130 F. OMORi : 



being to observe only seismic and similar slight movements. 

 The writing pointer and the record-receiver are precisely similar 

 to those described in the preceding section, with the difference 

 that the diameter of the wooden drum, which makes one revolu- 

 tion in about 40 minutes, is here 23 -^ cm. 



IV. 



12. I shall now describe some typical diagrams given b}^ 

 my Horizontal Pendulums, the observations having been made, 

 unless otherwise stated, with the instruments set up in the 

 " Earthquake-proof House." 



The ground is generally in movement of one kind or other, 

 even when there is no earthquake, but sometimes it is almost 

 still. (See fig. 19.) The duration of the calm epoch varies 

 from a few days to a month or more. 



13. The movements of the ground recorded by the Horizon- 

 tal Pendulums may be divided into two classes, according as 

 they are or are not of seismic origin. A non-seismic disturbance 

 consists of small pulsatory oscillations, or movements of slow 

 period', which varies usually between 4 and 8 seconds, the 

 maximum range of motion yet observed by me being about 0.2 

 mm. in each horizontal component. 



It is to be noted that in some pronounced storms of pulsatory 

 oscillations, the range of motion is quite as wide as in small 

 earthquakes, although these oscillations are quite imperceptible 

 to the senses, on account of the slowness of period. It is also 

 remarkable that the ground is so often disturbed by such storms. 



1. The tenn period is used throughout the paper in the sense of the complete period. 



