On October 2d, the comet was 26° 33' from the perihelion; its heliocentric longitude was 41° 53′ and latitude 720 1'; having two days before passed the higher part of its orbit, or 90 degrees from the node. The Earth at the same time was in about 90 of Aries; and the geocentric longitude of the comet was 174° 37', and its geocentric latitude 54° 5'. The comet's distance from the Sun was 102,532,550, and from the Earth 120,413,930 miles. The comet was nearest the Earth on the 11th of October, when its distance was 113,630,450 miles, its apparent motion in longitude at this time was nearly four degrees in twenty-four hours. On the 12th, the comet was 370 33' from the perihelion, having a rapid geocentric motion in longitude, the direction of the Earth and comet conspiring to produce that effect. Its geocentric longitude was 2030 46', and latitude 61° 39'; the Earth at the same time was 180 40 in the sign Aries. The comet's distance from the Sun was 108,342,464, and from the Earth 113,948,225 miles. On January 1st, 1812, the comet was 89° 11′ from the perihelion; its heliocentric longitude was 328° 15′, and latitude 230 33. The Earth was about 10° 21' in Cancer; the greatest geocentric longitude of the comet was 3120 2', and latitude 17° 18'. Its distance from the Sun was 190,520,000, and from the Earth 259,614,500 miles. See, for a delineation of a portion of this comet's orbit, Squire's Astronomy.

Though the real or heliocentric motion of this comet was not within the sphere of the Earth's orbit, yet its geocentric track, when referred to the ecliptic, crossed the orbit of the Earth; hence, the apparent place of the comet, during the greater part of the time it was visible, was towards the opposite part of the heavens to its true place.

From the true and apparent places of the comet, given above, for particular days, its real and visible path may be traced upon the celestial globe. Dr. Herschel makes the planetary body of this comet not more than 428 miles in diameter; but the real diameter of the head he makes to be about 127,000 miles.

The apparent motion of this comet was direct, yet very unequal, for when it first became visible after passing the ascending node, it was nearly stationary, and the same about the time of its disappearance, but when nearest the Earth it equalled that of Mercury.

This comet was visible a longer time than almost any other upon record, and therefore none has ever afforded such certain means of information with respect to its orbit. Had its heliocentric motion been direct it would have been visible

much longer, and would have passed within 44,485,850 miles of the Earth, had it crossed the line of its nodes at the same time. The comet would then have appeared a large nebulous body, but without a tail, as that appendage would have been projected in a direct line from behind its body.

6. The most striking phenomena, that makes the comets objects of attention to all mankind, is the tail of light which they often exhibit. When approaching the Sun, a nebulous tail of light is seen to issue from them, in a direction opposite to the Sun. This, after having increased, again decreases till it disappears. The stars are visible through it.

The nebulosity with which those comets are almost always surrounded, seems to be formed by the vapours which the solar heat raises on their surface. It is imagined that the great heat which they experience towards their periheion, rarifies the particles which have been congealed by the xcessive cold of the aphelion.

It appears also that the trains of comets are only these va pours elevated to a considerable height by this rarefaction combined either with the solar rays or with the dissolution of those vapours in the fluid, which reflects the zodaical light to us. This seems to result from the direction of their trains, which are always beyond the comets, relatively to the Sun, and which only becoming visible near their perihelion, are not at a maximum till after their passage through this point, when the heat communicated to the comet by the Sun, is increased by its duration, and by the proximity to this luminary.

Dr. Hamilton supposes the tails of comets, the aurora borealis, and the electric fluid to be matter of the same kind. He supports this opinion by many strong arguments, which are found in his ingenious essay on the subject. According to this hypothesis, it would follow that the tails are hollow; and there is every reason to suppose this, from the scarcely perceptible diminution of the lustre of the stars seer through them. He supposes that the electric matter, which continually escapes from the planets, is brought back by the assistance of the comets.

But much is yet to be known on this subject. Objectiore

may be made to his hypothesis, although so ingeniously supported. According to the opinion of Kepler, the rays of the Sun carry away some gross parts of the comets, which reflect other rays of the Sun, and give the appearance of a tail.

CHAPTER XVIII.

Of the Firmament of Fixed Stars.

1. The number of stars visible to the naked eye, as has already been remarked, is not more than 2000. We observe, says Dr. Brinkley, about 3000 stars visible to the naked eye, very irregularly scat tered over the concave surface of the heavens. There are seldom above 2000 visible at once, even on the most favourable star-light night.

