Page:Popular Science Monthly Volume 69.djvu/129

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a point on the opposite side of the earth from the origin in about 20 minutes; the latter are supposed to pass directly through the earth. In computing the velocities I have taken provisionally as the average

PSM V69 D129 San francisco earthquake recorded by observatory in cheltenham md.png

Fig. 2. Record of the San Francisco Earthquake on the Eschenhagen Magnetograph at the Magnetic Observatory, Cheltenham, Maryland, reduced 2 times. [The hours as marked are approximately local mean time. The earthquake effect will be noticed on the three magnetic elements, horizontal intensity, declination and vertical intensity in the shape of a trumpet formation between 8 and 9 a. m., local mean time. The range or double amplitude of the disturbance was about 1,1000 part of the horizontal intensity and about 1/3000 part of the vertical intensity. On account of the intersecting of the curves, the range in the magnetic declination cannot be given.]

time of the shocks on the Pacific Coast which gave rise to the effects recorded at distant places as occurring at 5h12m Pacific time or 13h 12m, April 18, Greenwich time. There may have been earlier preliminary shocks.[1]

  1. Professor George Davidson, of the University of California, determined the time of first shock at his home in San Francisco by counting the number of seconds it took him upon awakening and going to his watch and noting the time. Owing to his large experience in the work of the U. S. Coast and Geodetic Survey, the time which he gives he deems to be correct within two seconds, via., 5: 12: 00 Pacific time.

    Professor A. O. Leuschner, of the University of California, according to his article in the Berkeley Reporter, of Berkeley, Gal., April 20, 1906, says: "The best record of the beginning of the heaviest shocks is furnished by the standard clock of the Student Observatory, which stopped at 5h 12m 38s Pacific standard time, while less severe shocks were recorded by Mr. S. Albrecht some 35 seconds earlier. The principal part of the earthquake came in two sections, the first series of vibrations lasting about 40 seconds. The vibration diminished considerably during the following 10 seconds and then continued with renewed vigor for about 25 seconds more. But even at this writing, about noon, the disturbance has not as yet subsided, as slight shocks are being recorded at frequent intervals on the Ewing seismograph, which has been restored to working order. [This seismograph was thrown out of action at the beginning of the earthquake; however, a fairly complete record was obtained with the duplex instrument.] The principal direction of motion was from south-southeast to