It is very difficult for one to conceive how such a delicate structure, as the system of rings appears to be, can keep together in equilibrium and avoid destruction from the powerful attraction of the planet on one side and the disturbing influence of the satellites on the other. To explain it, several hypotheses have been advanced. The rings were first supposed to be solid, and upon this supposition Laplace determined the necessary conditions for their equilibrium; the most important of which require that the cross section of the rings should be an ellipse of irregular curvature, and having its major axis directed towards the centre of the planet, and also that the system should rotate upon an axis perpendicular to the plane of the rings. This theory was superseded by another, which supposed the rings to be fluid. This one was soon rejected for a third, assuming the system to be composed of vapors or gases; and more recently, all these theories were considered untenable, and replaced by a fourth, which supposes the system of rings to be made up of a congregation of innumerable small, independent bodies, revolving around Saturn in concentric zones. Naturally, such a divergence of opinion can only result from our comparative ignorance of the subject, and sufficiently indicates our inability to explain the phenomena; and it must be admitted that, so far, nothing is certainly known about this strange system. We shall probably remain in the same uncertainty until the rotation of the rings is ascertained by direct observations. It is pretty certain, however, that none of these theories account for the observed phenomena in their details, although a partial explanation may be obtained by borrowing something from each hypothesis.
It has been conjectured, and a theory has been advanced, that the breadth of the whole ring system is gradually increasing inwards, and that it will come in contact with the planet in about 2,150 years; but the question seems to have been settled in the negative by the elaborate measurements of the English observers. It is likely that the increase is only in the defining power of the instruments.
[COMETS]
PLATE XI
Among the celestial phenomena, none are more interesting than those mysterious apparitions from the depths which unexpectedly display their strange forms in our familiar constellations, through which they wander for a time, until they disappear like phantoms.
A comet, with its luminous diffused head, whence proceeds a long vapory appendage gradually fading away in the sky, presents an extraordinary aspect, which may well astonish and deeply impress the observer. Although these visitors from infinite space do not now inspire dread, as in by-gone times, yet, owing to the mystery in which the phenomenon is still involved, the apparition of a large comet, even in our days, never fails to create a profound sensation, and in some cases that unconscious fear which results from the unknown.
The effect of such a spectacle largely depends upon its rarity; but since the telescope has been applied to the sounding of the heavens, it has been found that the appearance of comets is by no means an unusual occurrence. If so few comets, comparatively, are seen, it is because most of them are telescopic objects, and are therefore invisible to the naked eye. Most of the telescopic comets are not only too faint to be perceived by the unaided eye, but are insignificant objects, even when observed through the largest telescopes.
It was Kepler's opinion that comets are as numerous in the sky as fishes are in the ocean. Undoubtedly the number of these bodies must be great, considering that we can only see them when they come into the neighborhood of the Earth, and that many even here remain invisible, or at least pass unperceived. That many of them have passed unperceived heretofore, is proved by the fact that the number of those observed becomes greater every year, with the increase of the number of instruments used in their search. The number of comets observed with the naked eye during historic times is nearly 600, and that of telescopic comets, which, of course, all belong to the last few centuries, is more than 200, so that we have a total number of about 800 comets of which records have been kept. From theoretical considerations, Lambert and Arago estimated their entire number at several millions, but such speculations have generally no real value, since they cannot be established on a firm basis.
Comets remain visible for more or less time, according to their size and the nature and position of their orbits, but in general, the large ones can be followed with the telescope for several months after they have become invisible to the naked eye. The comet of 1861, for example, remained telescopically visible for a year, and that of 1811, for 17 months after disappearing from ordinary sight.
While a comet remains visible, it appears to revolve daily about us like the stars in general; but it also moves among the constellations, and from this movement its orbit may be computed like that of a planet. From the apparent diurnal motion of a comet with the heavens, result the changes of position which it seems to undergo in the course of a night. The direction of the head and tail of a comet, of course, has only changed in regard to the horizon, but not in regard to the sky, in which they occupy very nearly the same position throughout a given night, and even for many nights in succession.
The movements of the comets in their orbits are, like those of the planets, in accordance with Kepler's laws, the Sun occupying one of the foci of the orbit they describe; but the orbits of comets differ, however, in several points from those of the planets. Their eccentricity is always great, being sometimes apparently infinite, in which case the orbit is said to be parabolic, or hyperbolic; but the smallness of the portion of a cometary orbit which can ordinarily be observed, makes it difficult to determine this with certainty. Again, while the planetary orbits are usually near the plane of the ecliptic, those of comets frequently have great inclinations to that plane, and even when the inclination is less than 90°, the comet may have a retrograde movement, or, in other words, a movement contrary to the course in which all the planets revolve about the Sun.