412 SCIENTIFIC RECORD FOR 1885. 



Saturn : The density of Saturn's ring. — " M. PoiDcar6 supplies a short 

 note on tbe stability of Saturn's ring in the IsTovember number of the 

 Bulletin Astronomique. Laplace had shown that the ring could only be 

 stable if it were divided into several concentric rings revolving at dif- 

 ferent speeds. M. Tisserand had confirmed this result, and had recog- 

 nized that a single ring must, in order to exist, possess a much higher 

 density than the planet, and had calculated the maximum breadth of 

 each elementary ring in terms of its density and mean radius. M. 

 Poincar^ has carried this investigation a step further, and shown that 

 if the density of a ring be less than a certain amount, it will, under the 

 influence of the slightest perturbation, no longer break up into a num- 

 ber of narrower rings, but into a great number of satellites, and that 

 if the rings be fluid and turn each as a single piece, the density of the 

 inner ring must be at least |, and of the outer ring ^ that of the planet. 

 For a ring of very small satellites (not for a fluid ring, as M. Poincar^ 

 erroneously states), Maxwell has shown the condition to be that the 

 density should not exceed 3-^ part of that of Saturn. 



" We do not at present know the actual density of the ring from ob- 

 servation sufBciently accurately to make therefrom any certain infer- 

 ence as to its physical condition. Bessel's determination from the 

 movement of the peri-saturnium of the orbit of Titan gave the reciprocal 

 of the mass of the ring as compared with that of Saturn as 118, which 

 since the A'olume of the ring — adopting Bond's value of 40 miles for its 

 thickness — is about 4^ that of the planet, would make its density about 

 3-4 times greater than the planet's. Bessel's value is, however, clearly 

 too great, as he neglected the influence of the equatorial protuberance 

 of Saturn on the movement of the apsides. Meyer's determination of 

 the secular variation of the line of apsides of Titan, viz d7r=n26"-5, 

 gives the reciprocal of the mass of the ring as 26,700, but from all the 

 six brighter satellites as 1,960 ; the latter value closely agreeing with 

 Tisserand's. It does not, however, seem to have been noticed that even 

 the smallest value for the mass considerably'^ exceeds the highest per- 

 missible in accordance Avith Maxwell's result, since that would make 

 the mass of the rings only ^ ^ q^q q ^ part of the i)lanet's, an amount we 

 cannot hope to detect with our present resources." {JSTature, January 

 28, 1886, vol.33: p. 303.) 



Sur la Variahilite des Anneaux de Saturne. — " Observers have as yet 

 discovered no fact that could lead them to suppose the rings of Saturn to 

 have as a whole a rotary motion about the planet. Now if such a motion 

 exists, it is inconceivable that it should have th^^s far escaped observa- 

 tion. But if these rings do not revolve about the planet, we cannot 

 admit that they are solid, liquid, or even gaseous, for it is evident that 

 such frail structures could not resist the force exercised upon them by 

 the constantly variable attraction in the direction of the satellites. If 

 it is true that without rotation the rings could be neither solid, liquid, 

 nor gaseous, we must seek some explanation that shall satisfy the neces- 



