year, viz., five solar and two lunar; but the most asual number is four. There can never be less than two in a year; in which case, both must be of the Sun. Eclipses both of the Sun and Moon recur in nearly the same order, and at the same intervals, at the expiration of a cycle of 223 lunations, or 18 years of 365 days and 15 hours. This cycle is called the Period of the Eclipses. At the expiration of this time, the Sun and the Moon's nodes will sustain the same relation to each other as at the beginning, and a new cycle of eclipses begins.

454. In a total eclipse of the Sun, the heavens are shrouded in darkness, the planets and stars become visible, the temperature declines, the animal tribes become agitated, and a general gloom overspreads the landscape. Such were the effects of the great eclipse of 1806. In a lunar eclipse, the Moon begins to lose a portion of her light and grows dim, as she enters the Earth's penumbra, till at length she comes in contact with the umbra, and the real eclipse begins.

455. In order to measure and record the extent of eclipses, the apparent diameters of the Sun and Moon are divided into twelve equal parts, called digits; and in predicting eclipses, astronomers usually state which "limb" of the bou is to be eclipsed-the southern or northern-the time of the first contact, of the nearest approach of centers, direction, and number of digits eclipsed.

456. The last annular eclipse visible in the United States, occurred May 26, 1854. The next total eclipse of the Sun will be August 7, 1869.

Some of the anciers, and all barbarous natious, formerly regarded eclipses with amazement and fear, as upernatural events, indicating the displeasure of the gods. Combus is said

How many of each? Least number, and which? Usual number f What said of the order of eclipses? Time of cycle? 454. Describe the effects of total eclipse of the Sun. The process of a lunar eclipse? 455. How are eclipses mea ared and recorded? 456. When the next annular eclipse visible in this country? The next total? How have

to have made a very happy use of this superstition, as already stated on a previous page. (Art. 433.)

457. Eclipses can be calculated with the greatest precision, not only for a few years to come, but for centuries and ages either past or to come. This fact demonstrates the truth of the Copernican theory, and illustrates the order and stability that everywhere reign throughout the planetary regions.

The following is a list of all the solar eclipses visible in Europe and America from 1838 to the close of the present century. To those visible in New England, the number of digits is annexed.

The eclipses of 1869, 1875, and 1900 will be very large. In those of 1873, 1875, and 1880, the Sun will rise eclipsed.

That of 1875 will be annular. The scholar can continue this table, or extend it back wards, by adding or substracting the Chaldean period of 18 years, 11 days, 7 hours, 51 minutes, and 31 seconds.

CHAPTER VI.

PRIMARY PLANETS CONTINUED-MARS AND THE

ASTEROIDS.

458. MARS is the first of the exterior planets, its orbit lying immediately without, or beyond, that of the Earth, while those of Mercury and Venus are within. He appears, to the naked eye, of a fine ruddy complexion; resembling, in color, and appa

the ignorant and superstitious regarded eclipses? 457. What said of the calculation of eclipses? What does this demonstrate and illustrate? 458. Position of Mars' orbit? How does he appear to the naked eye? When most brilliant? When least?

rent magnitude, the star Antares, or Aldebaran, near which it frequently passes. It exhibits its greatest brilliancy about the time that it rises when the Sun sets, and sets when the Sun rises; because it is then nearest the Earth. It is least brilliant when it rises and sets with the Sun; for then it is five times farther removed from us than in the former case.

459. Its distance from the Earth at its nearest approach is about 50,000,000 of miles. Its greatest distance from us is about 240,000,000 of miles. In the former case, it appears nearly 25 times larger than in the latter. When it rises before the Sun, it is our morning star; when it sets after the Sun, it is our evening star.

The distance of the interior planets from the earth, varies within the limits of the diameters of their respective orbits; for when a planet is in that part of its orbit which is nearest the Earth, it is evidently nearer by the whole diameter of its orbit, than it is when at a point opposite, on the other side of its orbit. The exterior planets vary in distance within the limits of the diameter of the Earth's orbit.

460. Mars is sometimes seen in opposition to the Sun, and sometimes in superior conjunction with him; sometimes gibbous, but never horned In conjunction, it is never seen to pass over the Sun's disc, like Mercury and Venus. These prove not only that its orbit is exterior to the Earth's orbit, but that it is an opaque body, shining only by the reflection of the Sun.

461. The motion of Mars through the constellations of the zodiac is but little more than half as great as that of the Earth; it being generally about 57 days in passing over one sign, which is at the rate of a little more than half a degree each day. Thus, if we know what constellation Mars enters to-day, we may conclude that two months hence it will be in the next constellation; four months hence, in the next; six months, in the next, and

so on.

