comet. o is supposed to be a variable star. is a double star. μ is a triple star.

THE ENCKE COMET.

This comet was first observed in the year 1786 by two French philosophers, Messier and Mechain; it was then traversing the constellation Aquarius. It was again noticed in 1795 by Miss Herschel, its situation was then in Cygnus, destitute of tail or nucleus, having merely a nebulous appearance, not well defined, and about 3' in diameter; in calculating its elements, at this time, it was found very difficult to make the places by observation correspond with a parabolic orbit, and the results of astronomers differed considerably with each other; this might, in some measure, have been owing to there being no bright stars in that part of the heavens through which the comet was pursuing its course, favorable for a comparison with it, and if the places of the stars that were employed were not correctly determined, that alone would have tended to vitiate the whole calculation.

M. Pons, who has been very successful in tracing this, as well as many other comets, saw it October 20th, 1805; it then appeared as a star of the fourth magnitude, with a nucleus, and a very faint tail two and a half degrees in length; it was, at this time, observed in Úrsa Major.

No idea had been entertained that the comet of 1786, 1795, and 1805, was the same body; this was principally to be attributed to the inaccuracy of the observations, and, consequently, the calculations founded on them, which prevented its identity being discovered till

the year 1819, when it was observed in the constellation Pegasus; it appeared tolerably bright, having no appearance of a tail, its diameter 5' or 6', and could be discerned in the evening twilight when only 5o above the horizon. As the parabolic orbit, computed for it by M. Bouvard, did not represent the observations with sufficient accuracy, M. Encke was induced to assume an elliptical one, which satisfied the observed places with the greatest precision; he soon discovered its identity by correcting the former observations, and so accurately calculated its elements, as to merit the honors conferred on him by learned societies; he discovered that the largest axis of its orbit is a little smaller than that of the asteroid Vesta,-that in its aphelion it is midway between the orbits of the small planets and Jupiter, at its aphelion passes within the orbit of Mercury, that its greatest distance is twelve times its least distance, and its period about 1207 days, completing its revolution in rather less time than the asteroids.

From an investigation of the intervals elapsed between the times of this comet being visible, it was soon discovered that there had been three revolutions between 1786 and 1795, and an equal number between 1795 and 1805, and that between 1805 and 1819 there had been four revolutions.

Only one revolution in three is favorable for observing it in the northern hemisphere: when the comet passes its perihelion, between the months of October and February, it is so much to the north of the ecliptic as to render it visible in the northern hemisphere; at opposite seasons of the year it would be unfavorable to us, but favorable for observing it in the southern hemisphere.

This was the case when it next returned; it was invisible here, but re-discovered at Paramatta, New South Wales, 2nd of June, 1822, by M. Rumker, who observed it in Gemini, when it had passed its perihelion; and the places, as calculated by Professor Encke, agreed so accurately with those observed by M. Rumker, as fully to justify the honour conferred on Encke by assigning his name to the comet.

On its next return, in 1825, it was recognized in both hemispheres, at the Observatory at Paramatta, and at Marseilles, by M. Pons; its situation in the heavens was near Pollux in Gemini; it had no tail, and was considered equal in brightness to the nebula in the head of Aquarius.

The interest excited at its former returns, since 1819, was considerably increased in the year 1828, when its course in the heavens was peculiarly favorable for observation in the northern hemisphere; most of the observatories in Europe were vigilantly engaged in witnessing its return. It was seen early in October, on the continent, appearing exceedingly dim; about the middle of November it was satisfactorily observed in this country; its course was traced at Deptford, from the 24th of November till the middle of December. Its figure was circular, approaching to an oval; a condensation of the nebulous matter was observed towards the centre, but without any star-like nucleus or tail. Scarcely an evening passed without evidences of its pellucid nature, very small stars being seen through it.-(See plate of the Encke comet.)

The following table exhibits the elliptic elements, answering to five passages of the perihelion, between 1804 and 1830.