ASTEROIDS.

SINCE the nineteenth century commenced, FOUR very small planets have been discovered, between the orbits of Mars and Jupiter. Their names are Ceres, Pallas, Juno, and Vesta.

Their real magnitudes are not yet accurately ascertained; they are estimated from 80 miles, to 2000 miles, in diameter. Probably neither of them is 1500 miles in diameter. Their density is something more than twice the density of water. Their mean distances from the Sun, vary from 225 millions, to 266 millions of miles. The light enjoyed at them, about one ninth as great as at the earth. One performs its revolution in less than 4 years; another in 4 years and 4 months. Two perform their revolutions with only 1 day's variation; 4 years, 7 months, and 10 days, and 11 days. Two of the Asteroids have their orbits much more inclined to the Ecliptic, and comparatively much more eccentric, than other planets.

Their nodes, and their points of nearest approach to the Sun, being in the same general direction in the heavens, together with their apparent irregularities, have originated the hypothesis that they have once been a single planet. The great atmospheres of two are conjectured to have been derived from the Comet of 1770, the disappearance of which is supposed to be otherwise unaccountable. In the Tabular view, the largest estimate of two of them is from Schroeter, the smallest from Herschel.

When were the Asteriods discovered? What are their names? What their magnitudes ?-densities ?-distances from the Sun?-and periods of revolution?

What facts are supposed to favour the hypothesis, that they once constituted but one planet? What is thought of the atmospheres of two of them?

CHAPTER V.

JUPITER.

RECEDING from the Sun, we next arrive at Jupiter, the largest of the planets; with an apparent diameter of about 40 seconds, and a real diameter of more than 89 thousand miles. Its density is about 14 the density of water; it turns on its axis in less than 10 hours, and revolves round the Sun in less than 12 years, at the distance of 490 millions of miles. Compared with the Earth, it has nearly 14 hundred times as much bulk, and about 300 times as much matter. The velocity with which it turns on its axis, must occasion an apparent motion of the Sun and stars, to organs of vision like ours.

The inclination of Jupiter's orbit, to the plane of the Ecliptic, is very small; and the Sun is constantly vertical at its Equator, allowing no variety in the length of its days, and little vicissitudes in seasons. The amount of light enjoyed there, is about one twenty-fifth part of what is enjoyed at the earth.

Light diminishes, as the squares of the distance increase. Call the distance of the Earth from the Sun, one part in five, of the distance of Jupiter. The Asteroids are about three parts of this distance, and three times three are 9, therefore, they have one-ninth of the light of the Earth. Five times five are 25, therefore, as Jupiter is five such parts distant from the fountain of light, it has only one twenty-fifth part as much light by day as the Earth.

What is the apparent diameter of Jupiter? What is its real diameter? What is its density?--its distance from the Sun ? -the time of its rotation? and of its revolution? How much more bulk than the Earth? and how much more matter? What effects result from the velocity of its rotation? What from a constantly vertic. Sun at its equator? What amount of light does it enjoy? At what rate does light diminish? How can you illustrate this?

The quick rotation of Jupiter on its axis, is connected with its being much flattened at its poles. Its disc, when seen through a telescope, is diversified with alternate streaks of light and shade called belts; as seen in Plate i. Figure 1. These vary

their size and position so rapidly, as to lead to the opinion, that they are occasioned by clouds in its atmosphere, formed in regular strata by the velocity of its diurnal motion.

SATELLITES OF JUPITER.

Two of

JUPITER has four satellites or moons. them are estimated as nearly the magnitude of the Earth, and two as larger. Their densities are like their primary. The times of their revolution vary from 14 days to 16 days.

Three of them revolve so near the plane of the primary's orbit, as to eclipse the Sun, and to be eclipsed, to eyes, on Jupiter, at every revolution.

