vessel to accomplish the circumnavigation of our globe in a year, it would require upwards of ten years to perform at the same rate a similar voyage around him. His true diameter is nearly ninety thousand miles, eleven times that of the earth; and his volume is thirteen hundred times greater. But as his density is about one-fourth that of the earth, but little exceeding that of water, the quantity of matter in that vast orb is only 331 times greater than what our globe contains. In two years and thirty-five days it would descend upon the sun, if left to the influence of his attraction.

From the immense velocity of the equatorial regions of Jupiter, and the intensity of the centrifugal force, a considerable deviation in his figure from a perfect sphere is to be expected. This is open to observation. The form of the planet, even on a careless view through a good telescope, appears that of an oblate spheroid. According to the observations of Struve and others, while the polar diameter of the earth is only 35 less than the equatorial, that of Jupiter amounts to, or, in round numbers, his diameter at his poles is six thousand miles less than that at his equator. He is more orange-shaped than any of the other planets except Saturn, owing to his great axical speed. The rapid rotation of the planet is a beneficial physical arrangement. Removed to a distance from the source of light and heat which is 5% that of the earth, the sun's apparent size will be what it is to us, supplying to him only of the light and heat that we receive. But before the earth has once rolled round upon its axis, he has accomplished two rotations upon his, and thus his surface is brought under the solar influence after a much shorter interval of suspension than with us, compensating, to some extent, for its diminished power through his distance. That change of seasons which we experience is a diversity unknown to this planet. This arises from the axis being perpendicular, or nearly so, to the plane of the orbit, so that the days and nights are constantly of equal length, and the direction of the sun's rays constantly uniform, oblique towards the poles, and perpendicular at the equator. We have here undoubtedly another instance of wise arrangement, for otherwise the regions towards the poles would have been alternately immersed in the darkness of a six years' wintry night. It is not, however, to be understood that one uniform season prevails over his surface, but that the same parallels of latitude north and south of his equator enjoy uniformly the same season, whatever that season may be. It is perpetual summer in his equatorial regions, and perpetual winter at his poles; but the rapid rotation upon his axis is a security against intense heat accumulating in the former through the invariably direct action of the solar rays, and against the intensity of cold that would prevail at the latter, if his influence was absent for any considerable interval.

Jupiter, when viewed through a telescope, exhibits a series of zones, or bands, familiarly called belts, stretching across his surface in a direction parallel to his equator, and, generally, to each other. They were first observed at Naples by the Jesuits Zappi and Bartoli, about the year 1633. These belts are variable both in number and breadth. Sometimes eight have been seen, sometimes only one, but more usually three. Instead of being uniformly regular in their shape, they have frequently a lacerated appearance, and, while some continue in the same form for months, others change in a few hours. The aspect of the planet has been sketched by many observers. The views given by Cassini, Dr. Hook, Sir W. Herschel, and Dr. Long are here inserted. In Hook's drawing, taken in May, 1664, a spot appears upon one of the dark belts, by which Cassini ascertained the period of Jupiter's rotation in the following year. This ancient spot, as it is called, has repeatedly appeared and vanished. It was not seen between the years 1708 and 1713, but in the winter of 1834 it was distinctly visible. Other similar spots have been observed, generally situated in the belts. These appearances open a wide field for speculation. The belts have been deemed alterations upon the surface of the planet

caused by great physical convulsions. A far more probable and generally received opinion is, that the dark bands are the actual body of the planet, and the bright bands compact and undisturbed strata of clouds and vapour. It is also supposed that currents similar

to our trade winds set in from the poles

to the equator of Jupiter, which assume a
parallel direction as the equator is ap-
proached, owing to the prodigious ve-
locity of his equatorial regions, and
thereby cause those parallel aggregations
of vapour, through the interstices of
which the opaque body of the planet is
seen. Of course, this explanation is
purely hypothetical, but it is the best
that has been offered. Admitting the
explanation, it follows that Jupiter has
clouds, rain, wind, water, evaporation,
and seems thus fitted up to be the ha-
bitation of vegetable productions and
animal life.

