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he watched its place in the heavens, and saw it appear again the succeeding year, nine months after its disappearance. It has been since found to be every year very regular in its period, except that in 1672 it was missed by Hevelius, and not seen again till 1676. Bullialdus determined the periodical time between this star's appearing in its greatest brightness, and returning to it again, to be about 333 days; observing further, that this star did not appear at once in its full magnitude and brightness, but by degrees arrived at them.

Three changeable, or re-apparent stars have been discovered in the constellation of the Swan; the first was seen by Jansonius, in 1600; the second was discovered in 1670; the third by Kirchius, in 1686.

In the latter end of September, 1604, a new star was discovered, near the heel of the right foot of Serpentarius. Kepler, in describing it, says, that it was precisely round, without any kind of hair or tail; that it was exactly like one of the stars, except that in the vividness of its lustre, and the quickness of its sparkling, it exceeded any thing he had ever seen before. It was every moment changing into some of the colours of the rainbow, as yellow, orange, purple, and red; though it was generally white, when it was at some distance from the vapours of the horizon. Those in general who saw it, agreed that it was larger than any other fixed star, or even any of the planets, except Venus; it preserved its lustre and size for about three weeks; from this time it grew gradually smaller. Kepler

supposes that it disappeared some time between October, 1605, and the February following, but on what day is uncertain.

Besides these several re-apparent stars, so well characterized and established by the earliest among the modern astronomers, there have been many discovered since, by Cassini, Maraldi, and others; Mr. Montanere speaks of having observed above onė hundred changes among the fixed stars.

The star Algol, in Medusa's head, has been observed long since to appear of different magnitudes, at different times. The period of it has been lately settled by J. Goodrick, Esq. of York. It periodically changes from the first to the fourth magnitude : the time employed from one greatest diminution to the other, was, anno 1783, at a mean 2 days, 20 hours, 49 minutes, 3 seconds.

The causes of these appearances cannot be assigned at present, with any degree of probability; perhaps they have some analogy to the spots on the sun, which at some times appear in greater numbers than at others, some of them bigger than the whole earth; or perhaps they are owing to some real motions of the stars themselves.

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There are several stars that appear single to the naked eye, which are, on examination with a telescope, found to consist of two, three, &c. The number of double stars observed before the time of Dr. Herschel, was but small; but this celebrated astronomer has noted upwards of four hundred ; among these, some that are double, others that are

treble, double double, quadruple, double treble, and multiple; his catalogue gives the comparative size of these stars, their colour, as they appeared to him, with several other very curious particulars.

OF NEBULÆ, AND OF HERSCHEL'S IDEAS RESPECTING THE CONSTRUCTION OF THE UNIVERSE.

Besides, those appearances of the fixed stars, already noticed, there is another which deserves particular attention; namely, the nebula, or parts of the heavens which appear brighter than the rest. The most remarkable among these is, that large irregular zone, or band of whitish light, which crosses the ecliptic in Cancer and Capricorn, and is inclined thereto in an angle of about 60 degrees; it is a circle bisecting the celestial sphere, irregular in breadth and brightness, and in many places divided into double streams. The principal part runs through the Eagle, the Swan, Cassiopea, Perseus, and Auriga: it continues its course by the head of Monoceros; along by the greater Dog, through the Ship, under the Centaur's Feet; till having passed the Altar,the Scorpion's Tail, and the Bow of Aquarius, it ends at last where it began.

This curious appearance is owing to a multitude of small stars, which are too minute to be distinguished by the naked eye; yet, blending their light together, form that whiteness which occupies so large a tract of the heavens. The milky-way may be

considered as a constellation of telescopic stars; a sea of them of great breadth, and of a whitish colour, encompassing the whole heavens; even before astronomy reaped any benefit from improvements in optics, Democritus considered it as formed of clusters of small stars.

Mr. Herschel's large telescope completely resolved the whitish appearance of the milky-way into stars. Having viewed and gauged this bright zone in all directions, he found it composed of shining stars, whose number increases and diminishes in proportion to its apparent brightness to the naked eye.

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The portion of the milky-way that he first observed, was that about the hand and club of Orion. Here he found an astonishing multitude of stars, which he attempted to number. By estimating the number contained in the field of his telescope at once, and computing, from a mean of these, how many might be contained in a given portion of the milky-way, in the most vacant places, about that part, he found 63 stars; other six fields contained 110, 60, 70, 90, 70, and 74 stars: a mean of these, gives 79 for the number of stars in each field; so that, allowing 15 minutes for the diameter of his field of view, a belt of fifteen degrees long, and two degrees broad, could not contain less than 50,000 stars, large enough to be distinctly numbered; besides which, he suspected twice as many more, which could be seen only now and then by faint glimpses, for want of sufficient light.

In the most crowded parts of the milky-way, he

has had a field of view of 588 stars, and these continued for many minutes; so that, in one quarter of an hour's time, not less than 116,000 stars have passed through the field of his telescope. He endeavours to shew, that the powers of his telescope are such, that it will not only reach the stars at 497 times the distance of Sirius, so as to distinguish them; but that it also shews, the united lustre of the accumulated stars that compose a milky nebulosity, at a far greater distance. From these considerations, it is highly probable, that as his twenty feet telescope does not shew such a nebulosity in the milky-way, it goes already far beyond the extent thereof; and therefore a more powerful instrument would remove all doubt, by exposing a milky nebulosity beyond the stratum, which could then no longer be mistaken for the dark ground of the heavens.

To a spectator, placed in indefinite space, all very remote objects appear to be equally distant from the eye. To judge of the milky-way, only, from phenomena, we must, of course, consider it as a vast ring of stars, scattered promiscuously round the celestial regions; but a more perfect view of the subject will shew us, that the appearance, &c. of this beautiful object, arise from our eccentric view. Mr. Wright, in his "Original, Theory of the Universe, 1750;' and Dr. Herschel, since, in "The Philosophical Transactions," have shewn, that this appearance may be accounted for, by assuming its figure as much more extended towards the apparent zone of illumination, than in any other direction.

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