in 1807, by Olbers. These bodies are nearly at equal distances from the sun, their periodic times are therefore nearly the same. The eccentricities of the orbits of Juno and Vesta, and the position of their nodes are nearly the same. These small planets are much disturbed by the proximity and vast magnitude of Jupiter and Saturn, and the series which determine their perturbations converge slowly, on account of the greatness of the eccentricities and inclinations of their orbits. The inclination of the old planets is so small, that they are all contained within the zodiac, which extends 8° on each side of the ecliptic, but those of the new planets very much exceed these limits. They are invisible to the naked eye, and so minute that their apparent diameters have not yet been measured. Sir William Herschel estimated that they cannot amount to the fourth of a second, which would make the real diameter less than 65 miles. However, Juno, the largest of these asteroids, is supposed to have a real diameter of about 200 miles. Jupiter. 651. Jupiter is the largest planet in the system, and with his four moons exhibits one of the most splendid spectacles in the heavens. His form is that of an oblate spheroid whose polar diameter is 35".65, and his equatorial 38".44; he rotates in 9 hours 56 minutes about an axis nearly perpendicular to the plane of the ecliptic. The circumference of Jupiter's equator is about eleven times greater than that of the earth, and as the time of his rotation is to that of the earth as 1 to 0.414, it follows that during the time a point of the terrestrial equator describes 1o, a point in the equator of Jupiter moves through 20.41; but these degrees are longer than the terrestrial degrees in the ratio of 11 to 1, consequently each point in Jupiter's equator moves 26 times faster than a point in the equator of the earth. In the beginning of 1801 the secular variations of his orbit were, Saturn. 652. Viewed through a telescope Saturn is even more interesting than Jupiter he is surrounded by a ring concentric with himself, and of the same or even greater brilliancy; the ring exhibits a variety of appearances according to the position of the planet with regard to the sun and earth, but is generally of an elliptical form: at times it is invisible to common observation, and can only be seen with superior instruments; this happens when the plane of the ring either passes through the centre of the sun or of the earth, for its edge, which is very thin, is then directed to the eye. On the 29th September, 1832, the plane of the ring will pass through the centre of the earth, and will be seen with a very high magnifying power like a line across the disc of the planet. On the 1st December of the same year, the plane of the ring will pass through the sun. Professor Struve has discovered that the rings are not concentric with the planet. The interval between the outer edge of the globe and the outer edge of the ring on one side is 11".037, and on the other side the interval is 11".288, consequently there is an eccentricity of the globe in the ring of 0.215. In 1825 the ring of Saturn attained its greatest ellipticity; the proportion of the major to the minor axis was then as 1000 to 498, the minor being nearly half the major. Stars have been observed between the planet and his ring. It is divided into two parts by a dark concentric band, so that there are really two rings, perhaps more. These revolve about the planet on an axis perpendicular to their plane in about 10h 29m 17, the same time with the planet. The form of Saturn is very peculiar. He has four points of greatest curvature, the diameters passing through these are the greatest; the equatorial diameter is the next in size, and the polar the least; these are in the ratio of 36, 35, and 32. Besides the rings, Saturn is attended by seven satellites which reciprocally reflect the sun's rays on each other and on the planet. The rings and moons illuminate the nights of Saturn; the moons and Saturn enlighten the rings, and the planet and rings reflect the sun's beams on the satellites when they are deprived of them in their conjunctions. The rings reflect more light than the planet. Sir William Herschel observed, that with a magnifying power of 570, the colour of Saturn was yellowish, whilst that of the rings was pure white. Saturn has several belts parallel to his equator: changes have been observed in the colour of these and in the brightness of the poles, according as they are turned to or from the sun, probably occasioned by the melting of the snows. Saturn's motions are disturbed by Jupiter and Uranus alone; the secular variations in the elements of his orbit were as follows, in the beginning of 1801. 653. This planet was discovered by Sir William Herschel, in 1781. The period of his sidereal revolution is 30687 days. If we judge of the distance of the planet by the slowness of its motion, it must be on the very confines of the solar system; its greatest elongation is 103°.5, and its apparent diameter 4": it is accompanied by six satellites, only visible with the best telescopes. The only sensible perturbations in the motions of this planet arise from the action of Jupiter and Saturn; the secular variations in the elements of its orbit were, in 1801, as follow: 654. It is remarkable that the rotation of the celestial bodies is from west to east, like their revolutions; and that Mercury, Venus, the Earth, and Mars, accomplish their rotations in about twenty-four hours, while Jupiter and Saturn perform theirs in of a day. On the Atmosphere of the Planets. 