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POPULAR SCIENCE MONTHLY
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/cf/PSM_V69_D128_San_francisco_earthquake_recorded_by_observatory_in_cheltenham_md.png/540px-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.]