. The Adolfo Stahl lectures in astronomy, delivered in San Francisco, California, in 1916-17 and 1917-18, under the auspices of the Astronomical Society of the Pacific. ensational descriptions of suchencounters by professional writers who have that little knowl-edge which has been called a dangerous thing. The Earth has undoubtedly encountered comets tails scoresand scores of times since the advent of man, and with no bane-ful effects; and in the light of present-day knowledge of thestructure and chemical composition of comets there is no dangerwhatever that our atmosphere will be poisoned by
. The Adolfo Stahl lectures in astronomy, delivered in San Francisco, California, in 1916-17 and 1917-18, under the auspices of the Astronomical Society of the Pacific. ensational descriptions of suchencounters by professional writers who have that little knowl-edge which has been called a dangerous thing. The Earth has undoubtedly encountered comets tails scoresand scores of times since the advent of man, and with no bane-ful effects; and in the light of present-day knowledge of thestructure and chemical composition of comets there is no dangerwhatever that our atmosphere will be poisoned by such anencounter. It is true that a collision between the Earth andthe head of a comet could happen, but we see no reason toquestion the accuracy of the estimates made by mathematicalastronomers that such encounters will not occur more thanonce in fifteen or twenty million years, on the average! It is byno means certain that such an encounter, should one everoccur, would be a serious matter for the Earth. Its effectsmight be confined to a brilliant shower of meteors, such as thepeoples of the Earth have observed many times. Geologists Delivered December 8, PLATE \l. Halleys Comet, May 1. 1910: Head axd Beginning OF Tail. Photograph by H. D. Curtis. What We Know About Comets 27 are of the opinion that the outcropping strata of the Earthwhich they have been able to study have required a period ofapproximately one hundred million years for their strata, embracing the entire land area of the Earth, havegiven only one bit of evidence that the Earths surface hasbeen affected by a collision with an outside body. In centralArizona is a cup-shaped hole in the ground, about threequarters of a mile in diameter and several hundred feet deep,which has been formed, with little doubt, by the descent of agreat meteorite, or of a great cluster of small meteorites:thousands of small iron meteorites have been found in and allaround the hole, and there are no evidences of vo
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectastronomy, bookyear19
