Page:Popular Science Monthly Volume 69.djvu/127

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THE SAN FRANCISCO EARTHQUAKE 123

smoked sheet of paper on which the record is made, makes one com- plete revolution in an hour, each sheet whether of the X.-S. or of the K.-W. component containing a whole day's record (24 lines). The distance between two of the dots represents one minute. In order to get a convenient size for the figure, it was necessary to omit about one third of the total length of the sheet, so that the distance from S. to S. or E. to E. represents about two thirds of an hour. On the original, the pointer or the recording stylus multiplies the motion ten times, hence in the reproduction the magnification is about three times. The maximum amplitude of motion was not recorded, the pointer stri- king the damping brushes. In deducing the actual displacement of an earth-particle at Cheltenham, it is necessary to take into account the period of the pendulum which for the X.-S. component was about 25 seconds and for the E.-YV. component about 20 seconds and the period of the recorded earth-movement about 2 to -f seconds for the preliminary tremors and aboitt 10 to 20 seconds for the principal portion. A rough calculation would give the total recorded horizontal displacement of the earth-particle, back and forth, of about 1/5 of an inch, which on account of the comparatively long period would not be felt by the human being. These explanations will doubtless be sufficient for the elucidation of the figure; for a description of the instrument the reader is referred to Dutton: ' Earthquakes.'

Passing next to the times recorded by the magnetograph (D stands for declination, H for horizontal intensity and Z for vertical intensity ) , it is seen that the effect in this instance did not begin at the four observatories where a record was obtained — Honolulu, Sitka, Baldwin and Cheltenham — until the arrival of the principal portion (long or surface waves) recorded on the seismographs, and that for this phase the agreement between seismograph and magnetograph is most satis- factory. It will also be noted that the time at Baldwin is inter- mediate between San Francisco and Cheltenham, so that the record of this observatory is a most desirable acquisition. Xote also that the time is nearly the same as at Sitka, Baldwin being just a trifle farther from San Francisco than Sitka.

Xext are found in the table the velocities of the various transverse waves — longitudinal, transverse and surface — computed along the paths indicated in the column on the extreme right. For the region embraced it will be seen that the longitudinal waves, which were the first to arrive, traveled at an average velocity of six miles per second, the transverse waves at an average velocity of 3V. miles, whereas the surface waves had a velocity of about 2 1/3 miles per second according to seismograph and magnetograph. It takes about 3 hours and 20 minutes for these waves to pass around the earth completely, whereas the preliminary tremors, phase I (longitudinal waves) reach

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