"And even

such, that the centrifugal and other forces balance each other. should an accumulation of disturbances, of which the absence of inequalities lessens the probability, bring the rings together, the velocities at the point of contact will be very nearly equal, and the two will coalesce without disastrous consequences." "If, in its normal condition, the ring has but one division, as is commonly seen, under peculiar circumstances it might be anticipated that the preservation of their equilibrium would require a separation in some regions of either the inner or outer ring; this would explain the fact of occasional subdivisions being seen. Their being visible but for a short time, and then disappearing to the most powerful telescopes, is accounted for by the removal of the sources of disturbance, when the parts thrown off would reunite."

"Finally, a fluid ring, symmetrical in its dimensions, is not of necessity in a state of unstable equilibrium, with reference either to Saturn or the other rings."*

At the meeting at Cincinnati of the American Association for the Advancement of Science, Professor B. Peirce read a memoir on the constitution of Saturn's ring, containing the same general views which he submitted to the American Academy of Arts and Sciences at Boston, on the 15th of April, 1851. Mr. Peirce arrives at the same results by analysis as those which Mr. Bond had derived from observations, illustrated and combined by his own ingenious computations. Mr. Peirce differs in opinion from Laplace in regard to the efficacy of an irregular figure in sustaining Saturn's ring. He considers this statement of Laplace, which his successors have blindly adopted, as a careless suggestion, and not the ripened fruit of his usual rigid examination. "I maintain," he says, "unconditionally, that there is no conceivable form of irregularity, consistent with an actual ring, which would serve to retain it permanently about the primary, if it were solid." Any tendency of the ring to fall upon the planet appears as a repulsion between the centre of the ring and the centre of the planet. The rotation of the ring on its axis will prevent the irregularity of figure from sustaining the ring, because this protuberance will come as frequently to the place nearest to the planet as to that most remote from it; and the tendency to fall will be more increased in the first position than it is diminished in the last position. If the times of rotation and revolution were the same, the protuberance might remain permanently at the place most remote from the planet. But this position of the protuberance is an unstable one. The only case, therefore, in which an irregularity would help to support the ring is an impossible one. A number of protuberant spots will not help the matter. "In any case, the result is essentially the same, that they will not permanently support the ring, that a solid ring would soon be destroyed, and that Saturn's ring must, therefore, be fluid. It consists, in short, of a stream, or rather of streams, of a fluid somewhat denser than water, flowing around the planet." Mr. Peirce believes that his analysis will confirm and ↑ Ibid., II. 17.

* Astronomical Journal, II. 9, 10.

illustrate Mr. Bond's idea of the opening and closing of new spaces in the material of the ring, by which the discrepancies of observation are ingen iously reconciled. The maximum number of rings is fixed at twenty by an approximate calculation. The definite width of the separating rings finds some analogy in the constant size of liquid drops dripping from a vial.

The stability of the ring does not depend on the attraction exerted on it by the planet. In the circulation of the fluid annulus around Saturn, the velocity is least at the greatest distance. Hence the matter accumulates at the most remote point of the ring, and to such an extent that the quantity of matter balances the distance, and the attraction exerted by the ring and the planet on each other is the same in every direction. The ring is held together by the attraction of the primary; but it is not sustained as a whole by the primary. It is sustained by the satellites. The satellites disturb it, and sustain it by a delicate equipoise of disturbances. Something like this restorative action had been hinted at by Sir J. F. W. Herschel.* But the remedial power is insufficient to sustain a solid ring.. It follows that no planet can have a ring unless richly provided with menials to hold it. Saturn alone of all the planets seems competent to preserve a ring when once bestowed. The planets are not well arranged around the sun for sustain. ing a grand solar ring. The only chance for it is within Jupiter and Saturn. But here it would be greatly disturbed, and perhaps finally dashed up against Mars, and splintered into asteroids. "The orbits of planets, formed under such circumstances, must have been characterized by great eccentricity." Had the sun been surrounded by a ring of light material, such as the zodiacal light is supposed to be, and considerably inclined to the ecliptic, so as to be beyond the derangement of the planets, the centre of gravity of this ring would begin to move; and, as it moved, the matter would accumulate at the places in the ring most remote from the sun; and accumulate more and more, until the whole matter of the ring would be collected at one point just as the opposite point had approached so as to touch the sun, if any matter had been left at that part. "The experiment of Tantalas would have been performed upon a grand scale, and the ring would have been instantaneously transformed into a comet in its aphelion."

