282 EXPLORATION GEOPHYSICS 



ing through the telescope sees, at each successive opening of the shutter, 

 that the moving image changes its position relative to the fixed image. This 

 shift takes place because the period of the pendulum is just slightly more 

 or less than the time between the openings of the shutter. 



Under these conditions, with the shutter openings representing seconds 

 beats of the chronometer, and the pendulum likewise swinging with a very 

 slightly different (less or more) time interval, there will be recurring 

 intervals when the image lines will coincide. By carefully watching the 

 flashes, noting the chronometer reading when a coincidence of the lines 

 takes place, and at a later time again noting and recording the chronometer 

 reading for another coincidence, the time interval in seconds between coin- 

 cidences can be measured. 



In the interval between two successive coincidences a standardized J^- 

 second (period) pendulum will have made one more or one less than twice 

 as many oscillations as the chronometer has beat seconds. Between any 

 two coincidences, the number of pendulum oscillations is twice the number 

 of seconds (s) plus or minus the number of coincidence intervals (n). The 

 time, or period, of one oscillation (T) is : 



T = Y^ (26) 



the plus or minus being used when the pendulum has a period which is less 

 or more, respectively, than its standardized 3^-second value. The smaller 

 or greater value of the period is, in turn, dependent on the local effect 

 upon g, as can be seen from T = 27r y/l/g. 



This can be illustrated by an example. A coincidence might be recorded at 10 :00 

 A.M. for a given observation. It would be noted as 10** OO" 00'. (h =z hours, m = 

 minutes and s =z seconds). The time of the next coincidence might be 10*" 03™ 01°, or 

 a coincidence interval of 03™ 01* = 181 seconds. 



T = — ^^ = 0.50138 + seconds. 

 361—1 



In the above example, the local effect was such as to decrease g, which in turn 

 increased the period (7"). Experimentally, the procedure was to compare the pendulum 

 swings with the seconds beats of the chronometer, noting that the number of oscilla- 

 tions was less by one than twice the number of seconds. This implies a T greater than 

 standard (J^-second), which in turn denotes a decreased g. 



An error in noting the exact time of the coincidence produces a relatively 

 small error in the period, as the time interval between about 10 coincidences 

 is recorded. For this reason the method is almost independent of observa- 

 tional errors. 



The field procedure for determining the relative gravity by pendulum 

 methods, in certain types of equipment, is based on the use of two or more 

 pendulum instruments. The beats of the pendulum at a base station are 

 transmitted by portable radio transmitter to the field station. There they 



