diameter of the exterior ring is 176,000 miles. The rings are extremely thin; their thickness, according to Sir J. Herschel, does not exceed 100 miles.

When the rings are examined by telescopes of moderate power, they appear as one, the interval between them not being perceptible; but this interval becomes distinctly seen when those of high power are used, appearing as a black line or narrow band, as represented in the figure.*

327. Inclination and rotation of the Rings. It has been ascertained that the rings coincide, or very nearly so, with the plane of Saturn's equator. They must, therefore, be inclined to the plane of the ecliptic in the same angle that the axis of the planet is inclined to the axis of the ecliptic; that is, in an angle of 28° 40′ (325). It is also found that the plane of the equator and rings, and, consequently, the line in which it intersects the plane of the ecliptic, remain parallel to themselves as Saturn makes his revolution in his orbit. From this it follows, that the axis of Saturn, like that of the earth, continues parallel to itself

From observations of some parts of the rings less bright than others, it has been inferred that they revolve in their own plane, making a revolution in about 10h. 29m. It is worthy of remark, that this is nearly the time in which a satellite, at a distance from Saturn, corresponding to the middle of the rings, would revolve round the planet.

328. Varying appearance of the Rings and their disappearances. As the plane of the rings continues parallel to itself, and the angle of their inclination to the ecliptic is not large, the face of the rings can never be turned directly to the earth, or very nearly so; and they do not, therefore ever present to us a circular appearance. Being seen obliquely, they must like all circular rings, when thus viewed, appear elliptical; the degree of ellipticity varying

* Some observers have thought they have detected lines of divisions in the exterior ring, indicating, that instead of one, there were several. But the existence of these divisions is not fully established.

according to the greater or less obliqueness of their position, which, in consequence of the motions of Saturn and the earth, is continually changing.

Let S, Fig. 54, be the sun, eae' the orbit of the earth, and ABCD the orbit of Saturn, which we may here suppose to coincide with the plane of the ecliptic; and let the parallel lines in the figure be the lines in which the plane of the rings intersects the plane of the ecliptic, in the positions of Saturn, to which they are drawn. Then, it is evident that when Saturn is in either of the positions A and C, the plane of the rings must pass through the sun, and only the edge of the exterior ring is illuminated. In these positions, the longitudes of which are 170° and 350°, the rings, in consequence of their being extremely thin, are invisible, except with a telescope of the very highest power. With such an instrument a fine line of light has been perceived, extending to some distance on each side of the planet.

It is not only at the positions A and C, that the rings are invisible. They usually disappear twice about each of these positions, remaining invisible some weeks at each disappearance. To understand this, suppose that as Saturn approaches A, the earth is moving in the part e'ea of its orbit. There must then be a time at which the line es, joining the earth and Saturn will become parallel to CA. At this time, the plane of the rings must pass through the earth, and only the edge being towards it, they are invisible. After this, while the earth is moving from e to some position a, and Saturn from s to A, the plane of the rings passes between the sun and earth, and the enlightened face is turned from the earth. Hence, as during this period, only the edge of the enlightened part of the rings is towards the earth, they remain invisible. When the planet has passed the position A, the sun and earth are both on the same side of the plane of the rings, the illuminated face is towards the earth, and the rings are again visible. This continues to be the case till the earth and planet attain the positions e' and s', when the plane of the rings again passes

through the earth, and the rings become invisible. They continue so till the earth and planet arrive at the positions e" and s", when the plane of the rings a third time passes through the earth. After this, the illuminated face is turned towards the earth and the rings are visible till the planet approaches the opposite position C, when two other disappearances usually take place.*

The illuminated face of the rings must, obviously, be most turned towards the earth when the planet is at or near the positions B and D, midway between A and C; and the rings must then appear most open. They have then nearly the appearance represented in Fig. 53.

While Saturn is in the part ABC of his orbit, that is, from 170° to 350° of longitude, the northern face of the rings is illuminated, and in the other part, the southern face.

329. Period of the disappearances of the rings. As the period of Saturn's revolution is about 29 years, nearly 15 years must elapse from the time he is at A till he is at C or from C to A, and this must be nearly the period from one set of dissappearances to the next. The last two took place about the position A, in the latter part of 1832 and towards the middle of 1833; the next will occur in 1847.

330. Saturn's Satellites, The seven satellites of Saturn revolve round him in periods varying from about 1 day to 79 days, and at distances varying from about 3 to 64 radii of the planet. The seventh satellite is the most conspicuous; that, and the sixth, may be discerned with telescopes of moderate power. The third, fourth and fifth can only be seen with a telescope of much higher power; and the first and second only with a telescope of great power.

The seventh satellite, like those of Jupiter, exhibits periodic defalcations in its light, from observations of which,

* The figure and illustration are adapted to the last two disappearances. It is obvious that the order and durations of the disappearances will be affected by the position of the earth when the plane of the rings first intersects the earth's orbit.

it has been inferred that it revolves on its axis in the same. time that it makes a revolution round the planet.

URANUS AND HIS SATELLITES.

331. General Remarks. Uranus is, so far as is known, the remotest planet of the solar system.* His distance is so great that though a large planet, he is barely discernible by the sharpest sight, without the aid of a telescope. His apparent diameter, which varies but little, is about 4".

332. Period, Distance, &c. of Uranus. Uranus revolves round the sun, in about 84 years, at the distance of 1800 millions of miles. His diameter is about 35,000 miles, and his bulk about 80 times that of the earth.

333. Satellites of Uranus. According to the observations of Sir William Herschel with his great telescope, Uranus is attended by six satellites revolving with retrograde motions in circular orbits nearly perpendicular to the plane of the ecliptic. These anomalies in their motions and in the positions of their orbits, led some to doubt the correctness of the observations. But Sir J. F. W. Herschel has lately confirmed his father's observations with regard to two of them.

VESTA, JUNO, CERES AND PALLAS.

334. General Remarks. The planet Ceres was discovered on the first day of the present century, and the other three, in the course of a few following years. These four planets, though far less distant than some others, are so extremely small, that but little knowledge has been obtained in regard to them, except the elements of their orbits.

335. Periods and Distances. Vesta and Pallas revolve

Uranus was discovered by Sir William Herschel in 1781, and was named by him, Georgium Sidus, in honour of his patron, King George III. By the French it was for a time called Herschel. It is now generally known by the name in the text.

round the sun in about 3 and 4 years, at the distances of 225 and 263 millions of miles. The periods and distances of Juno and Ceres, are between the two former.

CHAPTER XVII.

ON COMETS.

336. General Remarks. Comets revolve about the sun, shine by reflecting his light, and are retained in their orbits by his attraction. But, in almost every thing else, they differ widely from the planets. They are not confined to the zodiac or adjacent regions, but traverse all parts of the heavens, and in all directions; the motions of some being direct, and others retrograde. They generally move in elliptical orbits of great eccentricity, alternately approaching comparatively near to the sun, and receding to immense distances; and some of them moving off into the boundless regions of space, never return. They continue visible only for a few weeks or months; and some only for a few days; being only within the reach of observation, while in those parts of their orbit that are adjacent to their perihelions. They are not solid bodies like the planets, but seem to consist mainly of masses of vaporous matter.

Some comets have presented very splendid appearances; and others, though less brilliant, have still been conspicuous objects; but, by far the most numerous class are barely discernible by the naked eye, or can only be seen by the aid of the telescope.

337. Appearance of a Comet. A comet of the more conspicuous class, usually consists of a small, bright, central part or nucleus, enveloped to a considerable extent by an ill-defined, nebulous mass of light, called the coma; the