Mr. Peirce concludes his memoir with the following weighty paragraph : "Were the ring, however, supposed to be a large gaseous mass of a circular figure, the condensation which would occur at the point of aphelion might easily lead to chemical action. Precipitation might ensue, and the necessary consequence would seem to be a continually accelerated accumulation at this point, which would terminate in the formation of a planet. Under this modification, the nebular hypothesis may possibly be free from some of the objections with which it has been justly assailed. But in approaching the forbidden limits of human knowle e, it is becoming to tread with caution and circumspection. Man's speculations should be subdued from all rashness and extravagance in the immediate presence of the Creator. And † Astr. Journ., II. 19.

* Outlines, 320.

a wise philosophy will beware lest it strengthen the arms of atheism, by venturing too boldly into so remote and obscure a field of speculation as that of the mode of creation which was adopted by the Divine Geometer.” *

The planet Saturn, already eminently endowed, has acquired additional embellishment by the discovery of an eighth satellite. This discovery occurred during the season of careful examination to which the ring was subjected throughout its last series of disappearances. The Messrs. Bond of Cambridge noticed, on the evening of September 16th, 1848, a small star nearly in the plane of the ring. This object again attracted their attention on the 18th. But up to this moment, its real nature was scarcely suspected. It was seen on the 19th to partake of the retrograde motion of Saturn. On the 21st, it had changed its place sensibly, in reference to Saturn, and so also on the 22d and 23d. On the 25th, the discovery of a new satellite was announced by letter. It soon appeared that the same object had been seen by Mr. Lassell, ‡ near Liverpool, on the 18th. At that time he supposed it to be a fixed star. On the 19th he was struck with its motion, apparently towards the planet. "This suggested the idea that x must be a new satellite." "The conclusion is inevitable: z is a satellite hitherto undiscovered." This new satellite was seen by the American astronomers two days earlier than elsewhere. But its real character was recognized simultaneously and independently here and in England. The discoverers, on both sides the Atlantic, have united in recommending the name of Hyperion for the new satellite, and astronomers have been unanimous in approving of the name. This satellite is the seventh in order of distance from Saturn.

Much confusion has prevailed hitherto in distinguishing the individuals of the Saturnian system. One simple method would be, to number them in order, according to their distance from the primary, directly or inversely. Or astronomers might assign to them numbers according to the date of their discovery. The former plan renders astronomers liable to a change in some of the numbers whenever a new satellite is discovered. In fact, the rule of counting from the planet towards the outside was adopted when five satellites only were known. On the discovery of the two innermost satellites, some altered the old numbers so as to suit the old rule; others retained the old numbers for the old satellites, and, contrary to the rule, gave higher numbers to the two satellites nearest to Saturn. Thus Lamont § speaks of the satellite nearest but one to Saturn as the sixth. The designations of the satellites were at sixes and sevens, when Sir J. F. W. Herschel proposed to give the satellites of Saturn proper names, similar to those which describe the primaries. The general idea was not new. Simon Marius, who pretended to have discovered the moons of Jupiter, gave them the names of lo, Europa, Ganymede, and Callisto, "but for these mytho* p. 19. ↑ Mem. Amer. Acad., III. 289, 290. Compt Rend., XXVII. 341; XXVIII 322. : Phil Mag, XXXIII. 477. Compt. Rend., XXVII. 341.

§ Astr. Nachr., No. 316.

Results, &c., at Cape, p. 415.

logical designations, Galileo's nomenclature substituted the family names of the ruling house of Medici,-Catherina, Maria, Cosimo the elder, and Cosimo the younger." * A writer in the Literary Gazette furnished a list of classical names for the satellites of Jupiter, Saturn, and Uranus. The discovery of the eighth satellite between two of the old ones has aggravated the difficulty of numbering the satellites, and will contribute, no doubt, to establish in general use the names proposed by Herschel. In justification of his nomenclature he makes the following remarks. "As Saturn devoured his children, his family could not be assembled round him, so that the choice lay among his brothers and sisters, the Titans and Titanesses. The name Japetus seemed indicated by the obscurity and remoteness of the exterior satellite, Titan by the superior size of the Huyghenian, while the three female appellatives class together the three intermediate Cassinian satellites. The minute interior ones seemed appropriately characterized by a return to male appellatives, chosen from a younger and inferior (though still superhuman) brood. Should an eighth satellite exist, the confusion of the old nomenclature will become quite intolerable. I am not aware that a distant satellite of Jupiter (analogous to Japetus and our moon) has ever been looked for. Would it not be worth a search? "t

Names, &c., of the Satellites of Saturn.

