of detection, and can only be seen with telescopes of the first quality and on nights when the atmospheric conditions are very favourable. Other traces of division have not continued to be recognizable, and are therefore probably not permanent. It has been remarked that "if each division thus detected were considered as a satisfactory indication of a permanent division through a complete circumference, it would follow that the system consists not of two or three, but rather of thirty or forty concentric rings. Strange as such a conclusion might appear, and manifold as are the conditions of instability the complexity of such a system would introduce, we should have no resource (on the assumption of the solidity of the rings) but either to accept this solution of the question, or to reject the testimony of most accurate and skilful observers-of such men as Encke, the Struves, Captain Kater and Jacob, Mr. Dawes, and the astronomers of the Collegio Romano. The telescopes also through which such divisions have been repeatedly seen, have been among the most celebrated instruments of modern times."

But the most singular discovery yet made respecting this remarkable ring-system remains to be described. On November 15th, 1850, Bond, of America, discovered a dark ring inside the inner bright one; and a few days later Dawes, in England, independently discovered this ring. The colour of the dark ring is a deep purple; or rather, since, as we shall presently see, the ring is semi-transparent, and therefore a portion of its apparent colour is that which it transmits, we may say, without committing ourselves to

any theory as to the true seat of the colour, that the region occupied by the ring presents a deep purple colour. The nearer part of the dark ring can be traced over the disc of the planet, but the outline of the planet can be recognized through the ring. This portion of the ring does not shew a purple tinge, but has been compared to a crape veil. A division has at times appeared in the dark ring, which also appears at times to be separated from the neighbouring bright ring.

Professor Bond, of America, noticed at about the same time a very remarkable darkening of the inner bright ring, on its inner edge, close to the dark ring. The peculiarity about this darkening was, that instead of being exactly similar in shape to the outlines of the several rings, its outline formed a longer oval, as though the darkened part were wider in those places which lie upon the seeming longer axis of the rings. If the rings were really oval, as they appear through the effects of foreshortening, this peculiarity would be explicable; but as the rings are circular, and every part in turn comes into the position indicated, for the rings rotate and moreover Saturn himself carries them into varying positions as seen from the Earth, the appearance is altogether inexplicable as a mere shade or darkening. If, between the bright ring and the dark ring, there were a ring of an intermediate tint, that ring, being concentric with the others (else it could not rotate), would present similar outlines, whether the whole system were more or less foreshortened. This not being the case, one outline of the darkened part being always more

elliptical than the other, we must, in explaining the darkening, find an interpretation which will explain this peculiarity. I believe the explanation enforced upon us by this consideration, is simply that the inner part of the bright ring is transparent, or rather, that we can see through this part, for, as will presently appear, we have no reason for believing that the actual substance of any part of the ring-system is transparent in the same sense that glass or crystal is transparent. I have shewn in my treatise on Saturn that if the inner part of the bright ring consists of a multitude of concentric zones between which the dark sky beyond can be seen from our terrestial station, the observed appearances would necessarily be seen. The explanation, to be adequately understood, requires such an illustration as is given in the ninth plate of that work; but its general principle may be understood by any one accustomed to drawing, who will attend to the following description. Suppose several white hoops to be lying one within the other (and quite concentrically) on a dark flat surface; the hoops being not shaped like those used by girls, but like the iron hoops which boys use. (A piece of cane bent into a circle would make such a hoop.) Now, if we looked at such a set of hoops from above, we should see so many white concentric circles on a dark ground. But let the point of view be not directly above, so that we look slantwise at the set of hoops. Then we shall see a number of similar white ovals on a dark ground. Now, if these ovals were mere oval lines, that is if the hoops were mere threads, the dark

spaces between them would seem to grow narrow where the ovals were narrowed, and in just the same degree; but as the ovals are formed by stout hoops the case is altered, the dark spaces are more narrowed where the ovals are flattened. Nay, if the hoops are pretty close to each other, a very little foreshortening will make them seem actually to touch where the ovals are flattened, while where the ovals are lengthened out the dark background is visible. (If the reader will draw such a set of hoops, as they would actually be seen, he will at once perceive that this is so.) Now, without supposing that the rings of Saturn are composed of such hoops, I find myself led to the conclusion that the matter forming the rings runs into hooplike shapes, and that where Bond saw the darkening, above described, these hoop-formed portions were far enough apart to let the dark sky beyond be seen where the ovals of the rings were most lengthened, the spaces closing up by foreshortening where these ovals were most narrowed. It is demonstrable, indeed, that, if the appearance observed by Professor Bond was not a mere illusion (which in the case of so practised an observer is altogether unlikely), it can be explained in this way and no other. So that the peculiar darkening seen by Bond is an independent proof of the multiple nature of the ring-system-a proof as complete as though Bond's telescope had been powerful enough to reveal the several rings forming this part of the bright inner ring.

Saturn's ring-system, regarded as a whole, is a structure so remarkable that we seem invited to consider it with a

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special degree of attention, in order that if possible we may form some idea of its real nature. If there were no other circumstance remarkable about it but its mere vastness, it would even then be well deserving of our closest scrutiny. But that a ring system so symmetrical and beautiful should girdle a planet completely about, that it should accompany the planet on its path around the Sun, that it should be as definite a portion of the planet's system as the belts on the planet's real globe, or as the satellites which circle about that orb, these are circumstances which render the study of the ring-system specially interesting. For we cannot but recognize the fact that such a system, swayed as it must be by the mighty attraction of Saturn's mass, must present a number of relations of a very complex and perplexing kind.

Let us in the first place briefly consider the dimensions of the rings and of the globe round which they circle.

Saturn's globe is somewhat compressed, its greatest diameter being 73,000 miles, its least diameter 66,000 miles. Remembering that the Earth's mean diameter is about 7900 miles, we see how vastly the planet exceeds the Earth in volume. Roughly the volume of Saturn is seven hundred times that of the Earth, his mass about ninety times the Earth's, for the materials of which he is constructed are on the average of much smaller specific gravity than those forming the Earth's globe.

The utmost span of the rings-that is, the diameter of the outer boundary of the outer ring-is about 167,000 miles, the breadth of this ring about 10,000 miles. The