarely to be met with even in observatories. Sir William 

 Herschel discovered the two that are farthest from the planet, 

 and ascertained their approximate periods, which his son after- 

 wards determined to. be 13< ll h 7 m 12 8 6 and 8 d 16 h 56 m 28 8 '6 

 respectively. The orbits of both seem to have an inclination of 

 about 101*2 to the plane of the ecliptic. The two interior 

 satellites are so faint and small, and so near the edge of the 

 planet, that they can with difficulty be seen even under the most 

 favourable circumstances : however, Mr. Lassell has ascertained 

 that the more distant of the two revolves about Uranus in 4 days, 

 and that nearest to the planet in 2^ days, and from a long and 

 minute examination he is convinced that the system only con- 

 sists of four satellites. Soon after Neptune was seen Mr. Lassell 

 discovered the only satellite known to belong to that planet. 

 The satellites of Uranus and Neptune, the two planets on the 

 remotest verge of the solar system, offer the singular and only 

 instance of a revolution from east to west, while all the planets 

 and all the other satellites revolve from west to east. Ketrograde 

 motion is occasionally met with in the comets and double stars. 



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