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 



