254 SEKIYA AND OMORl. 



It should bo «tated tliat the «urface-grouud and the ])it instriunent« 

 were interchanged with each otlier in June, 1888. 



The (j nanti ties calcuJated lor the different earthquakes are : — 



(1). The number of waves in 10 seconds, marked n. 

 (2). Amplitude, (r), or semi-range of motion in mm. 

 (3). Complete Period, (T), or the time taken to make a complete 

 for-and-back moticjn of the u'round in sec. 



2 TT r 



(4). Maximum ^ elocity in mm. per sec, (V), or — ™ — . 



V- 

 (5). Maximum Acceleration in mm. per sec. per sec, ÇA) or — ^. 



In (4) and (5), it is assumed as usual that the motion of the 

 ground is simple-harmonic It is rare, however, that any complete 

 Avave presents a very good simple-harmonic character during the 

 whole of its course, but usually differs in extent of motion and in the 

 corresponding time of describing it in the first and second semi-])hases 

 of the motion, and so in some cases we have calculated V and A for 

 the two diffèrent semi-phases oï a wa\'e. Sometimes also we give the 

 maximum period during the 10 seconds interval. 



The East-West and Xorth-South components of the horizontal 

 motion are not compounded, but the same components in the pit and 

 on the siu'face are compared separately. It is a well known fact tbat 

 motions of very quick periods and of small amplitudes generally occur 

 at the beginning of earthquakes, and in the diagrams appear superposed 

 on the principal imdulations. In severe earthquakes, such as those of 

 January 15th, 1887, and of Feln-uary 18th, 1889, these ripples are 

 very prominent ; and, being very (juick in period, though small in am- 

 plitude, they have maxinuun accelerations very much greater than 

 those of the principal waves, which are longer in period though greater 

 in amplitude. We have also made calculations on some of these ripples, 

 which can sometimes be identified in the two sets of diagrams. As 



