somewhat the appearance of a comet, forming, according to Schroeter, an atmosphere 675 miles in height."

At

Ceres, as has been said, was the first discovered of the asteroids. her discovery, astronomers congratulated themselves upon the harmony of the system being restored. They had long wanted a planet to fill up the great void between Mars and Jupiter, in order to make the system complete in their own eyes; but the successive discoveries of Pallas and Juno again introduced confusion, and presented a difficulty which they were unable to solve, till Dr. Olbers suggested the idea that these small anomalous bodies were merely the fragments of a larger planet, which had been exploded by some mighty convulsion. Among the inost able and decided advocates of this hypothesis, is Dr. Brewster, of Edinburgh.

PALLAS, the next planet in order after Ceres, perform's her revolution around the Sun in 4 years, 7 months, at the mean distance of 264 millions of miles, moving in her orbit at the rate of 41 thousand miles an hour. Her diameter is estimated at 2025 miles, which is but little less than that of our Moon. It is a singular and very remarkable phenomenon in the solar system, that two planets, (Ceres and Pallas,) nearly of the same size, should be situated at equal distances from the Sun, revolve about him in the same period, and in orbits that intersect each other. The difference in the respective distances of Ceres and Pallas is less than a million of miles. The difference in their sidereal revolutions, according to some astronomers, is but a single day!

The calculation of the latitude and longitude of the asteroids, is a labour of extreme difficulty, requiring more than 400 equations to reduce their anomalous perturbations to the true place. This arises from the want of auxiliary tables, and from the fact that the elements of the star-form planets, are very imperfectly determined. Whether any of the asteroids has a rotation on its axis, remains to be ascertained.

JUPITER.

JUPITER is the largest of all the planets belonging to the solar system. It may be readily distinguished from the fixed stars, by its peculiar splendour and magnitude; appearing to the naked eye almost as resplendent as Venus, although it is more than seven times her distance from the Sun.

How high, according to Schroeter, is the atmosphere formed by this nebulous light} Why did astronomers congratulate themselves at the discovery of this planet? What again introduced confusion and difficulty into their system? How were they at length enabled to solve the difficulty? What planet is the next in order after Ceres ? In what time does she complete her revolution around the Sun? What is her mean distance from him? What is the rate of her motion in her orbit per hour? What is her diameter? How great is it compared with the diameter of the Moon? What is the difference between the respective distances of Ceres and Pallas from the Sun? What is the difference between the times of their sidereal revolutions? Why is the calculation of the latitude and longitude of the asteroids a labour of extreme difficulty? Have any of the asteroids rotations on their axes? Which is the largest planet of the solar system. How may Jupiter be readily distinguished from the fixed stars? How much farther is he from the Sun than Venus?

When his right ascension is less than that of the Sun, he is our morning star, and appears in the eastern hemisphere before the Sun rises; when greater, he is our evening star, and lingers in the western hemisphere after the Sun sets.

Nothing can be easier than to trace Jupiter among the constellations of the zodiac; for in whatever constellation he is seen to-day, one year hence he will be seen equally advanced in the next constellation; two years hence, in the next; three years hence, in the next, and so on; being just a year, at a mean rate, in passing over one constellation.

The exact mean motion of Jupiter in its orbit, is about one twelfth of a degree in a day; which amounts to only 30° 20′32′′ in a year.

For 12 years to come, he will, at a mean rate, pass through the constellations of the zodiac, as follows:

Jupiter is the next planet in the solar system above the asteroids, and performs his annual revolution around the Sun in nearly 12 of our years, at the mean distance of 495 millions of miles; moving in his orbit at the rate of 30,000 miles an hour.

The exact period of Jupiter's sidereal revolution is 11 years, 10 months, 17 days, 14 hours. 21 minutes, 25 seconds. His exact mean distance from the Sun is 495.533,837 miles; consequently, the exact rate of his motion in his orbit, is 29,943 miles per hour.

He revolves on an axis, which is perpendicular to the plane of his orbit, in 9 hours, 55 minutes, and 50 seconds; so that his year contains 10,471 days and nights; each about 5 hours long.

