had generally been supposed single, consists of two exactly in the same plane, and that these both revolve on their axes in the same time as Saturn, and in the plane of Saturn's equator. He also saw the ring when it had disappeared to other observers, either from the reflection of the edge, or from the dark side enlightened by the reflection of Saturn, as we see the whole moon near new moon. He observes that the ring is very thin, compared with its width, its thickness being only about 1000 miles. The outside diameter of the larger ring is

200000

At the mean distance of Saturn, the apparent diameter of the larger ring is 47".

153. Dr. Herschel tells us, he suspects two rings to the Georgium Sidus, perpendicular to each other, but at present can only hint at so curious a circumstance.

ON COMETS.

154. Comets are luminous bodies, occasionally appearing, and generally in the part of the heavens, not far from the sun. They are not so bright as the planets, but have somewhat of a nebulous appearance. They do not appear long together; some are seen only for a few days, and those that appear longest, only for a few months. It is probable that they receive their light from the sun, although this cannot be exactly proved. In the direction of their motion about the sun, they differ from the planets, some being direct, and others retrograde. Their paths, with respect to the ecliptic, are also very different: some move in a

a It ought to be noticed that this is doubted by Harding and Schroeter. See Conn. des. Temp. 1808, p. 429.

direction nearly perpendicular to it. But the most striking phænomenon, and what makes them objects of attention to all mankind, is the tail of light which they often exhibit. When approaching the sun, a nebulous tail of light is seen to issue from them in a direction opposite to the sun : this, after having increased, again decreases till it disappears. The stars are visible through it.

155. Very many comets have been recorded in history; the motions of at least one hundred have been computed. It may be sufficient to observe here, that they move about the sun in eccentric ellipses, the sun being in one of the foci. The other considerations of their orbits and motions, are deferred till after the account of the discoveries of Kepler. Little would have been known on this subject but for the discoveries of Kepler and Newton; and although the discoveries of Kepler might by analogy have led to a knowledge of the motion of comets, yet nothing of consequence was done till Newton himself illustrated the subject.

156. The appearance of one comet has been several times recorded in history, viz. the comet of 1680. The period of this comet is 575 years. It exhibited at Paris a tail 62o long, and at Constantinople one of 90°. When nearest the sun it was only

part of the diameter of the sun distant from his surface; when farthest, its distance exceeded 138 times the distance of the sun from the earth.

157. When the theory of the motion of comets was understood, Dr. Halley examined the comets that had been previously recorded in history, and been observed by astronomers. In general, he found the circumstances so vaguely delivered, or the observations so inaccurately made, that he was able to determine with much probability the identity of only one comet. He supposed also that the comets observed in 1532 and 1661 were the same, and, that therefore it might be expected again

in 1789; but it did not appear. However Dr. Halley was very doubtful of their identity, on account of the imperfection of the observations of Apian in 1532. Farther notice will be taken of this, when we mention more particularly the return of comets. The comet, which Dr. Halley predicted with a degree of confidence, returned in 1759. It had been previously observed with accuracy, in 1682 and 1607, and had also been noticed in 1531, 1456, and 1305. Its return was anxiously looked for by astronomers, and some curious circumstances attending it will be afterwards noticed. With what satisfaction it was received by the scientific part of mankind may easily be conceived, and how strikingly contrasted with the reception of the same comet in 1456, when all Europe beheld it with fear and amazement. The Turks were then engaged in the successful war, in which they destroyed the Greek empire; and Christians in general thought their destruction portended by its appearance. We may nearly certain that this comet will re-appear again in 1834.5

be

158. With respect to the tails of comets, little satisfactory can be offered, in recording the various opinions on this subject. According to Sir Isaac Newton, they arise from a thin vapour, sent out from the comet, by the heat of the sun, and supported in the solar atmosphere.

This hypothesis has been controverted by several authors, and very ably by Dr. Hamilton, late Bishop of Ossory.

Dr. Hamilton supposes the tails of comets, the aurora borealis, and the electric fluid, to be matter of the same kind. He supports this opinion by many strong arguments, which are found in his ingenious essay on the subject. According to his hypothesis, it would follow, that the tails are hollow; and there is every reason to suppose this, from the scarcely perceptible diminution of the lustre of the stars seen through them. He supposes that the electric matter, which continually escapes from the planets, is brought back by the assistance of the comets.

But much is yet to be known on this subject. Objections may be made to his hypothesis, although so ingeniously supported. According to the opinion of Kepler, the rays of the sun carry away some gross parts of the comet, which reflect other rays of the sun, and give the appearance of a tail.

CHAPTER IX.

CONSIDERATIONS ON THE SOLAR SYSTEM AND FIXED STARS.

159. MANY of the principal phænomena have now been examined, and the chief steps gone over, by which we arrive at the true arrangement, and motions of the bodies, that, on first viewing the heavens, are considered as all placed in the imaginary concave surface. The true motions have been distinguished from the apparent, and the magnitudes of the sun, moon, and planets have been ascertained, as also their situation with respect to the planet on which we live. This arrangement, that, with reference to the Sun, ought strictly to be called the Solar, is usually called the Copernican system. To give due honour to the memory of the discoverer, this name ought to be preserved; but, in retaining it, especial care should be taken, that the name attached may not occasion it to be ranked as a system of conjecture. It is not a system of hypothesis, but the system of nature.

160. The next steps in the science are the considerations of those observations, by which the motions of the celestial bodies may become more accurately known. An accurate knowledge of the laws of their motions is necessary to point out their places at any future period, and predict those phænomena which are a source of delight to the learned, and of fear to the ignorant. Long since mankind applied the motions of the celestial bodies to assist the sciences of Geography and Navigation. In more modern times it has been found, that the improved state of these