7 
the Ring of Saturn. 
That there should be a small difference in the periods of the 
rotation of the two rings, is highly probable from their dif- 
ferent dimensions; and now, that the rotation is known, the 
division of it into two parts seems to be a very natural conse- 
quence of its construction. For, when the extreme thinness 
is taken into consideration, 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 conse- 
quently this ring in its divided state, supposing the rotation of 
the parts to favour the construction, 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 two parts 
of the ring. For, notwithstanding we cannot but set the 
highest value upon the excellent theories that have been lately 
delivered in the Memoirs of a learned Society, of which I also 
have the honour to be a member, we must refer entirely to 
observation for the necessary data on which to found our sub- 
sequent computations. 
The memoir to which I allude * refers to observations of 
many divisions of the ring of Saturn. This must lead us to 
consider the question, whether the construction 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 
one or two circular planes only ? Now, without entering into 
a discussion, the mind seems to revolt, even at first sight, 
* See Histoire de V Academie Roy ale des Sciences de Paris, 1787, page 249. 
