400 Prof. J. A. Ewing. Seismometric Measurements [June 21 7 



cylindrical hole in the bracket j, which was firmly fixed to the 

 (motionless) top of the table. When the multiplication given by 

 this lever g is arranged to be the same as that given by the seismo- 

 graph the two records should be identical, except for error caused by 

 the " steady-point " of the seismograph wandering through friction, 

 or because of the stability of the suspended mass, and except for those 

 errors which both the seismograph and the testing lever are liable to 



Fig. 3. — Arrangement for testing the Duplex Pendulum Seismograph. 



through backlash at the joints and want of rigidity in the lever and 

 index arm. In practice the agreement between the records is most 

 satisfactory. Fig. 4 gives examples of the result of this test as 

 applied to the seismograph which was used upon the Tay Bridge, 

 when the shaking was made to imitate such movements as the 



