Immagini della pagina
PDF
ePub

ville, cane within the distance of 25 miles. An instance is given by Dr. Halley of a meteorite that exploáed at an elevation of 69 miles, with a report like that of

a cannon.

509. VELOCITY. The velocity of these bodies is gen erally somewhat more than 300 miles per minute, though many cases have occurred of far greater speed; the meteorite just mentioned, that came within 25 miles of our earth, moved at the rate of 1200 miles per minute.

510. If a body in the atmosphere is seen at the same time by two observers upon the earth at different sta tions, and its angular elevation taken at both stations; its height in miles and feet is easily ascertained by the aid of trigonometry, when the distance between the two stations is known. If the body is in motion, and its position noted at the moment of its appearance and disappearance, the distance it travels in this interval, or the length of its visible path can be obtained, when its height has first been computed. The speed is ascertained by dividing the length of the visible path by the number of seconds during which the body is seen. The magnitude is easily obtained by trigonometrical calculations, when the distance of the body and its angular diameter is known. In this manner computations are made upon meteorites, shooting-stars and the aurora borealis.

511. From their sudden appearance and extreme velocity, all observations upon these phenomena are liable to great inaccuracy, and estimates of magnitude, velocity and height, derived from such observations must be received with much allowance, and are to be regarded only as approximations, more or less near to the truth.

AEROLITES.

512. FORM. The greater number of aërolites, according to Schreibers, have always the same general form, which is that of an oblique or slanting pyramid.

In what manner are calculations made respecting the size, altitude, speed, &c., of bodies high above the earth?

What is said as to estimates of this kind?

What is the form and external appearance of aërolites ?

They are also alike in external appearance, presenting to view a black, shining crust, as if the body had been coated with pitch. This crust is not greater than the two-hundredth part of an inch in thickness, its composition is identical with that of the mass, it bears the marks of fusion, and strikes fire with the flint. When broken, the surface of the fracture displays the color of an ashgrey.

"Distinct aërolites," says Berzelius, the celebrated chemist," are frequently so like one another in color and external appearance, that we might believe them to have been struck out of one piece."

513. COMPOSITION. According to Berzelius, aërolites consist of eighteen elementary substances. A nineteenth has since been discovered, and perhaps two others. They are remarkable for containing malleable metallic iron, nickel, and chrome. Their specific gravity varies from 3.35 to 4.28.

514. These common characteristics indicate a common origin, and this we are led to seek beyond the earth, inasmuch as the composition of aërolites is totally dif ferent from that of any stony mass, forming a part of the crust of the globe. Malleable metallic iron is rarely, if ever, found in terrestrial substances, nickel is extremely scarce and has never been discovered on the surface of the earth and chrome is, if possible, still more rare.

515. It sometimes happens, though seldom, that the aërolite consists almost entirely of metallic iron. On the 26th of May, 1751, a meteorite burst with a tremendous report, over Hradschina, in the district of Agram, in Upper Sclavonia. Two fragments were seen rushing to the earth, the largest of which struck deep into the soil. This mass weighed 71 lbs., exhibited evident traces of fire, and, upon being analyzed, gave, out of every 100 parts, 95.5 of iron and 3.5 of nickel. A portion of the

What is their composition?

What do these common characteristics indicate?

Where must it be sought-and why?

Of what does the aërolite sometimes consist?

Give the instance.

iron being finely polished, and corroded with acids, a most beautiful crystaline structure was revealed, branching in every direction over the surface. This peculiarity belongs to most of the specimens of the iron of meteorites. (See Fig. 41.)

Fig. 41.

[graphic]

CRYSTALINE STRUCTURE OF THE METEORIC IRON OF TEXAS.

(Copy of an Impression taken from the Iron.)

516. METEORIC IRON. From the peculiar constitution and structure of aërolites, we are enabled to detect the meteoric origin of masses of iron which are occasionally found scattered over the surface of the earth, in all quarters of the globe. For since they possess the same elements as the iron of aërolites, combined in the same manner, and as no such masses have ever been taken from mines, we must necessarily conclude, that they were once exploded from a meteorite, though no record exists of their fall.

517. Humboldt relates, that in Mexico, near the envi

What structure did this iron possess?

Why are certain masses of iron supposed to have a meteoric origin?

rons of Durango, is an enormous mass of malleable iron and nickel, which possesses exactly the same composition as the fragment that fell at Agram.

A mass of metallic iron, weighing 1544 lbs., was discovered by Prof. Pallas, in 1771, at Krasnojark in Siberia. It was regarded by the Tartars as a sacred object, and according to their traditions had fallen from heaven.

The famous mass of malleable iron which was found in Texas in 1808, and is now in the cabinet of Yale College, weighs 1635 lbs. It contains nickel.

518. During an expedition in South Africa, Sir James Alexander discovered, near the Great Fish river, a considerable tract of country, over which fragments of metallic iron were scattered in profusion; a specimen analyzed by Sir John Herschel, was found to possess wickel, thus proving conclusively the meteoric origin of the masses.

519. ORIGIN OF METEORITES. Natural philosophers have advanced five hypotheses, to account for the origin of these extraordinary bodies.

1st. That they are ejected from terrestrial volcanoes. 2d. That they are produced in the atmosphere, being formed from the gases exhaled from the earth.

3d. That they are thrown from lunar volcanoes. 4th. That they are terrestrial comets revolving about the earth like the moon.

5th. That they are celestial bodies revolving about the sun like the planets, and encountered by the earth in its annual progress.

520. FIRST HYPOTHESIS. The first supposition cannot be maintained, since it is impossible for the volcanoes of the globe to hurl to the height of twenty miles masses of the size of meteorites; besides, the composi

Illustrate from the several instances given.

How many hypotheses have been advanced, to account for the origin of meteorites, and what are they?

What are the objections to the first hypothesis?

tion of the latter is entirely different from all volcanic products.

521. SECOND HYPOTHESIS. The second is likewise untenable. Nickel, according to high chemical authorities, has never been raised in vapor; even under the intense heat of volcanoes. A mass of matter formed in the air, must therefore be destitute of nickel, an element which meteorites invariably possess. Moreover, such a body, in its descent, would fall perpendicularly to the ground by the action of gravity, and not sweep along, as did the Weston meteorite, in a direction nearly parallel to the surface of the earth.

522. THIRD HYPOTHESIS. In regard to the third hypothesis, it has been shown, by calculations of La Place, and other eminent mathematicians, that a mass proJected from the moon, with a velocity of 10,660 feet per second, would pass beyond the point of the moon's attraction, and either fall to the globe in the space of tw: days and a half by the force of gravity, or revolve about the earth like the moon. It is not therefore impossible, that such an event might occasionally occur, but it is utterly improbable that meteorites originate in this manner.

523. Omitting other objections to this hypothesis, the size and number of meteorites constitute an insuperable difficulty. It requires a strong faith to believe, that such masses, as have been described, could be hurled from a lunar volcano, at the rate of not less that 10,000 feet per second; a speed five times greater than the highest velocity of a cannon-ball.

524. The number of meteorites must be also very great; for they have been seen from the earliest ages and in all inhabited quarters of the globe occasionally traversing the heavens, and those which have been noticed are probably only a part of the actual number that have visited the earth. Many must have passed unseen over the broad expanse of ocean, or crossed vas

What to the second?

What argument is advanced in favor of the third hypothesis?
What arguments against it?

« IndietroContinua »