tracts of uninhabited lands, leaving no trace of their existence, except those masses of meteoric iron, which from time to time are brought to light. If therefore the lunar theory is adopted, we can scarcely avoid the conclusion, that the moon has been ejecting for ages, so many, and such vast masses of matter, as must have sensibly diminished her bulk, and occasioned derangements in her system-results at variance with all observations.

525. FOURTH HYPOTHESIS. The fourth hypothesis, which is that of President Clap, of Yale College, affords a more reasonable explanation of the phenomena of these extraordinary bodies than any of the preceding. Under this view the earth is supposed to possess a system of comets like the sun. The solar comets revolve about their primary in very extended orbits; at one part of the course approaching so near the sun as almost to strike its surface, and during the remainder, sweeping far out of sight beyond the path of the planets, continuing invisible for years and even ages. In like manner meteorites are supposed to revolve about the earth; their size and periods of revolution being proportioned to the smallness of their primary. Moving also in very elliptical or oval orbits, they are too distant to be visible during the greater part of their course, but at one point of their path approach very close to the earth, and enter its atmosphere.

On account of the immense velocity of the meteorite, the air is imagined to be condensed before it to such a degree, that heat is evolved of sufficient intensity to inflame the mass at its surface, while during this combustion gases are generated, which by their expansive energy, produce explosions. By the strength of this disruptive force, glowing fragments are detached from the surface and fall to the ground, while the meteorite itself passes onward on its course.

526. It has been calculated, that the velocity of a

Which hypothesis affords a more reasonable explanation ?
Explain it fully.

body revolving about the earth must not be less than 300 miles per minute, nor greater than 420. Were it less than 300 miles, the mass would fall to the earth by the action of gravity; and if the rate exceeded 420 miles, it would pass away from the globe and never return. Within these limits, allowance being made for the motion of the earth in its orbit, and the resistance of the air, the body would revolve around the earth like the moon, approaching very near to its surface at stated periods.

527. In support of this hypothesis it is urged, that the velocity of meteorites, in general, is somewhat more than 300 miles per minute, though doubtless cases have occurred in which their speed was far greater.

The combustion of the meteorite, through the agency of a condensed atinosphere, is by no means improbable; for though the medium in which it moves is exceedingly rarefied, yet the velocity of the body is amazing; and it can easily be shown by calculation, that from the condensation thus effected, an intensity of heat would be developed of which we have no conception. (Art. 551.) Moreover, as silica, magnesia, and potassa are found in meteorites, it has been conjectured, that they may originally exist there in the state of pure metals; and, that when the meteorite enters our atmosphere, combustion arises from the extraordinary affinity of these substances for oxygen.

In those instances where meteorites move at a greater rate than 420 miles per minute, they are supposed either to revolve about the sun, and that the earth occasionally meets them in her annual progress; or to wander through space, until they come within the superior attraction of some other orb, and are then compelled to revolve around it.

What calculation has been made in respect to a body revolving about the earth?

What facts and suggestions are adduced in support of Pres. Clap's hypothesis?

What is said of meteorites moving at a greater rate than 420 miles per minute ?

528. FIFTH HYPOTHESIS. The last hypothesis is that of Chaldni, and is explained in Art. 555. In this the ignition and explosion of the meteorite are attributed to precisely the same causes as those assigned in the fourth hypothesis.

CHAPTER II.

OF SHOOTING-STARS AND METEORIC SHOWERS.

529. Shooting-stars or meteors differ from meteorites in several particulars. They commonly possess a superior velocity, and their altitude is generally greater : bursting from the clear sky, they dart along the heaven like a rocket, consuming themselves in their course, and leaving behind a luminous train, which gradually vanishes in a short time. Unlike the meteorite they usually pass away without any explosion, and no portion of the body ever reaches the earth. Besides, they are far more numerous and frequent; appearing almost every night, and at times descending in such multitudes that the heavens are illumined for hours with their glowing trains.

530. ALTITUDE. In order to investigate the phenomena of shooting-stars, Brandes and Benzenberg, two German philosophers, made a series of simultaneous. observations in the fall of the year 1798. On six evenings, between September and November, 402 shootingstars were beheld, and of these twenty-two were identified, that their altitudes, at the moment of their extinction, could be readily computed. They were found to be as follows:

7 disappeared at altitudes under 45 miles.

SO

Of what does chapter second treat?

In what particulars do shooting-stars and meteors differ from meteorites? Relate the account of the observations of Brandes and Benzenberg, for determining the altitudes of shooting-stars?

Give their results.

The least and greatest elevations were six miles and one hundred and forty.

531. In 1823, the investigation was renewed by Brandes, at Breslau and the neighboring towns, on a more extended scale. Between April and October, 1800 shooting-stars were seen at the different stations. Out

of this number, 98 were observed simultaneously at more than one station, and afforded the means of estimating their respective altitudes. The results were as follows:

4 disappeared at altitudes under 15 miles.

15

66

66

between 15 and 30 miles.

Out of the last eleven, two vanished at an elevation of 140 miles, a third at 220 miles, a fourth at 280 miles, and a fifth at 460 miles.

The height of four shooting-stars noticed by Professors Loomis and Twining, in December, 1834, varied

from 54 miles to 94.

532. Similar observations were made in Switzerland, on the 10th of August, 1838, by Wartman and others. A part of the observers stationed themselves at Geneva, and the rest at Planchettes, a village about sixty miles to the north-east of that city. In the space of seven hours and a half, 381 shooting-stars were seen at Geneva, and in five hours and a half 104 at Planchettes. All the circumstances attending their appearance were carefully noted, and their average height was computed at five hundred and fifty miles.

533. VELOCITY. In the first series of observations made by Brandes and Benzenberg, only two shootingstars afforded the means of determining their speed; one possessed a velocity of 1500 miles per minute, and

Give those of Loomis and Twining.

Give those of Wartman, at Geneva.

What is their velocity according to the observations of Brandes and Benzenberg, and Quetelet?

that of the other was between 1020 and 1260 miles per

minute.

In the second series, undertaken in 1823, the estimated rate of motion varied between 1080 and 2160 miles per minute. At Belgium, in 1824, M. Quetelet obtained observations upon six of these singular bodies, from which he was enabled to deduce their respective velocities, which were found to range from 600 to 1500 miles per minute.

534. COURSE. Of thirty-six stars, whose paths were ascertained by Brandes, the motion in twenty-six cases was downward, in one horizontal, and in the remaining nine, more or less upward; nor did they always move in straight lines; for the paths of some were curved, either upwards or sideways; while others proceeded in a serpentine course. Their general direction was from north-east to south-west. Several examples have been given by Chaldni, where the luminous body described a semicircle, first rising and then falling.

535. MAGNITUDE. The size of shooting-stars is variable. Fire-balls, which are regarded as nothing more than large meteors, have sometimes appeared of a magnitude almost incredible. During the remarkable shower of meteors, on the 12th and 13th of November, 1833, luminous globes, apparently as large as Jupiter and Venus, were seen darting through the air in all directions. About three o'clock on the morning of the 13th, a splendid body which appeared equal in size to the full moon, swept across the heaven from east to west. If the distance of this meteor was only eleven miles, its diameter must have been 528 feet, or one tenth of a mile. Amid the shower of stars that occurred in 1799, meteors were observed by Humboldt, apparently twice the size of the moon.

536. On the evening of the 18th of May, 1839, a meteor of extraordinary magnitude passed over the

What is said respecting the course of shooting-stars?

What of their magnitude?

Relate the account of the meteor of the 18th of May.