136 F. OMORi : 



less distinct slow undulations. The duration of sucli shocks is 

 short, being, when recorded by ordinary seismographs, usually 

 less thnn 1 minute. But, when recorded by the Horizontal 

 Pendulums, their duration is found generally to be longer than 

 10 minutes. 



In local shocks, movements of a very long period, say much 

 above 10 seconds, seem not to exist. 



19. Fig. 19 gives the NS component diagrams of three 

 small shocks, marked A, B and C, which happened in the 

 night of the 15tli and in the morning of the 16th Sej^tember, 

 1898. 



A. The 16th September, 1898; 8.82.31 a.m. The total 

 duration was about 12 minutes, and the duration of the preliminary 

 tremor about 16 seconds. At first quick movements were super- 

 posed on slow ones, the average period of vibration in the end 

 portion of the shock being 3.8 seconds. The maximum range of 

 motion was 0.3 mm. in each horizontal component. 



B. The 16th September, 1898; 4.48.23. a.m. The total 

 duration was about 17 minutes, and the duration of the preliminary 

 tremor about 27 seconds. In the principal portion of the earth- 

 quake, there seems to have existed traces of slow undulations, whose 

 period was 5.3 seconds. The average periods of vibration for three 

 successive series of 40 vibrations, measured from about 3-2- minutes 

 after the beginning, were found to be 3.5, 3.9 and 5.5 seconds. 

 The maximum range of motion was 0.4 mm. in the EW and 0.5 

 mm. in the NS component. 



C. The 15th September, 1898 ; 4.57.35 p.m. A very small 

 shock, whose total duration was 8— minutes. The average period 

 of the prevailing waves was 4.3 seconds, these being superposed 

 by others still smaller. 



