Page:Popular Science Monthly Volume 69.djvu/87

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In earthquake studies one of the most fascinating lines of research is the investigation of the relative intensities of the different shocks. Of the various evidences, that afforded by the resulting disturbances of the surface conditions is most reliable. Accounts of those who have felt the shocks are unreliable in determining intensities, since the feelings experienced at such a time are largely dependent upon nervous temperament, and upon previous experiences with earthquakes. One feeling a shock for the first time is often seriously disturbed by tremblings to which a resident of an earthquake country would not pay the slightest attention.

 The large stumps without enlarged butts are the old trees killed by the New Madrid earthquake. The living trees with enlarged butts are mainly subsequent growth.

A comparison of the effects produced upon artificial structures and upon the earth's surface by our three great earthquakes, New Madrid, Charleston and San Francisco, seems to show that of the various types of phenomena associated with earthquakes, nearly all were more strongly developed at New Madrid than at either of the other localities.

The length of the period of marked disturbance at San Francisco was only a few minutes during the eighteenth of April of this year. The Charleston earthquake occurred, after a preliminary tremor a day or two before, but not felt in the city on August 31, 1886, the severe shocks being confined to a few hours, although not entirely ceasing for three months. In New Madrid, on the other hand, the vibrations,