ON THE STABILITY OF THE MOTION OF SATURN'S RINGS. 373 



Taking the first case, we found that in an indefinite number of possible 

 cases the mutual perturbations of two rings, stable in themselves, might mount 

 up in time to a destructive magnitude, and that such cases must continually 

 occur in an extensive system like that of Saturn, the only retarding cause being 

 the possible irregularity of the rings. 



The result of long-continued disturbance was found to be the spreading 

 out of the rings in breadth, the outer rings pressing outwards, while the inner 

 rings press inwards. 



The final result, therefore, of the mechanical theory is, that the only system 

 of rings which can exist is one composed of an indefinite number of unconnected 

 particles, revolving round the planet with different velocities according to their 

 respective distances. These particles may be arranged in 'series of narrow rings, 

 or they may move through each other irregularly. In the first case the destruc- 

 tion of the system will be very slow, in the second case it will be more rapid, 

 but there may be a tendency towards an arrangement in narrow rings, which 

 may retard the process. 



We are not able to ascertain by observation the constitution of the two 

 outer divisions of the system of rings, but the inner ring is certainly transparent, 

 for the limb of Saturn has been observed through it. It is also certain, that 

 though the space occupied by the ring is transparent, it is not through the 

 material parts of it that Saturn was seen, for his limb was observed without 

 distortion ; which shows that there was no refraction, and therefore that the 

 rays did not pass through a medium at all, but between the solid or liquid 

 particles of which the ring is composed. Here then we have an optical argument 

 in favour of the theory of independent particles as the material of the rings. 

 The two outer rings may be of the same nature, but not so exceedingly rare 

 that a ray of light can pass through their whole thickness without encountering 

 one of the particles. 



Finally, the two outer rings have been observed for 200 years, and it appears, 

 from the careful analysis of all the observations by M. Struve, that the second 

 ring is broader than when first observed, and that its inner edge is nearer the 

 planet than formerly. The inner ring also is suspected to be approaching the 

 planet ever since its discovery in 1850. These appearances seem to indicate 

 the same slow progress of the rings towards separation which we found to be 

 the result of theory, and the remark, that the inner edge of the inner ring is 