This may at first appear incredible to some, because ai first sight they seem to be innumerable; but the deception arises from looking upon them hastily, without reducing them into any kind of order. For let a person look steadily for some time upon a large portion of the heavens, and count the number of stars in it, and he will be surprised to find the number so small. And if the moon be observed for a short interval of time, she will be found to pass very few in her way, although there are as many about her path as in any other part of the heavens. Flamstead's Catalogue contains only 3000 stars, and many of those are not visible without a telescope. But although the number of stars may be small when examined with the naked eye, yet when examined with a powerful telescope, the number exceeds all computation, For a good telescope, directed to almost any part of the heavens, discovers multitudes that are lost to the naked eye. In some places, however, they are crowded together; and in others, there are considerable spaces where no stars can be seen. In the small group called the Pleiades, in which only 6 or 7 stars can be seen by the naked eye, Dr. Hook, with a telescope of 12 feet long, discovered 78 stars. And F. de Rheita affirms, that he has observed more than 2000 stars in the constellation Orion; and above 188 in the Pleiades,

That which appears only a single star in Orion's sword, Huygens found, by the telescope, to consist of 12 stars very near together. Galileo found 80 in the belt of Orion's sword, 21 in the nebulous star of his head, and about 500 in another part of the constellation within the compass of one or two degrees, and more than 40 in the nebulous star Præsepe. Others, even in the best telescopes, appear still as small luminous clouds. There is a very remarkable one in the constellation Orion, which the best telescopes show as a spot uniformly bright. It is a singular and beautiful phenomenon.

So great is the number of telescopic stars in some parts of the milky way, that Dr. Herschel observed 588 stars in his telescope at the same time, and they continued equally numerous for a quarter of an hour. In a space of about 10 degrees long, and 25 degrees wide, he computed 258,000 stars. Phil. Trans. 1795.

2. The appearance of the stars seen in a telescope, is very different from that of the planets. The latter are magnified and show a visible disc. The stars appear with an increased lustre, but with no disc. Some Some of the brighter fixed stars appear most beautiful objects, from the vivid light they exhibit.

Dr. Herschel tells us that the brightness of the fixed stars, of the first magnitude, when seen in his largest telescope, is too great for the eye to bear. When the bright star Sirius was about to enter the telescope, the light was equal to that on the approach of sun-rise, and upon entering the telescope, the star appeared in all the splendour of the rising sun, so that it was impossible to behold it without pain to the eye. The apparent diameter of a fixed star is only a deception

arising from the imperfections of the telescope. The brighter stars appear sometimes in bad telescopes to subtend an angle of several seconds, and this has led astronomers into mistakes respecting their apparent diameters.

The more perfect the telescope, the less this irradiation of light. We know with certainty that some of the brighter stars do not subtend an angle of 1", from the circumstance of their instantly disappearing, on the approach of the dark edge of the moon. Dr. Herschel attempted to measure the diameter of Vega in Lyra, and imagined it to be about of a second.

That the diameter of the sun may appear less than

3

second, it must be removed 1900 times farther from us than at present; which is an argument in favour of the vast distance of the fixed stars.

Although the superior light of the sun effaces that of the stars, yet by the assistance of telescopes we can observe the brighter stars at any time of the day. The aperture of the telescope collects the light of the star, so that the light re ceived by the eye, from the star, is greater than when the eye is unassisted. The darkness in the tube of the telescope also in some measure assists. See Dr. Brinkley's Astronomy.

3. Some stars appearing single to the naked eye, when examined with a telescope, appear double or triple; that is, consisting of two or three stars very close together: such are Castor, a Hercules, the Pole Star, &c. Seven hundred, not noticed before, have been observed by Dr. Herschel.

In viewing these double stars a singular phenomenon discovers itself, first noticed by Dr. Herschel; some of the double stars are of different colours, which, as the images are so near each other, cannot arise from any defect in the telescope. a Herculis is double, the larger red, the smaller blue; & Lyræ is composed of four stars, three white and one red; y Andromedæ is double, the larger reddish white, the smaller a fine sky blue. Some single stars evidently differ in their colour. Aldebaran is red, Sirius a brilliant white.

From a series of observations on double stars, Dr. Herschel has found that a great many of them have changed their situations with regard to each other; that the one performs a revolution round the other, and that the motion of some is direct, while that of others is retrograde. He has observed that there is a change in more than 50 of the double stars, either in the distance of the two stars, or in the angle made by a line joining them with the direction of their daily motion.

Theowing are the observations that have been published relative to six double stars, a Gemenorum, (Castor,) y Leonis, s Boötes, Herculis, & Serpentis, y Virginis. In respect to Castor, the first of these, Dr. Herschel thinks it highly probable, that the orbits in which the two stars move Found their common centre of gravity are nearly circular, and at right angles to the line in which we see them; and that the time of a whole apparent revolution of the small star