OF COMETS.

Besides these planets already mentioned, there are some other bodies which revolve round the sun, called comets. They move in very eccentric ellipses, and their periods of revolution are so long, and so uncertainly known, that few are ever observed twice. They are only seen by us when they are in that part of their orbit which is nearest to the sun, and then they move so fast, that they soon become again invisible to us; the number of comets is unknown; numbers of small ones have been discovered by telescopes. Their distances are inconceivably great, and most of them move entirely beyond the planetary orbits, though some have descended below Mars. Their appearances are very different. Some appear only a faint vapour; others have a nucleus, or solid part, in the middle. When they approach the sun, they put forth the appearance of a beard, or tail, of luminous matter, which is sometimes of astonishing length. These tails are always directed from the sun. There are three comets, viz. of 1680, 1744, and 1759, that deserve to have a farther account given of them. The comet of 1680 was remarkable for its near approach to the sun; so near, that in its perihelion it was not above a sixth part of the diameter of the luminary from the surface thereof. The tail, like that of other comets, increased in length and brightness as it came nearer to the sun; and grew shorter and fainter, as it went farther from him and from the earth, till that and the comet were too far off to be any longer visible. The comet of 1744 was first seen at Lausanne, in Switzerland, December 13, 1743, N. S. From that time it increased in brightness and magnitude as it was coming nearer to the sun. Its diameter, when at the distance of the sun from us, measured about one minute, which brings it out equal to three times the diameter of the earth. It came so near Mercury, that if its attraction had been proportionable to its magnitude, it was thought probable it would have disturbed the motion of that planet. Mr. Betts, of Oxford, however, from some observations made there, and at Lord Macclesfield's observatory at Sherburn, found, that when the comet was at its least distance from Mercury, and almost twice as near the sun as that planet was, it was still distant from him a fifth part of the distance of the sun from the earth, and could therefore have no effect upon the planet's motions. He judged the comet

to be at least equal in magnitude to the earth. He says, that in the evening of January 23, this comet appeared exceedingly distinct and bright, and the diameter of its nucleus nearly equal to that of Jupiter. Its tail extended above 16 degrees from its body; and was in length, supposing the sun's parallax 10", no less than 33 millions of miles. Dr. Bevis, in the month of May, 1744, made four observations of Mercury, and found the places of that planet, calculated from correct tables, differed so little from the places observed, as to shew that the comet had no influence upon Mercury's motion. The nucleus, which had before been always round, on the 10th of February appeared oblong, in the direction of the tail, and seemed divided into two parts, by a black stroke in the middle. One of the parts had a sort of beard, brighter than the tail: this beard was surrounded by two unequal dark strokes, that separated the beard from the hair of the comet. The odd phenomena disappeared the next day, and nothing was seen but irregular obscure spaces like smoke in the middle of the tail: and the head resumed its natural form. February 15, the tail was divided into two branches; the eastern part about seven or eight degrees long, the western 24. On the 23d the tail began to be bent; it showed no tail till it was as near to the sun as the orbit of Mars; the tail grew longer as it approached nearer the sun; and at its greatest length was computed to equal a third part of the distance of the earth from the sun. The comet of 1759 did not make any considerable appearance, by reason of the unfavourable situation of the earth all the time its tail might otherwise have been conspicuous; the comet being then too near the sun to be seen by us; but deserves our particular consideration, as it was the first that ever had its return foretold. With respect to the real nature and use of the comets in the system, we are entirely unacquainted.

OF THE FIXED STARS.

The fixed stars are so called, because they are observed not to change their places in the heavens, as the planets do. They appear of an infinite variety of sizes; yet, for convenience, it is usual to class them into six or seven magnitudes; thus, they are called stars of the first, second, &c. magnitude. To the naked eye they appear innumerable, but this is only the consequence of their being scattered in so

confused a manner, and our not being able to see them all at one view. The whole number of stars visible to the naked eye is about 3186. But seldom above one-third of that number can be seen at one time. From the earliest ages they have been divided into groups, or constellations, which have been called by the names of various animals and objects, from a supposed resemblance to them; such as the Great Bear, the Little Bear, the Swan, &c. The fixed stars are placed at a distance from us so great, that it cannot be ascertained by any means yet known: hence, they must shine by their own light, and not by the light which they receive from our sun, as the planets do. Though it has been formerly mentioned that the relative situations of the fixed stars do not vary, yet in the course of several ages some variations have been observed among them. Some of the larger stars have not the same precise situations that ancient observations attribute to them, and new stars have appeared, while some others, which have been described, are now no longer to be found. Some stars are likewise found to have a periodical increase and decrease. Many of the fixed stars, upon examination with the telescope, are found to consist of two. Besides the phenomena already mentioned, there are many nebula, or parts of the heavens, which are brighter than the rest. The most remarkable of these is a broad irregular zone or belt, called the Milky-way. There are others much smaller, and some so small, that