The discovery of the moons of Jupiter,
four in number, was one of the first-fruits
gathered from the use of the telescope.
An opinion has indeed been current that
they may be discerned by a strong un-
assisted eye, but it
appears to be er-
roneous. Sir John Herschel remarks
that Dr. Wollaston, who had a keen eye,
never succeeded in so observing them,
though he cut off the light of the planet
by hiding the body behind a distant ob-
ject. The satellites are named after their

respective position in relation to the primary, the nearest to him being the first. Their comparative distances and magnitudes may be thus expressed;

The largest of these bodies is thus the third in point of distance; the next in magnitude is the fourth; the third in magnitude is the first; and the smallest is the second, being rather less than our own moon. It is in striking accordance with the case of our satellite, that the moons of Jupiter always turn the same face towards him, and thus make one rotation upon their axes while accomplishing one orbital revolution. These attendants are obviously designed to give him a splendid night in compensation for a day less lustrous than that which we enjoy; and it deserves notice that their orbital motions are so arranged that they can never be all new moons, and consequently invisible, at the same time. The position of the satellites with respect to each other, and to their primary, as seen from the earth, is very variable. Sometimes they appear ranged in a line on each side of the planet; at other times they are all grouped on the same side; and on one occasion, Nov. 2, 1681, Jupiter appeared deserted by his guards, three being on his disk, and one behind his body.

All the satellites are eclipsed once in every revolution by passing through the shadow of the planet, with the exception of the fourth, which sometimes escapes, because of the greater magnitude and inclination of its orbit. But with reference to the first satellite, it is never the case that both the immersion and the emersion can be observed, owing to its being so near the planet, and this is generally true of the second. On the other hand, the satellites pass between Jupiter and the sun in their revolution round him, and then project their shadows upon the enlightened portion of his disk, causing a solar eclipse to that part of his surface. They pass also between the planet and the earth, and then exhibit the phenomena of transits; and, lastly, they pass directly behind the body of the planet in opposition, and are then occulted. At A B C D the earth is represented in different parts of its orbit; and at J Jupiter is seen surrounded by his four satellites, whose orbits are marked 1, 2, 3, 4. The first satellite is shown entering the shadow of the planet at a; b is the point of emergence from it; and clearly, the interposing body of the planet will prevent the emersion from being visible to a spectator on the earth at

B

2

D, who sees the immersion, and that will be invisible from the same cause to a terrestrial observer at B, who sees the emergence. The satellite at c and e has its greatest eastern and western elongations to a spectator on the earth at A; and it appears projected as a small dark spot upon the disk of Jupiter at d. The following table expresses the general elements of the satellites, their distances and dimensions, times of revolution, and the duration of their eclipses :

Owing to the eclipses of the satellites taking place so frequently, they are of great use in the determination of terrestrial longitudes, those of the first being of the most importance on account of their more rapid recurrence. For this purpose they are carefully calculated, predicted, and registered in the Nautical Almanack, a book which forms one of the finest examples we have of the power of the human mind. There is nothing more simple than this method of finding the longitude. Let us suppose an individual to observe the immersion of the first satellite to take place where he is situated at ten o'clock at night, and that the Nautical Almanack registers its occurrence at Greenwich at six, it follows, that, as the sun apparently travels fifteen degrees an hour, the longitude of the observer is 4h=60° east of Greenwich; for faster time shows that he is east, and slower that he is west. At sea, great practical difficulty besets this method of finding the longi

tude, owing to the unsteadiness of the observer's station on board a vessel; and more convenient and accurate modes have superseded it in navigation. To the inhabitants of Jupiter, the opportunity will occur of witnessing upwards of four thousand lunar eclipses, and as many solar, in the course of their year. To us, the planet with his moons, constituting what is technically called the Jovian system, exhibits a miniature picture of the great solar scheme. The laws which govern the planets in their revolution round the

sun, govern the satellites in their revolution round their centre. They move in elliptical orbits, and, like the larger bodies, travel in a direction from west to east. Though insignificant in point of magnitude when compared with their primary, their united bulk is equal to thirteen of our moons. The first satellite will be a conspicuous object in the Jovian firmament, while, to it, the great planet will be exhibited on a scale of inconceivable magnificence, presenting every forty-two hours the varying forms of a crescent, a half and full moon, and a gibbous shape, appearing a thousand times larger than our moon appears to us in her corresponding phases.

