272 



NA TURE 



[January 20, i8c8 



RECENT SEISMOLOGY} 

 II._Unfelt Movements of the Earth's Crust. 



THE records obtained from seismographs often showed, 

 as we have ah-eady explained, preliminary vibra- 

 tions performed with a rapidity which has even reached 

 fifteen complete back and forth movements per second, a 

 shock or shocks, and lastly a number of irregular jolts or 



Mirror 



Q Watch 



Stand 



Boom 



Fig. 7. — Type of instrument adopted by the Seismological In\estigation 

 Committee of the British Association, and established at many stations 

 round the world. (Milne.) 



undulations which died away with an increasing period. 

 What seismologists had captured was what they could 

 feel, and they were left to speculate as to what came at 

 either extremity of the seismic spectrum. It seemed toler- 

 ably certain that if instruments were constructed capable 

 of responding to exceedingly rapid but minute elastic 

 vibrations, then a seismogram might show the move- 

 ments preceding but forming 

 portions of the preliminary 

 vibrations. These would ac- 

 company the sound waves which 

 in rocky districts so commonly 

 outrace movements of a more 

 pronounced character. These 

 sound phenomena, which never 

 extend far beyond an epifocal 

 area, are in certain districts 

 isolated phenomena which fre- 

 quently recur. Without calling 

 up the " spooks of Ballechin," 

 or the Ghost of Long Witten- 

 ham, which on New Year's Eve 

 rapped on the walls of many 

 houses, in two instances we are 

 assured that they have been 

 sufficient to explain noises 

 which had been regarded as 

 supernatural, and it is not un- 

 likely that the fully-equipped 



ghost hunter may be the discoverer of new paths in 

 seismic science. When a volcano explodes, the resultant 

 vibrations to which ordinary seismographs or ourselves 

 are sensible, seldom reach to any great distance ; but is 

 it not possible that elastic tremors, the result of a 

 powerful impulse, may even reach and be focussed at 



1 Continued from p. 249. 



N). 1473. VOL. 57] 



the antipodes of their origin ? In the Perry-tromometer 

 we have an apparatus which will automatically record 

 elastic vibrations of the order here considered, but up to 

 the present we are without the observer who is able to 

 isolate himself at a site suitable for its installation. In 

 the Isle of Wight the writer found that this instrument 

 recorded the firing of cannon at a distance of six miles, 

 the movements of trains at a distance of nearly one mile, 

 the rumbling of carriages at the distance of a quarter of 

 a mile, whilst it kept an excellent tally of the back and 

 forth journeys of eleven gravel carts 

 worked by a neighbouring contractor. 



Investigations on the other end of the 

 seismic spectrum have been more suc- 

 cessful, and we now know that waves 

 which have a period of one or two 

 seconds within the area where they are 

 appreciable to the senses, after these 

 have radiated to great distances, for 

 example over or through a quadrant of 

 the earth, their period may exceed twenty seconds. In 

 consequence of the slowness of the movements, together 

 with the flatness of the waves, which are probably miles 

 in length with heights measured by a few inches, every 

 city in the world is olten rocked slowly to and fro, and 

 yet to the unaided senses motion is imperceptible. 



A form of instrument used by the writer to record un- 

 felt earthquakes, diurnal waves, tremors and other move- 

 ments is shown in Fig. 7. On the outer end of the boom 

 of the pendulum there is a small aluminium plate in 

 which there is a slit. This is free to float to the right or 

 left over a slit in the lid of a box in which clockwork 

 drives a band of bromide paper. Light from a lamp 

 passing through the two crossed slits reaches the paper 

 as a point, and gives a line with extremely fine definition 

 (see Fig. 8, under the words September 21). At each 

 half-hour the minute hand of a watch crosses one end of 

 the slit, and by eclipsing the light gives the hour marks 

 shown in Fig. 8. 



In the photograms of these unfelt earthquakes we see 

 that the duration of the preliminary tremors is apparently 

 connected with the distance the disturbance has travelled, 

 and in this way a record has impressed upon it what is 

 partially equivalent to the post-mark of its origin. For 

 example, a disturbance originating in Japan would at a 



Fig. S— This earthquake of September 21, 1897, as recorded in the Isle of Wight, shows preliminary tremors 

 of about 43 minutes, which indicate an origin at a distance of 112°, say, east of Borneo. It disturbed 

 niagnetographs in Batavia. A similar earthquake ten hours earlier is not known to have done so. (Milne.) 



distance of, say, 200 km., be heralded by tremors which 

 an ordinary seismograph would show to have a duration 

 of ten or twenty seconds. A seismogram of the same 

 shock taken in Europe shows that the same move- 

 ments have a duration of about half an hour. The 

 maxima phases of movements, which may be waves of 

 distortion modified by gravity, travel at approximately 



