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 1878, 187 hu .880, the Sun will rise eclipsed.

That of 1875 will te annular. The scholar can continue this table, or extend it backwards, by adding se substracting the Chaldean period of 18 years, 11 days, 7 hours, 51 minutes, and 81 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' orbiti 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 33,000,000 of miles. about 245,000,000 of iniles. chan 50 times as large as in Sun, it is our morning star; evening star.

Its greatest distance from us is In the former casc, it appears more the latter. When it rises before the when it sets after the Sun, it is our

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 whiel 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, 25 hours, 80 minutes, 41.4 seconds. Its synodical revolution is performed in 779.986 sola 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 140,000,000 of miles; moving in its orbit at the mean rate of 53,000 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 t Their apparent diameters? 460. Is Mars ever in opposition to the Sun? In conjunclion Its phases? Does it ever transit the Sun? What do these facts prove? What is his rate of motion through the constellations? What calculation based upon it? 468 His periodic time? Distance from the Sun? Mean rate of motion per hour? I'm as rotation on his axis? How does his day compare with ours?

461.

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 55 is to 56, nearly. Its diameter is 4,300 miles. Its bulk, therefore, is 7 times less than that of the Earth; and being nearly 50,000,000 of miles farther from the Sun, it receives from him less than half as much light and heat. 464. The inclination of its axis to the plane of its orbit, is about 283°. 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 a 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 planet seems to retrograde is called the Arc of Retrogradation. The more remote the planet the less the arc, and the longer the timo 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? 66. What said of the motions of Mars and the other planets? What constitutes ha

RETROGRADE MOTION OF THE EXTERIOR PLANETS.

B

H

Suppose the Earth at A, and the planet Nepture 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 apparen 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 ( 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. Ins iddle 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 through a telescope? Dr. Herschel's opinion of its polar regions? How contrmed in

He was the more confirmed in these opinions by observing that after the exposure of the luminous zone about the north pole to a summer of eight months, it was considerably decreased, while that on the south pole, which had been in total darknese during eight months, had considerably increased. He observed, farther, that when this spot was most luminous, the disc of Mars did not appear exactly round, and that the bright part of its southern limb seemed to be swollen or arched out beyond the proper curve.

468. The extraordinary height and density of the atmosphere of Mars, are supposed to be the cause of the remarkable redness of its light. It has been found, by experiment, that when a beam of white light passes through any colorless transparent edium, its color inclines to red, in proportion to the density of the medium, and the space through which it has traveled. Thus the Sun, Moon, and stars, appear of a reddish color when near the horizon; and every luminous object, seen through a mist, is of a ruddy hue.

This phenomena may be thus explained :-The momentum of the red, or least refrangible rays, being greater than that of the violet, or most refrangible rays, the former wil! make their way through the resisting medium, while the latter are either reflected or absorbed. The color of the beam, therefore, when it reaches the eye, must partake of the color of the least refrangible rays, and this color must increase with the distance. The dim light, therefore, by which Mars is illuminated, having to pass twice through its atmosphere before it reaches the Earth, must be deprived of a great proportion of its violet rays, and consequently then be red. Dr. Brewster supposes that the difference of color among the other planets, and even the fixed stars is owing to the different heights and densities of their atmospheres.

THE ASTEROIDS, OR TELESCOPIC PLANETS.

469. Ascending higher in the solar system, we find, between the orbits of Mars and Jupiter, a numerous cluster of small planets, which present a variety of anomalies that distinguish them from all the older planets of the system. The first of these, namely, Ceres, was discovered by Piazzi, at Palermo, January 1, 1801; and three others, namely, Pallas, Juno, and Vesta, have been known since 1807. More than one hundred of these planets have been discovered since that time, the greater part of them since 1853. [See Table, p. 231.]

470. The scientific Bode entertained the opinion, that the planetary distances, above Mercury, formed a geometrical series, each exterior orbit being double the distance of its next interior one, from the Sun; a fact which obtains with remarkable exactness between Jupiter, Saturn, and Uranus. But this law seemed to be interrupted between Mars and Jupiter. Hence he inferred, that there was a planet wanting in that interval; which is now tala opi. ion? 465. Supposed cause of the ruddy color of Mars? Philosophical expla nation? Dr. Brewster's opinion? 469. Position and numi er of the asteroids? When discover d? 470. Bode's theory? What seeming interruption? What conclusion!