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further researches, he became satisfied of the correctness of the supposition he had made, and concluded that the variation in period must have been produced by the attractions of the other heavenly bodies. Having, therefore, made a rough calculation of the effect which the attraction of Jupiter would produce on the revolution the comet was then performing, he ventured to predict its return in the latter part of 1758, or early part of 1759. Subsequently, Clairaut, an eminent French mathematician, calculated the effects. of the attractions of both Jupiter and Saturn, and determined the time of the return to the perihelion, to be in the middle of April, 1759. It arrived there about a month prior to that time. In consequence of its return, nearly according to Halley's prediction, it has received his name.

With more ample means for correct computations, furnished by the observations during its appearance in 1759, and by the improvements in analysis, the recent return of Halley's comet in 1835, was much more accurately predicted. It arrived at the perihelion of its orbit, within less than two days of the time assigned for its return, by Pontécoulant, a distinguished French as


The least distance of Halley's Comet from the sun is 56 millions, and its greatest distance 3,350 millions of miles. The eccentricity of its orbit is 0.97 and its inclination to the ecliptic is 17° 44'. The motion of this Comet is retrograde.

345. Encke's Comet. The periodical character of this small comet, was discovered in 1819, by Professor Encke of Berlin, who identified the comet of that year with those that had been observed in 1786, 1795 and 1805, and which had been supposed to be different comets. He found its period to be only about 1207 days, or nearly 3 years; and he predicted its return in 1822, which was verified by observation. Its subsequent returns have been predicted and observed.

This comet is sometimes called the comet of short period. Its perihelion distance is 31 millions, and its aphelion distance 390 millions of miles. The eccentricity of its orbit is 0.854, and the inclination to the plane of the ecliptic is 13° 22′. Its motion is direct.

346. Resisting Medium. The observations of the successive returns of Encke's comet, show that its period is subject to a small, but continued diminution. It also appears, that this diminution is not produced by the actions of the planets. Encke, therefore assumes, that instead of a perfect vacuum in space, there must exist an exceedingly rare medium, which, opposing no perceptible obstruction to the motions of dense bodies, sensibly resists the motion of a mere mass of vapour like that of the comet. The obvious effect of such a resistance would be a diminution of the comet's velocity, in consequence of which, it would move nearer the sun, and perform its revolution in less time.*

347. Biela's Comet. This is a very small comet without the least appearance of a nucleus. Captain Biela, of Josephstadt, discovered its periodic character on its appearance in 1825.

M. Gambart of Marseilles also made the discovery, and the comet is therefore frequently called Gambart's. It performs its revolution in about 6 years, in an ellipse, whose eccentricity is 0.75; the least and greatest distances of the comet from the sun being 86 millions and 586 millions of miles. The inclination of its orbit is about 13°, and its motion is direct.

At the return of this comet in 1832, which was predicted with considerable precision, some alarm was created by the announcement that it would pass very near the earth's orbit. It did, on the 29th of October, pass within a few thousand miles of a point at which the earth arrived about one month later. Owing to its proximity to a conjunction with the sun, during this perihelion passage, it was only visible through powerful telescopes. And, in

* Encke infers, from the observations made on the comet, that the resistance, is only sensible in a portion of space round the sun, not extending beyond the orbit of Venus. He thus accounts for the fact, that the motions of Halley's comet, and another periodical one, noticed in the next article, have not indicated any resistance; for, but a very small part of the orbit of the former, and none of that of the latter, are within that distance of the sun.

The positions of Encke's comet at its returns in 1842 and 1848, as observed on several evenings, by the editor, at the observatory of the Central High School in Philadelphia, were found to be within 30" of space of its positions as given in Encke's Ephemeris, previously computed. This affords a striking evidence of the accuracy of the investigations and computations of its orbit and motion.

1839 it could not be observed at all. But, its last return was under more favourable circumstances, and it was observed from the 26th of November, 1845, until the 22d of April, 1846. On this occasion it presented the singular phenomenon of a double comet, or, two distinct comets moving through space, side by side. At first one was extremely small as compared with the other, but the smaller gradually increased, so that on the 13th of January the ratio of their magnitudes was as 1 to 8, and by the middle of February they were nearly equal in size; after which the variable comet began to diminish, and in about a month disappeared; while the other continued visible several weeks longer as a single comet.

