398 E. S. Holden — Earthquakes in California. 



(III?), Dr. Trembley. It waked sleepers in Oakland (V?), E. 

 Booth. — Berkeley ; slight. 



September 15? — Lick Observatory : the seismograph started 

 at 6:15 A. m., but as the record was not like that of a shock, 

 Mr. Keeler (in charge of the instrument) supposes the tremor 

 which started the instrument to have been due to a high wind. 



September 17, 3:51 a. m. — Lick Observatory : The seismo- 

 graph gives the following records (magnified l - 6 times for the 

 vertical, 3*3 times for the horizontal components). At 3 

 seconds after an assumed zero second, the vertical component 

 began its trace with a wave of period about 1|- seconds. The 

 amplitude (on the trace) is hard to estimate but is probably not 

 less than 5 mm for the first semi- wave, then about l ram for a full 

 wave, and after this mere tremors until about 40 seconds. The 

 N. and S. component (magnified) was as follows: 



At 4-3 seconds the earth moved S. of the neutral line 5 mm 



5-7 " N. " 2 



5-9 " on to " — 



6-1 " N. " 2£ 



6-4 " S. " 1£ 



6-9 " N. " 1 



7-5 " S. " 1| 



8-9 " N. " If 



and tremors occasionally as large as | mm continued until about 

 40 seconds. 



The E. and W. component (magnified) was as follows : 



At 4'3 seconds there was strong movement of the earth west 

 of about 3 mm ; this was followed by a wave of period about 1 

 second double amplitude 2 mm ; and this again by another of 

 period f second double amplitude l mm . After this tremors 

 continue for about 30 seconds. 



The strata of which Mt. Hamilton is composed lie at a high 

 angle to the horizon and the direction of the stratification is 

 nearer .N. and S. than E. and W. The earthquake instruments 

 are at the very summit of the mountain. This may account 

 for the fact that (at least for the shocks so far observed) the 

 vertical component is relatively large, and that the 1ST. and S. 

 component (in the general direction of the stratification) is 

 usually far larger than the E. and W. component. The record 

 of this shock on the duplex seismometer is very interesting, 

 but it gives no information additional to the above. 



We may then assume as a basis of computation for this 

 shock : 



Number of waves in 10 seconds =6 or 7, say 6^. 

 Period, T, of the representative wave =0'5 sec. 

 Amplitude of the representative wave (magnified) =2 , 5 mm . 

 a = 0'8 mra . 



