1888.] of the Vibration of the new Tay Bridge. 401 



ground executes in small and in large earthquakes. Tests of this 

 kind no!: only demonstrate the accuracy of the seismograph, but are 

 a convenient means of finding experimentally the ratio in which the 

 recording index multiplies the motion of the ground. 



Fig. 4. — Comparison diagrams to test accuracy of Duplex Pendulum Seismograph. 



For an exhaustive examination of the vibration of a structure 

 under "live" loads, the more elaborate type of seismograph might 

 be used, which records linear components of the motion on a surface 



