Page:Popular Science Monthly Volume 69.djvu/128

124  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.]