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excess or defect of Jupiter's distance from the earth above or below its average amount, and such that a difference of distance of one diameter of the earth's orbit should correspond to 16m 26.6 of time allowed. Speculating on the probable physica cause, he was naturally led to think of a gradual instead of an instantaneous propagation of light. This explained every particular of the observed phenomenon, but the velocity required (192000 miles per second) was so great as to startle many, and, at all events, to require confirmation. This has been afforded since, and of the most unequivocal kind, by Bradley's discovery of the aberration of light (art. 329). The velocity of light deduced from this last phenomenon differs by less than one eightieth of its amount from that calculated from the eclipses, and even this dif ference will no doubt be destroyed by nicer and more rigorously reduced observations.

(546.) The orbits of Jupiter's satellites are but little excentric, those of the two interior, indeed, have no perceptible excentricity. 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.

(547.) The satellites of Saturn have been much less studied than those of Jupiter, being far more difficult to observe. The most distant has its orbit materially inclined (no less than 12° 14′)* to the plane of the ring, with which the orbits of all the rest nearly coincide. Nor is this the only circumstance which separates it by a marked difference of character from the system of the six interior ones, and renders it in some sort an anomalous member of the Saturnian system. Its distance from the planet's centre exceeds in the proportion of nearly three to one that of the most distant of all the rest, being no less than 64 times the radius of the globe of Saturn, a distance from the primary to which our own moon (at 60 radii) offers the only parallel. Its variation of light also in differ

*Lalande, Astron. Art. 3075.

ent parts of its orbit is very much greater than in the case of any other secondary planet. Dominic Cassini indeed (its first discoverer, A. D. 1671) found it to disappear for nearly half its revolution, when to the east of Saturn, and though the more powerful telescopes now in use enable us to follow it round the whole of its circuit, its diminution of light is so great in the eastern half of its orbit as to render it somewhat difficult to perceive. From this circumstance (viz. from the defalcation of light occurring constantly on the same side of Saturn as seen from the earth, the visual ray from which is never very oblique to the direction in which the sun's light falls on it) it is presumed with much certainty that this satellite revolves on its axis in the exact time of rotation about the primary; as we know to be the case with the moon, and as there is considerable ground for believing to be so with all secondaries.

(548.) The next satellite in order proceeding inwards (the first in order of discovery)* is by far the largest and most conspicuous of all, and is probably not much inferior to Mars in size. It is the only one of the number whose theory and perturbations have been at all inquired into further than 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. The three next satellites still proceeding inwards‡ are very minute and require pretty powerful telescopes to see them; while the two interior satellites which just skirt the edge of the ring§ can only be seen with telescopes of extraordinary power and perfection, and under the most favourable atmospheric circumstances. At the epoch of their discovery they were seen to thread, like beads, the almost infinitely thin fibre of light to which the ring then seen edgeways was reduced, and for a short time to advance off it at either end, speedily to return, and hastening to their habitual concealment behind the body.||

*By Huyghens, March 25, 1655.

By Bessel, Astr. Nachr. Nos. 193, 214.

Discovered by Dominic Cassini in 1672 and 1684.

§ Discovered by Sir William Herschel in 1789.

Considerable confusion prevails in the nomenclature of the Saturnian system, owing to the order of discovery not coinciding with that of distances. Astronomers have not yet agreed whether to call the two interior satellites the 6th and 7th (reckoning inward) and the older ones the 1st, 2d, 3d, 4th, and 5th, reckoning outward; or to commence with the innermost and reckon outwards, from 1 to 7. This

(549.) Owing to the obliquity of the ring and of the orbits of the satellites to Saturn's ecliptic, there are no eclipses, occultations, or 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

confusion has been attempted to be obviated by a mythological nomenclature, in consonance with that at length completely established for the primary planets. Taking the names of the Titanian divinities, the following pentameters afford an easy artificial memory, commencing with the most distant.

Iapetus, Titan; Rhea, Dione, Tethys; (pron. Těthys)
Enceladus, Mimas

It is worth remarking that Simon Marius, who disputed the priority of the disco. very of Jupiter's satellites with Galileo, proposed for them mythological names, viz : -Io, Europa, Ganymede, and Callisto. The revival of these names would savour of a preference of Marius's claim, which, even if an absolute priority were conceded (which it is not), would still leave Galileo's general claim to the use of the telescope as a means of astronomical discovery intact. But in the case of Jupiter's satellites there exists no confusion to rectify. They are constantly referred to by their numerical designations in every almanack.

⚫ September 14th to November 9th, 1847.

† October 8th to December 10th, 1847.

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.

(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 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 eclipses 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 Struvef 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.

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 ASTONOMY, AND ITS REASONS.-REMARKS ON COMETARY ORBITS IN GENERAL.

(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. 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. distinct and satisfactory 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.

No

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

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