the centre of the earth and moon, would not fall again to the surface of the moon, but would become a satellite to the earth. Its primitive impulse might, indeed, be such as to cause it even to precipitate to the earth. The stones, which have fallen from the air, may be accounted for in this manner.

Satellites of Jupiter.

266. By the aid of the telescope we may discover four satellites revolving round Jupiter. The sidereal revolutions of these bodies are given in the following table: together with their mean distances from Jupiter, the semi-diameter of that planet's equator being considered as unity; and likewise their masses, compared with Jupiter considered also as unity.

1d 18h 27' 33",5

1d 769137788148

3 551181017849

III.
IV.

7

16

5.812964 0000173281

3 13 13 42,0
9 2486790000232355
3 42 33,4 7 154552783970 14 7524010000884972
16 31 49 7 16 688769707084 25.9468600000426591

The satellites of Jupiter are liable to be eclipsed by passing through his shadow; and on the other hand, they are frequently seen to pass over his disk, and eclipse a portion of his surface. This happens to the first and second satellite, at every revolution; the third very rarely escapes in each revolution; but the fourth (on account of its great distance and inclination) is seldom obscured.

These eclipses are of great utility in enabling us to determine the longitude of places, by their observation; and they likewise exhibit some curious phænomena with respect to light.

From the singular analogy, above alluded to, it follows that (for a great number of years at least) the first three satellities cannot be eclipsed at the same time: for in the simultaneous eclipses of the second and third, the first will always be in conjunction with Jupiter, and vice versa.

Satellites of Saturn.

267. Seven satellites may be seen by means of the telescope, to revolve about Saturn; the elements of which are but little known, on account of their

great distance. The following table will show the duration of their sidereal revolutions, and their mean distances in semi-diameters of Saturn.

Obs. The orbit of the first six satellites appear to be in the plane of Saturn's ring: whilst the seventh varies from it very sensibly.

Satellites of Uranus.

268. Six Satellities revolve round Uranus; which, together with their primary, can be discovered. only by the telescope. The following table will shew their sidereal revolutions, and mean distances in semi-diameters of the primary.

Obs. All these satellites move in a plane which is nearly perpendicular to the plane of the planet's orbit, and contrary to the order of the signs!

SECTION VI.

OF COMETS.

269. COMETS are certain dark or opaque bodies, like the planets, and move round the sun, but in very eccentric orbits, being sometimes so far from him, that their cold must be excessive, and sometimes so near him that their heat must be so intense, as would prove altogether intolerable to an inhabitant of this earth; and would even destroy, or at least vitrify, the earth itself.

Sir Isaac Newton computed the heat of the comet that appeared in the year 1680, when nearest the sun, to be two thousand times hotter than red hot iron, and that, being thus heated, it must retain its heat till it comes round again, although its period should be more than twenty thousand years; it is computed to be only five hundred and seventyfive.

It is believed that there are at least twenty-one comets belonging to our system, moving in different directions. All those which have been observed, have moved through the etherial regions and the orbits of the planets, without suffering the least sensible resistance in their motions, which sufficiently proves that the planets do not move in solid orbits.

Of all the comets, the periods of three only are known with any degree of certainty, being found to return at intervals of 75, 129, and 575 years: and of these, that which appeared in 1680 is the most remarkable.

This comet, at its greatest distance, is about eleven thousand two hundred millions of miles from the sun, while its least distance from the centre of that luminary is about four hundred and ninety thousand miles. In that part of its orbit which is nearest the sun, it flies with the amazing velocity of eight hundred and eighty thousand miles in an hour; and the sun, as seen from it, appears one hundred degrees in breadth, and consequently forty thousand times as large as he appears to us. The tail of this comet was at least an hundred millions of miles long; and that of 1812 was thirty millions of miles long.

Our earth was out of the way when this comet last passed near her orbit; but it requires a more perfect notion of the motion of the comet to be able to judge if it will always pass by us with so little effect. The comet, in one part of its orbit, approaches very near to the orbit of our

earth; so that, in some revolution, it may approach near enough to have very considerable if not fatal effects upon it. One of the comets was expected to return in 1789, but it has not yet appeared.

Comets are always attended with long transparent trains or tails, issuing from that side of them which is turned away from the sun.

Comets were formerly supposed to be prodigies or portents, and to foretel some great event or revolution, such as the fall of empires, or the death of some eminent and distinguished personage; but they are now known to have no more connection with the civil or political affairs of the world, than any other of the heavenly bodies.

The comet of 1811-12.

270. The chief particulars relating to this splendid comet may be arranged as follows.

1. The planetary body in the head of a comet as seen with the naked eye, presents a luminous appearance not unlike a star ; but that within its densest light there was an extremely small bright point, entirely distinct from the surrounding glare, and which by geometrical calculation was found to be from 428 miles to 120 thousand miles in diameter.

2. The eccentricity, colour, and atmosphere of the planetary body. The bright point was not in the middle of the head, but more or less eccentric at different times; and the colour of the planetary disk was of a pale, ruddy tint, like that of such equally small stars, as are inclined to red; and Dr. Herschel infers that it was visible by rays emitted from its own body, yet that since the central illumination, which, moderately magnified, was pretty uniform, became diluted into a gradual decrease from the middle towards the outside, the comet was surrounded by a transparent and elastic atmosphere. And this atmosphere was more than 507 thousand miles in diameter,

3. The tail of the comet. The most brilliant phenomenon that accompanies a comet is the stream of light which we call the tail.

The greatest real length of the tail was 100 millions of miles, and the real breadth was 15 millions of miles. This tail had a curved shape or flexure, and in its general appearance it seemed to be inclosed at the sides by two streams or branches arising from the sides of the head. And the tail of the comet being, on Nov. 9, very near the milky way, the appearance of the one compared to that of the other, in places where no stars could be seen in the milky way, was perfectly

alike. And that the tail is a hollow cone we may infer from the fact that the inside showed a comparative darkness, whereas had it been a cone of solid luminous matter, the brilliancy would have increased toward the centre, instead of diminishing.

SECTION V.

OF THE FIXED STARS.

271. THE fixed stars comprehend all the other heavenly bodies, except the sun, planets, and comets. They are distinguished by the naked eye from the planets, by being less bright and luminous, and by continually exhibiting that appearance which we call the twinkling of the stars. (Obs. 4. p. 46.) This arises from their being so apparently small, that the interposition of the least body, of which there are many constantly floating in the air, deprives us of the sight of them. When the interposed body changes its place, we again see the star; and this succession being perpetual, occasions the twinkling.

But a more remarkable property of the fixed stars, and that from which they have obtained their name, is their never changing their situation, with regard to each other, as the planets change their situations.

The stars which are nearest to us seem largest, and are therefore called of the first magnitude. Those of the second magnitude appearless; and so proceeding on to the sixth magnitude, which includes all the fixed stars that are visible without a telescope.

Of the number of fixed Stars.

272. As to their number, though in a clear winter's night, without moonshine, they seem to be innumerable, which is owing to their strong sparkling, and our looking at them in a confused manner, yet when the whole firmament is divided, as it has been done by the ancients, into signs and constellations, the number that can be seen at a time, by the naked eye, is not above a thousand.