Page:Popular Science Monthly Volume 69.djvu/125

Rh earlier record than the seismograph. The effect is such a characteristic 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 between 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.

Next are given the Greenwich mean times (0 to 24 hours, midnight to midnight) of the occurrence of the various phases of the seismic disturbance 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 particularly 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 Francisco 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 Francisco. 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 obtained at the Cheltenham Magnetic Observatory. The recording cylinder of the Bosch-Omori seismograph, around which is wrapped the