The comet returned again in 1852, but under such unfavorable circumstances, as precluded the possibility of extended observations. Both nuclei were, however, observed by several persons in August and September. Fluctuations of relative brightness were noticed, similar to those of 1846, but much greater; so great, indeed, that for several days the two comets were alternately visible,

one nucleus being observed one day, and the other the next. Professor Hubbard, after a thorough discussion of all the observations made on this mysterious object in 1846 and 1852, found that the distance of the two nuclei apart, during their visibility in 1846, was about 200,000 miles, with but little variation from the 20th of January, to the 5th of March; after which, they sensibly approached each other until one disappeared, when their distance was 170,000 miles; whilst in 1852, they were nearly 1,800,000 miles apart. Professor Hubbard was unable to decide with certainty which of the nuclei of 1852 was identical with the principal one of 1846, but concluded, with a high degree of probability, that their relative apparent direction was reversed. By tracing the orbits back, he found that the separation probably occurred about 500 days before the perihelion passage of 1846.*

348. Faye's Comet. In 1843, M. Faye of the Paris Observatory discovered a comet and determined its orbit to be an ellipse with the surprisingly small eccentricity of 0.55. He found the period to be about 7 years. This comet is remarkable as having an orbit more closely resembling those of the planets in form than any other cometary orbit thus far known.

The Imperial Academy of Sciences of St. Petersburg has offered a prize of 300 ducats for the best essay on the orbit of this remarkable comet, and the relation which the two parts bear to each other.

349. De Vico's Comet.

This comet was discovered in 1844 by Sr. De Vico, Director of the Observatory at Rome. Its orbit was found to be an ellipse, with an eccentricity of 0.62, whose plane almost coincides with the ecliptic. Its period of revolution was computed to be about 5 years. Le Verrier pronounced this comet probably identical with one which appeared in 1678.


350. Lexell's Comet. In the year 1770, a remarkable comet appeared, moving in an ellipse with the short period of 51⁄2 years. By tracing back its motion, it was found that, early in 1767, it was very near to Jupiter, and that previous to that time it had been moving in an orbit requiring 50 years for a revolution. This change in its orbit was produced by the action of Jupiter. Again, in 1779 the comet passed so near to Jupiter that his attraction for it was 200 times greater than the Sun's, in consequence of which, its orbit was changed into one of long period. Some suppose that this comet and Faye's are identical.

351. The Great Comet of 1843. Of all the comets of recent years, no other has excited so much astonishment as did the one known as the Great Comet of 1843. It was first seen in many parts of the world on the 28th of February, in the day time, as a brilliant body quite near the Sun. Its distance from the nearest limb of the Sun, as measured with a sextant at 3 o'clock P. M. was 3° 36'. Soon after this it became visible after sun set as a very conspicuous object in the southwest. The apparent length of its tail varied from 50° to 70°, and the greatest real length was about 110 millions of miles. It continued visible to the naked eye but a short time, and the last telescopic observation of it was made on the 10th of April, at the Philadelphia High School Observatory. This comet passed its perihelion on the afternoon of the 27th of February, at which time it almost grazed the Sun's disc, being only 530,000 miles from his centre. According to the computations of Sir John Herschel, the heat it received when it was nearest the Sun must have been 47,000 times that received by the earth from a vertical sun. This will account for the intense brilliancy of this comet on the 28th of February.

The probable identity of this comet with that of 1668 is generally admitted by astronomers.


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352. Classification of the stars. The stars are divided into classes, according to their apparent magnitudes or brightness. The most conspicuous stars form the first class, and are called stars of the first magnitude; those that are markedly less bright, form the second class, and are called stars of the second magnitude ; and thus on, down to stars of about the sixteenth magnitude, which are the smallest that are distinctly visible with the most powerful telescopes. The stars that are visible to the naked eye, are included in the first six or seven magnitudes; principally, however, in the first six.

The magnitudes are denoted by the numbers 1, 2, 3, &c. A star that is regarded as intermediate in brightness between those of two consecutive classes, so as to render it doubtful in which it would be more appropriately placed, is frequently distinguished by two numbers with a point between them. Thus, 1.2 denotes a star intermediate between those of the first and second magnitudes.

353. Number of stars in some of the classes. The distribution of the stars into magnitudes, is arbitrary, and it has not been made on any definite principles. There is, therefore, some diversity in the distribution; different astronomers having differed in the magnitude they have attached to the same star.* On the whole,

*Also, from want of care in forming the constellations (121), some of them have been made more or less to overlap one another, so that the same star or stars are frequently included in two different constellations. The inconvenience resulting from these causes, has claimed the attention of the British Association, a scientific body, that meets annually in Great Britain; and it is probable that, ere long, we shall have a revision of the nomenclature of the stars, and of their division into constellation.

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