Page:The evolution of worlds - Lowell.djvu/143

Rh new phenomenon detected there, that of a dark core in the shadow the rings threw across the planet's face, an explanation suggested itself to account for both them and it: to wit, that the rings were not really flat, but tores; rings, that is, like an anchor ring, any cross-section of which would be of the nature of an oval flattened on its inner side. The cogency of the explanation consisted in its solution not only of the appearances but of the cause competent to bring those appearances about.

For measurement showed that the knots were permanent in position, which, since the ring revolved, indicated that they extended all round it in spite of their not seeming to do so, and that their distances from Saturn were just what this cause should produce.

The action observed was a corollary from the important principle of commensurability of orbital period. As we saw in the case of the asteroids, if two bodies be travelling round a third and their respective periods of revolution be commensurate, they will constantly meet one another in such a manner that great perturbation will ensue and the bodies be thrown out of commensurability of period.

What has happened to the asteroids has likewise occurred in Saturn's rings. The disturber in this case has been, not Jupiter, as with them, but one or other