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known) until after the observations of Messrs. Bond and Dawes. The most remarkable feature of this singular discovery is, that subsequent observations, from many quarters, have concurred in showing the new ring to consist of semitransparent materials through which the limb of the planet may be seen up to the edge of the interior bright ring. Dark lines (apparently of a transitory nature) have been observed on the bright rings parallel to the permanent dark interval dividing them, and appear to indicate a fluid (possibly a vaporous) constitution of these wonderful appendages.*]
(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 satellites— four at least, probably five or six—whose orbits (as will be seen in the next chapter) offer remarkable peculiarities.
(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. To two observers it has afforded strong suspicion of being surrounded with a ring very highly inclined. And from the observations of Mr. Lassell, M. Otto Struve, and Mr. Bond, it appears to be attended certainly by one, and very probably by two satellites — though the existence of the second can hardly yet be considered as quite demonstrated.
(525.) If the immense distance of Neptune precludes all hope of coming at much knowledge of its physical state, the minuteness of the ultra-zodiacal planets 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 Schrceter 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. But of such speculations there is no end.
* The passage of Saturn across any considerable star would afford an admirable opportunity of testing the existence of fissures in the rings, as it would flash in succession through them. The opportunity of watching for such occultations— when Saturn traverses the Milky-Way, for instance — should not be neglected.
(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. 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 fourfifths 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 148; the Earth, in 7 minutes; Mars, in 4m 48"; Jupiter, 2h 56m; Saturn, in 3h 13m; Uranus, in 2h 16m; and Neptune, in 3h 30m.
OP THE SATELLITES.
OP THE MOON, AS A SATELLITE OP THE EAJITH. — GENERAL PROXIMITY OF SATELLITES TO THEIR PRIMARIES, AND CONSEQUENT
SUBORDINATION OF THEIR MOTIONS. MASSES OF THE PRIMARIES
CONCLUDED FROM THE PERIODS OF THEIR SATELLITES. MAINTENANCE Of Kepler's Laws In The Secondary Systems. — Of Jupiter's Satellites. — Their Eclipses, Etc. — Velocity Op
Light Discovered By Their Means. Satellites Of Saturn
Of Uranus — Of Neptdne.
(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 of 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 that the monthly orbit described by the earth's center about the common center of gravity is comprehended within a space less than the size of the earth itself. The effect is, nevertheless, sensible, in producing an apparent monthly displacement of the sun in longitude, of a parallactic kind, which is called the menstrual equation; whose greatest amount is, however, less than the sun's horizontal parallax, or than 8"6".
(529.) The moon, as we have seen, is about 60 radii of the earth distant from the center of the latter. Its proximity, therefore, to its center of attraction, thus estimated, is much greater than that of the planets to the sun; of which Mercury, the nearest, is 84, and Uranus 2026 solar radii from its center. It is owing to this proximity that the moon remains attached to the earth as a satellite. Were it much farther, the feebleness of its gravity towards the earth would be inadequate to produce that alternate acceleration and retardation in its motion about the sun, which divests it of the character of an independent planet, and keeps its movements subordinate to those of the earth. The one would outrun, or be left behind the other, in their revolutions round the sun ( by reason of Kepler'smthird law ), according to the relative dimensions of their heliocentric orbits, after which the whole influence of the earth would be confined to producing some considerable periodical disturbance in the moon's motion, as it passed or was passed by it in each synodical revolution.
(530.) At the distance at which the moon really is from us, its gravity towards the earth is actually less than towards the sun. That this is the case, appears sufficiently from what we have already stated, that the moon's real path, even when between the earth and sun, is concave towards the latter. But it will appear still more clearly if, from the known periodic times*in which the earth completes its annual and the moon its monthly orbit, and from the dimensions of those orbits, we calculate the amount of deflection, in either, from their tangents, in equal very minute portions of time, as one second. These are the versed sines of the arcs described in that time in the two orbits, and these are the measures of the acting forces which produce those deflections. If we execute the numerical calculation in the case before us, we shall find 2'233: 1 for the proportion in which the intensity of the force which retains the earth in its orbit round the sun actually exceeds that by which the moon is retained in its orbit about the earth.
* R and r radii of two orbits (supposed circular), F and p the periodic times; then the arcs in question (A and a) arc to each other as p to -; and since the versed sines are as the squares of the arcs directly and the radii inversely, these are to each other as —^ to ,; and in this ratio arc the force* acting
on the revolving bodies in cither case.
(531.) Now the sun is 399 times more remote from the earth than the moon is. And, as gravity increases as the squares of the distances decrease, it must follow that at equal distances, the intensity of solar would exceed that of terrestrial gravity in the above proportion, augmented in the further ratio of the square of 400 to 1; that is, in the proportion of 355000 to 1; and therefore, if we grant that the intensity of the gravitating energy is commensurate with the mass or inertia of the attracting body, we are compelled to admit the mass of the earth to be no more than 577353 of that of the sun.
(532.) The argument is, in fact, nothing more than a recapitulation of what has been adduced in Chap VIII. (art 448.) But it is here re-introduced, in order to show how the mass of a planet which is attended by one or more satellites can be as it were weighed against the sun, provided we have learned from observation the dimensions of the orbits described by the planet about the sun, and by the satellites about the planet, and also the periods in which these orbits are respectively described. It is by this method that the masses of Jupiter, Saturn, Uranus, and Neptune have been ascertained. ( See Synoptic Table.)
(533.) Jupiter, as already stated, is attended by four satellites, Saturn by seven; Uranus, certainly by four, and perhaps by six; and Neptune by two or more. These, with their respective primaries (as the central planets are called), form in each case miniature systems entirely analogous, in tho