His form is that of an oblate spheroid, whose polar diameter is to its equatorial, as 13 to 14. He is therefore considerably more flattened at the poles, than any of the other planets, except Saturn. This is caused by his rapid rotation on his axis; for it is a universal law that the equatorial parts of every body, revolving on an axis, will be swollen

In what case is he our morning star, and in what our evening? How may he be traced among the constellations of the Zodiac? In what constellation will he be, each year, for twelve years to come? What is his position in the solar system? What is his mean distance from the San? What is the rate per hour of his motion in his orbit? What is the exact period of his sidereal revolution? What is his exact mean distance from the Sun? What the exact rate per hour of his motion in his orbit? What is the posi tion of his axis with respect to the plane of his orbit? How many days and nights does his year contain? How long are they, each? What is his form? What is the ratio between his polar and equatorial diameters? What is the cause of his being more flattened at the poles than any of the other planets ?

Gut, in proportion to the density of the body, and the rapidity of its motion.

The difference between the polar and equatorial diameters of Jupiter, exceeds 6000 miles. The difference between the polar and equatorial di ameters of the Earth, is only 26 miles. Jupiter, even on the most careless view through a good telescope, appears to be oval; the longer diameter being parallel to the direction of his belts, which are also parallel to the ecliptic.

By this rapid whirl on his axis, his equatorial inhabitants are carried around at the rate of 26,554 miles an hour; which is 1600 miles farther than the equatorial inhabitants of the Earth are carried, by its diurnal motion, in twentyfour hours.

The true mean diameter of Jupiter is 86,255 miles; which is nearly 11 times greater than the Earth's. His volume is therefore about thirteen hundred miles larger than that of the Earth. (Compare his magnitude with that of the Earth. Plate I.) On account of his great distance from the Sun, the degree of light and heat which he receives from it, is 27 times less than that received by the Earth.

When Jupiter is in conjunction, he rises, sets, and comes to the meridian with the Sun; but is never observed to make a transit, or pass over the Sun's disc; when in opposition, he rises when the Sun sets, sets when the Sun rises, and comes to the meridian at midnight, which never happens in the case of an interior planet. This proves that Jupiter revolves in an orbit which is exterior to that of the Earth.

As the variety in the seasons of a planet, and in the length of its days and nights, depends upon the inclination of its axis to the plane of its orbit, and as the axis of Jupiter has no inclination, there can be no difference in his seasons, on the same parallels of latitude, nor any variation in the length of his days and nights. It is not to be understood, however, that one uniform season prevails from his equator to his poles; but that the same parallels of latitude on each side of his equator, uniformly enjoy the same season, whatever season it may be.

About his equatorial regions there is perpetual summer; and at his poles everlasting winter; but yet equal day and equal night at each. This arrangement seems to have been kindly ordered by the beneficent Creator; for had his axis been inclined to his orbit, like that of the Earth, his polar winters would have been alternately a dreadful night of six years darkness.

What is the difference between his polar and equatorial diameters? What does his form appear to be, through a good telescope? What is the direction of his longer diameter? At what rate per hour are his equatorial inhabitants carried by his motion on his axis? How much farther is this than the equatorial inhabitants of the Earth are carried in 24 hours? What is Jupiter's true mean diameter? How much greater is it than the Earth's? What is his volume, compared with the Earth's? What is the degree of light and heat which he receives from the sun, compared with that received by the Earth? How do we know that Jupiter's orbit is exterior to that of the Earth? What is the arrangement of Jupiter's seasons, and of his days and nights? Had his axis been inclined to the plane of his orbit, like that of our Earth, how long would his polar nights have been?

TELESCOPIC APPEARANCES OF JUPITER.

Fig. 17.

Jupiter when viewed through a telescope, appears to be surrounded by a number of luminous zones, usually termed belts, that frequently extend quite around him. These belts are parallel not only to each other, but, in general, to his equator, which is also nearly parallel to the ecliptic. They are subject, however, to considerable variation, both in breadth and number. Sometimes eight have been seen at once; sometimes only one, but more usually three. Dr. Herschel once perceived his whole disc covered with small belts.

