any other part of her disk. The mountainous ridges which encircle the cavities, reflect the greatest quantity of light: and from their lying in every possible direction, they appear, near the time of the föll moon, like a number of brilliant radiations, issuing from the small spot called Tycho.

“ It is difficult to explain, with any degree of probability, the formation of these immense cavities; but we cannot help thinking, that our earth would assume the same figure, if all the seas and Jakes were removed ; and it is therefore probable, that the lunar cavities are either intended for the reception of water, or that they are the beds of lakes and seas which have formerly existed in the moon. The circumstance of there being no water in the moon is a strong coufirmation of this theory."

Sect. 6. Of Mars. 29. Next to the earth is the planet Mars. It revolves in its orbit in little less than two years, at the distance of 144 millions of miles from the sun; and turns on its axis in little less than 25 hours. The light reflected by Mars is remarkably red. Spots and sometimes belts have been seen on the disk of this planet, some of which are permanent and others variable. These variations are supposed to arise from clouds and vapours floating in the atmosphere. The degree of heat and light at Mars is something less than one half what we enjoy.

Rrer

Sect. 7. Of Vesta, Juno, Pallas, and Ceres. 30. Next to Mars in the solar system is Vesta. It was discovered by Dr. Olbers, of Bremen, March 29, 1807. Its light is pure and white; and renders the planet visible to the naked eye. It revolves round the sun at the mean distance of about 223 millions of miles, in about 3 years and 8 months. The time of turning on its axis is not known.

31. Juno, the planet next to Vesta, was discovered by Mr. Harding, near Bremen, September 1, 1804. Its

colour is red, and its atmosphere appears cloudy. Its mean distance from the sun is about 223 millions of miles. Its orbit is very elliptical; so that its greatest distance from the sun is nearly double its least distance, and the time of passing through one half its orbit is about double the time of passing through the other half. It completes its revolution in about 4 years and 4 months, and is supposed to turn on its axis in about 27 hours.

32. Pallas was discovered by Dr. Olbers, March 28, 1802. It appears to have a dense cloudy atmosphere. It revolves round the sun in an orbit nearly as elliptical as that of Juno, in about 4 years and 7 months, at the mean distance of 263 millions of miles. The time of turning on its axis is not known.

33. Ceres was discovered, at Palermo, in Sicily, by Piazza, January 1, 1801. Its mean distance from the sun is about the same as that of Pallas; but its orbit is less elliptical. It is of a ruddy colour. It revolves round the sun in very nearly the same time that Pallas does ; and, what is very remarkable, its orbit intersects that of Pallas. All these planets undergo various changes in appearance and size; so that their real magnitude is not ascertained with any certainty.

These four planets have been very recently discovered, and but little is known of them as yet. They are certainly very small. In the Table at the close of this CHAP. their probable size is given, except that of Vesta. It is a remarkable fact, that some irregulari. ties, observed in the motions of the old planets, induced some astronomers to suppose that a planet existed between the orbits of Mars and Jupiter; a supposition that arose long previous to the discovery of the four new planets, which we have just noticed. The opinion has been advanced, that these four small bodies originally composed one larger one, which, by some unknown force or convulsion, burst asunder. This opinion is maintained with much ingenuity and plausibility by Dr. Brewster in the Edinburgh Encyclopedia, Art. ASTRONOMY. Dr. Brewster further supposes, that, the bursting of this planet may have occasioned the phenomena of the meteoric stones; that is, stones which have fallen on the earth from the atmosphere.

SECT. 8. Of Jupiter. 34. Jupiter revolves at the mean distance of 490 millions of miles from the sun. It completes its revolution in little less than 12 years, and turns on its axis in the short time of 9 hours and 56 minutes. It is the largest planet yet discovered in the solar system, being 89,000 miles in diameter. It reflects a beautiful light, and is the most brilliant of the planets, except Venus. The degree of heat and light at Jupiter is about 25 times less than at the earth.

35. When viewed through a telescope, Jupiter exhibits an appearance somewhat different from any of the above planets. Generally several belts or bands are distinctly seen, sometimes extending across his disk, and sometimes interrupted and broken. These belts are variable in distance and position as well as number. They are generally dark, but white ones have been

Both bright and dark spots have been seen in them; some of which revolve faster than others, which shows that they cannot be permanent spots on the body of the planet.

36. Jupiter is accompanied by 4 moons or satellites. These moons revolve round Jupiter as the moon does round the earth. Their revolutions are completed in different times; the shortest being less than 2 days, and the longest less than 17 days. These satellites often pass behind the body of the planet, and also into its shadow, and are eclipsed. These eclipses are of use in ascertaining the longitude of places on the earth, as will be shown hereafter. For this reason, astronomers have taken great pains to calculate the precise time when they take place at London. By these eclipses it is also ascertained that light is about 8 minutes coming from the sun to the earth. For an eclipse of one of these

seen.

satellites appears to us to take place 16 minutes sooner, when the earth is in the part of her orbit nearest Jupiter, than when in the part farthest from him. Hence light is 16' in crossing the earth's orbit, and of course gi in coming from the sun. The satellite nearest to the primary is reckoned 1st, and the others, 2d, 3d, &c. as they are farther from the primary. The 1st satellite is somewhat less than the 2d, and the 2d somewhat less than the 4th, which is about as large as our moon; but the 3d is about twice the size of our moon.

37. On account of the immense distance of this planet from the sun, and also from Mercury, Venus, the Earth, and Mars, observers on Jupiter, with our eyes, could never see either of the above named planets, for they are always immersed in the sun's rays. They would direct their observations to planets which lie beyond; and here we know not the advantages of a position on Jupiter over one on the earth. know not how many planets belonging to our system, within or beyond the orbit of Saturn or of Uranus, are distinctly visible at Jupiter, whose feeble light forever precludes their discovery by us.

For we

Sect. 9. Of Saturn. 38. Saturn, at the mean distance of 900 millions of miles, completes a revolution round the sun in little less than 30 years. It turns on its axis in little more than 10 hours. The light reflected by this planet is less brilliant than that of Jupiter. The degree of heat and light from the sun at Saturn is 80 times less than at the earth.

39. Saturn is remarkably distinguished from all the other planets in the solar system.

When viewed through a telescope, it appears encompassed by a large