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numerous; 450 are supposed to belong to our solar system.

It is supposed, that those comets, which go to the greatest distance from the sun, approach the nearest to him at their return.

Their motions in the heavens are not all direct, or according to the order of the signs, like those of the other planets. The number of those which move in a retrograde manner, is nearly equal to those whose motion is direct.

The orbits of most of them are inclined in very large angles, to the plane of the ecliptic.

The velocity with which they move is variable in every part of their orbit: when they are near the sun, they move with incredible swiftness; when very remote from him, their motion is inconceivably slow.

When they appear, they come in a direct line towards the sun, as if they were going to fall into his body; and after having disappeared for some time, in consequence of his extreme brightness, they fly off on the other side as fast as they came, continually losing their splendour, till at last they totally disappear. Their apparent magnitude is very different; sometimes seeming not bigger than the fixed stars, at other times equal in diameter to Venus. Hevelius observed one in 1652, which was not inferior to the moon in size, though not so bright: its light pale and dim, its aspect dismal.

A greater number of comets are seen in the hemisphere towards the sun, than in the opposite; and

are generally invisible at a smaller distance than that of Jupiter. Mr. Brydone observed one at Palermo, in July, 1770; which, in twenty-four hours, described an arch in the heavens, upwards of fifty degrees in length; so that, if it was far distant from the sun, it must have moved at the rate of upwards of sixty millions of miles in a day.

They differ also in form from the other planets, consisting of a large internal body, which shines -with the reflected light of the sun, and is encompassed with a very large atmosphere, apparently of a fine matter, resembling that of the aurora borealis : this is called the head of the comet, and the internal part the nucleus. When a comet arrives at a certain distance from the sun, an exhalation arises from it, which is called the tail.

The tail is always directed to that part of the heavens which is directly or nearly opposite to the sun, and is greater and brighter after the comet has passed its perihelium. The tail of the comet of 1680 was of a prodigious size, extending from the head, to a distance scarcely inferior to that of the sun from the earth.

No satisfactory knowledge has been acquired concerning the cause of that train of light which accompanies the comets. Some philosophers imagine that it is the rarer atmosphere of the comet, impelled by the sun's rays. Others, that it is the atmosphere of the comet rising in the solar atmosphere, by its specific levity: while others imagine that it is a phenomenon of the same kind with the aurora borealis, and

that this earth would appear like a comet to a spectator placed in another planet.

The number of the comets is certainly very great, considerably beyond any estimation that might be made from the observations we now possess.

Though astronomers have bestowed much labour in calculating the periods of comets, and much attention to account for their phenomena, yet experience bears no testimony in favour of their opinions, nor have modern calculators had better success. Indeed the immense distances to which they are supposed to run out, are entirely hypothetical.

There are, who do not think the present astronomy of comets well established; and as so many small ones are frequently seen, they think that nothing can be determined with certainty, till some better marks are discovered for distinguishing one from another, than any at present known; and that even the accomplishment of Dr. Halley's prediction is uncertain: for it is very singular, that out of four years, in which three comets appeared, the only one in which no comet was to be seen, should be that very year in which the greatest astronomers that ever existed had foretold the appearance of one; and in accounting for its non-appearance, Mr. Clairault would have been equally supported by cometic evidence, whether he had concluded the comet to have been retarded or accelerated by the action of Jupiter and Saturn. A comet appeared in 1757, as well as in 1755; and had he determined the retardation of the comet to be twice as great as

he did, another appeared in 1760 to have verified his calculations*.

* A comet, the most remarkable that has appeared in our hemisphere these 40 years, was first observed in London the end of September, 1807. The following are the most important particulars that we could infer from its apparent figure and motion : On the 28th of September, its declination was about 5° S. and right ascension 218°. It was conspicuous about 7 o'clock in the evening, with the brightness of a star of the second magnitude, with a short bright bushy tail, and a nucleous distinct and bright. It had passed the perihelion of its orbit, before it was any where observed in this country. It continued moving northerly at a daily rate of more than a degree, till about the end of the year. Its tail first diminishing in brightness and size, and lastly, the nucleous and comet itself. Its greatest length of tail did not exceed 1 dcgree; and its greatest brightness was during the month of October, while receding from the sun and the earth, or approaching towards its aphelion. On account of the oblique position of the axis of the comet's orbit, it could not be seen on its approach to its perihelion, or in its motion towards the sun. Its first appearance was near the constellation Mons-Menaulus, and it disappeared near Cygnus, or the Swan. Its figure is tolerably well represented in plate 4, fig. 6. Dr. Herschel estimated the expanse over space of the tail, to be about nine million of miles.

It was most clearly magnified and defined by the power of the common large aperture two-feet night telescope, with three glasses, of a power about eight times. Such phenomenon as this, with feeble light or lustre, require small powers and large apertures in a refracting telescope, which is the most suitable instrument. For more ample accounts, as published by Dr. Herschel, and other skilful astronomers, see "The Philosophical Transactions, for 1808;" and "The Philosophical Magazine," Vols. 29 and 30.

EDIT.

OF THE TELESCOPIC APPEARANCE OF THE PLANETS.

Though by the telescope we have been led onward in our advances towards a more powerful know -ledge of the heavenly bodies, and astronomy being raised from little more than a catalogue of stars into a science; yet by this instrument men have been led into errors, and astronomers have indulged in speculations that equally deviate from sound reason, and the plain dictates of common sense.

The generality of mankind, in all ages, have considered the sun as a mass of pure elementary fire, subsisting from the creation, and supported by some unknown cause, cause, without any occasion for the gross fuel necessary for supporting our terrestrial fires. The conjectures of astronomers have neither been so simple nor so rational; limited in their conceptions, they have not been able to perceive how fire of any kind could subsist without fuel, and have therefore supposed the sun and the earth to be of a similar substance; and, consequently, that the earth itself would be a sun if set on fire. Sir Isaac Newton has even proposed it as a query, whether the sun and fixed stars are not great earths made vehemently hot, whose parts are kept from fuming away by the vast weight and density of their superincumbent atmosphere, and whose heat is preserved by the prodigious action and re-action of their parts? Others have imagined the sun to be a body of quite a different.

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