76 SATURN AND BIS BINGS. [SECT. ix. 



mean distance of the interior part of this double ring from 

 the surface of the planet is about 22,240 miles, it is no less 

 than 33,360 miles broad, but, by the estimation of Sir John 

 Herschel, its thickness does not much exceed 100 miles, so 

 that it appears like a plane. By the laws of mechanics, it is 

 impossible that this body can retain its position by the adhe- 

 sion of its particles alone. It must necessarily revolve with 

 a velocity that will generate a centrifugal force sufficient to 

 balance the attraction of Saturn. Observation confirms the 

 truth of these principles, showing that the rings rotate from 

 west to east about the planet in ten hours and a half, which 

 is nearly the time a satellite would take to revolve about 

 Saturn at the same distance. Their plane is inclined to the 

 ecliptic, at an angle of 28 P 10' 44"'5; in consequence of 

 this obliquity of position, they always appear elliptical to us, 

 but with an excentricity so variable, as even to be occasionally 

 like a straight line drawn across the planet. In the begin- 

 ning of October, 1832, the plane of the rings passed through 

 the centre of the earth ; in that position they are only visible 

 with very superior instruments, and appear like a fine line 

 across the disc of Saturn. About the middle of December, 

 in the same year, the rings became invisible, with ordinary 

 instruments, on account of their plane passing through the 

 sun. In the end of April, 1833, the rings vanished a second 

 time, and reappeared in June of that year. Similar phe- 

 nomena will occur as often as Saturn has the same longitude 

 with either node of his rings. Each side of these rings has 

 alternately fifteen years of sunshine and fifteen years of dark- 

 ness. A dark line has been seen in the outer ring, supposed 

 to indicate a subdivision. 



It is a singular result of theory, that the rings could not 

 maintain their stability of rotation if they were everywhere 

 of uniform thickness ; for the smallest disturbance would 

 destroy the equilibrium, which would become more and more 

 deranged, till, at last, they would be precipitated on the 

 surface of the planet. The rings of Saturn must therefore 



