47. The number of comets belonging to the solar system is unknown. Above 500 have appeared since the commencement of the Christian era; and accounts of many more are extant. The orbits of the comets being very elliptical, their velocity in one part is much greater than in another. They are also turned out of their course, retarded and accelerated by the attraction of the planets. These circumstances, together with the difficulty of obtaining the elements of their orbits, render all calculations of their periodical times extremely uncertain.

Dr. Halley and Professor Encke are the only astronomers who ever successfully predicted the return of a comet; and these in single instances only. Of three sanguine calculations of Dr. Halley, one has proved correct, one has entirely failed, and one remains to be tested. Professor Encke, of Seeberg in Germany, made observations on a comet visible in 1819, and calculated its periodical time to be about 1200 days only. He predicted its return in 1822; but owing to its position it would not be visible in Europe or in the United States. According to his prediction it appeared in 1822, and was visible at the Islands in the South Pacific ocean. It is but a small body, passing in its perihelion within the orbit of Mercury, and in its aphelion, midway between the orbits of the newly discovered planets and that of Jupiter. It is not improbable that this body will ere long be classed with the planets.

The orbits of 98 comets, up to the year 1808, have been calculated from observations of the times at which they most nearly approached the sun; their distance from the sun and from the earth at those times; the direction of their movements; the places at which their orbits cut the ecliptic, and their inclination to it. The result is, that of these 98, 24 passed between the Sun and Mercury, 33 between Mercury and Venus, 21 between Venus and the Earth, 16 between the Earth and Mars, and 4 between Mars and Jupiter; that 50 of these comets moved from east to west; and that their orbits inclined at every possible angle to the ecliptic.

When comets are nearest to the sun, they often move with incredible velocity. Newton calculated the velocity of the comet of 1680, when nearest the sun, to be 880,000 miles an hour; and Mr. Squire, from data obtained since the days of Newton, has computed its motion to be 1,240,108 miles an hour.

The comet of 1758, the return of which was predicted by Dr. Halley, was looked upon with great interest by astronomers, because its return was predicted. But four revolutions before, in 1456, it was looked upon with the utmost horror. Its long tail spread consternation over all Europe, already terrified by the rapid success of the Turkish arms. Pope Callixtus, on this occasion, ordered a prayer, in which both the comet and the Turks were included in one anathema.

SECT. 12. Of the Stars.

48. All the heavenly bodies, of which we have not treated, are called stars; and except comparatively a few, which in a course of years, appear to change their places, they appear to be fixed, retaining the same sitution in relation to each other. Their number is unknown; but we are commonly very much deceived in the number visible to the naked eye. It is seldom that so many as 1000 are visible at once in the clearest night; but by looking at them confusedly, we imagine them to be much more numerous. They are classed into six magnitudes; the largest are of the first magnitude, and the smallest that can be seen by the naked eye, are of the 6th.

49. We have no certain means of ascertaining the distance of any body from the sun, which exceeds 200 thousand times that of the earth. But none of the stars come within that limit; so we cannot determine their real distance. It is generally supposed that a part, if not all the difference in the apparent magnitude of the stars is owing to a difference in their distances; the smallest being farthest off. Though the stars generally appear fixed, yet they all may have motion. For their distance being so immensely great, (in no instance less than 200 thousand times that of the earth, probably much

more in general,) a rapid motion might not perceptibly change their relative situation in two or three thousand years.

50. As telescopes are improved, other stars become perceptible, which before were invisible. Many stars also, which, to the naked eye, appear single, when seen through a telescope appear double, treble, or even quadruple. Some stars are subject to periodical variations in apparent magnitude, at one time being of the second or third, and at another of the fifth or sixth. Some have been noticed alternately to appear and disappear; being visible for several months, and again invisible. Several stars mentioned by ancient astronomers are not now to be found; and some are now observed, which are not mentioned in the ancient catalogues.

51. In a clear autumnal evening, a remarkably light broad zone is visible in the heavens, passing from northeast to south-west. This appearance is usually called the Milky-way, or Galaxy. It is generally supposed that this appearance is owing to an immense number of stars, which, from their apparent nearness, cannot be distinguished from each other. Dr. Herschel, in the course of of an hour, saw the astonishing number of 116,000 stars pass through the field of view of his telescope, while it was directed to the milky-way. Many whitish spots or tracts, called nebulæ, are visible in different parts of the heavens, which are supposed to be milky-ways at an inconceivable distance.

