608 



SEISMIC METHODS 



[Chap. 9 



number of rollers past a cylindrical lens and a time-marking mechanism. 

 This consists of a pendulum which interrupts a light beam at periodic 

 intervals and projects dashes every 1/10 second on the paper. The 

 paper is started by pressing an idler against one of the rollers driven 

 by the spring motor. By the above combination of mechanical and 

 optical magnification, high over-all magnification is obtained. If V is 



Fig. 9-112. Mintrop mechanical seismograph and recorder. 



the geometric magnification of the lever, r the radius of the mirror 

 spindle, and D the focal length of the lens in front of the seismograph 

 mirror, the over-all magnification is 2Z)V/r. In this manner magnifications 

 from 15,000 times and up are readily obtained. At a receiving station 

 seismograph and camera are generally set up in a small tent. These 

 are supplemented by a mirror device on a tripod next to the seismograph 

 for the recording of the shot instant transmitted by radio. 



Another type of mechanical seismograph that was widely used for refrac- 



