great physical revolutions which are perpetually changing the surface of the planet.

In 1780, May 28th, the disk of Jupiter was observed by Dr. Herschel, covered with small curved belts, or rather lines, not contiguous, as in plate 3d, figures 8 and 9. Parallel belts, however, as represented in plate third, fig. 10, are most common.

The axis of Jupiter is so nearly perpendicular to his orbit, that he has no sensible change of seasons, which is a great advantage, and wisely ordered by the Author of nature; for if the axis of this planet were inclined any considerable number of degrees, just so many degrees round each pole would in their turn, be almost six of our years together in darkness, and, as each degree of a great circle on Jupiter contains 778 of our miles at a mean rate; judge ye what vast tracts of land would be rendered uninhabitable by any considerable inclination of his axis.

The difference between the equatorial and polar diameters of this oblate spheroid is computed at 6,230 miles; for his equatorial diameter is to his polar, as 13 is to 12; consequently his poles are 3,115 miles nearer his centre than his equator. This results from his rapid motion round his axis, for the fluids together with the light particles which they can carry, or wash away with them, recede from the poles, which are at rest towards the equator, where the motion is more rapid, until there be a sufficient number of such particles accumulated to make up the deficiency of gravity occasioned by the centrifugal force, which arises from a quick motion round an axis; and when the deficiency of weight or gravity of the particles is made up by a sufficient accumulation, the equilibrium is restored, and the equatorial parts rise no higher. The orbit of Jupiter is inclined to the ecliptic one degree and twenty minutes. His north node is in the 7th degree of Cancer, and his south node in the 7th degree of Capricorn. His mean apparent diameter as seen from the earth is 39 seconds, and as seen from the Sun, 37 seconds of a degree.

This planet being situated at so great a distance from the Sun, does not enjoy that degree of light emanating from his rays, which is enjoyed by the earth. To supply this deficiency, the great Author of our existence has provided four satellites, or moons, to be his constant attendants, which revolve around him, in such manner, that scarcely any part of this large planet but is enlightened during the whole night, by one or more of these Moons, except at his poles, where only the farthest Moons can be seen; there, however, this light is not wanted; because the Sun constantly circulates in or near the horizon, and is very probably kept in view of both poles by the refraction of his atmosphere. The first Moon, or that nearest to Jupiter, performs a revolution around him in one day, 18 hours, and 36 minutes, of our time, and is 229 thousand miles distant from his centre: the second performs his revolution in three days, 13 hours, and 15 minutes, at a distance of 364 thousand miles: the third in seven days, 3 hours, and 59 minutes, at the distance of 580 thousand miles: and the fourth, or farthest from Lis centre, in 16 days, 18 hours, and 30 minutes, at the distance of one million of miles from his centre. The angles under which these satellites are seen from the earth, as its mean distance from Jupiter, are as follow: -The first three minutes and 55 seconds: the second six minutes and 15 seconds: the third 9 minutes and 58 seconds, and the fourth 17 minutes and 30 seconds. This planet when seen from its nearest Moon, must appear more than one thousand times as large as our Moon does to us.

The three nearest Moons to Jupiter, pass through ris shadow, and are eclipsed by him, in every revolution, but the orbit of the fourth is so much inclined, that it passes by its opposition to Jupiter without entering his shadow, two years in every six. By these eclipses, Astronomers have not only discovered that the Sun's light is about 8 minutes in coming to us; but they have also determined the longitude of places on this earth with greater cer tainty and facility, than by any other method yet known.

OF SATURN.

Saturn is the most remarkable of all the planets; it is calculated at 9 hundred millions of miles from the Sun, and travelling at the rate of 21,900 miles every hour, and performs its annual circuit in 29 years, 167 days, and 2 hours of our time: which makes only one year to that planet. Its diameter is computed at 79,042 miles, and performs a revolution on its own axis once in ten hours, 16 minutes and two seconds. Its mean apparent diameter as seen from the earth, is 18 seconds, and as seen from the Sun, 16 seconds of a degree; its axis is supposed to be 60 degrees inclined to its orbit.

