Its present appearance is like that of a small round nebula, without any tail. It is rather brighter at the centre than at the edges; but still its light is so faint, that it is not seen without difficulty. It does not influence any other body of the solar system; but itself suffers some perturbations from the attractions of the planets. Hence we infer that this, like most of the other comets, is little more than a vaporous globe of matter, without any specific solidity. This comet would be comparatively insignificant, but for the aid which it affords in establishing the undulatory theory of light. According to this theory, space is considered to be filled, apart from the atmospheres of planets, with an extremely rare medium termed ether, by the undulations of which light is propagated. This medium, though offering no sensible resistance to the motions of the planets, was thought to be likely, if existing, to impede to a certain extent the passage of so airy a thing as a comet. Accordingly, Professor Encke, having calculated the return of this comet upon the consideration of the vacuum of space, the actual time of the comet's return differed two days from the calculated return, in consequence, as asserted, of a resisting operation upon the motion of the comet in its path. But the ultimate effect of this retarding influence was discovered in hastening, not in deferring, the time of the appearance of the comet! for, as the all-pervading ether caused the comet to advance somewhat more slowly, the centrifugal force of the comet was diminished, whereby the solar attraction for it was increased; so that, by being drawn gradually nearer to the Sun, the time for its completing its revolution is, in consequence, dimi nished. In the course of time, therefore, this comet must fall to the Sun. Like reasonings have been made to apply to other comets, and also to the Earth and the other planets, in order to convey to our minds the great moral lesson, that all things have an end. The Psalmist thus addresses the Almighty, (Ps. cxix. 96,) "I have seen an end of all perfection, but thy commandment is exceeding broad;" implying thereby, that all created nature is bounded by limits which, however wide, are within the grasp of the Maker of all things. Biela's comet had been noticed in 1772 and 1806, and when it appeared in 1825, Biela, a German astronomer of Josephstadt, calculated the period of its revolution, and predicted its return in 1832:-this prediction was fulfilled, and the period of the revolution of this comet is considered to be established at 2440 days. Its last appearance was in 1832. It is spoken of as a small, insignificant comet, without a tail, or any appearance of a nucleus. The inclination of its orbit to the ecliptic is about 131°; and so near does its path approach that of the Earth at the intersection of the planes, that it is considered that, in 1832, if the Earth had been one month in advance of its annual journey, it would have passed through this comet. These are the only comets of whose periods we have any certain knowledge. There are, however, two others whose periods have been stated; but the times are not yet arrived for testing the accuracy of the predictions. One of these comets appeared in 1264, and again in 1556—an interval of 292 years; and it is expected again in 1848. The other comet, seen in the engraving, was observed by Newton in 1680, and he predicted its return in 575 years: a long period must therefore elapse before the truth of this calculation, or conjecture, can be verified. The tail of this comet, which is several times noticed in history, was of such vast extent, that when the head of the comet was in the horizon, the extremity of its tail was in the zenith. This observation was made at Constantinople. Of such sights as these, well might Dr. Young say— Hast thou ne'er seen the comet's flaming flight? The illustrious stranger passing terror sheds Of length enormous. A remarkable comet appeared in the year 1770, which was found to revolve in a period of about five years; but the prediction of its return was not verified, in consequence of its getting too near the satellites of Jupiter, which, though undisturbed themselves by the comet, so deranged the motions of the comet as to thrust it from its original path, and it has never been seen again. If this be correct, it shows that the mass of the comet is spare indeed. The comet of Encke is observed to grow less as it approaches the Sun, and to amplify at its departure from the solar presence; owing, it is said, to the vaporizable matter being converted into gas. The tail of the splendid comet of 1680, noticed by Newton, 120,000,000 miles in length, was observed to issue forth from the head of the comet in two days. This tail, being opposed to the Sun, is supposed to have been caused by the Sun to be emitted. The tail of the comet of 1769 was 48,000,000 miles in length, and that of 1811 was 108,000,000 miles in length. The diameter of the body of this last was 50,000 miles. It is thought that the matter of the comets is discharged at their tails, by the agency of the Sun; and that the gravitating power of the comet is not sufficient to re-collect it. CHAPTER XII. FIXED STARS. GENERAL PARTICULARS. POSITIONS OF HEAVENLY BODIES. HOW TO DESCRIBE OR FIND THEM. CLASSIFICATIONS AND DISTINCTIONS OF STARS. THEIR PARALLAX. LUNAR DISTANCES, AND THE LONGITUDE. THE GALAXY, OR MILKY WAY. DOUBLE AND TRIPLE STARS, &c. NEBULE. STARS CONSIDERED TO BE SUNS. POLE-STAR. CONSTELLATIONS OF THE GREAT AND LITTLE BEARS, &c. THE CROSS. SIDEREAL DAY. OCCULTATIONS. SHOOTING STARS. When I behold this goodly frame, this world, Their distance argues, and their swift return Around this spacious earth, this punctual spot, For ought appears, and on their orbs impose That better might with far less compass move, |