THE SAN FRANCISCO EARTHQUAKE 127 



thai at Baldwin there was a Blight actual magnetic effect 3-4 hours 

 before the shock was felt at San Francisco, to which no counterpart 

 has yet been found on the Cheltenham records, indicating that this 

 effect observed at Baldwin was not a cosmic one, but was due to some 

 local circumstances. To associate' it with the San Francisco earth- 

 quake is not at present warranted. 



Owing to the optical arrangement of the magnetograph, in order 

 to produce an effect which will be evident on the recording sheet, it is 

 necessary to have a turning movement of the suspended magnet. Any 

 parallel displacement of the magnet — sidewise or up and down — will 

 give no observable effect, an actual turning or rotary movement of the 

 magnet must take place and for this purpose a turning couple must in 

 some manner be introduced. Such a couple is produced when the 

 magnet is drawn out of its normal direction with the aid of a bit of 

 iron which is then quickly removed ; the earth immediately acts on 

 the magnet with equal and opposite forces applied near the extremities. 

 and after performing a number of vibrations about its mean position 

 the magnet settles down and then takes up the course pursued before 

 the artificial disturbance. The effect thus produced is very similar to 

 some of the earthquake effects. Were an earthquake accompanied by 

 the generation of magnetic forces, the explanation of the observed 

 effects would thus be very simple. 



When the seismic motion is such as to produce a tilting or rocking 

 of the support, it can readily be shown that because the suspended 

 mass is a magnet, a turning couple is brought into play by the earth's 

 magnetism causing the rotary, vibratory motion of the magnet about 

 its mean position. Were the suspended material a non-magnetic mass 

 of sufficient weight, no such turning would take place, but the mass 

 would act more or less as a ' steady point.' However, it is quite pos- 

 sible that with the very light magnets weighing but i/o gram, and 

 short suspensions used, we may also have to deal with a form of 

 pendulum seismograph, in which the period of the pendulum is suf- 

 ficiently small as to more readily respond to certain micro-seismic 

 motions than either type of instrument at present in use in this country. 



It would seem therefore that seismologists might be assisted in the 

 solution of some of the problems as to the precise character of the earth 

 movements recorded on seismographs by a careful study of the seismic 

 effects recorded on magnetographs, especially if the effects both in the 

 horizontal and in the vertical plane be considered, and if furthermore 

 the record be obtained on a more open time scale, so as to be com- 

 parable in this respect with the best seismograms. 



Whether the San Francisco earthquake caused a change in the dis- 

 tribution of the earth's magnetism within the affected region is at 

 present under investigation. 



