Holden — Earthquake- Intensity in San Francisco. 431 



unknown intensities to have had between 48 and 49 units of 

 intensity each, or 10460 units in all. This amounts to sup- 

 posing our average shock to be of intensity IV. 

 In this way the table will stand : 



Units of Acceleration. 



2 1 7 shocks of unknown intensity give 1 0460 



200 " " known intensity give . _ .22900 



417 shocks recorded (1808-1888) give 33360 



The average shock is of intensity IV corresponding to 80 

 units or to y^-g-d -part of the acceleration due to gravity. The 

 total intensity of 33360 units has been experienced in 80 years 

 and corresponds to 3*4 the acceleration due to gravity. That 

 is if all the earthquake force which has been expended in San 

 Francisco during the past 80 years were concentrated so as to 

 act at a single instant, it would be capable of producing an 

 acceleration of 34 times that of gravity or about 109 feet per 

 second. 



The total earthquake intensity during the 80 years is nearly 

 equal to the intensity of 28 separate shocks as severe as that of 

 1868, but it has been doled out so gently and gradually that 

 we have scarcely known of it. 



On the average 392 units of intensity have been developed 

 during each one of the 80 years (1808-88). This will allow for 

 six shocks of intensity III per year or one every two months. 

 In fact 417 shocks have been recorded in the 960 months. 



I believe that my earthquake catalogue as printed and the 

 present note, contain nearly all the precise information which 

 can be extracted from our past records, at this time. 



The automatic earthquake registers now in use at the Uni- 

 versity of California, Berkeley (under the care of Professors 

 Le Conte and Soule) and at the Lick Observatory, Mount 

 Hamilton, will afford valuable data after a few years. 



I am greatly in hopes that the chiefs of the U. S. Geological 

 Survey and of the TJ. S. Signal Bureau may find it practicable 

 to establish and care for seismometric stations in the state. 

 The cost of such stations is small. I find that the excellent 

 duplex-pendulum instrument of Professor Ewing can be satis- 

 factorily duplicated for $15. The California Electric Works, 

 35 Market street, San Francisco, is now prepared to furnish 

 such instruments at that price. If a sufficient number of 

 stations can be established in California, it seems to me that we 

 may look forward to the collection of data of real theoretical 

 and of some practical importance within comparatively few 

 years. 



