the slightest disturbance would bring the ball in contact with the rings, just as an egg-shell floating in the middle of a tea-cup is in a state of unstable equilibrium from capillary attraction, so that the slightest disturbance of its central position brings it in contact with the sides of the cup. The satellites exercise such a disturbing force, so that if the movement once began, it would necessarily continue till the contact was effected; and, if the rings once touched the planet, they would be in a state of stable equilibrium, so that they would never rise from it. It was not a satisfactory reply that the rings rotated in the same time that a satellite, at the same distance, would. A satellite, making the appropriate number of revolutions in a given time, is in no danger of falling upon the primary, but the case is totally different with a ring. Should a comet come into collision with a satellite, and throw it a little out of its course, no further harm would result. The disturbance would not be progressive. The orbit might be altered, but then it would continue permanent in that altered course. If a comet, on the other hand, struck a ring so as to bring one side nearer the ball than the other, the motion would be progressive. It would be like that of a stone moved from the top of a hill, which does not stop till it reaches the bottom. The ring, once pushed by a satellite, virtually starts down the hill till it reaches the body of the planet, and it can no more rise from its prostrate position, than a stone can of itself move
