442. Stability of the rings.-One of the circumstances attending this planet, which has excited most general astonishment, is the fact that the globe of the planet, and two, not to say more, stupendous rings, carried round the sun with a velocity of 22,000 miles an hour, subject to a periodical variation not inconsiderable, due to the varying distance of the planet from the sun, should nevertheless maintain their relative position for countiess ages undisturbed, the globe of the planet remaining still poised in the middle of the rings, and the rings, two or several, as the case may be, remaining one within the other without material connection or apparent contact, no one of the parts of this most marvellous combination having ever gained or lost ground upon the other, and no apparent approach to collision having taken place, notwithstanding innumerable disturbing actions of bodies external to them.

443. Cause assigned for this stability.—The happy thought of bringing the rings under the common law of gravitation, which gives stability to satellites, has supplied a striking and beautiful solution for this question. The manner in which the attraction of gravitation, combined with centrifugal force, causes the moon to keep revolving round the earth without falling down upon it by its gravity on the one hand, or receding indefinitely from it by the centrifugal force on the other, is well understood. In virtue of the equality of these forces, the moon keeps continually at the same mean distance from the earth while it accompanies the earth round the sun. Now it would be easy to suppose another moon revolving by the same law of attraction at the same distance from the earth. It would revolve in the same time, and with the same velocity, as the first. We may extend the supposition with equal facility to three, four, or a hundred moons, at the same distance. Nay, we may suppose as many moons placed at the same distance round the earth as would complete the circle, so as to form a ring of moons touching each other. They would still move in the same manner and with the same velocity as the single

moon.

If such a ring of moons were beaten out into the thin broad flat rings which actually surround Saturn, the circumstances would be somewhat changed, inasmuch as the periods of each concentric zone would vary in a certain ratio, depending on its distance from the centre of Saturn, so that each such zone would have to revolve more rapidly than those within it, and less rapidly than those outside it. But if the entire mass were coherent, as the component parts of a solid body are, the complete ring might revolve in a periodic time less than that due to its exterior, and longer than that due to its interior parts. In fact,

the period of its revolution would be the period due to a certain zone lying near the middle of its breadth, exactly as the time of oscillation of a compound pendulum is that which is proper to the centre of oscillation (M. 506). Indeed, the case of the oscillation of a pendulum, and the conditions which determine the centre of oscillation, afford a very striking illustration of the physical phenomena here contemplated.

444. Rotation of the rings. Now the observations of Sir William Herschel on certain appearances upon the surface of the rings, led to the discovery that they actually have a revolution round their common centre and in their own plane, and that the time of such revolution is very nearly equal to the periodic time of a satellite whose distance from the centre of the planet would be equal to that of the middle point of the breadth of the ings.

But if the principles above explained be admitted, it would follow that each of the concentric zones into which the ring is divided would have a different time of revolution, just as satellites at different distances have different periodic times; and it is extremely probable that such may be the case, because no observations hitherto made afford results sufficiently exact and conclusive as to either establish or overturn such an hypothesis. It appears, therefore, that the stability of the rings is explicable upon the same principle as the stability of a satellite.

445. Excentricity of the rings. The fact that the system of rings is not concentrical with the planet resulted from some observations made by Messrs. Harding and Schwabe; after which the subject was taken up by Professor Struve, who, by delicate micrometric observations and measurements executed with the great Dorpat instrument, fully established the fact, that the centre of the rings moves in a small orbit round the centre of the planet, being carried round by the rotation of the rings.

446. Arguments for the stability founded on the excentricity. Sir John Herschel has indicated, in this deviation of the centre of the rings from the centre of the planet, another source of the stability of the Saturnian system. If the rings were "mathematically perfect in their circular form, and exactly concentric with the planet, it is demonstrable that they would form (in spite of their centrifugal force) a system in a state of unstable equilibrium, which the slightest external power would subvert — not by causing a rupture in the substance of the rings, but by precipitating them, unbroken, on the surface of the planet. For the attraction of such a ring or rings on a point or sphere excentrically situate within them is not the same in all directions, but tends to draw the point or sphere toward the nearest part of the

