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ence will no doubt be destroyed by nicer and more rigorously reduced observations.

(467.) The orbits of Jupiter's satellites are but little eccentric, those of the two interior, indeed, have no perceptible eccentricity; their mutual action produces in them perturbations analogous to those of the planets about the sun, and which have been diligently investigated by Laplace and others. By assiduous observation it has been ascertained that they are subject to marked fluctuations in respect of brightness, and that these fluctuations happen periodically, according to their position with respect to the sun. From this it has been concluded, apparently with reason, that they turn on their axes, like our moon, in periods equal to their respective sidereal revolutions about their primary.

(468.) The satellites of Saturn have been much less studied than those of Jupiter. The most distant is by far the largest, and is probably not much inferior to Mars in size. Its orbit is also materially inclined to the plane of the ring, with which those of all the rest nearly coincide. It is the only one of the number whose theory has been at all enquired into, further than suffices to verify Kepler's law of the periodic times, which holds good, mutatis mutandis, and under the requisite reservations, in this as in the system of Jupiter. It exhibits, like those of Jupiter, periodic defalcations of light, which prove its revolution on its axis in the time of a sidereal revolution about Saturn. The next in order (proceeding inwards) is tolerably conspicuous; the three next very minute, and requiring pretty powerful telescopes to see them; while the two interior satel. lites, which just skirt the edge of the ring, and move exactly in its plane, have never been discerned but with the most powerful telescopes which human art has yet constructed, and then only under peculiar circumstances. At the time of the disappearance of the ring (to ordinary telescopes) they have been seen * threading like beads

. * By my Father, in 1789, with a reflecting telescope four feet in aperture. the almost infinitely thin fibre of light to which it is then reduced, and for a short time advancing off it at either end, speedily to return, and hastening to their habitual concealment. Owing to the obliquity of the ring, and of the orbits of the satellites to Saturn's ecliptic, there are no eclipses of the satellites (the interior ones excepted) until near the time when the ring is seen edgewise.

(469.) With the exception of the two interior satellites of Saturn, the attendants of Uranus are the most difficult objects to obtain a sight of, of any in our system. Two undoubtedly exist, and four more have been suspected. These two, however, offer remarkable and, indeed, quite unexpected and unexampled peculiarities. Contrary to the unbroken analogy of the whole planetary system whether of primaries or secondaries—the planes of their orbits are nearly perpendicular to the ecliptic, being inclined no less than 78° 58' to that plane, and in these orbits their motions are retrograde; that is to say, their positions, when projected on the ecliptic, instead of advancing from west to east round the center of their primary, as is the case with every other planet and satellite, move in the opposite direction. Their orbits are nearly or quite circular, and they do not appear to have any sensible, or, at least, any rapid motion of nodes, or to have undergone any material change of inclination, in the course, at least, of half a revolution of their primary round the

sun. *

* These anomalous peculiarities, which seem to occur at the extreme limits of our system, as if to prepare us for further departure from all its analogies, in other systems which may yet be disclosed to us, have hitherto rested on the sole testimony of their discoverer, who alone had ever obtained a view of them. I am happy to be able, from my own observations from 1828 to the present time, to confirm in the amplest manner my Father's results, Author.

CHAP. X.

OF COMETS.

GREAT NUMBER OF RECORDED COMETS. — THE NUMBER OF UN

RECORDED PROBABLY MUCH GREATER. DESCRIPTION OF A COMET. -COMETS WITHOUT TAILS. -INCREASE AND DECAY OF THEIR Tails. — THEIR MOTIONS. — SUBJECT TO THE GENERAL LAWS OF PLANETARY MOTION.— ELEMENTS OF THEIR ORBITS. - PERIODIC RETURN OF CERTAIN COMETS. -HALLEY'S. ENCKE's. — BIELA's. — DIMENSIONS OF COMETS. — THEIR RESISTANCE BY THE ETHER, GRADUAL DECAY, AND POSSIBLE DISPERSION IN SPACE.

