74 EAKTHQUAKES. 



usually indicate that motion continues longer than is 

 perceptible to the senses. 



Period of Vibration, — When an earthquake contains 

 several prominent vibrations which might be called the 

 shocks of the disturbance, our feelings tell us that these 

 have occurred at unequal intervals. 



About the time which is taken for the complete back- 

 ward and forward oscillation of the ground which consti- 

 tutes the shock a little has already been said. This was 

 deduced from the records of disturbances as drawn by 

 seismographs. From the same sources we can readily 

 obtain the period of all the prominent vibrations in a dis- 

 turbance. 



In any given earthquake there are irregularities in 

 period, and different earthquakes differ from each other. 

 About the early attempts to determine the period of 

 earth vibrations something has been said in the chapter 

 on Earthquake Instruments. 



In the earthquake of March 11 (referred to on p. 70) 

 we find that both components commenced with a series 

 of small vibrations, about five or six to the second ; next 

 came the shock, consisting of two complete vibrations 

 executed in two seconds. In this it is to be observed 

 that the motion eastwards was performed much more 

 quickly than the motion westwards. Next, by reference 

 to the east and west component, it is seen that there are 

 a number of large vibrations, about one per second, on 

 which a number of smaller motions are superposed. As 

 the motion proceeds, these become less and less definitely 

 pronounced and more irregular in their intervals, until 

 finally the motion dies away. 



This earthquake, as recorded at the author's house in 

 Tokio, lasted about one and a half minute. 



The same earthquake, as recorded by Professor Ewing 



