within or beyond the orbit of Saturn or of Uranus, are distinctly visible at Jupiter, whose feeble light for ever precludes their discovery by us.

SECT. 9. Of Saturn.

38. Saturn, at the mean distance of 900 millions of miles, completes a revolution round the sun in little less than 30 years. It turns on its axis in little more than 10 hours. The light reflected by this planet is less brilliant than that of Jupiter. The degree of heat and light from the sun at Saturn is 80 times less than at the earth.

39. Saturn is remarkably distinguished from all the other planets in the solar system. When viewed through a telescope, it appears encompassed by a large luminous ring. This ring, in fact, consists of two, one exactly without or beyond the other. They are entirely detached from each other and from the body of the planet. (They are represented Pl. I, fig. 4, and in the wood-cut, exhibiting the relative sizes of the planets.) They cast a deep shadow, and appear even brighter than the planet; perhaps because they are above the region of mists and clouds in his atmosphere. They turn on the same axis with the planet, and in nearly the same time. Stars are sometimes seen between the rings, and also between the inner ring and body of the planet.

40. The surface of Saturn is sometimes diversified like that of Jupiter with spots and belts; which, like those, often vary. Saturn has 7 satellites, revolving at different distances, and in various times, from little less than 1 day to nearly 80. The nearest is reckoned 7th, the next 6th, the others 1st, 2d, &c. in order outward. The reason is, that the 7th and 6th are of recent discovery; the others have been long known,

SECT. 10. Of Uranus.

41. The planet Uranus was discovered by Dr. Herschel on the 13th March, 1781. Before that time, it had been seen by several astronomers. It was considered a small star, and was introduced as such into several catalogues of the stars. But Herschel first discovered it to be a planet. Its distance from the sun is about 1800 millions of miles. The time of performing a revolution is about 84 years. It is not known in what time it turns on its axis. Heat and light at Uranus are about 360 times less than with us. Uranus is scarcely visible to the naked eye.

42. This planet is attended by six satellites; all of which were discovered by Dr. Herschel, and revolve in orbits nearly perpendicular to that of their primary. Their motion is apparently retrograde; but this is probably an optical illusion, arising from the difficulty of ascertaining which part of their orbits inclines towards. the earth, and which declines from it. They are reckoned like those of Jupiter.

This planet is not uniformly designated by the name Uranus. Its discoverer called it Georgium Sidus, and it is often called Herschel. But on the continent of Europe it has obtained the name Uranus. Different writers on astronomy use different names.

43. It was stated (No. 24,) that the moon turns on its axis in precisely the same time that it performs its revolution round the earth. This is known from its always presenting to us the same side. For example, at its full it always exhibits the same spots in very nearly the same place. So also at the first or third quarter; that is, in quadrature. It has been observed, that when the seventh satellite of Saturn is to the eastward of that planet, its light becomes continually weaker till it is.

scarce perceptible; which circumstance must arise from dark spots or regions of a nature to reflect little or no light, which extend in a great degree over the side then presented to us. Now, that this phenomenon should always occur, when this satellite is precisely in this position, it is necessary that it revolve round its own axis in the same time that it revolves round Saturn. In like manner, by observing periodical changes in the intensity of the light of Jupiter's satellites, Dr. Herschel infers, that they turn on their axes in the same time that they occupy in moving round Jupiter. Hence it appears to be a general law of satellites, that they turn on their axes in the same time in which they revolve round their primaries.

44. On this account, the inhabitants of secondary planets observe some singular appearances, which the inhabitants of primary planets do not. Those who dwell on the side of a secondary planet next to the primary will always see that primary; while those who live on the opposite side will never see it. Those, who always see the primary, will see it constantly in very nearly the same place. For example, those who dwell near the edge of the moon's disk, will always see the earth near the horizon, and those in or near the centre will always see it directly or nearly over head. Those who dwell in the moon's south limb will see the earth to the northward; those in the north limb will see it to the southward; those in the east limb will see it to the westward; while those in the west limb will see it to eastward; and all will see it nearer the horizon in proportion to their own distance from the centre of the moon's disk. Similar appearances are exhibited to the inhabitants of all secondary planets.

It may be necessary for young pupils, that the instructer should illustrate the reason of these appearances.