HORIZONTAL PENDULUMS. 14B 



26. The earthquake of the 7th November, 1898 ; 2. 57.11 a.m. 



The origin of this earthquake is unknown, but its distance 

 from Tokyo was probably not above 1000 km. The total duration 

 was 20 minutes, while the duration of the preliminary tremor was 

 about 18 seconds. The average period of the prevailing slow 

 undulations, of which there were six, in the principal portion, 

 was 8.2 seconds. From about 2 minutes after the commencement, 

 waves became prominent, whose average period was 4.0 seconds, 

 and wdiose maximum range of motion was in the EW component 

 0.7 mm. and in the NS 0.5 mm. Near the end, the average 

 period was 6.6 seconds, the waves being superposed on others 

 with an average period of about 4 seconds. 



In fig. 225, is reproduced the EW component diagram 

 of the earthquake as given by the Horizontal Pendulum of the 

 portable form. It will be observed that the diagram, allowing for 

 some insignificant traces of the proper motion of the pendulum, 

 is identical with that in fig. 22a, which was given by the 

 EW component Horizontal Pendulum in the " Earthquake-proof 

 House." 



The question whether the slow undulations of earthquakes 

 are horizontal movements or due to tiltings of the ground may be 

 discussed from a point of view similar to that stated in §10 in 

 connection with non-seismic movements. Thus, the groups of 

 undulations marked a in the two diagrams are identical with one 

 another. Now the peiiods of free vibrations of the large and 

 the portable horizontal pendulums were respectively 28 and 

 20 seconds, while their struts were respectively 1 m. and 75 

 cm. long, the multiplying ratio of the writing pointer being 

 for each of the instruments equal to 10. If, therefore, the undu- 

 lations, a, be due to the tilting of the ground, the ranges of 



