1909.] 



REID— SEISMOLOGICAL NOTES. 



309 



period of the earth's vibration to that of the seismograph itself, and 

 upon the viscous damping. The damping ratio is the ratio of the 

 ampHtude of successive swings of the heavy mass, when it is 



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allowed to swing freely. If this ratio is nearly i : i, that is, if there 

 is very little damping and the amplitude of the swinging mass dies 



