52 2 THE SCIENTIFIC MONTHLY
of the earth which they have been able to study have required a period of approximately 100 million years for their formation. These strata, embracing the entire land area of the earth, have given only one bit of evidence that the earth's surface has been affected by a collision with an outside body. In central Arizona is a cup-shaped hole-in-the- ground, about three quarters of a mile in diameter and several hundred feet deep which has been formed, with little doubt, by the descent of a great meteorite, or of a great cluster of small meteorites : thousands of small iron meteorites have been found in and around the hole, and there are no evidences of volcanic activity in the neighborhood. Geo- logic and geographic surveys of the earth have revealed no other case of coUisional effects* in the records of a hundred million years. Man himself has lived upon the earth certainly many tens of thousands of years, and there are no traditions extant concerning injuries to earth or to man from comets. Why then should anybody worry about pos- sible injury from a comet in his short span of three score years and ten ?
The answer to our first question, where do comets come from, in- volves the question of their relationship to the solar system and to the great stellar system. It is essential that every auditor should under- stand certain prominent features of the solar and stellar systems ; and, at the risk of repeating what many members of the audience already know, I shall devote a few lines to a description of these systems.
Widely scattered throughout a great, but finite, volume of space occupied by our stellar system are tens of millions of stars. It is esti- mated that our largest refracting telescopes could show us about seventy million stars, and that the reflecting telescopes could photograph pos- sibly two or three times as many. Our own sun is just one of these scores of millions of stars. It seems very large, very bright and very hot because we on the earth are relatively close to it. It is our own star. Revolving around it are many planets, of which our earth is one. Probably the other stars in many cases, possibly in all cases, have planets revolving around them in the same way. We do not know that this is a fact because the nearest star, excepting our own star, is so far away that we should require telescopes at least twenty-five feet in diameter to see planets revolving about it, even though such planets be as large as Jupiter and Saturn, the largest planets revolving around the sun.
Now the sun and its planets and their moons are the chief members of an orderly system which we call the solar system. Ninety-nine and six sevenths per cent, of all the materials in the solar system is in the sun, and only one seventh of one per cent, is divided up to form the planets and the^r moons : Mercury, Venus, the Earth and its one moon. Mars and its two moons, the more than 800 minor planets which move
- Xeglecting the insignificant cavities produced by isolated small meteorites.
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