HORIZONTAL PENDULUMS. 139 



the average periods of five series of successive 60 complete 



vibrations — 



Successive values of the 

 average period. 



6 . 6 seconds. 



7.3 



8.1 



8.9 



10.7 



There is thus in this case an apparent arithmetical increase 

 in the average period of vibration, which was iu the beginning 

 about 6 seconds and at the end about 11 seconds, the rate oi 

 increase of the period amounting to about 0.8 second in every 

 60 complete vibrations, that is to say, to about 7.5 seconds every 

 hour of the duration of the disturbance. 8uch an increase 

 in period during an earthquake, which can only be definitely 

 ascertained in shocks of hjng duration, should be, 1 belive, hardly 

 perceptible at very great distances from the origin of disturbance. 



From the diagram of this earthquake, it is evident that 

 the movements of a great earthquake consist generally of slow 

 undulations superposed upon quick vibrations, different waves 

 being probably generated simultaneously at the centre of 

 disturbance. 



One of the after-shocks of this earthquake is also shown in 

 the diagram (fig. 16). The total duration was about 12 minutes, 

 and the average period of the prevailing vibrations in the principal 

 portion was 6.0 seconds, the maximum range of motion being 0.3 

 mm. Measuring 3 minutes after the commencement, the average 

 period of slow undulations was found to be 7.0 seconds. Near the 

 end the average period was 5.3 seconds.. 



