thereby made the comet visible. The comets which appeared in 1402 and 1532, are reported to have been bright enough to be seen in the sunshine of open day. In 1680 a comet appeared, which possessed many striking phenomena: of this comet, shown in the engraving, we shall speak further on. It had a nucleus, which is considered to be the real comet itself, and this was surrounded by an atmosphere, or luminous ring, which assumed the appearance of a beard, called the coma, (from the Latin for hair;) hence the word comet, -a hairy body. The coma, together with the nucleus, make up the head; and, lastly, came a long train or tail, which extended to a considerable distance across the heavens; according to some accounts as much as 80° and this is another remarkable characteristic of modern comets, that, together with their star-like appearance, their tails have likewise diminished in extent. A comet seen in the year 371 B. C., had a tail, which extended over 60° of the celestial sphere, That which appeared in the year 1618, is said to have cast a tail of 104° in extent.

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At the same time it must be observed that the tail does not necessarily accompany a comet. Many small comets have had immense tails, and many large and bright comets have had no tails. The variety is immense; for the flock of these celestial coursers seems very large, and even countless. Hundreds have been noticed in the records of the world; and, as these are the largest and brightest seen, it is presumed that many, which were small, escaped notice. Very many have been observed of late, and their number is deemed to be illimitable; for all, which are near the Earth, are not always seen, owing to their being on that part of

the heavens which is above the horizon during the day.

The comets of 1585, 1682, and 1763, had no tails. Indeed, the second of these three is described as being large, bright, and flaming, with an appearance like that of the planet Jupiter. But a comet is reported to have been seen in 1744, which had a tail made up of as many as six streams of light, all extending to the distance of 30° over the dome of the heavens.

Very little is known concerning the nature of comets: some have asserted that stars have been seen through the nucleus or head of a comet; this, however, is doubtful, as it is more probable that the light from the star is refracted through the surrounding atmosphere of the comet. It seems a more rational conjecture that the body of the comet is opaque; but that, when it approaches so near, as some of them do, to the Sun, all the substances which are vaporizable on its surface are converted into vapour, and form an immense atmosphere round it. This atmosphere, on account of the great velocity with which the comet moves, becomes left somewhat behind, and may, perhaps, furnish an origin for the coma, or the tail. Sir Isaac Newton supposed that the tail of a comet arose from a thin vapour elicited from the comet by the heat of the Sun. Another opinion is, that comets serve the purpose of bringing back to the planets of our system the electrical matter which they constantly throw off: it must be admitted, however," that all is pure conjecture on this point. It has been urged by those who advocate the hypothesis, that the body or nucleus of the comet is opaque, that it exhibits phases similar to those of the Moon, when viewed in different directions; but this

again is denied, for some comets have been watched, when about to cross over the disk of the Sun, and no spot was discoverable upon that luminary. The comet of 1680 approached so near to the Sun, that Newton estimated that it was exposed to a heat two thousand times greater than that of red-hot iron. Newton further conjectured that comets were bodies which gradudually approached nearer to the Sun, and finally fell upon it, and might thus repair the waste supposed to occur from the continual emission of light and heat from the Sun. The discovery of the elliptical orbits of comets, however, deprives this hypothesis of much of its value.

But, in order to come to some definite opinion about the nature or substance of this class of the heavenly host, we have to state that all comets, especially those of modern observation, present a vaporous sort of substance to terrestrial gaze. Their mass becomes denser towards the centre, which appears like a stellar point; and the light, whether encircling this point as a coma, or streaming from it as a tail, seems to have, as it were, the texture of the aurora borealis, and like the latter, to allow of the appearance of the stars through its filmy substance. The more prevalent opinion then, at present, respecting the nature of these beings is, that they are of a flimsy character, and that they are gradually absorbed by the Sun when they come into his neighbourhood: whence it is that every remarkable comet, whose re-appearance has justified the expectations of mankind, has disappointed the crowds who have sought to gaze upon the strange visiter in the skies. Its spare and attenuated aspect has made us wonder what our forefathers of old saw to fear, and to be astonished at;

for if astronomers had not told us when and where to look for them, we should not be aware of their pre

sence.

Comets have great variations in size during the time they are visible. Sometimes they appear as faint objects in the distance, but enlarge by degrees, and throw out a tail, and so increase in brightness and extent till they become lost in the light of the Sun. If, during the time of the comet's doubling the Sun, (so to speak,) it lose any of its substance, it seems usually to gain in brightness; the sun-beams seem then most chiefly to promote the splendour of the tail, until the comet, having receded far away from the Sun, disappears like a spirit of ether.

In speaking of the orbits of the various planets, we always refer them to the Earth's orbit as a standard by which their directions in the heavens can be understood; and we say that an orbit is inclined a certain number of degrees to the ecliptic, or Earth's orbit, according as the two orbits deviate much or little from coincidence with each other. Now it so happens that few the orbits of all the planets deviate but a very degrees from each other, while the orbits of the various comets which have been observed, deviate from the direction of the Earth's orbit by angles of almost every amount. A right angle is the largest angle that can exist between two lines, or two planes, and it is found that the orbits of the comets form almost every kind of angle with the ecliptic, from 0 degree (or coincidence) to 90 degrees (or a right angle): this then is one circumstance in which comets differ greatly from the planets. Another point of difference is the form of their orbits; the orbits of the planets are in reality elliptical, but the