The comet which appeared in 1770, seems to have been particularly affected by the action of the planets. After many unsuccessful attempts to determine its orbit from a number of accurate observations, Le:cell found that the observations were best represented by an ellipse, in which the periodical revolution of the comet was 5 years and As this comet, however, has never been seen since that time, M. Burckhardt, at the request of the National Institute, repeated all the calculations with great care, and obtained results nearly the same as those of Lexell. The orbit of this comet must therefore have experienced very remarkable changes ; for, as it never goes to a much greater distance than the orbit of Jupi ter, and approaches as near to the earth as the moon, it ought to have appeared about eight times since the year 1770.
Among the Tables at the end of this article, we have given the most complete Table that has yet been pub lished, containing the elements of 97 comets, which have been observed and calculated by different astronomers till the year 1808. It contains the most accurate infor mation that astronomers can obtain respecting these in teresting bodies, and enables us to draw some important conclusions respecting their situation and distribution in the planetary system. The Arabic figures in the first column accompanying the Roman figures, point out the resemblance of one comet to another ; thus the number 49, opposite to the years 1531, 1607, and 1782, shews that these comets are the same as the comet No. XLIX.
By comparing together the numbers in the Table alluded to, it will appear, 1. That 24 comets have pass ed between the sun and the orbit of Mercury ; 33 be-. tween the orbits of Mercury and Venus ; 21 between the orbits of Venus and the Earth ; 15 between the orbits of the Earth and Mars ; 3 between the orbits of Mars and Ceres ; and 1 between the orbits of Ceres and Jupiter. 2. That 32 comets have been seen between April and September, and 65 between September and April. 3. That the greater part of the comets have their perihelia nearest to their ascending nodes. 4. That forty-nine of the comets move from west to east, and 48 in the op posite direction. 5. That the orbits of the comets are not confined to any particular region of the heavens like the old planets, but seem to have every possible inclina tion. This will appear from the following Table, which contains the number of comets whose inclinations are be low every tenth degree.
Hence it appears, that the orbits are inclined in every possible angle ; and since there arc only 8 comets whose inclination is less than 10°, there is less danger of their interfering with the planetary bodies.
When we examine a comet with a good telescope, it appears like a mass of vapours surrounding a dark nu cleus, of different degrees of opacity in different comets. As the comet approaches the sun, its pale and cloudy light becomes more brilliant, and when it reaches its perihelion, it is often brighter than the planets. When the comet is near its perihelion, it is accompanied with a tail or train of light directly opposite to the sun, and in creasing in length as it approaches that luminary. The tails of comets are generally a little concave towards the sun ; the fixed stars are always visible through them, and sometimes they are so brilliant, that they have been distin guished during full moon, and even after the rising of the sun. The following Table contains the lengths of the tails of several comets : gress of science. When multiplied obsertations shall have added to the imperfect knowledge which we at pre sent possess respecting these singular bodies, it will then be time to indulge in ingenious speculation. The ap pearance of a comet is she•n in Plate XL. Fig. 7.
For farther information on the subject of this Book, See Aristotle's Aleteorolog. lib. i. cap. 6. Seneca Quest. :Vat. vii. Hcvclii Cometographia. Batley, Phil. Trans. 1705, vol. xxiv. p. 1882. Euler Nov. Com. Petrol:, torn. xis. p. 499. Euler, Men:. Acad. Berlin, 1746, p. 117. Dionis du Sejour, Essai sur les Cometes. Lexell, Mem. Acad. Par. 1776, p. 638. and Phil. Trans. 1779, p. 68. Clairaut, Mem. :lead. Par. 1760. Boscovich, Alen:. Say. Etrang. tom. vi. p. 198, 401. La Place, Men:. Say. Etrang. 1773, p. 503, and Mem. Acad. Par. 1780, p. 13. Fuss, Ilfern. Say. Etrang. vol. x. 1781, p. 65. La Grange, Mem. Say. Etrang. x. 1785, p. 65. Lambert, Mem— dead. Berl. 1771, p. 352. Bode, Mem. Aced. Berl. 1786, 1787. Sir H. Englefield On the Orbits of Comets, 4to, London, 1793. Olber's Abhandlung uber die leichteste and be guemste Met/:ode die bahn eines Corneten aus einigen Beo bachtungen zu berechnen, 8vo, Weimar, 1797. But par ticularly Pingre Cometogranhie, 2 vols. 4to, Paris, 1784 ; a work which contains the most complete information upon the subject of comets. See also COMET, and the references at the end of Physical ASTRONOMY.