His mean distance from the sun is above 890 millions of miles.

The rotation on his axis is performed in 10h 16′ 19′′,2: and the axis is inclined in an angle of 58° 41′ to the plane of the ecliptic.

His mean diameter is 76068 miles: consequently he is nearly ten times as large as our earth. The axis of his poles is to his equatorial diameter as 11 to 12.

The proportion of light and heat received from the sun is 0011; that received by the earth being considered as unity.

Saturn is sometimes marked by zones or belts ; which are probably obscurations in his atmosphere. And he is accompanied by seven satellites.

The most singular phenomenon, however, attending this planet, is the double ring with which he is surrounded.

This ring, which is very thin and broad, is inclined to the plane of the ecliptic in an angle of 31° 19′ 12′′,0; and revolves from west to east, in a period of 10h 29′ 16′′,8, about an axis perpendicular to its plane and passing through the centre of the planet.

The breadth of the ring is nearly equal to its distance from the surface of Saturn: that is about of the diameter of the planet.

The surface of the ring is separated in the middle by a black concentric band, which divides it into two distinct rings.

The edge of this ring, being very thin, sometimes disappears and, as this edge will present itself to the sun twice in each revolution of the planet, it is obvious that the disappearance of the ring will occur about once in 15 years; but under circumstances oftentimes very different.

The intersection of the ring and the ecliptic is in 5o 20° and 11. 20°; consequently, when Saturn is in either of those signs, his ring will be invisible to us. On the contrary, when he is in 2o 20° or 8$ 20°, we may see it to most advantage. This was the case towards the end of the year 1811.

Uranus.

259. Uranus was discovered by Dr. Herschel, March 13, 1781, who gave it the name of the Georgium Sidus. It performs its sidereal revolution in

30688 17 616",2; or, in about 84 Julian years: and it is probably situated at the confines of the planetary system.

Its distance from the sun is upwards of 1800 millions of miles and its apparent diameter is scarcely 3",9.

:

Six satellites accompany this planet; which move in orbits nearly perpendicular to the plane of the elliptic.

Vesta.

260. The next planet in our system is Vesta, for the knowledge of which we are indebted to Dr. Olbers of Bremen, being first discovered by him March 29th, 1807. Its distance from the sun is about 223 millions of miles, and its annual revolution in its orbit is performed in 3 years 7 months. But neither has its diameter, nor the duration of its diurnal rotation, been yet ascertained.

Juno *.

261. Juno, the next in order, is another new planet; discovered by Mr. Harding, at the observatory at Lilienthol, near Bremen, Sept 1st, 1804. The mean distance of this planet from the sun is estimated at two hundred and fifty-three millions of miles, and its annual revolution is performed in 4 years, 4 months, and 6 days; but its diameter, and the time of its revolving on its axis are unknown.

Pallas &.

262. The next superior planet above Juno, is Pallas, which was first observed by Dr. Olbers, March 8th, 1812: the mean distance of which, from the sun, is reckoned to be about two hundred and sixty-three millions of miles, and its revolution in its orbit is made in about 4 years, 7 months, and 10 days; but like the two former, its diameter and diurnal rotation have not as yet been ascertained.

Ceres Q.

263. Ceres is the next higher planet, in our system; which was first discovered by Piazzi, of Palermo, Jan. 1st, 1801. Its mean distance is nearly the same as that of Pallas, and consequently its annual revolution is performed in nearly the same time.

SECTION III.

SATELLITES.

264. THE number of satellites in our system, at present known, is eighteen: namely, the Moon which revolves round the earth; four which belong to Jupiter, seven to Saturn, and six to Uranus. The moon is the only one visible to the naked eye.

They all move round their primary planets, as their centre, by the same laws as those primary ones move round the sun: namely,

I. The orbit of each satellite is an ellipse, of which the primary planet occupies one of the foci.

II. The areas, described about the primary planet, by the radius vector of the satellite, are proportional to the times employed in describing them.

III. The squares of the times of the revolutions of the satellites, round their respective primary planets are to each other as the cubes of their mean distances from the primary.

Moon.

