118 HISTORY OF ASTRONOMY. 



the matter, of which the ring is composed, is in a solid 

 state, we may compute for any point on its surface the 

 sum of the attractions of the whole ring and of Saturn. 

 The centrifugal force generated by its rotation, may thus 

 be determined from the condition that the particle must 

 remain on the surface. Now, in the case of a solid ring, 

 particles on the inner and outer edges must have the 

 same period of rotation. This condition limits the 

 breadth of the ring ; for, if it be found necessary for the 

 inner and outer edges to have different times of rotation, 

 this can be accomplished only by a division of the ring 

 into two or more parts. From careful computations, he 

 has inferred the necessity of admitting a large number of 

 rings, provided they are solid. But there are numerous 

 objections to admitting a large number of small rings 

 near each other, and to avoid these difficulties, he adopts 

 the hypothesis of a fluid ring. If in its normal condition 

 the ring has but one division, as is commonly seen, 

 under peculiar circumstances it might be anticipated that 

 the preservation of their equilibrium would require a 

 separation in some regions of either the inner or outer 

 ring; this would explain the fact of occasional sub- 

 divisions being seen. Their being visible for but a short 

 time, and then disappearing, to the most powerful tele- 

 scope, is accounted for by the removal of the sources of 

 disturbance when the parts thrown off would re-unite. 



Professor Peirce has undertaken to show, from purely 

 mechanical considerations, that Saturn's ring can not be 



