The discovery of this singular appendage, an object unique in the solar system, naturally attracted fresh attention to the question of the stability of the rings. The idea was thrown out by the elder Bond that the new ring may be fluid, or even that the whole ring-system may be fluid, and the dark ring simply thinner than the rest. It was thought possible that the ring-system is of the nature of a vast ocean, whose waves are steadily advancing upon the planet's globe. The mathematical investigation of the subject was also resumed by Professor Benjamin Pierce, of Harvard, and it was satisfactorily demonstrated that the stability of a system of actual rings of solid matter required so nice an adjustment of so many narrow rings as to render the system far more complex than even Laplace had supposed. 'A stable formation can,' he said, 'be nothing other than a very great number of separate narrow rigid rings, each revolving with its proper relative velocity.' As was well remarked by the late Professor Nichol, 'If this arrangement or anything like it were real, how many new conditions of instability do we introduce. Observation tells us that the division between such rings must be extremely narrow, so that the slightest disturbance by external or internal causes would cause one ring to impinge upon another; and we should thus have the seed of perpetual catastrophes.' Nor would such a constitution protect the system against dissolution. 'There is no escape from the difficulties, therefore, but through the final rejection of the idea that Saturn's rings are rigid or in any sense a solid formation.' The idea that the ring-system may be fluid came naturally next under mathematical scrutiny. Strangely enough, the physical objections to the theory of fluidity appear to have been entirely overlooked. Before we could accept such a theory, we must admit the existence of elements differing entirely from those with which we are familiar. No fluid known to us could retain the form of the rings of Saturn under the conditions to which they are exposed. But the mathematical examination of the subject disposed so thoroughly of the theory that the rings can consist of continuous fluid masses, that we need not now discuss the physical objections to the theory.