Page:Popular Science Monthly Volume 69.djvu/128

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PSM V69 D128 San francisco earthquake recorded by observatory in cheltenham md.png

Fig. 1. Record of the San Francisco Earthquake as obtained on the Bosch-Omori Seismograph at the Magnetic Observatory, Cheltenham, Maryland, reduced three and one half times. [The records of the N-S component and of the E-W component were re arranged so as to bring the points of beginning of the preliminary tremors, phase I (A) in the same vertical line. B marks the beginning of the preliminary tremors, phase II and C the beginning of the long surface waves.]