Motion of the aphelion in longitude, for 100 years, 1° 50' 7". Diurnal rotation, 10 h. 16 m. Mean distance from the Sun, 900,000,000 miles. SATELLITES OF SATURN. Saturn has seven satellites, revolving about their primary, and accompanying him in his revolution round the Sun. The seventh satellite of Saturn is, by some, reckoned the fifth. This satellite is remarkably bright at its greatest western elongation, surpassing all the others but one in lustre. Very small at other times, it entirely disappears at its greatest eastern elongation.) This phenomenon was first observed by Cassini. It may arise from one part of the satellite being more luminous than the other parts. It was observed through all the variations of light by Dr. Herschel. He concluded that, like the satellites of Jupiter and our Moon, it revolved on its axis at the same time that it performed a revolution round its primary. "There is not, perhaps," says Dr. Herschel, "another object in the heavens, that presents us with such a variety of extraordinary phenomena, as the planet Saturn a magnificent globe, encompassed by a stupendous double ring; attended by seven satellites; ornamented with equatorial belts; compressed at the poles; turning upon its axis; mutually eclipsing its ring and satellites, and eclipsed by them; the most distant of the rings also turning upon its axis, and the same taking place with the farthest of the satellites; all the parts of the system of Saturn occasionally reflecting light to each other; the rings and the moons illuminating the night of the Saturnian; the globe and the satellites enlightening the dark parts of the rings; and the planet and the rings throwing back the Sun's beams upon the moons, when they are deprived of them at the time of their conjunctions.' How was Saturn formerly considered? With what light does he shine? What does the character of Saturn, h, represent? What have been discovered on the disk of Saturn? What kind of belts were seen by Herschel? In what time does Saturn turn on his axis? How are the Sun's light and heat at Saturn? What is the form of Saturn? What is the most remarkable phenomenon of Saturn? How did the ring first appear to Galileo? When did he announce his discovery? How did Huygens discover that the object seen by Galileo was a ring? Is the ring of Saturn single or double? How is it inclined to the ecliptic? When is the ring visible to us? How long is each half of Saturn, in succession, enveloped in the dark shadow of the ring? What substance did Dr. Herschel consider the ring of Saturn? How many satellites has Saturn? What is remarkable in the appearance of the seventh? What does Dr. Herschel say of the phenomena of Saturn ? SECTION XIII. Of Herschel. Herschel, Uranus, or Georgium Sidus, was unknown, as a planet, to the scientific world, till the year 1781.) On the 13th of March, in that year, it was discovered by the celebrated astronomer whose name it usually bears in this country. Before the discovery, it had probably been seen by astronomers, but had attracted no particular attention. Prior, in his Lectures, tells us, "it had been observed by Flamstead and Mayer, but was considered by them as a fixed star, and, as such, introduced into their catalogues." Viewing the small stars near the feet of Gemini, Dr. Herschel was struck with the appearance of one, less brilliant than the rest, but surpassing them in magnitude. He suspected it to be a comet.) Observing it with different telescopes, he found that, contrary to the fixed stars, its disk appeared to increase with the magnifying power of the glasses. He found also, by measuring its distance from some of the fixed stars, at different times, that it moved about 24′′ in an hour. That it was a planet, first occurred to Dr. Maskelyne The name Georgium Sidus, or Georgian star, was given to this planet by Dr. Herschel, in compliment to his patron George III., the then reigning king of Great Britain. It is often called Uranus, in European publications. Uranus, in heathen mythology, was the father of Saturn. This planet is so distant, it is scarcely visible to the naked eye. In a serene sky, however, it appears like a star of the sixth magnitude, shining with a bluish-white light, and a brilliancy between the splendor of the Moon and that of Venus. Such is the immense distance of this planet, that no observations have been made upon it, by which the time of its diurnal revolution can be determined. Herschel is denoted by this character, the initial of a name immortal as human science ;)" the horizontal bar being crossed by a perpendicular line, forming a kind of cross, the emblem of Christianity, denoting, perhaps, its discovery was made in the Christian era. The ball, however, represented as pendent from the H, may be a globe or planet, as hanging on the discovery of the astronomer Herschel. ELEMENTS OF HERSCHEL. Mean diameter, 35,112 miles. Mean diameter as seen from the Sun, 4". Tropical revolution, 83 y. 305 d. 7 h. 21 m. Sidereal revolution, 84 y. 8 d. 9 h. 33 m. Place of the ascending node, Gemini, 12° 59′ 4′′. Place of the descending node, Sagittarius, 12° 59′ 4′′. Motion of the nodes in longitude for 100 years, 26' 10". Retrograde motion of the nodes in 100 years, 57′ 22′′. Place of the aphelion, 11 s. 17° 48' 6". Motion of the aphelion in longitude for 100 years, 1° 28' 0". Mean distance of the planet from the Sun, 1800,000,000 miles. Eccentricity, 86,263,800 miles. SATELLITES OF HERSCHEL. Six satellites have been discovered, accompanying Herschel in his dark and tedious round. "It is remarkable," says Prior, "that these satellites revolve in a retrograde direction, or contrary to the order of the signs, in orbits lying nearly in the same plane, and almost perpendicular to the plane of the planet's orbit." This statement is corroborated by other accounts. The satellites of Herschel were all discovered by Dr. Herschel. When was the planet Herschel discovered? By whom? If it had been before seen, what had it been considered? How was Dr. Herschel employed when he discovered this planet? To whom did it first occur that it was a planet? Why did Dr. Herschel call it Georgium Sidus? How does it appear to the naked eye? Has its diurnal rotation been determined? Why is Herschel denoted by this character, H? How many satellites has Herschel? What is remarkable in the motion of these satellites? CHAPTER II. Causes of the Planetary Motion. MATTER is in itself inactive, and moves but as impelled by external force. An impulse being given to a body, it passes in a right line, till turned out of its course by a different impulse, not in direct coincidence or opposition to the former. Uninterrupted, it would forever move in the same direction, and at the same rate, or over equal distances in equal times. After every new impulse, it will take a new direction, and pass in a diagonal between its former course and the direction of the new impulse. Let the body at A, [Plate v. Fig. 9,1 be impelled by a momentum sufficient to carry it in a given time from A to B. It would, uninterrupted, move from B to C, and from C to D, equal distances in equal times. But if, at B, it receive an impulse in the direction BE, sufficient to carry it to E in the same time that the former motion would carry it to C, it would move in the diagonal B F, and be found at F at the same time that it would have arrived at C, unaffected by the im pulse last given.-See Enfield's Philosophy, Book II. Chap. iii. Proposition 14. Circular or elliptical motion is the effect, not merely of an impulse in one direction,) but of such an impulse and a continued action forcing a body from a right line towards a centre. The planets all move in ellipses, differing, however, but little from circles, except the orbits of Juno and Pallas. They are kept in their orbits by the projectile force given at their formation by the Creator, and the constant force of gravity, or the Sun's attraction. Let A, a planet, [Plate vi. Fig. 1,] be projected along the line A B C, meeting with no resistance, it would forever retain the same velocity, and the same direction. The force, which would carry it from A to B |