Sometimes these belts continue for months at a time with little or no variation, and sometimes a new belt has been seen to form in a few hours. Sometimes they are interrupted in their length; and at other times, they appear to spread in width, and run into each other, until their breadth exceeds 5,000 miles.

Bright and dark spots are also frequently to be seen in the belts, which usually disappear with the belts themselves, though not always, for Cassini observed that one occupied the same position more than 40 years. Of the cause of these variable appearances, but little is known. They are generally supposed to be nothing more than atmospherical phenomena, resulting from, or combined with, the rapid motion of the planet upon its axis.

Different opinions have been entertained by astronomers respecting the cause of these belts and spots. By some they have been regarded as clouds, or as openings in the atmosphere of the planet, while others imagine that they are of a more permanent nature, and are the marks of great physical revolutions, which are perpetually agitating and changing the surface of the planet. The first of these opinions sufficiently explains the variations in the form and magnitude of the spots, and the parallelism of the belts. The spot first observed by Cassini, in 1665, which has both disappeared and re-appeared in the same form and position for the space of 43 years, could not possibly be occasioned by any atmospherical variations, but seems evidently to be connected with the surface of the planet. The forin of the

Describe Jupiter's appearance, as seen through a telescope. What is supposed to be the cause of these phenomena? Relate some of the different opinions cntertained by astronomers on this subject.

belt, according to some astronomers, may be accounted for by supposing that the atmosphere reflects more light than the body of the planet, and that the clouds which float in it, being thrown into parallel strata by the rapidity of its diurnal motion, form regular interstices, through which are seen its opaque body, or any of the permanent spots which may come within the range of the opening.

Jupiter is also attended by four satellites or moons, some of which are visible to him every hour of the night; exhibiting, on a small scale and in short periods, most of the phenomena of the solar system. When viewed through a telescope, these satellites present a most interesting and beautiful appearance. The first satellite, or that nearest the planet, is 259,000 miles distant from its centre, and revolves around it in 421 hours; and appears, at the surface of Jup`ter, four times larger than our Moon does to us. His second satellite, being both smaller and farther distant, appears about the size of ours; the third, somewhat less; and the fourth, which is more than a million of miles from him, and takes 16 days to revolve around him, appears only about one third the diameter of our Moon.

These satellites suffer frequent eclipses from passing through Jupiter's shadow, in the same manner as our Moon is eclipsed in passing through the Earth's shadow. The three nearest satellites fall into his shadow, and are eclipsed, in every revolution; but the orbit of the fourth is so much inclined, that it passes by its opposition to him, two years in six, without falling into his shadow. By means of these eclipses, astronomers have not only discovered that light is 8 minutes and 13 seconds in coming to us from the Sun, but are also enabled to determine the longitude of places on the Earth with greater facility and exactness than by any other methods yet known.

It was long since found, by the most careful observations, that when the Earth is in that part of her orbit which is nearest to Jupiter, the eclipses appear to happen 8' 13" sooner than the tables predict; and when in that part of her orbit which is farthest from him, 8′ 13" later than the tables predict; making a total difference in time, of 16′ 26′′. From the mean of 6000 eclipses observed by Delambre, this disagreement between observation and calculation, was satisfactorily settled at 8' 13', while both were considered equally correct. Now when the eclipses happen sooner than the tables, Jupiter is at his nearest approach to the Earth-when later, at his greatest distance; so that the difference in his distances from the Earth, in the two cases, is the whole diameter of the Earth's orbit, or about 190 millions of miles. Hence, it is concluded that light is not instantane

How many satellites has Jupiter? How often are they visible to him? What is the distance from him of his first or nearest satellite? What is the time of its revolution ? What is its apparent magnitude at the surface of Jupiter, compared with the magnitude of the Moon, as seen oy us? What are the apparent magnitudes of his other satellites, as seen at his surface, compared with that of the Moon as seen at the Earth? What is the distance of his fourth satellite from him? What is the time of its revolution? How often are his three nearest satellites eclipsed? How often his fourth? Why is it not eclipsed as often as the others? What important purposes have these eclipses served to astronomers? State the method by which the progressive motion of light, and the time which it occupies in coming to us from the Sun, were discovered.