310

ASTRONOMY.

at right angles to it, nor indeed is it ever elevated more than aboil'30° above it: so that the ring, being commonly viewed at an ob-lique angle, appears of an oval form, and through very good tele-scopes double, as represented Plate XX111, tig. 22, and more ac-curately according to Dr. Ilerschel, in Plate XXV. tig. lb. Boththe outward and inward rim are projected into an ellipsis, moreor less oblong according to the different degrees ot obliquity withwhich it is viewed. Sometimes our eye is in the plane of thering, and then it becomes invisible, except through very powerfultelescopes; either because the outward edge is not lilted to reflecttiie Sun ’s light, or more probably because it has not a width suf-ficiently great to he seen at such a distance. As the plane oithis ring keeps always parallel to itself, that is, its situation in onepart of the orbit is always parallel to that in any other part, it dis-appears twice in every revolution of the planet, that is about oncein 15 years; at other limes, the distance betwixt the body of theplanet and the ring is very perceptible ; and Mr. M histon tells us,that Dr. Clarke’s father saw a star through the opening. WhenSaturn appears round, if our eye be in the plane ol the ring, it willappear as a dark line across the middle of the planet’s disk ; andif our eve be elevated above the plane of the ring, a shadowy beltwill be Visible, caused by the shadow of the ring as well as by theinterposition of part of it betwixt the eye and the planet. 1 heshadow of the ring is broadest when the Sun is mo.t elevated, butits obscure parts appear broadest when our eye is most elevatedabove the plane of it. When it appears double, the ring next thebody of the planet appears brightest; when the ring appears of anelliptical form, the parts about the ends of the largest axis are(Called the ansa:. These, a little before and after the di-appearingof the ring, are of unequal magnitude: the largest ansa is longervisible before the planet’s round phase, and appears again soonerthan the other. In the diagram, (Plate XXII1, tig. 18,) are deli-neated the phases of the rimj from its full appearance in 1782 to itsdisappearance in 1789, and its full re-appearance in 1790. Dr.Ilerschel has found that the ring is double, or that there are twoconcentric rings ; also that it has a motion of rotation in its ownplane, its axis of motion being the same as that of Saturn himselt,and its periodical time 10b. 32' 13",4: but he thinks it probablethat the concentric rings may not revolve in the same period. I heirdimensions, and the space between them, he states in the followingproportion to each other: miles.

Inner diameter of the smaller ring. 146345

Outside diameter of ditto. 184393

Inner diameter of the larger ring. 190248

Outside diameter of ditto,. 204883

Breadth of the inner ring. 20000

Breadth of the outer ditto. 7200

Breadth of the vacant space. 2839

Dr. Herschel concludes from his observations on the ring, that itsstructure, is such, as to allow it to remain permanently in its presentstate; nor does he think it at ail probable that the ring is otthat change-able nature w hich some persons have imagined. lie has also foundthat the figtire of this planet is singular, i. e. that the battening of thepolar regions is not in that gradual manner as with Jupiter : it seemsnot to begin till at a high latitude where the curvature is greatest,and from thence, it becomes more suddenly Hat towards the polesthan in Jupiter : this peculiar figure is accounted for by the attraclions of the ring. See Plate XXV. fig. lb. The same excellent astro-nomer from a series of observations on the belts of Saturn has con-cluded that he revolves upon his axis in 10b 16’ 0",4, that he has adense atmosphere, and that his polar diameter is to his equatorialone as 10 to 11.

(169.) Saturn has, besides his ring, seven little secondary planetsor satellites revolving round him.- One of them, which till latelywas reckoned the 4th in order from Saturn , was discovered byHuygens in 1655 by means oi a telcseope 100 feet long ; and theothers, viz. the 1st, 2d, 3d, and 5th, at different times bv Cassini,between 1671 and 16S4, by the help of glasses of 100 and 136 feet.The 6th and 7th have lately been discovered by Ilerschel with his40 feet relleclingtelescope in 1787 and 17S8. I hose he hascalledthe 6th and 7th satellites, though they are nearer to Saturn than theother live, that the names inav not he mistaken with regard to for-mer observations of them. The four first describe ellipses likethose of the ring, and are in the same plane ; their inclination tothe orbit is from 30° to 31°. The 5th describes an orbit inclinedfrom 17° to 18° to the orbit of Saturn , his plane lving between theecliptic and those of the other satellites. Dr. Ilerschel observes

that the 5th satellite turns round its axis once, exactly in the timein which it revolves round Saturn . In this respect it resembles ourMoon, and the satellites of Jupiter , which do the same tiling.

