is in this point of view, also, that it furnishes a link between the comets and the planets. The planet occupies the one pole of permanency, while the comet represents the opposite, of mutability; but Saturn occupies a middle position, exhibiting great organic changes, the progress of which is slow, compared to that of the comet. The gradual expansion of the rings towards the body of the planet, has been observed. The rings may be regarded as the iris of the eye; and as the iris contracts the pupil, so the rings close in upon the planet. This closing in is not effected by the contraction of the outer circumference of the system of rings, but only by the increase of the breadth of the rings. The strip of tissue paper, to which we have compared the system, is becoming broader, and probably thinner, but the outer edge remains of the same length. There is a process of stretching-out going on, but it is not the same for the whole breadth of the ring. The parts nearest the planet expand more than those more remote. This result is due to the researches of M. Otto Strüve, who has examined the measurements of the rings for two hundred years back, and by comparing them with those taken at the present day, he has been able to compute the rate of growth. He .calculates that, in about one hundred and twenty years, the inner edge of the system of rings will reach the planet. This conclusion has been questioned by Mr Main of the Royal Observatory, Green-