ring, or away from the centre. Hence, supposing the body to become, from any cause, ever so little excentric to the ring, the tendency of their mutual gravity is, not to correct but to increase this excentricity, and to bring the nearest parts of them together. Now, external powers, capable of producing such excentricity, exist in the attractions of the satellites: and in order that the system may be stable, and possess within itself a power of resisting the first inroads of such a tendency, while yet nascent and feeble, and opposing them by an opposite or maintaining power, it has been shown that it is sufficient to admit the rings to be loaded in some part of their circumference, either by some minute inequality of thickness, or by some portions being denser than others. Such a load would give to the whole ring to which it was attached somewhat of the character of a heavy and sluggish satellite, maintaining itself in an orbit with a certain energy sufficient to overcome minute causes of disturbance, and establish an average bearing on its centre. But even without supposing the existence of any such load — of which, after all, we have no proof- and granting, therefore, in its full extent, the general instability of the equilibrium, we think we perceive, in the periodicity of all the causes of disturbance, a sufficient guarantee of its preservation. However homely be the illustration, we can conceive nothing more apt in every way to give a general conception of this maintenance of equilibrium, under a constant tendency to subversion, than the mode in which a practised hand will sustain a long pole in a perpendicular position resting on the finger, by a continual and almost imperceptible variation of the point of support. Be that, however, as it may, the observed oscillation of the centres of the rings about that of the planet is in itself the evidence of a perpetual contest between conservative and destructive powers both extremely feeble, but so antagonising one another as to prevent the latter from ever acquiring an uncontrollable ascendency, and rushing to a catastrophe."

Sir. J. Herschel further observes, that since "the least difference of velocity between the planet and the rings must infallibly precipitate the one upon the other, never more to separate (for, once in contact, they would attain a position of stable equilibrium, and be held together ever after by an immense force), it follows either that their motions in their common orbit round the sun must have been adjusted to each other by an external power with the minutest precision, or that the rings must have been formed about the planet while subject to their common orbital motion, and under the full and free influence of all the acting forces.”

The rings must obviously form a most remarkable object in the

firmament to observers stationed on Saturn, and must play an important part in their uranography. The problem to determine their apparent magnitude, form, and position, in relation to the fixed stars, the sun, and Saturnian moons, has, therefore, been regarded as a question of interesting speculation, if not of great scientific importance. The subject has, accordingly, more or less engaged the attention of astronomers. The conclusion, however, at which they have arrived, and the views which have been generally expressed and adopted respecting it, are open to considerable doubt, if not altogether erroneous. It is not the object of this work to enter controversially on any disputed part of astronomical science, we must therefore leave the subject in the hands of those who are interested in a subject, which to say the least, may be considered speculative.

447. Satellites.— Saturn is attended by eight satellites, seven of which move in orbits whose planes coincide very nearly with that of the equator of the planet, and therefore with the plane of the rings. The orbit of the remaining satellite, which is the most distant, is inclined to the equator of the planet at an angle of about 12° 14', and to the plane of the planet's orbit at nearly the same angle.

448. Their nomenclature. In the designations of the satellites, much confusion has arisen from the disagreement of astronomers as to the principle upon which the numerical order of the satellites should be determined. Some name them first, second, third, &c., in the order of their discovery; while others designate them in the order of their distances from Saturn. It has been proposed to remove all confusion, by giving them names, taken, like those of the planets, from the heathen divinities. The following metrical arrangement of these names, in the order of their distances, proceeding from the most distant inwards, has been proposed, as affording an artificial aid to the memory:—

Iapetus, Titan; Rhea, Dione, Tethyst;
Enceladus, Mimas

449. Order of their discovery.- Since this was suggested, the eighth satellite situate between Iapetus and Titan has been discovered, and called Hyperion.

These prevailing errors respecting the uranography of Saturn, form the materials of a long and interesting paper by Dr. Lardner, published in the Memoirs of the Royal Astronomical Society, Vol. XXII. Those who feel interested in the consideration of this subject may consult this memoir with advantage. E. D.

Pronounced Tethys.

The order of their discovery was as follows:

Hyperion was discovered on the same night, the 19th of September, 1848, by Mr. Lassell of Liverpool, and Professor W. C. Bond of the University of Cambridge in the United States.

450. Their distances and periods.— The periodic times and mean distances of these bodies from the centre of Saturn, ascertained by the same kind of observations as already explained in the case of the satellites of Jupiter, are as follows:

-The greatest

451. Elongations and relative distances. elongations of the satellites from the primary, and the scale of their distances in relation to the diameters of the planet and its rings, are represented in fig. 73, assuming, for convenience, that the angular value of the semidiameter of the planet is equal to 10".

It appears, therefore, that the orbit of the most remote of the satellites subtends a visual angle of only 1286" at the earth, being about two-thirds of the apparent diameter of the sun or moon, and within this small visual space all the vast physical machinery and phenomena which we have here noticed are in operation, and within such a space have these extraordinary discoveries been made. The apparent diameter of the external edge of the rings is only 44", or the fortieth part of the apparent diameter of the sun or moon; yet within that small circle have been observed and measured the planet, its belts, atmosphere, and rotation, and the two rings, their magnitude, rotation, and the lineaments of their surface.

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