Page:Popular Science Monthly Volume 69.djvu/130

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126
THE POPULAR SCIENCE MONTHLY.

The amplification of this table to embrace the entire earth will be left to Professor Harry F. Reid, 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 Rico 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 geophysics 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 magnetograph 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 comparatively 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 13h 12m, Greenwich mean time, which is at present adopted. L. A. B.