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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 

 clue 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 earth- 

 quake 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 Cleodetic Survey Magnetic Observa- 

 tories; 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 Kico 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 earth- 

 quake 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 Becords of 

 the Guatemala Earthquake, April 18, 1902. 



