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112 of Saturn's own satellites. For when we calculate the problem, we find that Mimas, Enceladus, and Tethys have periods exactly commensurate with the divisions of the rings; in other words, these three inner satellites, whose action because of proximity is the greatest, have fashioned the rings into the three parts we know, called A, the outermost; B, the middle one; and C, the crepe ring, nearest to the body of the planet. Mimas has been the chief actor, though helped by the two others, while Enceladus has further subdivided ring A by what is known as Encke's division.

Such has been the chief action of the satellites on the rings: it has made them into the system we see. But if we consider the matter, we shall realize that a secondary result must have ensued—when we remember that the particles composing the rings must be very crowded for the rings to show as bright as they do, and also that, though relatively thin, the rings are nevertheless some eighty miles through.

Now it is evident that any disturbance in so closely packed a system of small bodies as that constituting Saturn's rings must result in collisions between the bodies concerned. Particles pulled out or in must come in contact with others pursuing their own paths, and as at each collision some energy is lost by the blow, a general falling in toward the planet results. At the same time, as the blow will not usually be exactly