The thickness of the rings around Saturn would seem to be very inconsiderable. This is plainly indicated by the circumstance that when the edge of the ring is turned towards the observer, it is generally found to be invisible, even when the most powerful tele scopes are directed towards it. From certain lucid protuberances which Sir William Herschel remarked upon the ring, lie was led to suspect that it had a rotary motion around the planet. A closer scrutiny of the phenomenon confirmed this impression, and he finally concluded that the ring effects a complete rotation around the planet in a period of 1011. 32m. 158.1.
On the 15th of November, 1850, Mr. Bond, Director of Harvard College Observatory, Cambridge, U.S., discovered a third ring around Saturn, situated between the interior of the two rings already dis covered, and the body of the planet. The same phenomenon was discovered by Mr. Dawes on the 19th of the same month, and, of course, before intelligence of Mr. Bond's discovery had reached Eng land. The new ring is much fainter than either' f the two exterior rings. Its breadth is Neal to about two-fifths of the interval between the interior bright ring and the planet.
It turned out, soon after the discovery of the faint ring by Bond and Dawes, that the phenomenon had been also seen by Dr. Gallo at the Berlin Observatory in the year I835; but its exact nature was not made out, and consequently, the circumstance did not receive any further at tent lon. It would seem, indeed, that Hadley observed the faint ring as early as the year 1723. In the course of his observations of the planet in that year, he remarked that the dusky line which in 1720 he observed to accompany the inner edge of the ring across the disc, continued close to the same, though the breadth of the ellipse had considerably increased since that time (' Phil. Trans.,' 1723, No. 378). He asserts. moreover, in reference to the hypothesis of the belt being produced by the shadow of the ring upon the planet, that when he considered the situation of the sun, with respect to the ring and the planet, he found that the belt could not arise from such a cause.
From an examination of the recorded measures of the rings of the extending from the time of I luyghens down to the present day, M. Otto Struve was induced to conclude that the rings are gradually approaching the body of the planet. Mr. Main, however, who recently subjected to a thorough discussion his own measures of the Saturnian system, has discovered no trace of the existence of such a movement.
The question with respect to the physical constitution of Saturn's rings, and the mechanical conditions which assure their stability, has naturally given rise to much speculation. Laplace, who investigated the question of stability, concluded that in order to prevent the rings from being precipitated on the body of the planet they must be irregular in their contour. Several succeeding astronomers and mathe maticians have considered the same subject. The most complete investigation of it is due to Professor Clerk Maxwell ; he concludes that the only system of rings which can exist mechanically is one com posed of an indefinite number of unconnected particles, revolving round the planet with different velocities according to their respective distances. These particles may be arranged, in series of narrow rings, or they may move through each other irregularly. In the first case the deatructiou of the system will be very slow ; in the second case it will be more rapid ; but there may be a tendency towards an arrange ment in narrow rings, which may retard the process.
We shall coucludo this article with referring to an admirable delineation of Saturn and his rings by Mr. De La Rue, founded on his own observations with his 13-inch reflector. Several valuable measures of the Saturnian system by Mr. Main, Mr. De Is Rue, Mr. Dawes, and other observers, will be found in the recent volumes of the Memoirs and Monthly Notices of the Royal Astronomical Society.. In the 28th volume of the Memoirs the reader will find an elaborate investigation of the orbits of the Satellites (except those of 3Iimas and Hyperion) by Captain Jacob.