SECT. IX. SATURN AND HIS RINGS. 67 



fluid ; even the luminous rings cannot be very dense, since the 

 density of Saturn himself is known to be less than the eighth 

 part of that of the earth. A transit of the ring across a star 

 might reveal something concerning this wonderful object. The 

 ball of Saturn is striped by belts of different colours. At the 

 time of these observations the part above the ring was bright 

 white ; at his equator there was a ruddy belt divided in two, 

 above which were belts of a bluish green alternately dark and 

 light, while at the pole there was a circular space of a pale colour. 

 (See Plate 2.) The mean distance of the interior part of the 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 impos- 

 sible that this body can retain its position by the adhesion 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 W 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 beginning 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 phenomena 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 darkness. 



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, 



