transits of these bodies or their shadows across the disc of their primary (the interior ones excepted), until near the time when the ring is seen edgewise, and when they do take place, their observation is attended with too much difficulty to be of any practical use, like the eclipses of Jupiter's satellites, for the determination of longitudes, for which reason they have been hitherto little attended to by astronomers.

(550.) A remarkable relation subsists between the periodic times of the two interior satellites of Saturn, and those of the two next in order of distance; viz. that the period of the third (Tethys) is double that of the first (Mimas), and that of the fourth (Dione)`double that of the second (Enceladus). The coincidence is exact in either case to about one-800th part of the larger period.

(551.) The satellites of Uranus require very powerful and perfect telescopes for their observation. Two are, however, much more conspicuous than the rest, and their periods and distances from the planet have been ascertained with tolerable certainty. They are the second and fourth of those set down in the synoptic table. Of the remaining four, whose existence, though announced with considerable confidence by their original discoverer, could hardly be regarded as fully demonstrated, two only have been hitherto reobserved; viz. the first of our table, interior to the two larger ones, by the independent observations of Mr. Lassell,' and 'M. Otto Struve, and the fourth, intermediate between the larger ones, by the former of these astronomers. The remaining two, if future observation should satisfactorily establish their real existence, will probably be found to revolve in orbits exterior to all these.

2

(552.) The orbits of these satellites 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 centre 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. When the earth is in the plane of their orbits, or nearly so, their apparent paths are straight lines or very elongated ellipses, in which

'September 14th to November 9th, 1847.

* October 8th to December 10th, 1847.

case they become invisible, their feeble light being effaced by the superior light of the planet, long before they come up to its disc, so that the observation of any edipses or occultations they may undergo is quite out of the question, with our present telescopes.

(553.) If the observation of the satellites of Uranus be difficult, those of Neptune, owing to the immense distance of that planet, may be readily imagined to offer still greater difficulties. Of the existence of one, discovered by Mr. Lassell,' there can remain no doubt, it having also been observed by other astronomers, both in Europe and America. According to M. Otto Struve2 its orbit is inclined to the ecliptic at the considerable angle of 35°; but whether, as in the case of the satellites of Uranus, the direction of its motion be retrograde, it is not possible to say, until it shall have been longer observed.

1 On July 8th, 1847.

"Astron. Nachr. No. 629, from his own observations, September 11th to December 20th, 1847.

CHAPTER XI.

OF COMETS.

GREAT NUMBER OF RECORDED COMETS. THE NUMBER OF THOSE UNRECORDED PROBABLY MUCH GREATER. GENERAL DESCRIPTION OF A COMET. COMETS WITHOUT TAILS, OR WITH MORE THAN ONE. - THEIR EXTREME TENUITY. — THEIR PROBABLE STRUCTURE. MOTIONS CONFORMABLE TO THE LAW OF GRAVITY. -ACTUAL DIMENSIONS OF COMETS. PERIODICAL RETURN OF SEVERAL. HALLEY'S COMET.

OTHER ANCIENT COMETS PROBABLY PERIODIC.

ENCKE'S COMET. - BIELA'S. — FAYE'S. —LEXELL'S. — DE VICO'S. —BRORSEN'S.—PETERS'S.-GREAT COMET OF 1843.-ITS PROBABLE IDENTITY WITH SEVERAL OLDER COMETS. GREAT INTEREST AT PRESENT ATTACHED TO COMETARY ASTRONOMY, AND ITS REASONS. REMARKS ON COMETARY ORBITS IN GENERAL.

Even

(554.) 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. 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 distinct and satisfactory account has yet been rendered of those immensely voluminous append. ages 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.

(555.) The number of comets which have been astronomically observed, or of which notices have been recorded in history, is very great, amount

1

ing 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, sixty-two 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 with the naked eye in the daytime, even at noon and in bright sunshine. Such were the comets of 1402, 1532, and 1843, and that of 43 B. C. which appeared during the games celebrated by Augustus in honour of Venus shortly after the death of Cæsar, and which the flattery of poets declared to be the soul of that hero taking its place among the divinities.

(556.) 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 imposing of all natural phenomena. Comets consist for the most part of a large and more or less splendid, but ill-defined nebulous mass of light, called the head, which is usually much brighter towards its centre, 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 most commonly close in and unite at a little distance behind it, but 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

1 See catalogues in the Almagest of Riccioli; Pingré's Cometographie; Delambre's Astron. vol. iii.; Astronomische 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, 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. See also notices of the Astronomical Society and Astron. Nachr. passim. A great many of the more ancient comets are recorded in the Chinese Annals, and in some cases with sufficient precision to allow of the calculation of rudely approximate orbits from their motions so described.

the tail. This magnificent appendage attains occasionally an immense apparent length. Aristotle relates of the tail of the comet of 371 в. 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 ac counts 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°; that of the comet of 1769 extended 97°, and that of the last great comet (1843) was estimated at about 65° when longest. The figure (fig. 2, Plate II.) is a representation of the comet of 1819-by no means one of the most considerable, but which was, however, very conspicuous to the naked eye.

(557.) 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 a few great comets have been entirely without them. Those of 1585 and 1763 offered no vestige of a tail; and Cassini describes the comets of 1665 and 1682 as being as round' and as well defined as Jupiter. 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 small comet of 1823 had two, making an angle of about 160°, the brighter turned as usual from the sun, the fainter towards it, or nearly so. The tails of comets, too, are often somewhat 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.

(558.) 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 somewhat oval vaporous masses, more dense towards the centre, where, however, they appear to have no distinct nucleus, or anything which seems entitled to be considered as a solid body. Stars of the smallest magnitudes remain distinctly visible, though covered by what appears to be the densest portion of their substance; although the same stars would be completely obliterated by a moderate fog, extending only a few yards from the surface of the earth. And since it is an observed fact, that even those

1 This description however applies to the "disc" of the head of these comets as seen in a telescope. Cassini's expressions are, "aussi rond, aussi net, et aussi clair que Jupiter," (where it is to be observed that the latter epithet must by no means be translated bright). To understand this passage fully, the reader must refer to the description given further on, of the "disc" of Halley's comet, after its perihelion passage

in 1835-6.