SEISMOMETRY. 25 



short distance, and then allowing it to fall back towards 

 its normal position. 



In connection with this subject we may mention the 

 pendulum seismographs of Kreil, Wagener, Ewing, and 

 Gray. 



In the bob of Kreil's pendulum there was clockwork, 

 which caused a disc on the axis of the pendulum to con- 

 tinuously rotate. On this continually revolving surface a 

 style fixed to the earth traced an unbroken circle. At the 

 time of an earthquake, by the motion of the style, the 

 circle was to be broken and lines drawn. The number 

 and length of these lines w^ere to indicate the length and 

 intensity of the disturbance. 



Gray's pendulum consisted of a flat heavy disc carrying 

 on its upper surface a smoked glass plate. This, which 

 formed the bob of the pendulum, was supported by a 

 pianoforte steel wire. When set ready to receive an earth- 

 quake, the wire was twisted and the bob held by a catch so 

 arranged that at the time of the earthquake the catch 

 was released, and the bob of the pendulum allowed to turn 

 slow^ly by the untwisting of the supporting wire. Resting 

 on the surface of this rotating disc were two multiplying 

 indices arranged to write the earth's motions as two com- 

 ponents. 



In the instruments of Wagener and Ewing, the clock- 

 work and moving surface do not form part of the pen- 

 dulum, but rest independently on a support rigidly 

 attached to the earth. In W^agener's instrumbut one index 

 only is used, while in Ewing's two are used for writing 

 the record of the motion. 



A difficulty which is apparent in all pendulum 

 machines is that when the bob of such a pendulum is de- 

 flected it tends to fall back to its normal position. To 

 make a pendulum perfect it therefore requires some com- 



