described by the innumerable disconnected masses of which the curious appendage is now generally believed to be composed. The opening and closing of its chasms, and other observed signs of its restless character, are calculated to give the impression that the extent to which it is altered during many years must be considerable. But, considering the conditions necessary for the phenomena it exhibits, it would seem that the rapidity with which its changes proceed might be approximately determined from the amount of heat which it radiates into space, and which must have been, for the most part, produced at the expense of its motion. Now, though the rings may be principally composed of water, and have a superficial temperature near the freezing-point, yet, their surfaces being over a hundred times as extensive as that of the earth, they may be reasonably supposed to lose by radiation about thirty times as much heat as our planet receives from the sun and allows to escape into space. Taking Bessel's estimate for the mass of the double girdle, it will be found that such an amount of heat might be generated by the conflicting movements of its parts without reducing their orbits more than one per cent, in ten thousand years. There is, indeed, reason to believe that, in this case, Bessel's results are unreliable, in consequence of the uncertain and defective character of the data with which they were obtained. If we assign to each ring the probable amount of matter in the neighboring moons of the gigantic planet, it would seem that their permanent change of size may be so considerable that it might be detected by the observations of a few centuries.
It is likely that, in the inner ring, especially at the zone nearest to the primary, the temperature is much higher than that which I have supposed, and that alterations in its condition might proceed at a rate sufficiently rapid to be discoverable by the telescope. More than twenty years ago Otto Struve, having carefully compared observations since the time of Huygens, announced as the result of his labors that the inner ring is changing its dimensions so rapidly that before two hundred years it will be united to the planet. Other astronomers have expressed a belief of the recent origin and of the mutable nature of the obscure or vapor ring which lies closer to Saturn. The conclusions of Struve, however, have been disputed; and indeed it is probable that they give an exaggerated picture of the transitory state of things in the Saturnian dominions; nor can the conflict of opinions on this point be settled by observation alone. But, though taking place too slowly to be at once detected in this way, the changes in question are still inevitable; and they give safe ground for tracing the history of past events in this part of the celestial regions. It is evident that the matter composing Saturn's wonderful appendage must have once moved in a wider zone, where it could exist only in the form of two secondary planets. I have shown in a previous article that a dismemberment and a conversion into a ring must be the general fate of every planetary body which, by a slow contraction of its orbit, revolves at last too close
