Meteors and shooting stars are luminous bodies which are frequently observed in our upper atmosphere and are sometimes seen to strike the earth. The luminosity of these bodies is thought to be due to friction against the atmosphere and that before entering the atmosphere they are cold and non-luminous.
Many of them are dissipated in the upper atmosphere, but probably the fragments in the form of invisible dust reach the earth in the course of time. Ten to twenty millions of meteors strike the earth's atmosphere every day.
It is thought by some that the earth has been formed by the aggregation of such particles, which would mean that unless the earth is losing matter in some way it is still increasing in weight.
Meteors which fall to the earth are called meteorites. They vary in size from very minute fragments to bodies of many tons in weight. The great Tent meteorite in New York City which Peary brought from Cape York, Greenland, weighs 36.5 tons. The Bacubirito meteorite in Mexico weighs about 27.5 tons. The Willamette meteorite, shown in fig. 6, weighs 15.6 tons.
Some are composed of stone, some of metals and some of both. About four out of every hundred are nearly pure iron with a little nickel. The source of meteors and meteorites is not definitely known. All material things so far as we know have a beginning, a period of growth, decline, and death.
This is not true of matter itself but of the forms which it takes. The fact is commonly recognized in regard to plants and animals but is probably no less true of many inanimate objects, except that the changes"go on so much more slowly that they are frequently not recognized. It is now known that the
hills are not *' everlasting. " They may be ''rock-ribbed" but they are not as ''ancient as the sun."
The mountains have a beginning, and a period of growth, after which they begin to dwindle and gradually disappear. So it is with the earth, the sun, and the solar system ; they did not always exist as such.
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