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than usually abundant on the 18th of October, the 6th and 7th of December, the 2d of January, the 20th of April, and from the 15th to the 20th of June.

550. ORIGIN. Prof. Olmsted, who was the first to present his views upon the extraordinary phenomenon, which occurred on the 12th of November, 1833, has arrived at the following conclusions from a very extensive examination of facts.

That the source of the meteors is a body possibly of great extent, composed of matter exceedingly rare like the tail of a comet. That it revolves about the sun within the orbit of the earth, its period of revolution being probably a little less time than a year,

That in consequence of its proximity on the night in question, the extreme parts of the body were detached and drawn towards our globe, by the force of gravity.

That its altitude above the surface of the earth, at its nearest point, was about 2238 miles; and that the descending fragments entered the atmosphere with a velocity ranging from about fourteen to twenty miles per second.

That these fragments were combustible, and in consequence of their amazing velocity, the air was so powerfully compressed before them, that they took fire, and were consumed before reaching the earth.

551. This last conclusion will appear by no means incredible, when the following considerations are taken.

into view.

By suddenly forcing down a solid piston to the bottom of a cylinder, in which it moves air-tight, sufficient heat can be evolved to ignite tinder; and this occurs, when the air within the cylinder is compressed to one-fifth of its original volume. Upon the supposition, that the descending fragments compressed the rarefied atmosphere at the height of 35 miles only to the density of common air, the amount of heat developed would be 46,080°

What is said of other periods?

Detail Prof. Olmsted's theory.

What is said respecting the amount of heat developed by the condensa on of the atmosphere?

Fah.; an intensity nearly three times greater than the highest temperature of a glass-house furnace, which is 16,000° Fah.

552. If the nebulous body revolves about the sun in a period somewhat less than a year, it tends to explain the occurrence of shooting-stars at all seasons (since the earth and the nebulous body would then be always comparatively near each other), and will also favor the explanation of the meteoric showers which have happened towards the end of April.

553. Prof. Olmsted has been led to suppose, from the whole course of his observations, that the nebulous body in which the meteors originated, might be identical with the zodiacal light. In a late article published by M. Biot, this distinguished philosopher also maintains, that meteoric showers are occasioned by the zodiacal light coming in periodic contact with the atmosphere of the earth.

It is not regarded by Prof. Olmsted as essential to the truth of his theory, that a shower of meteors should occur upon the 13th of every November

554. In order to account for shooting-stars in general, including alike their ordinary and extraordinary displays, and embracing the several epochs, the views of Chaldni have been adopted by Arago and other eminent philosophers

555. CHALDNI'S THEORY. This theory consists in supposing, that, besides the planets, millions of small bodies are constantly revolving about the sun, which become ignited when they enter the terrestrial atmosphere. They are not considered to be uniformly spread throughout space; but in some regions to be diffusely scattered, and in others grouped together in vast multitudes, forming zones or rings around the sun; many of which cross the path of the earth.

The ordinary, nightly phenomenon of shooting-stars,

If the nebulous body revolves about the sun in a little less time than a vear, what does it tend to explain ?

What is M. Biot's opinion? What is Chaldni's theory?

is then imagined to arise, when the earth, in her progress through the heavens, traverses those regions which contain only a few of these bodies; but when the zones are encountered, and the globe passes amid countless numbers, the display is proportionally greater, and the meteors occasionally descend in magnificent showers. Amid this vast collection solid masses of considerable size are supposed to exist, and should one of these enter the atmosphere of the earth, a meteorite with all its splendors sweeps across the sky.

Such at present is the general state of our knowledge in regard to shooting-stars.

CHAPTER III.

OF THE AURORA BOREALIS OR NORTHERN LIGHT.

556. THE Aurora Borealis is a luminous appearance in the northern sky, which presents, when in full display, a spectacle of surpassing splendor and beauty. It has in all ages been an object of wonder and mystery, and still continues so; for although many valuable facts have been brought to light by the investigations of science, the cause of this brilliant phenomenon is yet involved in obscurity.

557. CONSTITUTION. Notwithstanding its fantastic motions, and momentary changes in brightness and color, the aurora, according to the best observations, still preserves, amid all its fluctuations, certain invariable characteristics of form and position. It consists of a dark segment, an arch of light, luminous streamers,

and a corona or crown.

558. DARK SEGMENT. All observers in the high latitudes of Europe, agree in stating, that before the

What does chapter third treat of?

What is the Aurora Borealis ?

Of what does it consist?

Describe the dark segment.

aurora appears, the sky in the northern horizon assumes a darkish hue, which gradually deepens, until a circular segment is formed, bordered by an arch of light, extending from east to west. The segment presents the appearance of a cloud, its tint is light in the lower latitudes, and grows darker as we advance to the north, up to a certain limit; after this the reverse occurs, and when high latitudes are attained it becomes so faint as to be scarcely visible. At Upsal and Christiana it is sometimes black or of a deep gray, which changes into a violet.

During a splendid aurora, that occurred at Toronto in Dec. 1835, and which is described by Capt. Bonnycastle, a dark, black changing mass, was visible below the luminous arch, (fig. 42,) and in a remarkable phase of the aurora, when several bright bows were seen at once, the interval between the second and third assumed a blackness of the deepest intensity.

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559. A difference of opinion exists in regard to the nature of this segment. From numerous observations made at Dorpat in Russia, Struve infers, that the dark

Is it real or imaginary?

ness is simply the effect of contrast with the luminous arch; while, from equally extensive researches at Abo in Finland, Argelander concludes, that the segment is something real; since the portion of the sky it occupies, is darker than common, before the bright bow of the aurora appears.

560. ARCH OF LIGHT. The dark segment is bounded by a luminous arch or bow, varying in width from one to three apparent diameters of the moon. Its lower edge is clearly defined, but the upper is only so when the arch is narrow, for as the width increases, it gradually blends with the brightness of the sky. The color of the bow is a pale white, which becomes more pure and brilliant near the polar regions.

According to the most accurate observations, this arch has a tendency to place itself at right angles to the magnetic meridian, or in other words, to the direction of a compass-needle at rest. (C. 985.) This fact was particularly noticed by Lieutenant Hood, who accompanied Franklin in his northern expedition in 1819.

561. The centre of the auroral arch probably coincides with the north magnetic pole of the earth, which is situated in 70° N. Lat. In our own country, the compass-needle points to the north, and the arch crosses the heavens from east to west; but in some parts of Greenland, the needle is directed to the west, and the arch is then seen extending from north to south.

In the year 1838, when Simpson wintered at Fort Confidence, in 66° 54' N. Lat., he found the needle. always pointing to the north-east, and the auroral arches invariably spanning the heavens at right angles, from north-west to south-east.

At Melville Isle, in 74° 30′ N. Lat., the luminous arches were seen by Parry in the south; the north magnetic pole of the earth being then in that direction. 562. This beautiful bow of light is not stationary,

Describe the arch of light. Its color and position.
What is its position in some parts of Greenland?

What was its position at Fort Confidence and at Melville Isle ?
Is the arch of light stationary?

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