further diminution of the interval between the rings and the ball. The question, however, appears to be definitively settled in the negative by the elaborate micrometrical measures of M. Main, at the Royal Observatory at Greenwich, and by the discussions entered into by M. Kaiser. (522 b.) The rings of Saturn must present a magnificent spectacle from those regions of the planet which lie above their enlightened sides, being seen as vast luminous arches, spanning the sky from horizon to horizon, and holding an almost invariable situation among the stars. To a spectator situated any where in the axis of the planet, it is evident that their interior and exterior outlines must both appear as circles corresponding to parallels of declination, and must occasion a permanent eclipse of every heavenly body lying between these parallels. It is otherwise to a spectator situated on the planet's surface. To such a one the interior and exterior outline of each ring would, by the effect of perspective, be thrown into nonconcentric ellipses, so that (supposing he could see through the whole planet and obtain a view of the whole ring) it would appear broader on the side nearest to him than on that most remote. These ellipses, moreover, when traced along the heavens, would not coincide with parallels of declination ; but would deviate from such parallels towards the elevated pole, as is evident, if we consider that a perpendicular ST from any point S on the planet's surface to the plane of the ring AB is parallel to the axis of rotation; so that the right cone ASD, generated by the revolution of AS round ST, traces on the heavens a circle of decli * Mem. Ast. Soc. xxv. Ast. Soc. Notice, xvi. 66. The circumstances have been traced in minute detail by Dr. Lardner, who first, I believe, drew attention to the effect of situation on the surface of the planet in modifying the phænomena presented by the rings. nation, having the edge A of the ring for its upper culminating point: whereas the oblique cone ASB, tracing the visible course of the ring in the heavens, though coincident with the former at its upper culmination A, lies elsewhere wholly exterior to it, and has its inferior culmination B nearer to the elevated pole by the angle BSD, the difference of the angles of the two cones. The apparent course of either edge of the ring, then, is a curve touching the circle of declination at which that edge culminates, but receding from it towards the elevated pole, so as to allow stars or the Sun to be visible at certain seasons under the ring at their rising-to be eclipsed wholly or partially by it at its under edge, and again to emerge before setting. This will not prevent, however, some considerable regions of Saturn from suffering very long total interception of the Solar beams, affording, to our ideas, but an inhospitable asylum to animated beings, ill compensated by the feeble 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. (523.) 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 82 times that of the earth. It is attended by four satellites, whose existence may be considered as conclusively established (and more have been suspected). (524.) The discovery of Neptune is so recent, and its situation in the ecliptic at present so little favourable for seeing it with perfect distinctness, that nothing very positive can be stated as to its physical appearance. It was at first suspected to have a ring, but the suspicion has not been verified. It is attended by at least one satellite, the existence of which has been demonstrated by the observations of Mr. Lassell, M. Otto Struve, and Mr. Bond. (525.) If the immense distance of Neptune precludes all hope of coming at much knowledge of its physical state, the minuteness of the Asteroids is no less a bar to any enquiry into theirs. One of them, Pallas, has been 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. It is probable, however, that the appearance in question has originated in some imperfection in the telescope employed, or other temporary causes of illusion. In Vesta and Pallas only have sensible discs been hitherto observed, and those only with very high magnifying powers. Vesta was once seen by Schroeter with the naked eye. 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. From some recent researches of M. Leverrier, it appears that we shall be warranted in attributing to the totality of the Asteroids a quantity of matter quite insignificant. (525 a.) There is a remarkable division of the planetary system into two families or classes of planets, the large, and the small. To the latter family belong those interior to the orbits of Jupiter, viz., Mercury, Venus, the Earth, and Mars, with the Asteroids. To the former, all exterior to the orbits of the first class-Jupiter, Saturn, Uranus, and Neptune. The Asteroids themselves, however, may be considered as forming a family apart, their magnitudes being as much inferior to those of the interior planets as these are to the exterior, or in a still lower ratio. Not less remarkable is the circumstance that while all the interior planets revolve on their axes (so far as is known) in about the same time (24h), the exterior (as is certain in the case of Jupiter and Saturn at least) have periods of rotation less than half that length. In point of density, too, as we shall see further on, an equally marked distinction of specific character is preserved, all the interior ones having about the same density as the Earth, while that of all the exterior is very much less, not exceeding a quarter of the Earth, and agreeing (in the cases of Jupiter and Uranus) very closely with that of the Sun. (526.) 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 of 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; the Asteroids, 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; Uranus a full-sized cherry, or small plum, upon the circumference of a circle more than a mile and a half; and Neptune a good-sized plum, on a circle about two miles 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 14; the Earth, in 7 minutes; Mars, in 4m 485; Jupiter, 2h 56m; Saturn, in 3h 13m; Uranus, in 2h 16m; and Neptune, in 3h 30m.* In the "Penny Encyclopædia," vol. 22. p. 197, the diameters of the orbits of the planets here set down, are quoted as their distances from the center, and the size of the sun is enlarged to four feet, while the sizes of the planets are unaltered. A A CHAPTER X. OF THE SATELLITES. GENERAL PROX OF THE MOON, AS A SATELLITE OF THE EARTH. OF URANUS-OF NEPTUNE. (527.) IN the annual circuit of the earth about the sun, it is constantly attended by its satellite, the moon, which revolves round it, or rather both round their common center of gravity; while this center, strictly speaking, and not either of the two bodies thus connected, moves in an elliptic orbit, undisturbed by their mutual action, just as the center of gravity of a large and small stone tied together and flung into the air describes a parabola as if it were a real material substance under the earth's attraction, while the stones circulate round it or round each other, as we choose to conceive the matter. (528.) If we trace, therefore, the real curve actually described by either the moon's or the earth's centers, in virtue of this compound motion, it will appear to be, not an exact ellipse, but an undulated curve, like that represented in the figure to article 324., only that the number of undulations in a whole revolution is but 13, and their actual deviation from the general ellipse, which serves them as a central line, is comparatively very much smaller-so much so, indeed, that every part of the curve described by either the earth or moon is concave towards the sun. The excursions of the earth on either side the ellipse, indeed, are so very small as to be hardly appretiable. In fact, the center of gravity of the earth and moon lies always within the surface of the earth, so |