that 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 earthquake 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 possible that with the very light magnets weighing but 1⁄2 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 sufficiently 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 comparable in this respect with the best seismograms.
Whether the San Francisco earthquake caused a change in the distribution of the earth's magnetism within the affected region is at present under investigation.
