ON SEISMOLOGICAL INVESTIGATION. 



223 



Apparent Duration of Preliminary Tremors. 



These time intervals and their corresponding distances are shown 

 graphically in the following figure, in which the free curve indicates the 

 general result towards which the observations point : — 



Fig. 4. 



CO 



60 



CO 



20 



10 



ISO 160 140 no 100 eo eo 40 lo 



Intervals by which Preliminary Tremors have outraced Long-period Waves. 



