u6 POPULAR SCIENCE MONTHLY 



SEISMOGRAPH AND MAGNETOGRAPH RECORDS OF THE 

 SAN FRANCISCO EARTHQUAKE, APRIL 18, 1906 1 



By Dr. L. A. BAUER 



DIVISION OF TERRESTRIAL MAGNETISM, U. S. COAST AND GEODETIC SURVEY. 



HPHE San Francisco earthquake was one of several large earth- 

 -*- quakes recorded the world over since the beginning of this year. 

 The writer's prime interest in it as a magnetician is in the record it 

 left behind on the magnetographs at various magnetic observatories of 

 the United States Coast and Geodetic Survey. 



It has happened several times within the last few years that earth- 

 quakes have occurred in this country which were not recorded for one 

 reason or another, on the existing seismographs, but were indicated by 

 the record of certain magnetographs. The most notable instance was 

 the New England earthquake of March 21, 1904, at about eight minutes 

 after one o'clock in the morning, eastern time. Seismographs of the 

 Milne type at Toronto, Canada, and Baltimore, Maryland, and of the 

 Bosch-Omori type at the Weather Bureau, Washington, D. C, failed to 

 give any record of this earthquake, which was appreciably felt through- 

 out the New England States. The magnetograph at the observatory, 

 Cheltenham, Maryland, sixteen miles southeast of Washington, gave a 

 distinct record at l h 05 s to l h lT m eastern time. So there have been 

 a number of earthquakes recorded by the magnetograph at Baldwin, 

 Kansas, which were felt in the middle states and reported in the papers. 

 In fact, at this observatory, situated in a region where felt and unfelt 

 local and regional earthquakes are comparatively frequent — note for 

 example the many recent occurrences — more records of earthquakes are 

 obtained on the magnetograph than at any of the other magnetic ob- 

 servatories. 



This repeatedly authenticated fact made desirable a concurrent 

 study of seismograph and magnetograph records and hence seismo- 

 graphs have been installed within the last two years at all of the 

 magnetic observatories excepting at Baldwin, Kansas, which was 

 omitted because of its probable early removal on account of the pos- 

 sibility of disturbing influences from electric-car lines. So it happens 

 that the Coast and Geodetic Survey is able at present to contribute the 

 principal portion of the accurate observational data of earthquakes 

 obtained in this country. It was with the expectation that magnetic 

 observatories would also be excellent stations for the installation of 



