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the friction partly arrests their motion, and converts it into heat and light. The body thus becomes visible to us. Its size and direction determine its appearance. If very small, it is consumed in the upper regions, and leaves only the luminous trail of a shooting star. If of large size, it may sweep along at a high elevation, or plunge directly toward the ground. Becoming highly heated in its course, it sheds a vivid light, while, unequally expanding, it explodes, throwing off large fragments which fall to the earth as aerolites, or continue their separate course as meteors. The cinders of the portion consumed rain down on us as fine meteoric dust.
Meteoric Kings.—These little bodies, it is thought, do not generally revolve individually about the sun, but myriads of them are collected in several rings, and when the earth passes through one of these floating girdles, a star-shower follows. This would account for their regular appearance in certain seasons of the year. In the cut we see how one ring, intersecting the earth's orbit at two points, would account for the August and November showers. Another ring, more inclined to the earth's path, and crossing it nearer the aphelion point, would produce the April showers.
Recent investigators are inclined to the view that there are separate rings for each of the established periods, and that they are very elliptical. The November ring seems to have its perihelion near the
ecliptic, and its aphelion beyond the orbit of Uranus; while the August ring extends beyond the solar system. The day of the month in which the great November shower occurs is becoming later at each re
turn; hence it is believed that the nodes of that ring are slowly travelling eastward along the ecliptic. The meteoric bodies are supposed to be quite uniformly distributed through the August stream, but very un/
equally through the November one. On this account, the former star-showers are quite regular, while the latter vary in brilliancy through periods of 33£ years.
Eelation Between Meteors And Comets.—The orbit of the November shower is found to be almost identical with that of the comet of 1866; while the August stream is in the track of the comet of 1862. It is a popular theory that these comets are only clusters of meteors crowded so closely together as to be visible by the reflected light of the sun. The single meteors are too small to be seen, except when they plunge into the earth's atmosphere and take fire. On the other hand, Herschel thinks that meteors are the dissipated parts of comets torn into shreds by the sun's attraction.
Eaddint Point.—A star (n) in the blade of the sickle is the point from which the stars in the November shower seem to radiate, while one in Perseus (y) is the radiant point of the August shower. In the shower of 1866, two observers, who counted the falling stars at the rate of 2,500 per hour, saw only five whose paths, if traced back, would not meet in Leo.
Meteorological Effect.—The temperature of August and November is said to be considerably increased by this ring of meteoric bodies, which prevents the heat of the earth from radiating into space. A corresponding decrease of temperature in February and May is caused by the stream or ring of meteors coining between the sun and earth.
Height.—Herschel estimates the average height of shooting stars above the earth at 73 miles at their appearance and 52 at their disappearance.
Weight.—Prof. Harkness estimates that the average weight of shooting stars does not differ much from one grain.
We come now to notice a class of bodies the most fascinating, perhaps, of any in astronomy. The suddenness with which comets flame out in the sky, the enormous dimensions of their fiery trains, the swiftness of their flight, the strange and mysterious forms they assume, their departure as unheralded as their advent—all seem to bid defiance to law, and partake only of the marvellous. Superstitious fears have always been excited by their appearance, and they have been looked upon in every age as
"Threatening the world with famine, plague, and war;
Thus the comet of 43 B. 0., which appeared just after the assassination of Julius Caesar, was looked upon by the Eomans as a celestial chariot sent to convey his soul heavenward. An old English writer observes: "Cometes signifie corruptions of the ayre. They are signs of earthquakes, of warres, of changyng kyngedomes, great dearthe of corn, yea, a common death of man and beast." Another remarks: "Experience is an eminent evidence that a comet, like a sword, portendeth war; and a hairy comet, or a comet with a beard, denoteth the death of kings, as if God and nature intended by comets to ring the knells of princes, esteeming bells in churches upon earth not sacred enough for such illustrious and eminent performances."
Description.—The term comet signifies a hairy body. A comet consists usually of three parts;—the nucleus, a bright point in the centre of the head; the
coma (hair), the cloud-like mass surrounding the nucleus; and the tail, a luminous train extending generally in a direction from the sun. There are comets without the tail, and others with several, while some are deprived of even the nucleus. These last consist merely of a fleecy mass, known to be comets from