tible; 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 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

+ Appearances of other lines of division have been seen occasionally by several observers, and sometimes under circumstances which seemed to leave no room to doubt the existence of one or more subdivisions in each ring.

The most interesting discovery recently made in reference to the rings of Saturn, is that of a new ring, of an obscure, dusky appearance, interior to the inner, principal one. This was observed at about the same time in November, 1850, by Mr. Bond of Cambridge, United States, and by Mr. Dawes of England. The breadth of the obscure ring is estimated to be 1".7, or two-fifths of the interval between the surface of the planet and the inner, bright ring.

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

* 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.

turned towards the earth when the planet is at or near the posi tions 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 disappearances to the next. The two to which the above illustration refers, took place about the position A, in the latter part of 1832 and towards the middle of 1833; the next occurred in 1847.

330. Saturn's satellites. The eight satellites of Saturn revolve round him in periods varying from 1 day to 79 days, and at distances varying from 3 to 64 radii of the planet. The eighth 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, second, and seventh only with a telescope of great power.

The eighth satellite, like those of Jupiter, exhibits periodic defalcations in its light, from observations of which it has been inferred, that it revolves on its axis in the same time that it makes a revolution round the planet (323).

The discovery of the sixth of these satellites was made by Huygens, in 1655; that of the third, fourth, fifth, and eighth by Cassini between the years 1670 and 1685; that of the first and second by Sir William Herschel, in 1789; and that of the seventh by Mr. Bond of Cambridge, Mass., and Mr. Lassell of Liverpool, in 1848. Mr. Bond having seen it about 48 hours earlier than Mr. Lassell.

Some authors have distinguished the satellites of Saturn by reversing the order of the numbers above and calling the exterior one the first, &c. Sir John Herschel, in a work published in 1847, in alluding to the inconvenience arising from this uncertainty, says: "Should an eighth satellite exist, the confusion of the old nomen

clature will become quite intolerable," and proposes the following mythological names: 1st. Mimas; 2d. Enceladus; 3d. Tethys ; 4th. Dione; 5th. Rhea; 6th. Titan; 8th. Iapetus. In accordance with this system, the seventh satellite recently discovered has been named Hyperion.

URANUS AND HIS SATELLITES.

331. General remarks. Uranus was discovered by Sir William Herschel in 1781, and was named by him the Georgium Sidus, in honour of his patron, King George III., which name, abbreviated to the Georgian, was retained in the Greenwich Nautical Almanac until the year 1851. By the French it was for a time called Herschel and by others Uranus. It is now universally recognised by its mythological name. The distance of Uranus is so great that, though a large planet, he is barely discernable 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 in 1833, Sir John Herschel confirmed his father's observations with regard to two of them; and in 1847, Struve at Pulkova, and Lassell at Liverpool, observed these two with a third several times. On one occasion, Mr. Lassell believed he saw still another.

NEPTUNE.

334. General remarks. Neptune is, so far as is known, the remotest planet in the solar system. The distance of this planet is so great that, though next to Saturn in size, it can never be seen

with the naked eye, and through ordinary telescopes it has the appearance of a small star. Its apparent diameter is about 3" and it is only through telescopes of high power that it presents a measurable disc.

335. Period, distance, &c., of Neptune. Neptune revolves round the sun in about 1641 years, at the distance of 2850 millions of miles. Its diameter is not much less than 40,000 miles, and its volume about 100 times that of the earth.

336. Satellite of Neptune. Neptune is attended by at least one satellite, and analogy favours the presumption that there are several. This satellite was first seen on the 10th of October, 1846, by Mr. Lassell of Liverpool, and has since been observed many times. by its discoverer and also by Mr. Bond of Cambridge. It revolves round its primary in 5 days and 21 hours at the distance of about 230,000 miles. From the motions of the satellite, the mass of the planet has been deduced with considerable exactness. It is 19000 part of the sun's.

337. History of the Discovery of Neptune. Soon after the discovery of Uranus, by an examination of the catalogues of the fixed stars, it was found that the place of the planet had been recorded nineteen times, once as early as 1690, as that of a fixed star. In 1821, M. Bouvard of Paris published tables for computing the place of this planet (279), founded on the observations made since 1781, a period of about 40 years. In preparing these tables he discussed all the observations made during a period of 130 years; and, finding it impossible to represent the observed motion of Uranus during all this period by one set of elements and the perturbations (279) produced by the known planets, he rejected those made prior to 1781, attributing the discrepancies to imperfections in the ancient observations, or to "some extraneous and unknown influence which has acted on the planet." Soon after the construction of these tables, however, the planet was found to be departing from the path assigned by them to an extent that could not be ascribed to errors of observation. The difference between the observed and computed places amounted to nearly 1′.5 in 1840, by which time the belief in the existence of a trans-Uranian pla