Its mean sidereal revolution is performed in 686.9796458 solar days; or in 686 days, 23 hours, 30 minutes, 41.4 seconds. Its synodical revolution is performed in 779.936 solar days; or in 779 days, 22 hours, 27 minutes, and 50 seconds.

462. Mars performs his revolution around the Sun in one year and 10 months, at the distance of 145,000,000 of miles; moving in its orbit at the mean rate of 55,000 miles an hour. Its diurnal rotation on its axis is performed in 24 hours, 39

459. Its distance from the Earth? What effect upon its apparent magnitude? When morning and evening star? How do the distances of the planets from the Earth vary? Their apparent diameters? 460. Is Mars ever in opposition to the Sun? In conjunc tion? Its phases? Does it ever transit the Sun? What do these facts prove? 461. What is his rate of motion through the constellations? What calculation based upon it? 462. His periodic time? Distance from the Sun? Mean rate of motion per hour? Time of rotation on his axis? How does his day compare with ours?

minutes, and 21 seconds; which makes its day about 44 minutes longer than ours.

463. Its form is that of an oblate spheroid, whose polar diameter is to its equatorial, as 15 is to 16, nearly. Its diameter is 4,500 miles. Its bulk, therefore, is 7 times less than that of the Earth; and being 50,000,000 of miles farther from the Sun, it receives from him only half as much light and heat.

464. The inclination of its axis to the plane of its orbit, is about 28°. Consequently, its seasons must be very similar to those of the Earth. Indeed, the analogy between Mars and the Earth is greater than the analogy between the Earth and any other planet of the solar system. Their diurnal motion, and of course the length of their days and nights, are nearly the same; the obliquity of their ecliptics, on which the seasons depend, are not very different; and, of all the superior planets, the distance of Mars from the Sun is by far the nearest to that of the Earth; nor is the length of its year greatly different from ours, when compared with the years of Jupiter, Saturn and Uranus.

465. To a spectator on this planet, the Earth will appear alternately, as a morning and evening star; and will exhibit all the phases of the Moon, just as Mercury and Venus do to us; and sometimes like them, will appear to pass over the Sun's disc like a dark round spot. Our Moon will never appear more than a quarter of a degree from the Earth, although her distance from it is 239,000 miles. If Mars be attended by a satellite, it is too small to be seen by the most powerful telescopes.

When it is considered that Vesta, the smallest of the asteroids, which is once and s half times the distance of Mars from us, and only 269 miles in diameter, is perceivable in the cpen space, and that without the presence of a more conspicuous body to point it out, we may reasonably conclude that Mars is without a Moon.

466. The progress of Mars in the heavens, and indeed of all the superior planets, will, like Mercury and Venus, sometimes appear direct, sometimes retrograde, and sometimes he will seem stationary. The portion of the ecliptic through which a planeɩ seems to retrograde is called the Arc of Retrogradation. The more remote the planet the less the arc, and the longer the time of its retrogression. These retrograde movements and stations, as they appear to a spectator from the Earth, are common to all the planets, and demonstrate the truth of the Copernican system.

468. Form of Mars? Diameter? Bulk? Light and heat? 464. Inclination of his axis to the plane of his orbit? His seasons? Resemblance to our globe? 465. How would the Earth appear to a spectator upon Mars? Our Moon? Has Mars a satellite? 466. What said of the motions of Mars and the other planets? What coustitutes *hg

RETROGRADE MOTION OF THE EXTERIOR PLANETS.

Suppose the Earth at A, and the planet Neptune at B, he would then appear to be at C, among the stars; but as Neptune moves but a little from B toward F, while the Earth is passing from A to D, Neptune will appear to retrograde from C to E. Whatever Neptune may have moved, however, from B toward F, will go to reduce the amount of apparent. retrogression.

It is obvious from this figure, that the more distant an exterior planet is, and the slower it moves, the less will be its arc of retrogradation, and the longer will it be retrograding. Neptune appears to retrograde 180 days, or nearly half the year.

The following table exhibits the amount of arc and the time of the retrogradation of the principal planets:

The right-hand figure represents Mars as seen at the Cincinnati Observatory, August 5, 1845. On the 30th of the same month he appeared as represented on the left. Ine middle view is from a drawing by Dr. Dick.

467. The telescopic phenomena of Mars afford peculiar interest to astronomers. They behold its disc diversified with numerous irregular and variable spots, and ornamented with zones and belts of varying brilliancy, that form, and disappear, by turns. Zones of intense brightness are to be seen in its polar regions, subject, however, to gradual changes. That of the southern pole is much the most brilliant. Dr. Herschel supposes that they are produced by the reflection of the Sun's light from the frozen regions, and that the melting of these masses of polar ice is the cause of the variation in their magnitude and appearance.

Arc of Retrogradation? What do these motions prove? 467. How does Mars appear brough a telescope? Dr. Herschel's opinion of its polar regions? How con' red in