THE SAN FRANCISCO EARTHQUAKE 121 



earlier record than the seismograph. The effect is such a character- 

 istic one that when it has once been recognized it will not be mistaken 

 for any ordinary magnetic effect. Four types can be distinguished: 

 First, those in which the disturbance begins abruptly and quickly 

 reaches a maximum, dying down gradually (usually the case in a 

 near-by earthquake) ; second, those in which a small preliminary effect 

 precedes the principal portion, and in which there are often two or 

 more maxima; third — by far the most common — those consisting of 

 a small diamond-shaped' disturbance; and fourth, those in which the 

 trace is simply blurred and broadened. 



With these introductory statements as to the possible relation be- 

 tween seismology and terrestrial magnetism, let us now pass to the 

 consideration of the recent San Francisco earthquake. 



Table 3 contains the results of the records obtained up to date 

 (May 17) at the office of the Coast and Geodetic Survey, both from 

 magnetographs and from seismographs. It will be seen that the region 

 embraced extends from Honolulu on the west to Hungary on the east, or 

 about one third the way around the globe. All necessary data, such as 

 latitude, longitude, distances from San Francisco along the surface, 

 as well as along chord, chord depth, etc., etc., are found in the table. 



Xext are given the Greenwich mean times (0 to 24 hours, midnight 

 to midnight) of the occurrence of the various phases of the seismic dis- 

 turbance as recorded on the seismographs. For the preliminary 

 tremors, phase I (longitudinal waves), next phase II (transverse 

 waves), then principal portion (surface waves), etc., etc. It is par- 

 ticularly interesting to compare the times for Cheltenham, Washington 

 and Baltimore and to note how closely they agree. Owing to the slight 

 difference in distance of these three different stations from San Fran- 

 cisco the times should not of course be strictly the same, though the 

 difference should not be more than a few seconds. Considering the 

 totally different types of instrument (Milne at Baltimore and Bosch- 

 Omori at Washington and Cheltenham), certainly the comparison is 

 very satisfactory. 



It will be seen that the preliminary tremors were recorded by the 

 seismographs at Honolulu and Cheltenham at about the same time, 

 these two stations being at about the same distance from San Fran- 

 cisco. The reader will follow without assistance the progression of the 

 various waves from station to station as given in the table. [Since this 

 table was prepared many more records have been obtained which are 

 of interest, notably the seismograph and the magnetograph records 

 from the Toronto Magnetic Observatory.] 



In Fig. 1 we have a reproduction of the seismograph record ob- 

 tained at the Cheltenham Magnetic Observatory. The recording cylin- 

 der of the Bosch-Omori seismograph, around which is wrapped the 



