earth's orbit as to make the year 2 h. 48 m. longer than before. But the length of the year was not changed as

much as two seconds; whence we infer that the mass

must have been less than gooo that of the earth.

490. The great comet of 1843.-On the 28th of February, 1843, a comet appeared in the day-time, quite near the sun, the head and the beginning of the tail seeming like a dagger turned from the sun. In a few days it appeared after sunset, with a tail 65° in length. When its orbit was determined, it was found that its center had passed within 80,000 miles of the sun's surface, and that the two bodies were distant not more than 32,000 miles; the heat to which it was subjected was more than 47,000 times as intense as the solar heat received at the surface of the earth, and more than 25 times that required to melt and vaporize agate and rock-crystal. Through this intensest fire, the comet whirled at a rate which increased its distance from the sun tenfold in one day. Its tail was 150 millions of miles long and 3 millions broad. Its orbit is elliptic; some have deemed it identical with the comet of 1668, having a period of 175 years.

491. Other recent comets.-Donati's comet appeared on the 2d of June, 1858; in October, it was a very beautiful object in the northern sky. The nucleus was not large; the tail was about 50 millions of miles in length, very brilliant, and of very graceful form. Its period is about 2100 years.

The comet of 1861 was noted for its tail, which extended over more than 100°. Its period is about 450 years.

The comet of 1862 formed frequent bright jets, like jets of steam, directed toward the sun, or to the eastward, in a direction opposite to its motion. The material of each jet seemed to drift away in the direction of the tail.

Comets are nebulous masses which move about the sun in very eccentric orbits. They are composed of very rare material, and usually show nucleus, coma, and tail.

The tail is developed by some unknown repulsion in the sun as the comet approaches, and retracted as the comet recedes. It is always curved from the direction of motion, and is tubular. The head diminishes as the comet comes near the sun.

The path of a comet is always the curve of some one of the conic sections, and the motion conforms to the great laws of planetary motion.

The elements of a comet's orbit are: Inclination; longitude of perihelion; longitude of ascending node; perihelion distance; eccentricity. Comets which have the same orbital elements are deemed identical.

Halley's comet first returned in accordance with prediction. The periods of eight small comets are verified by returns.

Encke's and Faye's comets meet regularly some resistance, which shortens their periods.

Biela's comet appeared in two portions, moving side by side. Lexell's comet had its orbit twice changed by the attraction of Jupiter.

Ast. 22

CHAPTER XVI.

STAR-SHOWERS. THE ZODIACAL LIGHT.

493. Shooting-stars.-The bright objects which in a clear night suddenly glide along a portion of the sky, and as suddenly vanish, sometimes leaving a faint trail of light, are called shooting-stars. On a moonless night, a single observer may count an average of 8 an hour. As one person can see but about one-fourth of the sky at once, it follows that about 30 are visible in an hour, or more than 700 in a day, if none were obscured by sunlight. But the same observations may be made from more than 10,000 stations on the earth; whence 7 millions a day pass near enough to the earth to be seen. 50 times as many may be seen with the telescope, as without, and this number increases with the power of the instrument.

494. The November showers.-On the 12th of November, 1833, a brilliant display of these meteors was observed throughout the eastern half of North America. Humboldt saw in South America a similar shower on the same month and day in 1799. Records were found of at least twelve other great November showers, at dates which answer very nearly to periods of 33 years. These and other facts caused the belief that these displays are periodical, and that one would occur in 1866. In America, though more meteors were counted than are commonly seen, the shower bore little likeness to that of 1833; in Europe the scene was more brilliant, and fully confirmed the predicted return.