265. The motions of the moon are exceedingly eccentric and irregular. She performs her mean sidereal revolution in 274 7 43 11",5. But this period is variable: and a comparison of the modern observations with the ancient proves inconM 2

testably an acceleration in her mean motion. Her mean tropical revolution is 274 7 43' 4",7; and her mean synodical revolution is 29d 12h 44′ 2′′,8.

Her mean distance from the earth is 29.982175 times the diameter of the terrestrial equator; or above 237000 miles.

Her orbit is inclined to the plane of the ecliptic in an angle of 5° 9'; but this inclination is variable. The greatest inequality, which sometimes extend to 8′ 47′′, 1, is proportional to the co-sine of the angle on which the inequality of the nodes depends.

Her orbit, at the commencement of the present century, crossed the ecliptic in 0 15° 55′ 20′′,3; but the place of her nodes is variable. They have a retrogade motion, and make a sidereal revolution in about 18.6 Julian years. A synodical revolution of the nodes is performed in 346d 14h 52′ 43′′,6. The motion of the nodes is subject also to a secular inequality, dependent on the acceleration of the moon's mean motion.

The rotation of the moon on her axis is equal and uniform; and it is performed in the same time as the tropical revolution in her orbit, whence she always presents nearly the same face to the earth. But, as the motion of the moon in her orbit, is periodically variable, we sometimes see more of her eastern edge, and sometimes more of her western edge. This appearance is called the libration of the moon in longitude.

The axis of the moon is inclined to the plane of the ecliptic in an angle of 88° 29′ 49′′. In consequence of this position of the moon, her poles alternately become visible to, and obscured from us: and this phenomenon is called her libration in latitude.

There is also another optical deception arising from the moon being seen from the surface of the earth, instead of the centre. This appearance is called her diurnal libration.

The figure of the moon is that of an oblate spheroid like the earth. Her mean diameter is in the proportion to that of the earth, as 5823 to 21332; or as 1 to 3'665. Whence her mean diameter will be about 2160 miles. But the apparent diameter of the moon varies according to her distance from the earth. When nearest to us it is 33′ 31′′,1; and at her greatest distance it is 29′ 21′′,5. Hence her mean apparent diameter is 31′ 26′′,5.

The phases of the moon are caused by the reflection of

the sun's light; and depend on the relative positions of the sun, the earth, and the moon.

An eclipse of the moon can take place only at the time of her opposition to the sun; and is caused by her passing through the shadow of the earth. That shadow is 3 times longer than the distance between the moon and the earth: and its breadth, where it is traversed by the moon, is about 23 times greater than the diameter of the moon. The breadth of the earth's shadow, where it is traversed by the moon, is equal to the difference between the semi-diameter of the sun, and the sum of the horizontal parallaxes of the sun and moon. ' The moon cannot be eclipsed, however, if her distance from the place of her node, at the time of her opposition, exceeds 13° 21'; but if it is within 7° 47, there will certainly be an eclipse. The duration of the eclipse will depend on the apparent diameter of the moon, and on the breadth of the shadow at the point where she traverses it.

The sun cannot be eclipsed unless the moon be in conjunction; and then only when the centres of the sun and moon are in the same straight line with the eye of the spectator on the earth, in such case, if the apparent diameter of the moon be greater than that of the sun, the eclipse will be total; but, if it be less, it will be annular. Partial eclipses, however, may arise; as in the case of lunar eclipse.

The sun cannot be totally obscured for a longer period of time than four minutes; but the moon may be hid from our view for a much longer period.

The number of eclipses in a year cannot be less than two, nor more than seven.

Eclipses generally return in the same order and magnitude at the end of 223 lunations.

The atmosphere of the moon, if it has any, must be more rare than that which we can produce with our best airpumps.

The light of the moon is 300000 times more weak than that of the sun. Its rays, collected by the aid of powerful glasses, do not produce any sensible effect on the thermome

ter.

The refraction of the rays of light, at the surface of our earth, must be at least 1000 times greater than the surface of the moon.

Volcanoes and mountains are discovered on her surface, by the aid of the telescope.

A body projected from the surface of the moon, with a momentum that would cause it to proceed at the rate of about 8200 feet in the first second of time, and whose direction should be in a line which at that moment passed through