Page:Popular Science Monthly Volume 69.djvu/124

120 effect, as the latter would have been observed simultaneously at all stations. Or, if it was a magnetic effect, then in each case it was due to one of the possible local causes enumerated brought into action upon the arrival of the mechanical disturbance at the particular station.

In our study, however, it has been possible to differentiate much more closely and at times to separate the effects on the magnetic records into the various phases—preliminary tremors and principal portions, etc.—in a manner analogous to usual treatment of the seismograph records. A notable instance was the destructive Guatemalan earthquake of April 19, 1902, which, as may be recalled, preceded the Antillean volcanic eruptions of that period. At that time there were no seismographs at the Coast and Geodetic Survey Magnetic Observatories; however, an inspection of the table below will show that with the records obtained on the magnetographs at Cheltenham, Baldwin, Sitka and Honolulu (the Porto Rico Observatory did not then exist), it is possible to study the seismic effects on them—even down to the preliminary tremors—equally as well as on the seismic records obtained at Baltimore, Toronto and Victoria. The earliest notice of this earthquake was received at Baldwin, the nearest station to the origin—Guatemala. Here then we have a notable case where the magnets were affected by even the preliminary seismic tremors, this being a different, case from the European ones cited above, as these tremors travel with a velocity of about nine kilometers or more per second.

There have been many other similar instances and it has even occurred at times that the magnetic instruments have given a slightly

 Table 2. Seismograph and Magnetograph Records of the Guatemala Earthquake, April 18, 1902.