Propagation of .the Charleston Earthquake. 3 



nine seconds. Mr. Allan is authority for the statement that 

 its reading next morning was 9:51 exactly. He had received 

 the time signal on August 31st, but as the clock was' within 

 the limit of tolerance he did not correct it. Subject to this 

 limit he had no knowledge of the exact error of his clock and 

 his memory on this point did not serve him The second clock 

 was the regulator which controls the time of the North Eastern 

 Kail way. This clock was compared with the time signal on 

 August 31st, but was not corrected, its error being within the 

 limit of tolerance, which was eight seconds. It had been reset 

 two days previously. Its reading was 9:51:15. It was stopped 

 by the point of the pendulum catching behind the metallic arc 

 in front of which it properly vibrates. The third clock was 

 that which regulates the time of the Charleston & Savannah 

 Railroad. It had been reset two daj^s previously and compared 

 with the time signal on August 31st, and was within the limit 

 of tolerance, eight seconds. Its reading was 9:51:16 and it was 

 stopped in the same manner as the preceding one. The fourth 

 clock was that of the South Carolina Railroad. It had been 

 reset by the daily time signal on the day of the earthquake. 

 Its reading was 9:51:48. 



Although these records range through an interval of 48 

 seconds they may be reconciled. The azimuths of the planes 

 of oscillation of their pendulums were as follows : 



James Allan & Go's N. 85° E. 



North Eastern Railroad N. 40° E. 



Charleston & Savannah Railroad.. N. 66° E. 



South Carolina Railroad N. 30° W. 



These azimuths may be put into relation with what is now 

 known concerning the varying phases of the shocks, their 

 respective durations and directions of vibratory motion. The 

 earthquake at Charleston began as a light tremor, steadily in- 

 creasing in power through an interval estimated to be from 10 

 to 15 seconds' duration ; then suddenly or by swift degrees it 

 swelled into the full power of the first maximum, then sub- 

 sided to a minimum, then swelled suddenly to a second maxi- 

 mum and lastly died away gradually. The interval from the 

 beginning of the first maximum to the close of the second 

 maximum is estimated at from 35 to 55 seconds ; the subsid- 

 ing tremors are estimated at about 6 to 8 seconds : the total 

 duration from 55 to 75 seconds. It may be expressed graphi- 

 cally by the following curve in which the abscissas represent 

 time and the ordinates an arbitrary scale of intensity. 



In the first maximum the waves were mainly normal and 

 came from N. 30° W. In the second maximum the direction 

 of vibration was about at right angles with the foregoing or 



