pretty generally in Europe until the 10th or 11th of April. It was found, on its return from the sun, at the observatory, Cape of Good Hope, on May 2nd, and continued visible there for three weeks. It is generally described as round, bright, and so well defined, as almost to exhibit a planetary disc. This agrees with Professor Argelander's description of the comet's appearance in 1825. The strong evening twilight doubtless hid a great portion of the more feeble nebulosity surrounding the nucleus, only permitting the denser part in the immediate vicinity of the comet's centre to remain visible. The real diameter of this condensed portion appears to have been about 25,000 miles.

The next appearance in the summer of 1845, took place under the most unfavourable circumstances; nevertheless, a very accurately computed ephemeris of its geocentric path was published by M. d'Arrest, of Berlin. Though the attention of astronomers was closely directed to the comet, it was seen in Europe by one observer only, the late Professor de Vico, who, favoured by the clear sky of Rome, observed it on the nights of July 9th and 14th. In America, two observations were procured, the first at Philadelphia, on the 4th, and the second at Washington, on the 10th of the same month. The error of the predicted place was less than 45 seconds of space, a most satisfactory proof that the theory of the comet's movements was well understood.

The next return occurred in the autumn of 1848, when its apparent track in the heavens was such as to allow of extensive observation in these parts of the

globe. The comet arrived at perihelion only five days later than in 1805, so that its path amongst the stars was not very different in the two years, and, judging from previous experience, Prof. Encke considered there would be little chance of any exact observations before the commencement of September. It was found by Professor Bond, with the great telescope, at Cambridge U. S., on the 27th of August, and by several astronomers during the first week of September. It was last seen by Professor Bond, on the 26th of November. When first visible in large telescopes, it was faint and without any condensation of light. On September 24th, the nebulosity extended over eight minutes of arc, or about 140,000 miles. Two days later, a faint brush of light extending from the more condensed part towards the sun, was detected by Professor Bond, and on the 6th of the following month, the same observer found it just visible to the naked eye. On the 22nd it was still distinctly visible without the telescope: the general outline was elliptical, and the light appeared strongest on the side opposite the sun. Early in November, the comet exhibited a tail in the usual direction, extending over a space of between one and two degrees, while the same emanation from the head towards the sun, that had been remarked in September, was again observed. The last observation of the American astronomer, on November 26th, was taken after a very close approach to the planet Mercury. About midnight, on the 22nd of that month, the comet was separated from the planet, by only 0.038 of the earth's mean distance, or 3,600,000 miles, a

much nearer approximation than that of August, 1835, from the effects of which the mass of Mercury at present received, was investigated. The observations of 1852, and subsequent years, will, therefore, lead to a more correct value of this element, for they will contain in them, the effects of the planet's perturbations, expressed as the mathematician can read them on the comet's geocentric places.

The last perihelion passage occurred on March 14th, 1852, and the comet was seen in the second week of January, and remained visible two months. Early in March, it was very distinct in the strong evening twilight. Its appearance, when best seen this year, was similar to that presented in 1845; it resembled a nebulous star, with something of a planetary disc. In 1858, the track of the comet amongst the stars will be more favourable for observation than it was in 1852, or is likely to be in 1855. The dimensions of the orbit of Encke's comet are as follow:

:

CHAPTER VI.

COMETS OF SHORT PERIOD-continued.

The Comet of Biela.

ANOTHER very remarkable periodical comet is that usually called BIELA'S, though it has occasionally been termed Gambart's comet.

On the 8th of March 1772, M. Montaigne of Limoges, discovered a comet in Eridanus which he saw until the 20th of the same month, though for want of proper instruments he could observe it but very imperfectly.

M. Pons discovered a telescopic comet on the 10th of November, 1805, which was observed until the 9th of the following month. The elements very much resembled those of the comet of 1732, and the identity was hinted at by several astronomers, including Olbers and Gauss. The latter remarked that an ellipse with the greater semi-axis = 2·82 or longer, would agree better with the observations than a parabola, and his last calculation assigned as the most probable period 1732 days. Notwithstanding this circumstance it does not appear that any prediction of the return of the comet was ventured upon, and

the question remained in this state until 1826, when an unexpected discovery again drew attention to the subject.

On the evening of the 27th of February, 1826, M. Biela, of Josephstadt in Bohemia, detected a telescopic comet in the constellation Aries. It presented a small round nebulosity, with a feeble condensation of light towards the centre. The same comet was recognised independently by M. Gambart at Marseilles on the 9th of March, when it was situate in Cetus: there was no trace of tail or nucleus, and the diameter did not exceed 1. It was very soon found that the elements bore a striking similarity to those of the comets of 1772 and 1805, and what was more important, that no parabolic orbit would represent with reasonable accuracy the observed path of the comet. Clausen, Biela, and Gambart, determined the elements of elliptic orbits, the period differing but little from 67 years. The identity of the comets of 1772, 1805, and 1826, was therefore rendered extremely probable, notwithstanding some calculations by the late Professor Bessel, which appeared to throw a degree of uncertainty on this conclusion, as respects the former year: but as we have already remarked, the comet of 1772 was very imperfectly observed, and was only followed by M. Montaigne during a space of twelve days.

Supposing the period of revolution about 63 years, the comet must have appeared in 1778, 1785, 1792, 1799, 1813, and 1819, without being perceived, and such was no doubt the case, for the computations of MM. Clausen, Gambart, and others, showed that the