106 



GRAVITATIONAL METHODS 



[Chap. 7 



opposite phases, are photographed simultaneously. Since the passage of 

 the pendulum through the rest position, with reference to a radio time 

 signal, may be determined with an accuracy of about 2-10~ sec. and ten 

 successive passages are observed at intervals of about 40 minutes, the 

 period may be measured with an accuracy of about ±2.10~ seconds. 



Fig. 7-13. Stroboscopic coincidence record (after Martin). 



Table 19 

 EVALUATION OF STROBOSCOPIC COINCIDENCE RECORD" 



In the Vening Meinesz method, the coincidence record (Fig. 7-14) is 

 obtained by interrupting the light beam twice during a full swing. In the 

 actual record (see Fig. 7-15), more vibrations occur between successive 

 passages than indicated in Fig. 7-14, since the difference in period between 