655. Spots and belts are observed on the discs of some of the planets varying irregularly in their position, which shows that they are surrounded by an atmosphere; these spots appear like clouds driven by the winds, especially in Jupiter. The existence of an atmosphere round Venus is indicated by the progressive diffusion of the sun's rays over her disc. Schroëter measured the extension of light beyond the semicircle when she appeared like a thin crescent, and found the zone that was illuminated by twilight to be at least four degrees in breadth, whence he inferred that her atmosphere must be much more dense than that of the earth. A small star hid by Mars was observed to become fainter before its appulse to the body of the planet, which must have been occasioned by his atmosphere. Saturn and his rings are surrounded by a dense atmosphere, the refraction of which may account for the irregularity apparent in his form: his seventh satellite has been observed to hang on his disc more than 20' before its occultation, giving by computation a refraction of two seconds, a result confirmed by observation of the other satellites. An atmosphere so dense must have the effect of preventing the radiation of the heat from the surface of the planet, and consequently of mitigating the intensity of cold that would otherwise prevail, owing to his vast distance from the sun. Schroeter observed a small twilight in the moon, such as would be occasioned by an atmosphere capable of reflecting the sun's rays at the height of about a mile. Had a dense atmosphere surrounded that satellite, it would have been discovered by the duration of the occultations of the fixed stars being less than it ought to be, because its refraction would have re ndered the stars visible for a short time after they were actually behind the moon, in the same manner as the refraction of the earth's atmosphere enables us to see celestial objects for some minutes after they have sunk below our horizon, and after they have risen above it, or distant objects hid by the curvature of the earth. A friend of the author's was astonished one day on the plain of Hindostan, to behold the chain of the Himala mountains suddenly start into view, after a heavy shower of rain in hot weather. The Bishop of Cloyne says, that the duration of the occultations of stars by the moon is never lessened by 8" of time, so that the horizontal refraction at the moon must be less than 2": if therefore a lunar atmosphere exists, it must be 1000 times rarer than the atmosphere at the surface of the earth, where the horizontal refraction is nearly 2000". Possibly the moon's atmosphere may have been withdrawn from it by the attraction of the earth. The radiation of the heat occasioned by the sun's rays must be rapid and constant, and must cause intense cold and sterility in that cheerless satellite. The Sun. 656. The sun viewed with a telescope, presents the appearance of an enormous globe of fire, frequently in a state of violent agitation or ebullition; black spots of irregular form rarely visible to the naked eye sometimes pass over his disc, moving from east to west, in the space of nearly fourteen days: one was measured by Sir W. Herschel in the year 1779, of the breadth of 30,000 miles. A spot is surrounded by a penumbra, and that by a margin of light, more brilliant than that of the sun. A spot when first seen on the eastern edge, appears like a line, progressively extending in breadth till it reaches the middle, when it begins to contract, and ultimately disappears at the western edge: in some rare instances, spots re-appear on the east side; and are even permanent for two or three revolutions, but they generally change their aspect in a few days, and disappear: sometimes several small spots unite into a large one, as a large one separates into smaller ones which soon vanish. The paths of the spots are observed to be rectilinear in the beginning of June and December, and to cut the ecliptic at an angle of 7°20'. Between the first and second of these periods, the lines described by the spots are convex towards the north, and acquire their maximum curvature about the middle of that time. In the other half year the paths of the spots are convex towards the south, and go through the same changes. From these appearances it has been concluded, that the spots are opaque bodies attached to the surface of the sun, and that the sun rotates about an axis, inclined at an angle of 7° 20′ to the axis of the ecliptic. The apparent revolution of a spot is accomplished in twenty-seven days; but during that time, the spot has done more, having gone through a revolution, together with an arc equal to that described by the sun in his orbit in the same time, which reduces the time of the sun's rotation to 25d 9m 36". These phenomena induced Sir W. Herschel to suppose the sun to be a solid dark nucleus, surrounded by a vast atmosphere, almost always filled with luminous clouds, occasionally opening and discovering the dark mass within. The speculations of La Place were different: he imagined the solar orb to be a mass of fire, and that the violent effervescences and explosions seen on its surface are occa |