EARTHQUAKES ON THE PACIFIC COAST 31 



out of the State, for example; to Cleveland, Ohio (Warner and 

 Swasey), Washington, D. C. (U. S. Geological Survey), Carson, 

 Nevada (Professor C. W. Friend), Keadville, Mass. (Blue Hill Ob- 

 servatory), *Santiago de Chile (National Observatory), *Mexico 

 (Tacubaya) (National Observatory), *Cordoba, Argentine Republic 

 (National Observatory), *Greenwich, England (Royal Observatory). 

 It may be useful to print in this place the following brief in- 

 structions for setting up the Duplex Seismometer, which were pre- 

 pared by Dr. Joseph Le Conte and myself in 1887 and sent out 

 with the first instruments: 



Use of the Ewing Duplex Seismometer. 



" The object of the instrument is to automatically register on a 

 smoked glass plate the horizontal motions of the earth below its 

 base. 



" The best way to set up the instrument is to drive a post into the 

 ground about four feet. The top of the post should be sawed off 

 square as near to the ground as convenient, a piece of stout plank 

 spiked to it, and the three leveling-post screws of the instrument 

 placed on this. The screws should be turned until the two pen- 

 dulums hang freely at equal distances from the frame all around, 

 and until the pointer or index is near the centre of the glass plate. 

 A line marked N — S on this plate should be put in the north and 

 south line (N to the north). f If the instrument cannot be 

 placed at the surface of the ground, it should be placed as near to 

 the surface as possible (since it is desired to register the move- 

 ments of the ground and not the oscillations of any particular 

 house or part of a house), and it should always be placed on a post 

 firmly set in the ground when this can be done. If this is not 

 practicable, it should be placed in the best position attainable. It 

 is convenient to have the instrument protected by a glass case. 

 The glass plate should be smoked on one side by holding it above 

 the flame of a lajnp or candle (burning camphor gives the best film 

 of soot). The instrument is then ready for use. 



" When a shock occurs, the base of the machine will be moved and 



* Presented to the Observatory by Mateo Clark, Esq., of London. 



+ A line registered on the plate from the point of beginning towards N (if 

 caused by a shock) means that the earth has itself moved north below the 

 instrument. 