52. The stars are probably suns, around each of which revolve primary and secondary planets, as about our sun. It is certain that they do not reflect the light of the sun, as do the planets; for their distance is so great, that they would not in such case be visible. The sun, at the distance of a star, would certainly appear to us no larger than a star does. Stars are distinguishable from the planets by their twinkling.

53. The ancients, in reducing astronomy to a science, formed the stars into constellations, by applying names to particular clusters. This arrangement was effected very early, and is the most ancient monument of human skill. The choicest efforts of art, and the most wonderful productions of labour, the pride and ruin of empires of the greatest known antiquity, have passed away, while the constellations remain, telling of people still anterior. (Orion, in nearly the middle of which is the yard L, and the Pleiades, commonly called the 7 stars, are mentioned in the book of Job, the oldest book of which copies are extant with us.) The number of constellations among the ancients was about 50; the moderns have added about as many more. On the celestial globe, the largest star in each constellation is usually designated by the first letter of the Greek alphabet, and the next largest by the second, and so on. When the Greek alphabet is exhausted, the English alphabet, and then numbers, are used.

54. In the zodiac are 12 constellations, of the same names with the signs of the zodiac or ecliptic. (But these constellations and signs do not coincide; but each constellation is now just about 30° or a sign, eastward of the sign of the same name. For example, the constellation Aries is 300 eastward of the sign Aries, and the constellation Taurus, 30° eastward of the sign Taurus, and so on. Thus the sign Aries lies in the constellation Pisces, the sign Taurus in the constellation Aries, the sign Gemini in the constellation Taurus, and so on. Hence the importance of distinguishing between the signs of the Zodiac, and the constellations of the Zodiac. The cause of their difference will be noticed hereafter.

Our observations of the stars and nebula, are confined principally to those of the northern hemisphere. Of the constellations near the south pole, we know but little; while every region and point in the

northern hemisphere is as familiar to the astronomer, as the geogra phy of his native village. The following beautiful and interesting extract is from Humboldt's Personal Narrative :

"From the time we entered the torrid zone, we were never wearied with admiring, every night, the beauty of the southern sky, which, as we advanced the south, opened new constellations to our view. We feel an indescribable sensation, when, on approaching the equator, and particularly on passing from one hemisphere to the other, we see those stars, which we have contemplated from our infancy, progressively sink and finally disappear. Nothing awakens in the traveller a livelier remembrance of the immense distance by which he is separated from his country, than the aspect of an unknown firmament. The grouping of the stars of the first magnitude, scattered nebulæ, rivalling in splendour the milky way, and tracks of space remarkable for their extreme blackness give a particular physiognomy to the southern sky. This sight fills with admiration even those, who, uninstructed in the branches of accurate science, feel the same emotion of delight in the contemplation of the heavenly vault, as in the view of a beautiful landscape, or a majestic site. A traveller has no need of being a botanist, to recognise the torrid zone on the mere aspect of its vegetation; and without having acquired any notions of astronomy, without any acquaintance with the celestial charts of Flamstead and de la Caille, he feels he is not in Europe, when he sees the immense constellation of the Ship, or the phosphorescent clouds of Magellan, arise on the horizon. The heaven, and the earth, every thing in the equinoctial regions, assumes an exotic character.

"The lower regions of the air were loaded with vapours for some days. We saw distinctly for the first time the Cross of the south, in the sixteenth degree of latitude; it was strongly inclined, and appeared from time to time between the clouds, the centre of which, furrowed by uncondensed lightnings, reflected a silver light. If a traveller may be permitted to speak of his personal emotions, I shall add, that in this night I saw one of the reveries of my earliest youth accomplished.

"When we begin to fix our eyes on geographical maps, and read the narratives of navigators, we feel for certain countries and climates a sort of predilection, for which we know not how to account at a more advanced period of life. These impressions, however, exercise a considerable influence over our determinations; and from a sort of instinct we endeavour to connect ourselves with objects, on which the mind has long been fixed as by a secret charm. At a period when I studied the heavens, not with the intention of devoting myself to astronomy, but only to acquire a knowledge of the stars, I was agitated by a fear unknown to those who love a sedentary life. It seemed painful to me to renounce the hope of beholding those beautiful constellations, which border the southern pole. Impatient to rove in the equinoctial regions, I could not raise my eyes towards the starry vault without thinking of the Cross of the south.