tensive temperate zones. In passing from his equator to his poles, we meet every variety of climate, from the warmest to the coldest, with but slight variations in any latitude, from age to age. His days and nights are always nearly of the same length, as the sun is always near his equinoctial. His poles have alternately six years day and six years night.

205. The polar inclination and zones of Saturn differ but little from those of Mars; but his seasons are greatly modified by the length of his periodic time. This being about 30 years, his four seasons must each be about 7 years long; and his polar regions must have alternately 15 years day and 15 years night. The rings of Saturn, which lie in the plane of his equator, and revolve every 10 hours, are crossed by the sun when he crosses the equinoctial of the planet. During the southern declination of the sun, which lasts fifteen years, the south side of the rings is enlightened, and has its summer. It has also its day and night, by revolving in a portion of the planet's shadow. When the sun is at the southern tropic, it is midsummer on the south side of the rings, as the rays of light then fall most directly upon them. As the sun approaches the equator, the temperature decreases, till he crosses the equinoctial, and the long winter of fif teen years begins. At the same time, the north side of the rings begins to have its spring; summer ensues, and in turn it has fifteen years of light and heat. The influence of these wonderful rings upon the climate of Saturn must be very considerable. During the winter in each hemisphere, they cast a deep shadow upon some portion of his surface during the day; and in the summer, these immense reflectors so near the planet, and so bright in the sunlight, must contribute greatly to the light, if not to the warmth, of his summer evenings. The poles of Saturn are alternately 15 years in the light, and 15 years in darkness.

206. Of the inclination of the axes of Uranus and

205. Zones of Saturn, and why? Length of seasons? Rings-how on. lightened? Influence upon climate? Polar days and nights?

Neptune, respectively, we have no knowledge, and con sequently can form no opinion respecting their tropics, polar circles, zones, &c. If not too much incline, like Venus, they have but four seasons in their year, which would make each season of Uranus 22 years and 9 days long, and each season of Neptune 41 years and 564 days long; as these periods are, respectively, one-fourth of the periodic time of the planet (72).

Thus we see that tropics, polar circles, zones, and seasons are not peculiar to our globe, but are a necessary result of an inclined axis, and a revolution around the sun. The causes which produce our seasons are known to be in operation in other planetary worlds, and it would be unreasonable to deny that the effect was there also.

DISCOVERY OF THE DIFFERENT PLANETS.

207. The old planets, as they are called, viz., Mercury, Venus, Mars, Jupiter, and Saturn, have been known as planets, or "wanderers," from the earliest ages. Uranus was discovered by Sir William Herschel, March 13th, 1781. Neptune was demonstrated to exist before it had been seen, by M. Le Verrier, of France, August, 1846; and first seen by Dr. Galle, of Berlin, Sept. 23, 1846.

208. The discovery of Neptune is probably one of the greatest achievements of mathematical science ever recorded. By comparing the true places of Uranus with the places assigned by the tables, it was found that he was not where his known rate of motion required him to be; and after making all due allowance for the attraction of Jupiter and Saturn (65), by which pertur bations would be produced, it was found that there was evidently the effect of some other body, exterior to the orbit of Uranus, the attraction of which body helped to cause the perturbations of Uranus. From this effect, produced by an unknown and invisible world, lying far out beyond the supposed boundaries of the solar system, not only was the existence of its cause demonstrated, but its direction, distance, mass, and period were proximately ascertained.

206. What said of the seasons of Uranus and Neptune? Probable length of former? Latter? (Remark in note?)

207. What said of the "old planets?" Of Uranus? Neptune?

208. Describe the discovery of Neptune. Perturbations? Tables, &c.! (Describe successive steps in detail. What said of Mr. Adams ?)

1. On the evening of the 23d of September, 1846, Dr. Galle, one of the astronomers of the Royal Observatory at Berlin, received a letter from Le Verrier, of Paris, requesting him to employ the great telescope at his command in searching for the supposed new planet, and giving its position, as ascertained by calculation, as 325° 52′s' of geocen tric longitude. Dr. Galle, taking advantage of the very evening on which he received Le Verrier's letter, soon discovered an object resembling a star of the eighth magnitude, near the spot indicated by Le Verrier, as the place of the new planet. On consulting an accurate star chart, it was found that no such star was there laid down, and observations were at once commenced, with a view to detecting any change of place. In three hours time, it was seen to have moved; and by the next evening at eight o'clock, i was found to have retrograded more than four seconds of time (see 97 and cut)-a circumstance which proved it to be much nearer the earth than the fixed stars, and consequently a planet-the very planet which had caused the unaccountable irregularities of Uranus. The geocentric longitude of the planet, at midnight, September 23, 1846, was 325° 52'8'; which was less than 10 from the place assigned to it by Le Verrier! The reason why Le Verrier wrote to Dr. Galle was, that the former had no suitable telescope for conducting the search in which he was so deeply interested.

