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 4,222 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 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 240,000 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 open 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 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.

463. 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 constitutes the

H

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. The 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 through a telescope? Dr. Herschel's opinion of its polar regions? How confirmed 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 darkness 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 medium, 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 refrang!ble rays, being greater than that of the violet, or most refrangible rays, the former will 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 TELESCOPIO PLANETS.

469. Ascending higher in the solar system, we find, between the orbits of Mars and Jupiter, a cluster of twenty-seven small planets, which present a variety of anomalies that distinguish them from all the older planets of the system. Their names are Flora, Clio, Vesta, Iris, Metis, Eunomia, Psyche, Thetis, Melpomene, Fortuna, Massilia, Lutetia, Calliope, Thalia, Hebe, Parthenope, Irene, Egeria, Astræa, Juno, Ceres, Pallas, and Hygeia. These, and four others whose names have not yet been announced, have all been discovered during the present century.

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

this opiion? 468. Supposed cause of the ruddy color of Mars? Philosophical explanation? Dr. Brewster's opinion? 469. Position and number of the asteroids? When discovered? 470. Bode's theory? What seeming interruption? What conclusion?

happily supplied by the discovery of the numerous star-form planets, occupying the very space where the unexplained vacancy presented a strong objection to his theory.

According to Bode, the distances of the planets may be expressed nearly as follows: the Earth's distance from the Sun being 10.

Comparing these values with the actual mean distances of the planets from the Sun, we cannot but remark the near agreement, and can scarcely hesitate to pronounce that the respective distances of the planets from the Sun, were assigned according to a law, although we are entirely ignorant of the exact law, and of the reason for that law.Brinkley's Elements, p. 89.

471. The Asteroids are much smaller in size than the older planets-they all revolve at nearly the same distances from the Sun, and perform their revolutions in nearly the same periodstheir orbits are much more eccentric, and have a much greater inclination to the ecliptic-and what is altogether singular, except in the case of comets-some of their orbits cross each other; so that there is even a possibility that two of these bodies may, some time, in the course of their revolutions, come into collision.

The orbit of Vesta is so eccentric, that she is sometimes farther from the Sun than either Ceres, Pallas, or Juno, although her mean distance is many millions of miles less than theirs. The orbit of Vesta crosses the orbits of several other asteroids, in two opposite points.

The student should here refer to the Figures, Map I. of the Atlas, and verify such of these particulars as are there represented. It would be well for the teacher to require him to observe particularly the positions of their orbits, and to state their different degrees of inclination to the plane of the ecliptic.

472. From these and other circumstances, many eminent astronomers are of opinion, that these twenty-seven planets are the fragments of a large celestial body which once revolved between Mars and Jupiter, and which burst asunder by some tremendous convulsion, or some external violence. The discovery of Ceres, by Piazzi, on the first day of the present century, drew the attention of all the astronomers of the age to that region of the sky, and every inch of it was minutely explored. The consequence was, that in the year following, Dr. Olbers, of Bremen, announced to the world the discovery of Pallas, situated not many degrees from Ceres, and very much resembling it in size.

How substantially justified? 471. Size of the asteroids? Distance from the Sun? Periodic time? Forms of their orbits? Position with respect to the ecliptic? What other singularity in their orbits? What remarkable facts respecting the orbit of Vesta? 472. What conclusion has been drawn from these facts? Progress of discovery?

473. From this discovery, Dr. Olbers first conceived the idea that these bodies might be the fragments of a former world; and if so, that other portions of it might be found either in the same neighborhood, or else, having diverged from the same point, "they ought to have two common points of reunion, or two nodes in opposite regions of, the heavens through which all the planetary fragments must sooner or later pass."

474. One of these nodes he found to be in the constellation Virgo, and the opposite one in the Whale; and it is a remarkable coincidence that it was in the neighborhood of the latter constellation that Mr. Harding discovered the planet Juno. In order, therefore, to detect the remaining fragments, if any existed, Dr. Olbers examined, three times every year, all the small stars in Virgo and the Whale; and it was actually in the constellation Virgo, that he discovered the planet Vesta. Since that time, twenty-three additional asteroids have been discovered, and it is not unlikely that still additional fragments of a similar description will hereafter be discovered.

Dr. Brewster attributes the fall of meteoric stones to the smaller fragments of these bodies happening to come within the sphere of the Earth's attraction.

Meteoric stones, or what are generally termed aerolites, are stones which sometimes fall from the upper regions of the atmosphere upon the Earth. The substance of which they are composed, is, for the most part, metallic; but the ore of which it consists is not to be found in the same constituent proportions in any known substance upon the Earth. Their fall is generally preceded by a luminous appearance, a hissing noise, and a loud explosion; and when found immediately after their descent, they are always hot, and usually covered with a black crust, indicating a state of exterior fusion.

Their size varies from that of small fragments of inconsiderable weight to that of the most ponderous masses. They have been found to weigh from 300 pounds to several tons; and they have descended to the earth with a force sufficient to bury them many feet under the surface.

Some have supposed that they are projected from volcanoes in the Moon; others that they proceed from volcanoes on the Earth; while others imagine that they are generated in the regions of the atmosphere; but the truth probably is not yet ascertained. In some instances, these stones have penetrated through the roofs of houses, and proved destructive to the inhabitants.

If we carefully compute the force of gravity in the Moon, we shall find that if a body were projected from her surface with a momentum that would cause it to move at the rate of 8200 feet in the first second of time, and in the direction of a line joining the centers of the Earth and Moon, it would not fall again to the surface of the Moon; but would become a satellite to the Earth. Such an impulse might, indeed, cause it, even after many revolutions, to fall to the earth. The fall, therefore, of these stones, from the air, may be accounted for in this manner.

Mr. Harte calculates, that even a velocity of 6000 feet in a second, would be sufficient to carry a body projected from the surface of the Moon beyond the power of her attraction. If so, a projectile force three times greater than that of a cannon, would carry a a body from the Moon, beyond the point of equal attraction, and cause it to reach the Earth. A force equal to this is often exerted by our volcanoes, and by subterranean steam. Hence, there is no impossibility in the supposition of their coming from the Moon.

473. Theory of Dr. Olbers? 474. Where did he find these nodes? What remarkable coincidence? Dr. Olbers' efforts? Discoveries since? Dr. Brewster's idea respecting meteoric stones? What are meteoric stones? Circumstances of their fall? Size and weight? Supposed origin? Could they have fallen from the Moon? What computations?