22 EARTHQUAKES ON THE PACIFIC COAST ""• 



the glass plate will move with it. The double pendulum is so cou- 

 structed as to remain steady, or very nearly so; the pointer 

 over the glass plate remains steady also, and writes the 

 motion of the earth upon the moving plate. The motion of the 

 earth is magnified approximately four times. The line traced 

 on the plate will represent the direction of each shock, and the 

 length of the line gives a measure of the intensity. In any large 

 earthquake this line will be a looped curve. If the time of begin- 

 ning of the earthquake is also noted by the observer on his watch, 

 and if the watch is compared as soon as possible with the time of 

 the nearest railway station (time is received daily at noon, at all 

 railway stations, from the Lick Observatory), all the data are se- 

 cured which are necessary for the accurate study of the shock at 

 this one station. If the original glass plate is carefully packed 

 (so as to preserve the film) and sent to the Director of the Lick 

 Observatory, it will be measured at the Observatory, and a blue 

 print of the tracing will be returned to the sender, together with 

 the original glass plate. The memorandum relating to the time of 

 the shock should also be sent, with a statement of exactly how and 

 where the instrument is mounted. As soon as one glass plate is 

 removed, the spare plate furnished with the instrument should be 

 blackened and inserted. The only precautions necessary to be 

 taken in the use of this instrument are to keep it level and to keep 

 a freshly-smoked plate underneath the pointer." 



During the years 1888-1897 the large Ewing seismograph of the 

 Lick Observatory has been under the charge of Messrs. Keeler, Hill, 

 A. J. Burnham and Perrine, and certain improvements in its con- 

 struction have been suggested by experience. Some of these im- 

 provements have actually been made. The following memoran- 

 dum has been prepared at my request. 



Improvements in the Ewing Seismographs Suggested by 

 Experience. 



BY C. D. PEKKINE. 



" Our experience with the Ewing seismograph of the Lick Obser- 

 vatory has suggested some slight changes to improve its working. 

 The magnetic release for the driving clock has frequently failed to 

 act, as a heavy current was required to move the armature, which 



