HORTZONTAL PENDULUMS. IBü 



given by the Horizontal Pendulum of the portable form described in 

 section III, and the latter given by the EW component apparatus 

 in the '* Earthquake-proof House." It will be observed that the 

 range of motion is the same in the two diagrams, although the too 

 instruments had different periods of proper oscillation and different 

 lengths of struts, and conseqently were differently sensitive to 

 tilting movements. (See also § 26). 



Again, with respect to the Paschwitz Apparatus diagrams, 

 fig. 13 and fig. 14, the periods of free oscillation of the NS and 

 EW component pendulums were respectively 35.5 and 18.8 

 seconds, the ratio of the squares of these two numbers being nearly 

 as 4 : 1 ; but the range of motion of the pulsatory oscillations 

 was the same on the whole, in the two components. 



I conclude, therefore, that the pulsatory oscillations, at least 

 those of the kind here considered, are horizontal movements and 

 not tiltings of the ground. The direction of motion of these 

 oscillations is evidently changing from time to time. 



V. 



17. Seismic disturbances. Seismic disturbances, or the 

 movements due to earthquakes, are frequently recorded by 

 Horizontal Pendulums in Tokyo, which is evidently a very 

 favourable place for seismological observation. 



They may conveniently be divided into two classes, namely, 

 those of near origin, and those of distant origin. Some typical 

 examples of the former class shall first be given. 



18. Every small local shock, as shown in fig. 19, begins 

 with a preliminary tremor of a few seconds duration, the motion 

 consisting of vibrations of short period superposed on more or 