they can be seen only by telescopes. If the telescope be directed to these nebulæ, they are resolvable into clusters of stars, which appear as white clouds in instruments of less force. Dr. Herschel has rendered it highly probable, both from observation and well grounded conjecture, that the starry heavens is replete with these nebula or systems of stars, and that the Milky-way is that particular nebula in which our sun is placed. Reasoning analogically from the circumstances with which we are acquainted, we may deduce, that the universe consists of nebula or distinct systems of stars; that each nebula is composed of a prodigious number of suns or bodies, that shine by their own native splendour; and that each individual sun is destined to give light to numbers of worlds that revolve about it. What an august, what an amazing conception does this give of the works of the Creator? Instead of one world and one sun, we find thousands and thousands of suns ranged around us at immense distances, all attended by innumerable worlds, all in rapid motion, yet calm, regular, and harmonious, invariably keeping the paths prescribed them; and these worlds peopled with myriads of intelligent beings, formed for endless pro, ression in perfection and felicity. We shall now, in the form of a table, give the names of the constellations, and the number of stars observed in each by different astronomers.

him upon composing a catalogue of the fixed stars, that by it posterity might learn whether any of the stars perish, and others are produced afresh. After several ages, another new star appeared to Tycho Brahe, and the astronomers who were contemporary with him; which put him on the same design with Hipparchus, namely, the making a catalogue of the fixed stars. Of this, and other stars, which have appeared since that time, we have the following history by Dr. Halley. "The first new star in the chair of Cassiopeia was not seen by Cornelius Gemma on the 8th of November, 1572, who says, he, that night, considered that part of the heaven in a very serene sky, and saw it not; but that the next night, November 9, it appeared, with a splendour surpassing all the fixed stars, and scarce less bright than Venus. This was not seen by Ty. cho Brahe before the 11th of the same month; but from thence he assures us that it gradually decreased and died away; so as in March, 1574, after sixteen months, to be no longer visible; and at this day no signs of it remain. Its place in the sphere of fixed stars, by the accurate observations of the same Tycho, was ( 9° 17' a 1ma, with 30° 45′ north latitude. Such another star was seen, and observed, by the scholars of Kepler, to begin to appear on September 30, St. Vet. anno 1604, which was not to be seen the day before; but it broke out at once, with a lustre surpassing that of Jupiter, and like the former, it died away gradually, and in much about the same time disappeared totally, there remaining no footsteps thereof in January, 1605-6. This was near the ecliptic, following the right leg of Serpentarius; and by the observations of Kepler, and others, was in 7' 28° 00 a 1ma, with north latitude 1° 56′. These two seem to be of a distinct species from the rest, and nothing like them has appeared since. But between them, viz. in the year 1596, we have the first account of the wonderful star in Collo Ceti, seen by David Fabricius on the 14th of August, as bright as a star of the third magnitude, which has been since found to appear and disappear periodically; its period being precisely enough seven revolutions in six years, though it returns not always with the same lustre. Nor is it ever totally extinguished, but may at all times be seen with a six feet tube. This was singular in its kind, till that in Collo Cygni was discovered. It precedes the first star of Aries 1° 40', with 15° 57' south latitude. Another new star was

first discovered by William Jansonius in the year 1600, in Pectore, or rather in Eductione Colli Cygni, which exceeded not the third magnitude. This, having continued some years, became at length so small, as to be thought by some to have disappeared entirely; but in the years 1657, 1658, and 1659, it again arose to the third magnitude; though soon after it decayed by degrees to the fifth or sixth magnitude, and at this day is to be seen as such in 98 18° 38′ a 1mo op, * with 55° 29' north latitude. A fifth new star was first seen by Hevelius in the year 1670, on July 15, St. Vet. as a star of the third magnitude; but by the beginning of October was scarce to be perceived by the naked eye. In April following it was again as bright as before, or rather greater than of the third magnitude, yet wholly disappeared about the middle of August.

