028 James Elliot on certain 



difficulty from which it has never yet been satisfactorily freed. 

 It is agreed on all hands that the assumption and main- 

 tenance of the annular form is due to the centrifugal force 

 arising from its rotation; but the difficulty is of another kind; it 

 has arisen from a supposed demonstration by Laplace, in which, 

 as far as I am aware, all other astronomers have acquiesced — 

 that a uniform ring, revolving round a centre of attraction, will 

 be in equilibrium only when the attracting centre coincides ma- 

 thematically with the centre of the ring, — that, consequently, 

 the equilibrium is unstable ; so that, if either the attracting 

 object or the ring be displaced in the least, they will inevi- 

 tably approach each other till they come into collision. But, 

 though the planet Saturn w r ere poised, with mathematical 

 accuracy, in the centre of his ring (a circumstance without a 

 parallel in astronomy), the nice adjustment would not con- 

 tinue a single day. for it would be immediately disturbed by 

 the varying influence of the other planets and of its own sa- 

 tellites. And not only are there abundant and constant 

 causes to disturb that adjustment, if it existed, but it has been 

 shown, as Sir John Herschel states, " by recent micrometri- 

 cal measurements of extreme delicacy, that no such adjust- 

 ment exists, but that the centre of the rings oscillates round 

 that of the body, describing a very minute orbit." 



If, then, according to Laplace, there is no stability in the 

 equilibrium of a uniform ring, it follows that, unless there 

 were some preserving contrivance — some counteracting cir- 

 cumstance, that beautiful mechanism would inevitably fall to 

 pieces. For this purpose, Laplace has recourse to the expe- 

 dient of supposing that the ring must be loaded on one side. 

 That load, having of itself a tendency to describe an elliptic 

 orbit round the planet, like a satellite, will drag the rest of 

 the ring with it. The motion will thus belong to the load, the 

 ring, large as it is, being merely its encumbrance. 



To that hypothesis, there are serious objections. In the 

 first place, the load is almost hypothetical ; for although 

 some slight apparent inequality may be observed in the 

 different parts of the ring, yet nothing to justify Laplace's 

 idea, — nothing which could be regarded as sufficient to bring 

 about the result on which he calculates. In the second 