(170.) The Georgium Sidus, or Ilerschel, being more remot®than Saturn , is also less known. Its apparent magnitude is sosmall, that it can seldom be seen by the naked eye, and evenw hen viewed bv means of the telescope, it appears only about lourseconds in diameter. 1 his planet was discovered by Dr. Her.-chclin 1781 ; lie likewise discovered its six satellites. They are pro-bably not less than Jupiter ’s moons, and their orbits arc nearly per-pendicular to the ecliptic. This is a circumstance in which theydiffer from the attendants of all the other planets. Though Dr.Ilerschel was the first who discovered the Georgium Sidus to beone of the planets of the Solar System ; yet no doubt can be en-tertained of its haring been before observed as a fixed star. I lam-‘steed in 1690, Mayer in 1756, and Monnier in 1769, determinedthe places of three stars which cannot now be found. And M. LaPlace, in calculating this planet’s place ior these three respectiveperiods, according to his theory of Jupiter and Saturn , has foundthat the Georgium Sidus was exactly in those three points, atthose very times. These truly singular concurrences leave nodoubt of the identity of these stars with the new planet.

(171.) Besides the above, there are three very small planetslately discovered between the orbits of Mars and Jupiter . The firstin order from Mars , is Ceres Ferdinandca, discovered at Palermo,January 1, 1801, by Mr. Piazzi, an Italian astronomer ; it is notapparently larger than a fixed star of the eighth magnitude. Thenext is l’allas, first seen by Dr. Gibers, ot Bremen, on the 28thof March, 1802, very near the 20th star in the wing of the Vir-gin. Juno, is the third discovered by Mr. Harding, while makinghis observations at Lilienthal, near Bremen, September 1, 1804.These, Dr. Ilerschel, pro]loses to called asteroids. To these maybe added a fourth, first seen by Dr. Gibers, 011 the 291b of March,1807, in Virgo, which he calls Vesta. In appearance it was simi-lar to Ilerschel, and like a star of the 6th magnitude,

Sect. XIV. Gf the Phenomena of Comets.

(172.) Conu-ts are opaque, spherical, and solid bodies like theplanets, performing revolutions about the Sun in elliptical orbits,which have the Sun in one of the foci. The ancients were divi-ded in their opinions concerning them; some considering them aswandering stars; others, as meteors kindled in the atmosphere ofthe Earth , subsisting for a time, and then dissipated: others lookedupon them as prodigies. The ancient philosophers allowed them aplace in the heavens, and supposed them to be much further fromthe Earth than the Moon is. For Aristotle, Seneca, Plutarch , andethers, testify, that the Pythagoreans, and the whole Italian sect,maintained, that a comet w as a kind of planet or wandering star,which appeared again after a long interval of time. Hippocrates Chiuswas of the same opinion, as Aristotle informs us. Democritus heldalso the same opinion, as Seneca tells us in his “ Natural Questions, ”Book 7. chap. 3. “ For”, says he, “ Democritus , the most curiousand subtle of all the ancients, suspected that there were manymore stars which moved, meaning hv this expression the comets ;but fie neither established their number, or their nanus, thecourses of tlie live planets not having as yet been discovered.”Again, Seneca assures us, That Apollonius Myndius, one ofthe most skilful philosophers in the search of natural causes, didassert, that the Chaldeans reckoned comets among the other wan-dering stars, and that they knew their courses. Apollonius him-self maintained, that a comet was a star of its own kind, as theSun and-Moon are, but that its course was not yet known; thatby its motions it mounts very high in the heavens, and only ap-pears when it descends into the lower part of its orb. And Senecahimself embraces this opinion in the following truly philosophicalwords : “ 1 cannot believe”, says he, “ that a comet is a tire sud-denly kindled, but that it ought to be ranked among the eternal worksof nature. A cornel has its proper place, and is not easily movedfrom thence; it goes its course, and is not extinguished, hut runsoff from us. But you will say, if it were a wandering star it wouldkeep in the zodiac. But who can set one boundary to all the stars ?Who can restrain the works of the Divinity to a narrow compass ? Foreach of those bodies which you imagine to he the only ones thathave motion, have very different circles; why, therefore, maythere not be some that have peculiar ways of their own, w hereinthey recede far from the rest? But that their courses may beknown, it is necessary to have a collection of all the ancient ob-I nervations about comets; for their appearances are so rare, that

their