20 EARTHQUAKES ON THE PACIFIC COAST 



Another and distinct instrument is the duplex pendulum seis- 

 mograph, shown in Plate IV. A massive bob is.hung by three par- 

 allel wires from the top of a three-cornered box, and is reduced to 

 nearly neutral equilibrium by being coupled by a ball-and-tube 

 joint to the bob of an inverted pendulum below it. The two form 

 a system which can be made as nearly astatic as is desirable, and 

 so furnish a suitable steady-point for the horizontal part of earth- 

 quake movement in any azimuth. The motion is magnified* and 

 recorded by a vertical lever geared to the upper bob by a ball-and- 

 tube Joint, supported on gimbals from a bracket fixed to the box, 

 and furnished with a jointed index which wi'ites on a fixed plate of 

 smoked glass. Kecords of the kind which the duplex pendulum 

 gives are of course incomplete in two important particulars: they 

 show nothing of the vertical motion (which, however, is usually a 

 comparatively small part of the whole), and they show nothing of 

 the relation of time to displacement throughout the disturbance. 

 But they exhibit very clearly the change of direction which the 

 movements undergo, and the actual direction taken by any pro- 

 nounced element of the shock." 



These instruments have been kept in working order at Mount 

 Hamilton since June, 1888. 



The larger instrument is somewhat complicated and is not suit- 

 able for private establishments, where its care would require too 

 much time. The smaller seems to be what is wanted for a general 

 instrument to record (the horizontal components of) shocks of 

 average intensity. 



I have had a copy made of it, with some simplifications and 

 improvements, and such copies can be purchased from Paul 

 Seller's electrical works, 406 Market Street, San Francisco, for $15. 



Such copies have been set up in California at various places, 

 among others at the Cliff House, S. F. (Hon. A. Sutro), Kono 

 Tyee, Lakeport (Miss Floyd), Chabot Observatory, Oakland (Mr. 

 Charles Burckhalter), Students' Observatory, Berkeley (Professor 

 Frank Soule), Plighland Park, East Oakland (Mr. F. O. Blinn\ 

 University of the Pacific, San Jose (the Professor in charge of the 

 Observatory), Stanford University (Professor Branner), Mills Sem- 

 inary, Oakland (Professor Keep). Other copies have been sent 



* In the Lick Observatory instrument the horizontal components are mag- 

 nified 4 times. 