The fourth revolves without an eclipse one time in three. Galileo, when discovering them, by a telescope of his own invention, little more than two centuries ago, doubtless had no conception of the eminent utility to mankind, of that discovery. From their eclipses, longitude can be accurately computed, and the difficult problem, of the rate at which light moves, mathematically demonstrated. From the periods of their revolution, the density of Jupiter can be estimated, and the correctness

Is Jupiter more spheroidal than the Earth? Why? What are seen on its disc? What is the supposed cause? How many satellites has Jupiter? What are their magnitudes?-densities?distances from their primary? What eclipses result from their motions? By whom were they discovered? What advantages have resulted from their discovery?

of Kepler's rule, for computing the distances of the planets from the Sun, clearly proved.

The mean distance of Jupiter's satellites from their primary, and their periods of revolution, are as follows. The first is two hundred and sixty-six thousand miles distant, and revolves in one day and eighteen hours. The second is four hundred and twenty thousand miles distant, and revolves in three and a half days; the third and largest is six hundred and seventy thousand miles distant, and revolves in seven days; the fourth is more than a million of miles distant, and revolves in sixteen and a half days. The angles which their orbits make with the primary, as seen from the Earth in its mean distance from Jupiter, are from four minutes, to seventeen and a half minutes; about half the apparent diameter of the Moon's disc. Suppose an eclipse of one of these satellites, is accurately predicted to be seen in the meridian of Greenwich, England, precisely at twelve o'clock at noon, and an observer near Philadelphia, sees it precisely at seven o'clock in the morning, by a watch set to the meridian of that city, he then knows, that Philadelphia is 75 degrees west longitude from Greenwich, because the meridians of the two places, are found to vary five hours. Suppose an eclipse of one of these satellites, computed with perfect accuracy, if seen from the Sun. Now if the Earth were directly between the Sun and Jupiter, that eclipse would terminate eight minutes sooner than predicted; or if the Earth were exactly on the opposite side of the Sun from Jupiter, it would terminate eight minutes later than predicted. These facts would prove, that light coming across the whole diameter of the Earth's orbit, occupied sixteen minutes; and knowing that distance to be one hundred and ninety millions of miles, the velocity with which light moves, would be found to be nearly two hundred thousand miles the second.

CHAPTER VI.

SATURN.

HALF a century ago, Saturn was the remotest planet from the Sun, which had been discovered; How distant from the primary, and how long the period of the revolution of Jupiter's 1st satellite ?-2d satellite ?-3d sat ellite ?-4th satellite? What angles do their orbits make as seen from the Earth? How can Longitude be learned by their eclipses? How the velocity with which light moves estimated ?

and from its splendid appendages, it is still an object of intense interest, to the scientific observer. Its apparent mean diameter is 18 seconds; and its real diameter is 79 thousand miles. Its density is about half the density of water, making about one hundred times as much matter, and nearly one thousand times as much bulk in Saturn as in the Earth. Thus far the densities of planets diminish, as you recede from their centre of motion. Saturn turns on its axis in 10 hours; and requires almost 30 years to complete its revolution round the Sun, at the distance of 900 millions of miles, and moving 22 thousand miles the hour.

At its equator, it has more than 25 thousand days in one of its years; at its poles, but one day and one night. The orbit of Saturn inclines to the Ecliptic 2 degrees, and its axis has an inclination to its orbit, which makes the Sun shine vertically 30 degrees on either side of its equator. It is more spheroidal than Jupiter, and receives about one ninetieth part of the light enjoyed at the Earth. Infinite wisdom and benevolence has, however, wonderfully compensated for this deficiency.

SATURN'S APPENDAGES.

Two remarkable rings, encircle Saturn at its equator. The outermost, is more than 200 thousand miles in diameter, and about 7 thousand miles in breadth, towards the planet. A space occurs be

What planet is next without the orbit of Jupiter? What is its apparent diameter?-its real diameter ?-density? What is the period of its rotation?-revolution? What is its distance from the Sun ?-its velocity the hour? How near the Ecliptic is it always seen? How broad its torrid zone? What amount of light? What remarkable appendages has Saturn? What is the diameter of the outermost ring?-its breadth?