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same, or very nearly the same, angle to that of the orbit, and, therefore, to the ecliptic, viz. 28° 40'; and intersects the latter plane in a line, which makes an angle with the line of equinoxes of 170°. So that the nodes of the ring lie in 170° and 350° of longitude. Whenever, then, the planet happens to be situated in one or other of these longitudes, as at A B, the plane of the ring passes through the sun, which then illuminates only the edge of it; and as, at the same moment, owing to the smallness of the earth's orbit, E, compared with that of Saturn, the earth is necessarily not far out of that plane, and must, at all events, pass through it a little before or after that moment, it only then appears to us as a very fine straight line, drawn across the disc, and projecting out on each side,—indeed, so very thin is the ring, as to be quite invisible, in this situation, to any but telescopes of extraordinary power. This remarkable phænomenon takes place at intervals of 15 years, but the disappearance of the ring is generally double, the earth passing twice through its plane before it is carried past our orbit by the slow motion of Saturn. This second disappearance is now in progress *. As the planet, however, recedes from these points of its orbit, the line of sight becomes gradually more and more inclined to the plane of the ring, which, according to the laws of perspective, appears to open out into an ellipse which attains its greatest breadth when the planet is 90° from either node, as at CD. Supposing the upper part of the figure to be north, and the lower south of the ecliptic, the north side only of the ring will be seen when the planet lies in the semicircle AC B, and the southern only when in AD B. At the time of the greatest opening, the longer diameter is almost exactly double the shorter.

(443.) It will naturally be asked how so stupendous an arch, if composed of solid and ponderous materials,

The disappearance of the rings is complete, when observed with a reflector eighteen inches in aperture, and twenty feet in focal length. April 29, 1833. - Author,

can be sustained without collapsing and falling in upon the planet? The answer to this is to be found in a swift rotation of the ring in its own plane, which observation has detected, owing to some portions of the ring being a little less bright than others, and assigned its period at 10h 29m 17s, which, from what we know of its dimensions, and of the force of gravity in the Saturnian system, is very nearly the periodic time of a satellite revolving at the same distance as the middle of its breadth. It is the centrifugal force, then, arising from this rotation, which sustains it; and, although no observation nice enough to exhibit a difference of periods between the outer and inner rings have hitherto been made, it is more than probable that such a difference does subsist as to place each independently of the other in a similar state of equilibrium.

(444.) Although the rings are, as we have said, very nearly concentric with the body of Saturn, yet recent micrometrical measurements of extreme delicacy have demonstrated that the coincidence is not mathematically exact, but that the center of gravity of the rings oscillates round that of the body describing a very minute orbit, probably under laws of much complexity. Trifling as this remark may appear, it is of the utmost importance to the stability of the system of the rings. Supposing them 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 towards the nearest part of the ring, or away from the center. 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. (See Chap. XI.) Now, external powers, capable of producing such excentricity, exist in the attractions of the satellites, as will be shown in Chap. XI.; 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 center. 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 centers 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 antagonizing one another as to prevent the latter from ever acquiring an uncontrollable ascendancy, and rushing to a catastrophe.

(445.) This is also the place to observe, that, as the

smallest difference of velocity between the body and rings must infallibly precipitate the latter on the former, never more to separate, (for they would, once in contact, have attained 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 orbitual motion, and under the full and free influence of all the acting forces.

(446.) The rings of Saturn must present a magnificent spectacle from those regions of the planet which lie above their enlightened sides, as vast arches spanning the sky from horizon to horizon, and holding an invariable situation among the stars. On the other hand, in the regions beneath the dark side, a solar eclipse of fifteen years in duration, under their shadow, must afford (to our ideas) an inhospitable asylum to animated beings, ill compensated by the faint light of the satellites. But we shall do wrong to judge of the fitness or unfitness of their condition from what we see around us, when, perhaps, the very combinations which convey to our minds only images of horror, may be in reality theatres of the most striking and glorious displays of beneficent contrivance.

(447.) Of Uranus we see nothing but a small round uniformly illuminated disc, without rings, belts, or discernible spots. Its apparent diameter is about 4′′, from which it never varies much, owing to the smallness of our orbit in comparison of its own. Its real diameter is about 35,000 miles, and its bulk 80 times that of the earth. It is attended by satellites two at least, probably five or six whose orbits (as will be seen in the next chapter) offer remarkable peculiarities.

(448.) If the immense distance of Uranus precludes all hope of coming at much knowledge of its physical state, the minuteness of the four ultra-zodiacal planets

is no less a bar to any enquiry into theirs. One of them, Pallas, is said to have somewhat of a nebulous or hazy appearance, indicative of an extensive and vaporous atmosphere, little repressed and condensed by the inadequate gravity of so small a mass. No doubt the most remarkable of their peculiarities must lie in this condition of their state. A man placed on one of them would spring with ease 60 feet high, and sustain no greater shock in his descent than he does on the earth from leaping a yard. On such planets giants might exist; and those enormous animals, which on earth require the buoyant power of water to counteract their weight, might there be denizens of the land. But of such speculation there is no end.

(449.) We shall close this chapter with an illustration calculated to convey to the minds of our readers a general impression of the relative magnitudes and distances of the parts of our system. Choose any well levelled field or bowling green. On it place a globe, two feet in diameter; this will represent the Sun; Mercury will be represented by a grain of mustard seed, on the circumference of a circle 164 feet in diameter for its orbit; Venus a pea, on a circle 284 feet in diameter; the Earth also a pea, on a circle of 430 feet; Mars a rather large pin's head, on a circle of 654 feet; Juno, Ceres, Vesta, and Pallas, grains of sand, in orbits of from 1000 to 1200 feet; Jupiter a moderate-sized orange, in a circle nearly half a mile across; Saturn a small orange, on a circle of four-fifths of a mile ; and Uranus a full sized cherry, or small plum, upon the circumference of a circle more than a mile and a half in diameter. As to getting correct notions on this subject by drawing circles on paper, or, still worse, from those very childish toys called orreries, it is out of the question. To imitate the motions of the planets, in the above mentioned orbits, Mercury must describe its own diameter in 41 seconds; Venus, in 4m 14s; the Earth, in 7 minutes; Mars, in 4m 48s; Jupiter, in 2h 56m; Saturn, in 3h 13m; and Uranus, in 2h 16m.

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