276 



EXPLORATION GEOPHYSICS 



U.S. Naval Observatory at Annapolis, Maryland, were used to calibrate 

 chronometer rates at the field stations. A new type pendulum was also 

 introduced made of invar, (a nickel-steel alloy of 1 part Ni and 2 parts Fe), 



Fig. 136. — Base plate with pendulum tripod. (Courtesy of the Askania Corporation.) 



F — level screw 



B — holding stirrup for level screw 



KL — swing bolt for hood 



S — evacuation petcock 



KS — headpiece of pendulum tripod 



LS — agate bearing in head piece 



P — pendulum 



PK — pendulum head 



Sell — pendulum wedge 



PL — pendulum level 



K — crank for releasing and clamping 



HL — bearings in the arresting device 



G — amplitude limiting device 



H — adjusting lever for the amplitude limiting 



device 

 Ha — holder with thermometer and barometer 



having a coefficient of expansion about 1/15 as much as the bronze pre- 

 viously used. Pendulum gravity field equipment is shown in Figures 135, 

 136, 137, and 138. 



