lutions on the surface of that planet. The first of these hypotheses appears to explain the variations in the form and magnitude of the belts; but it by no means accounts for their parallelism, nor for the permanence of some of the spots. The spot first observed by the astronomer Cassini, in 1665, which has both disappeared and re-appeared in the same form within the space of fifty years, seems evidently to be connected with the surface of the planet. The form of the belt, according to some astronomers, may be accounted for by supposing that the atmosphere of Jupiter 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 his diurnal motion, form regular interstices, through which are seen the opaque body of Jupiter, or any of the permanent spots which may come within the range of the opening. TABLE, Showing the mean distance of Jupiter from the Sun, and the eccentricity of his orbit in miles; longitude of the ascending node, &c. Mean distance in miles Longitude of the perihelion at the same 3s. 8° 25′ 34" 0 11 8 35 0 Greatest equation of the centre 8 54 5 29 25 4 48 5 0 0 0 59 The secular motion of the apsides in longitude 1° 34′33.8", in consequentia, or according to the order of the signs. The direct secular motion of the nodes is 57′ 12.4". The greatest equation of the centre is subject to a decrease of 55" in a century. The great bulk of this planet, and the short interval of time in which it makes a revolution on its axis, cause the velocity of its equatorial parts to be prodiriously great; not less than 26 thousand miles per hour. 6. By directing the telescope to the planet Jupiter, it is found to be accompanied by four small stars, ranged nearly in a right line parallel to the plane of his belts. These small stars are the moons or satellites of Jupiter, which move round him in different periods, and at unequal distances from their primary. The discovery of these satellites was made by Gallileo in 1610; and this may be considered as one of the first fruits of the invention of the telescope. They cannot be seen by the naked eye, but are distinctly visible with a telescope of a moderate power. Their relative situation with regard to Jupiter, as well as to each other, is constantly changing. Sometimes they may be all seen on one side of Jupiter, and sometimes all on the other. They are designated by their distances from Jupiter, that being called the first whose distance from Jupiter is the least, when at the greatest elongation, and so on with the others. They are of very different magnitudes, some of them being greater than our Earth, while others are not so large as the Moon. Their apparent diameters being insensible, their real magnitudes cannot be exactly measured. The attempt has been made by observing the time they enter the shadow of Jupiter; but there is a great discordance in the observations which have been made to obtain this circumstance; and, of course, the result of these observations must be very discordant. The third, however, is the greatest; the fourth is the second in magnitude; the first the third in magnitude; and the second is the least. The first or nearest satellite of Jupiter, completes its mean sidereal revolution round that planet in 1 day, 18 hours, 27 minutes, and 33 seconds, at the mean distance of 264,490 miles from the centre of its primary; the second revolves in 3 days, 13 hours, 13 minutes and 42 seconds, at the mean distance of 420,815 miles; the third in 7 days, 3 hours, 42 minutes, and 33 seconds, at the mean distance of 671,234 miles; and the fourth in 16 days, 16 hours, 31 minutes, and 50 seconds, at the mean distance of 1,180,582 miles. The form of the orbits of these satellites is found to be nearly circular, especially those of the first, second, and third; and the velocity of their motions nearly uniform. In consequence of observing periodical changes in the intensity of the light of the satellites, Dr. Herschel inferred that they revolved on their axis, and that the period of their rotation is equal to the time of their revolution round Jupiter. The four moons or satellites of Jupiter must afford many curious phenomena to the inhabitants of that planet, in their nightly course through the heavens. Their apparent diameters as seen from Jupiter, are as follows: The apparent diameter of the first is 60′ 20′′ 'The app. mean diam. of the Earth's Moon 31 26 When the satellites are on the right hand, or west of Jupiter, approaching him, or east of Jupiter, receding from him, they are then in the superior parts of their orbits or farthest from the Earth. On the contrary, when the satellites are on the right hand, or west of Jupiter, receding from him, or east of Jupiter, approaching him, they are then in the inferior part of their orbits, or nearest the Earth. The satellites, like the inferior planets, are sometimes direct, stationary, and retrograde, as seen from the Earth. QUESTIONS. What is the mean hourly velocity of Jupiter in its orbit? In what time does Jupiter perform a revolution on its axis? What is the ratio of the equatorial diameter of Jupiter to its polar? What is the relative mean distance of Jupiter from the Sun, with respect to the Earth ? What is the inclination of Jupiter's orbit to the plane of the ecliptic? What is the duration of Jupiter's retrograde motion? nent ? Do the belts of Jupiter always appear permaHow many satellites or moons has Jupiter? CHAPTER XI. Of Saturn, of its Rings and its Satellites. 1. Saturn, the tenth planet in order of distance from the Sun, performs its sidereal revolution in 10,758 days, 23 hours, and 16 minutes, 34 seconds, which is the length of its year. The rotation of this planet on its axis, from west to east, is completed in 10 hours, 16 minutes, and 49 seconds, which is the length of its day. Saturn shines with a very feeble light, compared with that of Jupiter, partly on account of his great distance from the Sun, and partly from his dull red colour. 2. The diameter of Saturn is 77,680 miles; his shape is an oblate spheroid, like that of Jupiter, but still more elliptical, the equatorial diameter being to the polar as 12 to 11. The apparent mean diameter of this planet is about 17"; and its relative mean distance from the Sun is 95, that of the Earth being considered as 10. Saturn, next to Jupiter, is the largest planet in the solar system: its relative size is 974.78; its mass 93.271; and its density .095684; the size, mass, and density of the Earth being respectively considered as unity, or 1. And a body weighing 1 pound on the surface of the Earth, would weigh a little more than 14 pounds on the surface of Saturn. 3. The inclination of the orbit of Saturn to the plane of the ecliptic is 2° 30′ 18"; and the inclination of this planet's axis to its orbit is probably 60 degrees. The inclination of Saturn's orbit is subject to a small secular diminution of 15 seconds. su 4. The apparent motion of Saturn in its orbit is + to irregularities, similar to those of. Jupiter and Mars. It commences and finishes its retrograde motion when the planet before and after its opposition is about 108° 54' distant from the Sun. The arc of retrogradation is about 6° 18', and its duration is nearly 131 days. The synodic revolution of this planet, or the time from opposition to opposition, is 378 days, 2 hours, and 1 minute. TABLE, Showing the mean distance of Saturn from the Sun, &.c. The sidereal secular motion of the apsides is 32′ 17′′, but their tropical motion is 10 55' 47" in consequentia. The secular increase of the greatest equation is 1' 50". 5. Soon after the invention of telescopes, a remarkable appearance was observed about Saturn. After a considerable interval of time, Huygens having much improved them, discovered, by careful observations, a phenomenon unique, as far as we know, in the solar system. He found that Saturn is encompassed with a broad thin ring, inclined by a constant angle of about 30° to the plane of Saturn's orbit; and therefore at nearly the same angle to our ecliptic, and so always appearing to us obliquely. When the edge of the ring is turned towards us, it is invisible, on account of its thinness not reflecting light enough to be visible, except in the very best telescopes. When the |