This planet is surrounded by a thin broad ring, which nowhere touches its body, and when viewed by the aid of a good telescope appears double. In Plate 2d, Saturn and his double ring are represented as in the largest view when seen from the earth. In Plate 3d, fig. 11, he appears as if viewed by a spectator at right angles to the plane of the ring. In Plate 3d, fig. 12, the ring is represented very obliquely to the view of the observer. It is inclined 30 degrees to the ecliptic, and is about 21 thousand miles in breadth; which is equal to its distance from Saturn on all sides. This ring performs a revolution on its axis in the same space of time with the planet, namely, ten hours, 16 minutes and two seconds. -This ring seen from the planet Saturn, appears like a vast luminous circle in the Heavens, and, as if it does not belong to the planet. When we see the ring most open, its shadow upon the planet is broadest, and from that time the shadow grows narrower, as the ring appears to do to us, until by Saturn's annual motion, the Sun comes to the plane of the ring, or even with its edge; which being then directed towards the earth, it becomes to us invisible on account of its thinness.The ring nearly disappears twice in every annual revolution of Saturn, when he is in the 19th degree, both of Pisces and Virgo. But, when Saturn is in the 19th degree either of Gemini or Sagitarius, his ring appears most open to us, and then its longest diameter is to its shortest as 9 to 4.

This planet is surrounded with no less than seven satellites, which supply him with light during the absence of the Sun. The fourth of these was first discovered by Huygens, on the 25th of March, 1655.Cassini discovered the fifth in October, 1671. The third on the 23d of December, 1672: And the first and second in the month of March, 1684. The sixth and seventh were discovered by Dr. Herschel in the year 1789. These are nearer to Saturn than any of the others.

These Moons perform their revolutions round this planet on the outside of his ring, and nearly in the same plane with it. The first, or nearest Moon to Saturn, performs its periodical revolution around him in 22 hours and 37 minutes, at the distance of 121,000 miles from his centre; the second performs its periodical revolution in one day, 8 hours, and 53 minutes, at the distance of 156 thousand miles; the third in one day, 21 hours, 18 minutes, and 26 seconds, at the distance of 193 thousand; the fourth in two days, 17 hours, 44 minutes, and 51 seconds, at the distance of 247 thousand; the fifth in 4 days, 12 hours, 25 minutes, and 11 seconds, at 346 thousand; the 6th in 15 days, 22 hours, 41 minutes and 13 seconds, at the distance of 802 thousand; and the 7th or outermost in 49 days, 7 hours, 53 minutes, and 43 seconds, at the distance of two milJions, 337 thousand miles from the centre of Saturntheir primary.

When we look with a good telescope, at the body of Saturn, he appears like most of the other planets, in the form of an oblate spheroid, arising from the rapid rotation about his axis. He however appears more flattened at the poles, than any of the others, and although his motion on his axis is not equal to that of Jupiter, yet he does not appear to be in form, so near that of a globe as that planet. When we consider that the ring by which Saturn is encompassed, lies in the same plane of his

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equator, and, that it is at least equal if not more dense than the planet, we shall find no difficulty in accounting for the great accumulation of matter, at the equator of Saturn. The ring acts more powerfully upon the equatorial regions of Saturn, than upon any part of his disk; and by diminishing the gravity of these parts, it aids the centrifugal force in flattening the poles of the planet. Had Saturn indeed never revolved upon his axis, the action of the ring would of itself have been sufficient, to have given it the form of a spheroid.

The following, are the dimensions of this luminous

Breadth of the exterior ring,

7,200

Breadth of the dark space between the two rings,

2,839

Angle which it subtends when seen at the mean

M. S.

distance of the planet.

7 25

ON HERSCHEL, OR URANUS.

From inequalities in the motion of Jupiter and Saturn, for which no rational account could be given, and from the mutual action of these planets, it was inferred by some Astronomers, that another planet existed beyond the orbits of Jupiter and Saturn; by whose action these irregularities were produced. This conjecture was confirmed on the 13th of March, 1781; when Dr. Herschel discovered a new planet, which in compliment to his Royal Patron, he called Georgium Sidus, although it is more generally known by the name of Herschel, or Uranus. This new planet, (which had been formerly observed as a small star by Flamsted, and likewise by Tobias Mayer, and introduced into their catalogue of fixed stars,) is situated, one thousand, eight hundred millions of miles from the centre of the System, and performs its revolution round the Sun in 83 years, 150