SATURN. From the noblest of the planets in point of magnitude, we pass to the most extraordinary in architecture—an orb which would exhibit the most fascinating appearance to the eye but for its remoteness. An interval of space, nearly twice as great as the vast chasm between Jupiter and the sun, must be crossed to arrive at Saturn, whose mean distance from the solar body is about nine hundred millions of miles, and who never hails the terrestrials from any station nearer than eight hundred millions. He occupies a period of 10,759 solar days in accomplishing his circuit round the sun, having a mean daily motion among the stars of only about 2', the thirtieth part of a degree. If observed therefore entering a particular constellation of the zodiac, we may conclude that a period of two years and a half will elapse before he will bid it farewell. The year of the planet, extending to nearly thirty of ours, gives an age to his octogenarians, should there be any, parallel to that of a terrestrial born when the Jews were in Babylon, and surviving to be one of our contemporaries. His day is rather longer than that of Jupiter, but shorter by more than one half than our own, as he rotates upon his axis in 10h 29m. While appearing to the naked eye as a pale feeble point in the heavens, the Saturnian orb has an actual equatorial diameter of 79,160 miles, and a volume which is nine hundred times greater than that of the earth. Owing to his far removal from the central source of light and heat, his surface receives only the ninetieth part of these

H

elements compared with that we enjoy ; but it is computed, that even the ninetieth part of the solar light exceeds the illuminating power of three thousand of our moons at the full, and would itself be amply sufficient for the purposes of life. If arrested in his orbital course, and abandoned to the force of the solar attraction, the planet would drop to the sun in about five years and two months. The form of the body of Saturn is peculiar. Though not so swift upon his axis as Jupiter, his two diameters exhibit a greater difference, the polar being 6700 miles shorter than the equatorial, a degree of oblateness due to the cork-like lightness of his material in connection with his axical speed. It has been found however that his diameter is not the greatest at the equator, but at some distance from it, and that his north polar region is much more flattened than the south, a case of difference from that of the other planetary spheroids which may perhaps be referable to the anomalous local attraction to which he is subject.

Men had long been upon terms of acquaintance and familiarity with Saturn without suspecting the grandeur of his construction, or the remarkable apparatus with which he is furnished. The shepherd astronomers of Chaldea - the star-gazers of Egypt, Greece, and Rome the astrologers of the middle ages - Copernicus and Tycho Brahe- saw the planet only as a dull nebulous star slowly moving through the heavens. It was not until the earth had performed its annual circuit round the sun many thousand times, that the stately form and numerous attendants of the remote wanderer- hitherto deemed obscure and dreary were revealed. At length, in the year 1610, Galileo sent to Keppler the enormous word,

Smaiomrmilmepoetalevmibvnenvgttaviras

introduced in a former page, which veiled the Latin sentence to uninitiated eyes, and announced the most distant planet to be threefold. This was a glimpse caught of the luminous appendage of Saturn, which Huygens, with a more perfect instrument, found to be a ring, at the same time discovering one of the satellites. The planet is now more fully known to us; it occupies an illustrious place in the system, having a train of seven moons, with two concentric rings encompassing its body a peculiarity of structure without another example in the universe, as far as we are acquainted with the subject.

It is probable that this representation will prove to be a defective view of the wonderful architecture of Saturn. Perhaps, as mightier instrumental power is brought to bear upon his rings, they will be resolved into a greater number, as the one has already been into two. Indications appear of the outer ring being multiple, the determination of which may be reserved for the telescope of Lord Rosse. The singular fact has been recently ascertained, that the ball of the planet is not in the centre of the annulus,