HOKIZONTAL PENDULUMS. 145 



of the earthquake was about 2~ hours. The preliminary tremor, 

 whose duration was about 4 minutes, consisted of vibrations of 

 an average period of 8.4 seconds, with some traces of small move- 

 ments of a shorter period imposed upon it. Then followed 13 

 large slow undulations, whose maximum range of motion was in 

 the EW component 1.6 mm., and in the NS 3.0 mm., the average 

 period being 22 seconds. In the next epoch, the period became 

 shorter, the value averaged from 38 vibrations during about 9 

 minutes being 13 seconds. In the following portion of the shock, 

 the range of motion became small, the value of the period, 

 averaged from successive series of 50 vibrations, being 13, 13, 

 12, 10, 11, 11 and 10 seconds. Near the end, the average 

 period was 11 seconds. 



To estimate the lengths of the waves constituting this 

 earthquake, I shall assume the velocities of propagation of the 

 very first tremors and of the principal undulations to be 

 respectively 13.0 and 3.6 km. per second, these values being the 

 mean results deduced from observations in Europe of the 

 Japan Earthquake (origin in the Pacific) of the 20th Feburary 

 1897 and of the great Indian Earthquake of the 12th June, 

 1897. In our case, the period of waves in the preliminary 

 tremor and in the principal portion being respectively 8.4 and 

 22 seconds, the corresponding wave-lengths come out as about 

 110 and 80 km. 



December, 1898. Seismological Institute. 



