278 



EXPLORATION GEOPHYSICS 



The wedge is made of quartz and is mounted in the pendulum head. 



The metal mirror which is located in the pendulum head is ground in 

 such a way that errors due to displacements of the plane of the mirror are 

 minimized. The pendulum hood which covers the tripod and all its acces- 

 sories is so mounted that a close air-tight fit is assured. Two windows are 

 provided to read the thermometer and barometer and to permit the light 

 ray to travel from the point of observation to the optical bridge and return. 



Two levels, having a sensitivity of 10 sec. per division, are mounted so 

 that they can be conveniently lowered to the agate bearings, the exact level 

 position of which they control. Two heavy handles are provided for carry- 

 ing the instrument. 



During transport, the holder for the thermometer and barometer is 

 carried in a box. Suitable means are provided so that the barometer will 

 not be damaged while in transport, even if the mercury should be subject 

 to considerable movement. 



Abh.7 



Fig. 138. — Askania 4-pendulum apparatus ready for operation. 



a — oil pump 



b — instrument box 



PH — pendulum housing 



d — observation window 

 e — observation equipment 

 KL — locking screw 



GENERAL OUTLINE OF PROCEDURE AT EACH STATION FOR 

 PENDULUM OBSERVATIONS 



1. Selecting a Station 



To obtain a high accuracy of the pendulum observation (about 1 milligal) it is 

 necessary to select a place free from vibrations, preferably with a hard rock or cement 



