120 PHILOSOPHICAL TRANSACTIONS. [aNNO 179'2- 



to the observations of the spots by which the rotation of the ring was deter- 

 mined. The spot called a, for instance, which has been observed to revolve 

 with great regularity through upwards of 300 periods, between the 28th of July 

 and the 24th of December, 1789, (Philos. Trans,, v. 80) was certainly situated 

 pretty near the outer edge. The spot |3, as may be gathered from the observa- 

 tion of the l6th of September, and 25th of December, was most likely on the 

 very edge itself: nor could the spot S be far from it. This, without considering 

 the situation of y and s, is quite sufficient to determine us to assign the period 

 we have given to belong to the large, thin and narrow, outward ring. 



The spots y and £ were probably at some distance from the outer edge of the 

 outer ring; but this distance might possibly not exceed that of the inside edge of 

 the same ring. We may however admit them to have adhered to the inner 

 ring, whose rotation is perhaps not very different from that of the outer one; or 

 we may examine whether tliese 2 spots may not perhaps agree to some other 

 supposed revolution of the inner ring; but then the observations that are given 

 of them will hardly be sufficient for establishing the time of that ring's rotation 

 with accuracy, though they undoubtedly must amount to a proof that it also 

 revolves with great velocity on its axis. 



That there should be a small difference in the periods of the rotation of the 2 

 rings, is highly probable from their different dimensions; and now, that the 

 rotation is known, the division of it into 2 parts seems to be a very natural con- 

 sequence of its construction. For when the extreme thinness is taken into con- 

 sideration, we find by Kepler's law, of the periods of revolving bodies placed at 

 different distances, that it would be very wonderful for so thin, and so broad a 

 plane, to have adhesion enough to keep together; and that consequently this 

 ring in its divided state, supposing the rotation of the parts to favour the con- 

 struction, is more permanent than it would be otherwise. This however is only 

 mentioned as a collateral circumstance, and by no means intended either as a 

 proof of the division, or the different rotation of the 2 parts of the ring. For 

 though we cannot but set the highest value on the excellent theories that have 

 been lately delivered in the memoirs of a learned society, we must refer entirely 

 to observation for the necessary data on which to found our subsequent 

 computations. 



The memoir here alluded to,* refers to observations of many divisions of the 

 ring of Saturn. This must lead us to consider the question, whether the con- 

 struction of this ring is of a nature so as permanently to remain in its present 

 state ? or whether it be liable to continual and frequent changes, in such a 

 manner as in the course of not many years, to be seen subdivided into narrow 

 slips, and then again as united into 1 or 2 circular planes only .'' Now, without 

 • See Histoire de I'Academie Royale des Sciences de Paris, 1787, P- 249. 



