so many degrees round each pole would, in their turn, be almost six years in darkness; and as Jupiter is of such an amazing size, in this case, immense regions of land would be uninhabitable.

Jupiter is attended by four satellites, or moons; these are invisible to the naked eye; but through a telescope they make a beautiful appearance. As our moon turns round the earth, enlightening the nights, by reflecting the light she receives from the sun, so these also enlighten the nights of Jupiter; and move round him in different periods of times, proportioned to their several distances: and as the moon keeps company with the earth in its annual revolution round the sun, so these accompany Jupiter in its course round that luminary.

In speaking of the satellites, we distinguish them according to their places; into the first, second, and so on; by the first, we mean that which is nearest to the planet.

The outermost of Jupiter's satellites will appear almost as big as the moon does to us; five times the diameter, and twenty-five times the disc of the sun. The four satellites must afford a pleasing spectacle to the inhabitants of Jupiter; for sometimes they will rise all together; sometimes be all together on the meridian, ranged one under another, besides frequent eclipses. Notwithstanding the distance of Jupiter and his satellites from us, the eclipses thereof are of considerable use for ascertaining with accuracy the longitude of places. From the four satellites, the inhabitants of Jupiter will have four different

kinds of months, and the number of them in their year not less than 4,500.

An astronomer in Jupiter will never see Mercury, Venus, the Earth, or Mars; because, from the immense distance at which he is placed, they must appear to accompany the sun, and rise and set with him; but then he will have for the objects of observation, his own four moons, Saturn, his ring and satellites, and probably the Georgium Sidus.

Before the discovery of the Georgium Sidus, Saturn was reckoned the most remote planet in our system: he shines but with a pale feeble light, less bright than Jupiter, though less ruddy than Mars. The uninformed eye imagines not, when it is directed. to this little speck of light, that it is viewing a large and glorious globe, one of the most stupendous of the planets, whose diameter is nearly 78,000 miles. We need not, however, be surprised at the vast bulk of Saturn, and its disproportion to its appearance in the heavens; for we are to consider that all objects decrease in their apparent magnitude, in proportion to their distance; but the distance of Saturn is immense; that of the earth from the sun is 96,000,000 miles; of Saturn, 916,500,000 miles.

The length of a planet's year, or the time of its revolution round its orbit, is proportioned to its distance from the sun. Saturn goes round the sun in 29 years, 167 days, six hours; moving at the rate of

rather more than 22,298 miles per hour. His apparent diameter, afa mean distance from the earth, is 16'.

It has not yet been ascertained with certainty by astronomical observation, whether Saturn revolves or not upon his axis. The sun's disc will appear ninety times less to an inhabitant of Saturn, than it does to us; but notwithstanding the sun appears so small to the inhabitants of the regions of Jupiter and Saturn, the light that he will afford them is much more than would be at first supposed; and calculations have been made, from which it is inferred, that the sun will afford 500 times as much light to Saturn, as the full moon to us; and 1600 times as much to Jupiter. To eyes like our's, unassisted by instruments, Jupiter and the Georgium Sidus would be the only planets seen from Saturn; to whom Jupiter would sometimes be a morning, sometimes an evening, star.

One of the first discoveries by the telescope, when brought to a tolerable degree of perfection, was, that Saturn did not appear like other planets. Galileo, in 1610, supposed it composed of three stars or globes, a larger in the middle, and a smaller on each side; and he continued his observations till the two lesser stars disappeared, and this planet looked like the others. Further observation shewed that what Galileo took for two stars, were parts of a ring. This singular and curious appendage to the planet Saturn, is a thin, broad, opake ring, encompassing the body of the planet, without touching it, like the horizon of an artificial globe; appearing double when

viewed through a good telescope. The space between the ring and the globe of Saturn is supposed to be rather more than the breadth of the ring, and the greatest diameter of the ring to be in proportion to that of the globe, as seven to three; the plane of the ring is inclined to the plane of the ecliptic, in an angle of 50°, and is about 21,000 miles in breadth. It puts on different appearances to us, sometimes being seen quite open, at others only as a line upon the equator. It is probable, that it will at times cast a shadow over vast regions of Saturn's body. The ring of Saturn, considered as a broad flat ring of solid matter, suspended round the body of the planet, and keeping its place without any connection with the body, is quite different from all other planetary phenomena with which we are acquainted. Of the nature of this ring, various and uncertain were the conjectures of the first observers, though not more perplexed than those of the latest. Of its use to the inhabitants of Saturn, we are as ignorant as of its nature; though there are reasons for supposing that it would appear to them as little more than a white or bright-coloured cloud. Some of the phenomena of Saturn's ring will be treated of more particularly in another part of this Essay. Saturn is not only furnished with this beautiful ring, but has also seven attendant moons.

OF THE GEORGIUM SIDUS. Ḥ

From the time of Huygens and Cassini, to the discovery of the Georgium Sidus by Dr. Herschel, though the intervening space was long, though the number of astronomers was increased, though assiduity in observing was assisted by accuracy and perfection in the instruments of observation, yet no new discovery was made in the heavens; the boundaries of our system were not enlarged. The inquisitive mind naturally enquires, why, when the number of those that cultivated the science was increased, when the science itself was so much improved, in practical discoveries, it was so deficient? A small knowledge of the human mind will answer the question, and obviate the difficulty. The mind of man has a ́natural propensity to indolence; the ardour of its pursuits, when they are unconnected with selfish views, are soon abated; small difficulties discourage, little inconveniences fatigue it, and reason soon finds excuses to justify, and even applaud this weakness. In the present instance, the unmanageable length of the telescopes that were in use, and the continual exposure to the cold air of the night, were the difficulties that the astronomer had to encounter with and he soon persuaded himself, that the same effects would be produced by shorter telescopes, with equal magnifying power; herein was his mistake; and hence the reason why so few discoveries have been made since the time of Cassini. A similar instance of the retrogradation of science occurs in the history of