(470.) The extraordinary aspect of comets, their rapid and seemingly irregular motions, the unexpected manner in which they often burst upon us, and the imposing magnitudes which they occasionally assume, have in all ages rendered them objects of astonishment, not unmixed with superstitious dread to the uninstructed, and an enigma to those most conversant with the wonders of creation and the operations of natural causes. Even now, that we have ceased to regard their movements as irregular, or as governed by other laws than those which retain the planets in their orbits, their intimate nature, and the offices they perform in the economy of our system, are as much unknown as ever. No rational or even plausible account has yet been rendered of those immensely voluminous appendages which they bear about with them, and which are known by the name of their tails, (though improperly, since they often precede them in their motions,) any more than of several other singularities which they present.

(471.) The number of comets which have been astronomically observed, or of which notices have been recorded in history, is very great, amounting to several hundreds *; and when we consider that in the earlier ages of astronomy, and indeed in more recent times, before the invention of the telescope, only large and conspicuous ones were noticed ; and that, since due attention has been paid to the subject, scarcely a year has passed without the observation of one or two of these bodies, and that sometimes two and even three have appeared at once ; it will be easily supposed that their actual number must be at least many thousands. Multitudes, indeed, must escape all observation, by reason of their paths traversing only that part of the heavens which is above the horizon in the daytime. Comets so circumstanced can only become visible by the rare coincidence of a total eclipse of the sun,-a coincidence which happened, as related by Seneca, 60 years before Christ, when a large comet was actually observed very near the sun. Several, however, stand on record as having been bright enough to be seen in the daytime, even at noon and in bright sun. shine. Such were the comets of 1402 and 1532, and that which appeared a little before the assassination of Cæsar, and was (afterwards) supposed to have predicted his death.

(472.) That feelings of awe and astonishment should be excited by the sudden and unexpected appearance of a great comet, is no way surprising ; being, in fact, according to the accounts we have of such events, one of the most brilliant and imposing of all natural phænomena. Comets consist for the most part of a large and splendid but ill defined nebulous mass of light, called the head, which is usually much brighter towards its center, and offers the appearance of a vivid nucleus, like a star or planet. From the head, and in a direction opposite to that in which the sun is situated from the comet, appear to diverge two streams of light, which grow broader and more diffused at a distance from the head, and which sometimes close in and unite at a little distance behind it, sometimes continue distinct for a great part of their course ; producing an effect like that of the trains left by some bright meteors, or like the diverging fire of a sky-rocket (only without sparks or perceptible motion). This is the tail. This magnificent appendage attains occasionally an immense apparent length. Aristotle relates of the tail of the comet of 371 A. C., that it occupied a third of the hemisphere, or 60° ; that of A. D. 1618 is stated to have been attended by a train no less than 104° in length. The comet of 1680, the most celebrated of modern times, and on many accounts the most remarkable of all, with a head not exceeding in brightness a star of the second magnitude, covered with its tail an extent of more than 70° of the heavens, or, as some accounts state, 90°. The figure (fig. 2., Plate II.) is a very correct representation of the comet of 1819-by no means one of the most considerable, but the latest which has been conspicuous to the naked eye.

* See catalogues in the Almagest of Riccioli; Pingré's Cometographia : Delambre's Astron, vol. iii.; Astroncmische Abhandlungen, No. 1. (which contains the elements of all the orbits of comets which have been computed to the time of its publication, 1823); also, a catalogue now in progress, by the Rev. T. J. Hussey. Lond. & Ed. Phil. Mag. vol. ii. No. 9. et seq. In a list cited by Lalande from the 1st vol, of the Tables de Berlin, 700 comets are enumerated

(473.) The tail is, however, by no means an invariable appendage of comets. Many of the brightest have been observed to have short and feeble tails, and not a few have been entirely without them. Those of 1585 and 1763 offered no vestige of a tail; and Cassini describes the comet of 1682 as being as round and as bright as Ju. piter. On the other hand, instances are not wanting of comets furnished with many tails or streams of diverging light. That of 1744 had no less than six, spread out like an immense fan, extending to a distance of nearly 30° in length. The tails of comets, too, are often curved, bending, in general, towards the region which the comet has left, as if moving somewhat more slowly, or as if resisted in their course.

(474.) The smaller comets, such as are visible only in telescopes, or with difficulty by the naked eye, and which are by far the most numerous, offer very frequently no appearance of a tail, and appear only as round or some

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