126 POPULAR SCIENCE MONTHLY 



The amplification of this table to embrace the entire earth will be 

 left to Professor Harry F. Keid, a member of the San Francisco 

 Earthquake Commission. 



Why is it that in this severe earthquake the magnets responded 

 only to the long or surface waves and not to the preliminary tremors, 

 and why did the magnets at Porto Eico give no record at all? These 

 are the questions which I believe to be of concern not alone to the 

 magnetician, but also to the seismologist and to the student of geo- 

 physics in general. Of the many earthquake records already obtained, 

 there are a large number where the disturbance on the seismograph was 

 considerably smaller than the San Francisco one and yet the magneto- 

 graph responded to even the preliminary effects. Evidently we must 

 be getting a record of something on the magnetograph, not immediately 

 evident from the present seismograph records, which causes this 

 peculiar differentiation of seismic disturbances into the following 

 classes: (a) recorded by seismograph and not by the most delicate 

 magnetograph, (b) recorded by magnetograph and not recorded by 

 seismograph, (c) recorded by seismographs and magnetographs 

 partially (surface waves), (d) recorded completely by seismographs 

 and magnetographs. 



My present belief is that the effects recorded by suspended magnets 

 are chiefly mechanical ones due to the vibrating motion of the points 

 of support, though the possibility of a magnetic effect within a certain 

 prescribed region of the origin of the earthquake, brought about as 

 above explained, is not to be excluded. It is a notable fact that at 

 the Baldwin Magnetic Observatory, where, as stated, so many seismic 

 effects are being detected which are to be associated with the com- 

 paratively local earthquakes in the Middle States and which fail to 

 make any record on seismographs as far distant as Washington, the 

 effects corresponding in time to lightning discharges have also been 

 found which in many instances resemble very closely the seismic effects. 



In the case of the San Francisco earthquake, however, there can 

 apparently be no question that what was recorded by the magnetographs 

 was a mechanical effect (see Fig. 2). It is a matter of interest to note 



north-northwest. The remarkable feature of this earthquake aside from its 

 intensity was its rotary motion. The sum total of all displacements represents 

 a very regular ellipse and some of the lines representing the earth's motion 

 can be traced along the whole circumference." From this we deduce the time 

 of the first shock 5:12:03. 



At the Lick Observatory the first shock was recorded at 5:12:12. 



At the Ukiah Latitude Observatory the first shock was recorded according 

 to Dr. Townley at 5:13, correct within two or three seconds. 



At Eureka, California, the first shock as reported to Professor Davidson 

 was noted on a regulator owned by H. H. Buhne, who was awake at the time 

 at 5:11. 



As it is likely that the epicenter was somewhat west of San Francisco, but 

 at no considerable distance, owing to absence of tidal waves, it is probable 

 that the average time of the shocks at the origin which produced the records 

 at the distant observatories was not far from 13" 12'", Greenwich mean time, 

 which is at present adopted. L. A. B. 



