EMERSIONS. 1st Satellite, 2d day, 30 m. after 6 evening. The Moon and Venus will be in conjunction at 4 m. after 7 in the evening of the 3d of this month. Mercury will attain his greatest elongation on the 10th, and Venus on the 14th. Venus will also be in conjunction with a in Scorpio on the 13th; and the Moon with Pollux at 18 m. after four in the afternoon of the 21st. She will also be in conjunction with Mercury at four in the morning of the 29th ; and with in Scorpio at 12 m. after 9 at night of the 31st. On COMETS in general, and those of 1807 and 1811 in particular. [Continued from page 233.] The two comets which appeared in 1807 and 1811 were the most conspicuous of any that have lately visited our system; and not only created a very lively interest in astronomers, but excited a great curiosity in the mind of the public. Numerous observations were made upon the first of these by both English and foreign astronomers, for determining the elements of its orbit, as well as for ascertaining its physical constitution. This comet first appeared about the end of September; and Mr. Gavin Lowe made fourteen observations upon it from the 4th of October to the 12th of November. From the data obtained by those observations he calculated its ele ments, according to the rules given by Sir Henry Englefield, in his Treatise on Comets. These ele ments are, taking the mean distance of the Earth from the Sun for unity, Perihelion distance Longitude of the perihelion Longitude of the ascending node 0'64802 28° 41' in Scorpio. Time of passing the perihelion Sep. 18d 22h 10m 13° 11'Gemini. Time of passing the ascending node Sep. 29 18 48 24 43 Inclination of the orbit to the plane of the } 63 ecliptic 63 15 In these elements, it is astronomical time that is used, and therefore the time of passing the perihelion, if reduced to common time, would be at 10 m. after 10 in the morning of the 19th of September, and that of passing the ascending node would be 48 m. past 6 in the morning of the 30th. Dr. Herschel's observations upon this comet were principally directed towards ascertaining its physical constitution. They were begun on the 4th of October, 1807, and continued till the 21st of February, 1808. On the 19th of October, it appeared to be less than the third satellite of Jupiter, its apparent diameter being only about 1". According to his calculations, the following elements answered to that day, viz. Perihelion distance Distance from the Earth 0.647491 Its distance from the ascending node of 730 45' 44" its orbit And its diameter in English miles 538. The observations which Dr. Herschel made on the light of this celestial visitant were from the 4th to the 19th of October, during which period the comet uniformly preserved the appearance of a planetary disc fully enlightened by the Sun; it was every where equally bright, round, and well defined. Now, as that part of the disc which was then visible to us could not possibly have a full illumination from the Sun, but ought to have had a defalcation of light, the doctor inferred that it did not shine by the reflected rays of the Sun alone. He also con cludes that the nucleus had a real disc, and he thinks it was composed of dark and solid matter, like the planetary bodies. The doctor's last observation was made on the 21st of February, 1808, when the comet was still a very conspicuous object, though almost three times the mean distance of the Sun from the Earth, and nearly 2-7 from the Sun. The circumstance of its being so conspicuous at such a distance affords great support to the opinion of its self illumination; which property, if correct, constitutes a further distinction between cometary and planetary bodies than had previously been pointed out. The perihelion distance found by Dr. Herschel also agrees very nearly with that obtained by Mr. Lowe, as above stated. This comet was first discovered in the constellation of the Serpent, and travelled nearly at the rate of a degree a day through those of Hercules and the Lyre, till it ceased to be visible. M. Olbers, of Bremen, embraced all favourable opportunities of observing it till the 19th of February, 1808, when, his observations being interrupted by illness, M. Bessel continued his observations in the observatory of Lilienthal, near Bremen, till the 24th of the same month; and the following elements were calculated by him, from these observations and others taken at Bremen. The time here mentioned is mean time at Paris. Time of passing the perihelion, Sep. Perihelion distance Longitude of the ascending node Inclination of its orbit Period of its revolution 1900 years. 18 d. 7 h. 59 m. 48 s 9 s. 8 s. 26° 46' 0° 56' 0.64648 3" 63 10 53 The time of this revolution is by no means to be relied upon as that which will actually take place; for even admitting the calculation to have been made with all possible care and correctness, the perturbations this comet may experience in the course of so long a period may greatly alter its return. For besides the influence it may experience from the large gravitating bodies of our system, may it not be af fected by the bodies of other systems, into which the distant part of its orbit may enter in the course of so long a revolution? In that part, the velocity in its orbit will also be the least, and therefore the perturbations would be most easily effected. The comet which appeared so brilliant and beautiful in the autumn of 1811, moved in an orbit very favourably situated for astronomical observations; and Dr. Herschel availed himself of this circumstance to examine all parts of it with scrutinizing attention, by telescopes of every degree of requisite light, distinctness, and power. His observations were chiefly made with a view to ascertain the most interesting particulars of its physical construction. These observations were communicated to the Royal Society, and inserted in their volume for 1812; and from which the following account of the constitution of this celestial stranger is principally derived. In an early part of his observations, this eminent observer remarked something like a distinct luminous body near the centre of its head, or what some astronomers would call the nucleus. He observed this luminous part to change its relative situation in the head, sometimes appearing nearer to and at others more remote from the edge nearest the Sun; its brightness was also subject to considerable variation. Dr. H. was soon led to infer that the comet enveloped a real planetary body; and from a series of observations which he made on the 19th of October, at the time when the comet was 114 millions of miles from the Earth, with magnifying powers from 169 to 600, he ascertained that the diameter of the planetary body was about 428 miles, and that it was surrounded with a cometic atmosphere. Numerous observations induced him to conclude that this body was globular. Its colour was a pale ruddy tint, like that of such equally small stars as are inclined to red. The phasis of its illumination, at the time he made the observation, was to a full disc as 1.6 to 2. From this (he observes), as well as from the high magnifying power, which a point so faint could not have borne with advantage, had it shone with reflected light, we may infer that it was visible by rays emitted from its own body.' On the 19th of October, the apparent magnitude of this disc was only 0"-775. The colour of the head which enveloped this body was very remarkable. Its light appeared to be greenish or bluish green. Its apparent magnitude, when viewed with a 20 feet telescope, on the 6th of October, was 3′ 45′′. And from calculation it appeared that the real diameter of the head at that time was 127 thousand miles. This head was surrounded by a transparent cometic atmosphere, of about 15' apparent diameter, and more than 507 thousand miles in real diameter. The head was also partly surrounded by a train of yellowish light, which was kept at a considerable distance by an interval of comparative darkness. This light was apparently circular on one side of the head, and concentric with it; but, after extending about half round, it divided into two streams, one passing off by each side of the head. The apparent diameter of this envelope, considering the semicircle as completed, was about 19', and its real diameter more than 643 thousand miles. The greatest observed length of the tail was on the 15th of October; the atmosphere was very clear, and the length was 23 degrees, which gave a computed length of 100 millions of miles: its greatest breadth was about 15 millions. The appearance of the tail was exactly like that of the milky way, where it is destitute of stars. Having given a detailed account of the observations from which he obtained the preceding conclusions, Dr. Herschel enters on inquiries relative to the real construction of the various parts of the comet. From these he concludes that the figure of both the planetary body and the head, as well as that of its trans |