THE SCIEISTTIPIO MONTHLY
��DECEMBER, 1916
��WHAT WE KNOW ABOUT COMETS^
By Db. W. W. CAMPBELL
DIRECTOB OF THE LICK OBSBBVATOBY^ UNIYEBSITY OF CALIFOBNIA
THE startlingly sudden appearance of some great comets, the rapid growth of others to enormous sizes and their equally rapid dis- appearance have naturally excited the interest and, only too often, the fears of the human race. We are removed less than two centuries from the long-prevailing theological view that comets are flaming fire-balls hurled at the earth by an angry God, to frighten and punish a sinful world. Up to the time of my childhood the opinion was widespread among civilized peoples that comets are the forerunners of famine, pestilence and war. Did not the great comet of 1811 herald the war of 1812; the comet of 1843 the war of 1846; and Donati's comet of 1858 our Civil War? Even in the twentieth century the fear that a comet may collide with the earth and destroy its inhabitants comes to the surface, here and there, every time a comet is visible to the naked eye. This fear is not lessened by the highly sensational descriptions of such encounters by professional writers who have that little knowl- edge which has been called a dangerous thing.
The earth has undoubtedly encountered comets^ tails scores and scores of times since the advent of man, and with no baneful eflfects; and in the light of present-day knowledge of the structure and chemical composition of comets there is no danger whatever that our atmos- phere will be poisoned by such an encounter. It is true that a collision between the earth and the head of a comet could happen, but we see no reason to question the accuracy of the estimates made by mathemat- ical astronomers that such encounters will not occur more than once in fifteen or twenty million years, on the average! It is by no means certain that such an encounter, should one ever occur, would be a serious matter for the earth. Its effects might be confined to a bril- liant shower of meteors, such as the peoples of the earth have observed many times. Geologists are of the opinion that the outcropping strata
1 Retiring address of the first president of the Pacific Division of the Amer- ican Association for the Advancement of Science. San Diego, August 9, 1916.
VOL. III.— 36.
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