2. It is worthy of remark that Mr. Adams, of St. John's College, Cambridge. Englan had also calculated the place, &c., of the new planet, and had arrived at results similar to those reached by Le Verrier; but as the latter had published his conclusions first, the honor of the discovery is generally accorded to Le Verrier.

209. The Asteroids have all been discovered during the present century, and most of them since 1847. The first known was discovered by Professor Piazzi, of Palermo, on the first day of January, 1801, while searching for a star which he found mapped down in his star chart, but could not find in the heavens. He soon lost sight of it, however, on account of its nearness to the sun, but it was re-discovered by Dr. Olbers, of Bremen, in January, 1802. Pallas, Juno, and Vesta were discovered between 1802 and 1807, after which no additional asteroids were discovered for thirty eight years, or till 1845. From 1845 to 1855, thirty new asteroids were discovered, while fifty others were added to the list during the next ten years. From the probability that they are very numerous, and the rapidity with which they have recently been discovered, it is not unlikely that the list may yet be extended to hundreds. (For complete list, see page 247.)

209. How long have asteroids been known? When and by whom was the fir t discovered? The next three? How many from 1807 to 1845? From 1845 to 1855? From 1855 to 1865? From 1865 to the present time: What said of the probability of other discoveries?

2

CHAPTER V.

SECONDARY PLANETS -THE MOON.

210. THE Secondary Planets are those that revolve statedly around the primaries, and accompany them in their periodical journeys around the sun. Of these, the earth has one; Jupiter, four; Saturn, eight; Uranus, six; and Neptune, one-in all twenty. Besides these, there is a strong suspicion among astronomers that Venus is attended by a satellite, and that Neptune has at least two, instead of one.

Sir John Herschel says Uranus is attended "certainly by four, and perhaps by six, and Neptune by two or more." Outlines, Art. 533. In regard to Venus, Prof. Hind, of London, says: Astronomers are by no means satisfied whether Venus should be attended by a satellite or not. * It is a question of great interest, and must remain open for future discussion."

211. Though the secondary planets have a compound motion, and revolve both around the sun and around their respective primaries, they are subject to the same general laws of gravitation-of centripetal and centrifugal force-by which their primaries are governed. Like them, they receive their light and heat from the sun, and revolve periodically in their orbits, and on their respective axes. In the economy of nature, they seem to serve as so many mirrors to reflect the sun's light upon superior worlds, when their sides are turned away from a more direct illumination.

The design of all the secondaries may be inferred from what is said of the purposes for which our own satellite was created. "And God said, Let there be lights in the firmament of heaven, to divide the day from the night and let them be for signs, and for. seasons, and for days and years; and let them be for lights in the firmament of heaven, to give light upon the earth: and it was so. And God made two great lights; the greater light to rule the day, and the lesser light to rule the night: he made the stars also."-Gen. i, 14—16.

210. What are the Secondary planets? How many? How distributed? What supposition respecting Venus? Neptune? (Herschel's remark? Prof. Hind's?

211. What said of the laws by which the primaries are governed? Ligat and heat? Uses? (From what may we infer their design?)

212. To the inhabitants of our globe, the earth's satellite or moon is one of the most interesting objects in all the heavens. Her nearness to the earth, and consequent apparent magnitude, her rapid angular motion eastward, her perpetual phases or changes, and the mottled appearance of her surface, even to the naked eye, all conspire to arrest the attention, and to awaken inquiry. Add to this her connection with Eclipses, and her influence in the production of Tides (of both of which we shall speak hereafter in distinct chapters), and she opens before us one of the most interesting fields of astronomical research.

213. The Romans called the moon Luna, and the Greeks Selene. From the former, we have our English terms lunar and lunacy. In mythology, Selene was the daughter of Helios, the sun. Our English word selenography-a description of the moon's surface-is from Selene, her ancient name, and grapho, to describe.

214. The point in the moon's orbit nearest the earth is called Perigee, from the Greek peri, about, and ge, the earth. The point most distant is called Apogee, from apo, from, and ge, the earth. These two points are also called the apsides of her orbit; and a line joining them, the line of the apsides.

See the moon in apogee and perigee in the cut. The singular of apsides is apsis.

APOGEE

PERIGEE.

215. The mean distance of the moon from the earth's center is, in round numbers, 239,000 miles; or, more accurately, 238,650. The eccentricity of her orbit amounting to 13,333 miles, of course her distance must vary, and also her apparent magnitude (56). Her average angular

212. What said of our moon? Why specially interesting?

213. Latin name of the moon? Greek? Derivation of words from Luna! Who was Selene in mythology? Selenography?

214. Perigee and Apogee? Derivation? What other name for these two points? What is the line of the apsides? (Apsis ?) 215. Moon's distance? Does it vary?

Why? Eccentricity of orbit!