When the brightest of all these satellites, called Saturn's guard, was discovered by C. Huyghens in 1655, he was so imprudent as to intimate that no unknown satellites now existed in the solar system, since their number already equalled that of the known planets, and both together are expressed by the excellent number twelve: "Quo majorem post hac repertum non iri, prope est ut confirmare audeam. Certe jam majoribus illis ac primariis, inter quos Tellus hæc reponenda est, æquales multitudine minores existunt, et utrique illo, quem perfectum dicimus, numero continentur, ut consilio summi opificis modus hic præfinitus videri possit."§ The discovery of other planets as well as other satellites has betrayed the weakness of the argument. Huyghens lived to see four more satellites of Saturn brought to

* Cosmos, 703. + Results, &c., p. 415.

The radius of Saturn is the unit.

$ "Christiani Hugenii Systema Saturnium, sive de Causis mirandorum Saturni Phenome non et Comite ejus Planeta novo." Haga-Comitis, 1659. Opera, I. pp. 530, 531.

light by Cassini. Thirty years after this unfortunate prediction, C. Huyghens, in a letter to his brother on the Planetary Worlds, speaks thus:“Nay, I am afraid there are one or two more still behind, and not without reason. For between the fourth and fifth [Titan and Japetus] there is a distance not at all proportional to that between all the others. Here, for aught I know, may lurk a sixth gentleman; or perhaps there may be another without the fifth [Japetus] that may yet have escaped us, for we can never see the fifth but in that part of his orbit which is towards the west.'

The distance between Hyperion and Japetus still remains disproportionately large, and the space between Rhea and Titan is abnormal. It is safer to predict that other satellites will be discovered, than to assert, as Huyghens and others have done, in the case of planets as well as satellites, that discovery has exhausted the bounty of Nature. But the most unwise thing of all is to set bounds to science, and give a reason for it; as Harris did in 1729, when he predicted that no other would be added to the five satellites then seen to circulate round Saturn, "for I don't think our telescopes will be much further improved." A positive prediction is sometimes instantly and startlingly fulfilled, or it not now, there always lives the hope that it may be hereafter. A negative prediction is never fulfilled while time lasts, and it may at any moment mortify by its failure the vanity of him who made it. The world cherishes as its greatest benefactors, not those who are the harbingers of misfortune, so much as those who hold high the lamp of hope. The history of Saturn's satellites assures us how little reliance is to be placed on the fancied analogies of distance in the various members of the same system; harmonies, not very accordant at best, and liable at any moment to be turned to discord by the first sudden twinkling in the eye of some new star.

The planet Uranus was discovered by Sir William Herschel, on the 13th of March, 1781. It was at first supposed to be a comet; a year, however, did not pass before its true character was known. The attention of the discoverer was first drawn to it, not by its motion, which is sensible even from night to night, but by its disk. An examination of the star-catalogues, at the instigation of Bode, led to the discovery of two early observations of Uranus while yet considered a star, one by Flamsted in 1690, the other by Tobias Mayer in 1756. By means of these, with the observations assiduously made on the planet after the discovery of its true character by Herschel, an orbit was calculated which continued to represent faithfully the planet's path until 1788. In that year Lemonnier ascertained that he had observed Uranus as a star in 1764 and 1769. And in all, nineteen or twenty similar observations have been recovered, which are distinguished from ob

* Smyth's Cycle, I. 197. "Christiani Hugenii Cosmotheoros, sive de terris celestibus, earumque ornatu, conjecturæ ad Constantinum Hugenium fratrem, Gulielmo III. Magna Britanniæ Regi a secretis." Lib. II. Opera, I. p. 698.