The next year, in March, 1672, it was seen again, but not exceeding the sixth magnitude; since when it has been no further visible, though we have frequently sought for its return; its place is 9 30 17' a ima, and has lat. north 47° 28'. The sixth and last is that discovered by Mr. G. Kirch in the year 1686, and its period determined to be of 4044 days; and though it rarely exceeds the fifth magnitude, yet is very regular in its returns, as we found in the year 1714. Since then we have watched, as the absence of the moon and clearness of the weather would permit, to watch the first beginning of its appearance in a six feet tube, that, bearing a very great aperture, discovers most minute stars. And on June 15 last, it was first perceived like one of the very least telescopical stars; but in the rest of that month and July, it gradually increased, so as to become in August visible to the naked eye, and so continued till the month of September. After that, it again died away by degrees; and, on the 8th of December, at night, was scarcely discernible by the tube; and, as near as could be guessed, equal to what it was at its first appearance on June 25, so that this year it has been seen in all near six months, which is but little less than half its period; and the middle, and consequently the greatest brightness, falls about the 10th of September."

The late improvements in astronomy, and particularly those in the construction of telescopes, have now given astronomers an opportunity of observing the changes which take place among the stars with much greater accuracy than could be formerly done. In a paper in the

76th volume of the Philosophical Transactions, Mr. Edward Pigot gives a dissertation on the stars suspected by the astronomers of the last century to be change. able. For the greater accuracy in the investigation of his subject, he divides them into two classes; one containing those which are undoubtedly changeable, and the other those which are only suspected to be so. The former contains a list of 12 stars, from the first to the fourth magnitudes, including the new one which appeared in Cassiopeia in 1572, and that in Serpentarius in 1604: the other contains the names of 38 stars, of all magnitudes, from the first to the seventh. He is of opinion, that the celebrated new star in Cassiopeia is a periodical one, and that it returns once in 150 years. Keill is of the same opinion; and Mr. Pigot thinks, that its not being observed at the expiration of each period is no argument against the truth of that opinion; "since (says he) perhaps, as with most of the variables, it may, at different periods, have different degrees of lustre, so as sometimes only to increase to the ninth magnitude; and if this should be the case, its period is probably much shorter." For this reason, in September, 1782, he took a plan of the small stars near the place where it for merly appeared; but in four years had observed no alteration. The star in the neck of the Whale had also been examined by Mr. Pigot, from the end of 1782 to 1786, but he never found it exceed the sixth magnitude; though Mr. Goodricke had observed it on the ninth of August to be of the second magnitude, and on the third of September, the same year, it was of the third magnitude. Mr. Pigot deduced its period from its apparent equality with a smaller star in the neighbourhood, and thence found it to be 320, 328, and 337 days. The most remarkable of these changeable stars is that called Algol, in the head of Medusa. It had long been known to be variable; but its period was first ascertained by Mr. Goodricke, of York, who began to observe it in the beginning of 1783. It changes continually from the first to the fourth magnitude; and the time taken up from its greatest diminution to its least is found, at a mean, to be 2d 20h 49m and 3. During four hours it gradually diminishes in lustre, which it recovers during the succeeding four hours; and in the remaining part of the period, it invariably preserves its greatest lustre, and after the expiration of the term its diminution again commences. According to Mr. Pigot, the

degree of brightness of this star, when at its minimum, is variable in different periods, and he is of the same opinion with regard to its brightness when at its full ; but whether these differences return regularly or not has not been determined,

OF ECLIPSES.

When any one of the heavenly bodies is obscured or darkened by the shadow of another falling upon it, or by the interposition of any body, it is said to be eclipsed. The eclipses of the sun and moon are the most striking of any. They were formerly considered as ominous, and have often excited the dread and apprehension of the vulgar; but the improvement of science has shewn that they have no connection with future events; that they depend upon regular and invariable causes, and may be calculated and foretold with the greatest certainty. As the earth is an opaque body, enlightened only by the sun, it will cast a shadow towards that side which is farthest from the sun. If the sun and earth were of the same size, this shadow would be cylindrical, and would extend to an infinite distance: but as the sun is much larger than the earth, the shadow of the latter must be conical, or end in a point, (see fig. 6.) On the sides of this conical shadow, there is a diverging shadow, the density of which decreases in proportion as it recedes from the sides of the former conical shadow: this is called the penumbra. As the moon revolves round the earth sufficiently near to pass through the shadow of the earth, an eclipse must always take place when these three are all in one straight line. An eclipse of the moon can never happen but at the time of full moon; but, on account of the inclination of the moon's orbit to that of the earth, an eclipse cannot take place every full moon. When the moon passes entirely through the earth's shadow, the eclipse it total; but when only a part of it passes through the shadow, the eclipse is partial. The quantity of the moon's disc which is eclipsed (and the same thing is to be understood of that of the sun in a solar eclipse) is expressed by twelfth parts, called digits; that is, the disc is supposed to be divided by twelve parallel lines: then, if half the disc is eclipsed, the quantity of the eclipse is said to be six digits. When the diameter of the shadow through which the moon must pass is greater than the diameter of the moon, the quantity of the eclipse is said to be more than 12 digits;