THE SAX FRANCISCO EARTHQUAKE no 



of the mechanical vibrations. We know that a magnet subjected to 



strain undergoes changes in its magnetization and so the question 

 arises whether the earth's magnetized rocks may not likewise give some 

 indication of their state of strain during an earthquake by slight 

 magnetic fluctuations. Or, an earthquake may be accompanied by a 

 redistribution of the magnetic rocks or of the electric earth-currents 

 known to exist, and thus give rise to a possible magnetic effect. 



Enough has been said to show that a careful and exhaustive in- 

 vestigation of seismic effects recorded on magnetographs is certainly 

 one that merits undertaking. The Department of Terrestrial Magnet- 

 ism of the Carnegie Institution of Washington, in connection with the 

 study of the magnetic effect, recorded simultaneously over the entire 

 dobe coincident with the outbreak of Mt. Pele, on May 8, 1902, is 

 making a systematic study of the volcanic and seismic effects recorded 

 by magnetic instruments with the .cooperation of the Coast and Geo- 

 detic Survey and of the Canadian Meteorological Service. A paper by 

 Mr. J. E. Burbank, published in Vol. X., p. 113, of the journal, 

 Terrestrial Magnetism, brought the investigation up to the time of the 

 installation of the seismographs at the Coast and Geodetic Survey 

 Observatories two to three years ago ; a second paper, to be published in 

 the course of the year, will continue the research as based upon seismic 

 and magnetic instruments in operation at the same observatory. 



It had been noticed for some time that magnetic instruments re- 

 sponded to certain earthquakes, but the cases noted were of such a 

 class as to convey the first impression at once that the effects recorded 

 were mechanical ones. Milne in 1898 made quite an exhaustive in- 

 vestigation of this class of effects for the whole earth and covering the 

 period from 1889 to 1897. He likewise found that these effects were 

 not invariably recorded at every magnetic observatory. He considered 

 the results inconclusive and deemed it necessary to await the time when 

 both seismograph and magnetograph records could be had at the same 

 place. A recent notable contribution to the subject based on magnetic 

 records at one observatory, without, however, at the same time cor- 

 responding -seismological data, has been made by Dr. Messerschmitt, in 

 charge of the Munich Magnetic Observatory. 



Previous magneticians, such as Eschenhagen, Wild and Liznar, had 

 found that from a comparison of the effects recorded on magnetographs 

 at various European observatories the effects, in certain notable earth- 

 quakes, progressed from station to station with the velocity of about 

 three kilometers i. e. the rate of propagation of the long or surface 

 seismic waves. This measurable difference in time between any two 

 stations and its correspondence with the time interval required for the 

 transmission of the surface waves was a very good indication that a 

 purely mechanical effect had been recorded and not a distant magnetic 



