CONTRIBUTIONS TO SCIENCE. 505 



The Examiners give notice, that the following is the subject 

 for the prize to be adjudged in 1857 : 



The Motions of Saturn's Rings. 



* # * The Problem may be treated on the supposition that 

 the system of Kings is exactly, or very approximately, concentric 

 with Saturn, and symmetrically disposed about the plane of his 

 equator, and different hypotheses may be made respecting the 

 physical constitution of the Rings. It may be supposed (1) that 

 they are rigid ; (2) that they are fluid, or in part aeriform ; (3) 

 that they consist of masses of matter not mutually coherent. 

 The question will be considered to be answered by ascertaining, 

 on these hypotheses severally, whether the conditions of mechan- 

 ical stability are satisfied by the mutual attractions and motions 

 of the Planet and the Rings. 



It is desirable that an attempt should also be made to 

 determine on which of the above hypotheses the appearances both 

 of the bright rings and the recently discovered dark ring may be 

 most satisfactorily explained ; and to indicate any causes to which 

 a change of form, such as is supposed from a comparison of 

 modern with the earlier observations to have taken place, may 

 be attributed. 



In his essay, to which the prize was adjudged, and which 

 occupies sixty-eight pages of quarto, Maxwell considered in 

 the first place the hypothesis of Laplace, and showed that to 

 secure stability the irregularity of the ring must be enor- 

 mously great. Speaking of Laplace's conclusion that the 

 rings must be irregular, Maxwell says : 



We may draw the conclusion more formally as follows : 

 If the rings were solid and uniform, their motion would be 

 unstable, and they would be destroyed ; but they are not de- 

 stroyed, and their motion is stable, therefore they are either not 

 uniform or not solid. 



I have not discovered, either in the works of Laplace or in 

 those of more recent mathematicians, any investigation of the 

 motion of a ring either not uniform or not solid. So that, in the 

 present state of mechanical science, we do not know whether an 

 irregular solid ring, or a fluid or disconnected ring, can revolve 

 permanently about a central body ; and the Saturnian system 

 still remains an unregarded witness in heaven to some necessary 

 but as yet unknown development of the laws of the universe. 



