SECTION XI-OF SATURN.

Next beyond Jupiter in the solar system, is Saturn. Before the discovery of Herschel, Saturn was considered the most remote of the planets. It shines with a dim feeble light.

The most remarkable phenomenon of Saturn, is a broad ring with which he is encompassed. This ring consists of two concentric rings, detached from each other, and from the body of the planet. It is inclined to the ecliptic in an angle of 31°. It is visible to us, only when the sun and earth are on the same side of its plane. The two parts of the ring lie in the same plane, revolving about an axis perpendicular to that plane, in 10h. 32′ 15′′. It casts a deep shadow on that part of the body of Saturn, which is opposite to the sun. Each half of the planet in succession must be involved in this dark shadow, during one half of the planet's annual revolution, almost 15 of our years. For the same term, each in succession must enjoy the light of the double ring, a light more brilliant than that of the planet himself.

In Plate II, Saturn and his double ring are represented as in the greatest view, when seen from the earth; in Plate III. Fig. 22, as they would appear to a spectator, placed in a line at right angles to the plane of the ring. In Plate III. Fig. 23, a position of Saturn is represented, when the ring is very oblique to the observer.

So the books tell us. But if the breadth of the inner ring be equal in different parts, or measured at the same place with the diameters, why should it not be about 19,000 miles, instead of 20,000? A small eiror seems apparent in the stated breadth of the exterior ring.

Dr. Herschel considers the ring of this planet, not any shining matter, or aurora borealis, as some have supposed, but a solid body, of equal density with the planet. He is also of opinion, that the edge of the ring is not flat, but is spherical, or rather spheroidical in its form.

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The surface of Saturn is diversified with dark spots and belts. Five belts, nearly parallel to the equator, were observed by Huygens. Several, nearly parallel to the ring, were seen by Dr. Herschel. These belts appear more extensive, in proportion to the body of the planet, than those of Jupiter.

By spots on the surface of Saturn, changing their position, Dr. Herschel ascertained the period of Saturn's diurnal rotation to be 10h. 16' 0.44′′.

The sun's light and heat, to an inhabitant of Saturn, must be about 90 times less than they are to us.

Saturn, viewed with a good telescope, appears of a spheroidical figure. What is remarkable, the flatting at the poles does not seem to begin till a high latitude, 43° 20′. The proportion of his disk, according to Dr. Herschel, is, Diameter of the greatest curvature, Equatorial diameter,

Polar diameter,

36

35

32

ELEMENTS OF SATURN.

Saturn has seven satellites, which revolve about their primary, and accompany him round the sun.

The seventh satellite of this planet, reckoned by some the fifth, surpasses all the others but one in brightness, when at its greatest western elongation, but is very small at other times, entirely disappearing at its greatest eastern elongation. This phenomenon, first observed by Cassini, appears to arise from one part of the satellite being more luminous than the rest. "Dr. Herschel observed this satellite through all its variations of light, and concluded, that, like our moon and the satellites of Jupiter, it turned round its axis at the same time it performed a revolution round the primary planet.”—Dr. Brewster.

"There is not, perhaps," says Dr. Herschel," another object in the heavens, that presents us with such a variety of extraordinary phenomena, as the planet Saturn; a magnificent globe, encompassed by a stupendous double ring; attended by seven satellites; ornamented with equatorial belts; compressed at the poles; turning upon its axis ; mutually eclipsing its ring and satellites, and eclipsed by them; the most distant of the rings also turning upon its axis, and the same taking place with the farthest of the satellites ; all the parts of the system of Saturn occasionally reflecting light to each other; the rings and the moons illuminating the

nights of the Saturnian; the globe and the satellites enlightening the dark parts of the rings; and the planet and rings throwing back the sun's beams upon the moons, when they are deprived of them, at the time of their conjunctions."

SECTION XII.-OF HERSCHEL.

Herschel, Uranus, or Georgium Sidus, was discovered by Dr. Herschel, on the 13th of March, 1781. It had, probably, before been seen by astronomers, but was considered a fixed star. Dr. Herschel, when observing the small stars near the feet of Gemini, was struck with the appearance of one larger than the rest, but not so brilliant. Supposing it to be a comet, he observed it with telescopes of different magnifying powers, from 227, with which it was discovered, to 2010. Its apparent magnitude increased in proportion to the magnifying power, contrary to the fixed stars. By measuring its distance from some of the stars, and comparing its situation for several nights, he found, that it moved about 21 seconds in an hour. He wrote immediately to the royal society, that it might be observed by other astronomers. It was found and observed by Dr. Maskelyne, who almost immediately declared, he suspected it to be a planet. On the first of April, Dr. H. wrote to the astronomers of Paris an account of his discovery. It was soon observed by all the astronomers of Europe.

So distant is this planet, it can scarcely be discovered by the naked eye. In a serene sky, however, it appears like a star of the sixth magnitude, with a bluish white light, and a brilliancy between Venus and the moon.

Six satellites, accompanying Herschel, have been discovered.

"The most remarkable circumstance," says Rees' Cyclopædia, "attending these satellites, is, that they move in a retrograde direction, and revolve in orbits nearly perpendicular to the ecliptic, contrary to the analogy of other satellites; which phenomenon is extremely discouraging, when we attempt to form any hypotheses, relative to the original cause of the planetary motions."

*. The planet is denoted by this character as the initial of the name, the horizontal bar being crossed by a perpendicular line, forming a kind of cross, the emblem of christianity; denoting, perhaps, that its discovery was made in the Christian era."