1888,] on Earthqualces and how to Measure iliem. 363 



stand of the instrument, and drew two diagrams side by side upon 

 tlie glass plate — one, the record given by the seismograph itself, and 

 the other the record derived from a fixed piece which was held fast 

 in an independent support. The agreement of the two records with 

 one another proved how very nearly motionless the "steady-point" 

 of the seismograph remained during even a prolonged shaking re- 

 sembling an earthquake. This test w^as applied to the instruments 

 on the table, and the close agreement of the two diagrams was 

 exhibited by projecting them on the lantern-screen. A large number 

 of autographic records of Japanese earthquakes were thrown on the 

 screen,* and particulars were given of the extent of the motion, and 

 the velocity and rate of acceleration, in some represent itive examples. 

 To determine the rate of acceleration was of S2)ecial interest, because 

 it measured the destructive tendency of the shock. 



The lecturer explained that some of the seismograms exhibited on 

 the screen had been obtained since he had left Japan by his former 

 assistant, Mr. Sekiya, who now held the unique position of Professor 

 of Seismology in the Imperial Japanese University. Professor Sekiya 

 had recently taken the pains to construct a model rej)resenting, by 

 means of a long coil of copper wire carefully bent into the proper 

 form, the actual path j^ursued by a point on the earth's surface during 

 a j^rolonged and rather severe shaking. This model of an earthquake 

 had been made by combining the three components of each successive 

 displacement as these were recorded by a set of seismograjohs like 

 Ihose ujjon the lecture-table. The appearance of Professor Sekij'-a's 

 model (a description of wliich will be found in ' Nature,' vol xxxvii. 

 p. 297) was shown to the audience by means of the lantern. 



Professor Ewing drew attention to the small tremors of high fre- 

 quency which characterised the beginnings of earthquake motion, 

 and which were apparent in a number of the diagrams he exhibited. 

 These generally disappeared at a comparatively early stage in the 

 d'sturbance. In the early portion they were as a rule found at first 

 alone, preceding the larger and slow'er principal motions ; and then 

 when the principal motions began, small tremors might still be seen 

 for some time, sujDerposed upon them. In all ^probability these quick- 

 period tremors were normal vibrations, while the larger motions were 

 transverse vibrations; and a reference to the theory of the transmis- 

 sion of vibrations in elastic solids served to explain why the quick- 

 period tremors were the first to be felt. The whole disturbance w^ent 

 on for several minutes, with irregular fluctuations in the amplitude of 

 the motion, and with a protracted dying out of the oscillations, the 

 period of which usually lengthened towards the close. The record 

 of a single earthquake comprised some hundreds of successive move- 

 ments, to and fro, or round fantastic loops. Each single movement 

 usually occupied from half a second to two seconds. Earthquakes 



* Examples of these will be found in the lecturer's memoir on ' Earthquake 

 Measurement,' also ' Nature,' vol. xxx. p. 174, xxxi p. 581, xxxvi. p 107. 



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