GRAVITATIONAL METHODS 



279 



base, and not subject to large temperature fluctuations. Sufficient space must be pro- 

 vided for the setting up of the coincidence apparatus. (See p. 281.) 



2. Setting-up and Leveling of the Askania Equipment 



After the proper spot for the station has been located, the base frame is set on the 

 ground, oriented, leveled and grouted with plaster-of-Paris in order to furnish a firm 

 connection between base and ground. The pendulum apparatus is then mounted on the 

 frame with the window facing the observation equipment. The level screws F and the 

 pendulum levels PL (Figure 136) permit accurate leveling of the plane of the bearings. 

 (Such leveling is necessary to prevent the pendulums from slipping off the bearings 

 during oscillations, which last several hours.) 



3. Precautions to be Followed in Suspending the Pendulums 



In order to suspend the pendulums freely, it is first necessary to move the selector 

 disk and the oscillation knob to the fixed stop. The operator should wear gloves when 

 handling the pendulums in order to minimize temperature variations due to handling 

 and deposition of oily films. 



4, Control of the Light Path and of the Amplitudes 



Before placing the hood over the apparatus, the observation equipment must be 

 adjusted and the light path checked. Also, the magnitude of the oscillations is con- 

 trolled and corrected, if necessary. The proper operation of the selector disk and 

 oscillation knob is checked as well as the proper fitting of the hood. 



The pendulum hood is fastened to the tripod base by means of 6 swing bolts. 

 The equipment is then ready for evacuation of the air in the pendulum chamber. 



5. Evacuating the Apparatus 



For convenience, the oil pump is mounted on the shipping box (Figure 138) and 

 the hose is fastened to the apparatus. The manufacturers recommend reducing the 

 pressure to about 5 mm. of mercury and then admitting sufficient air to raise 

 the pressure to about 10 mm. of mercury. By following this procedure, the danger of dew 

 formation is practically eliminated. 



6. Adjustment of the Selector Disk: 

 Oscillating the Pendulums 



The pendulums are lowered so that 

 the wedge touches the bearings. (This 

 should only be done when the oscilla- 

 tion knob touches the fixed stop. Also, 

 the selector disk should be operated 

 only when the knob is in this position.) 



The various marked positions of 

 the knob permit oscillating one, two, 

 or all four of the pendulums at any 

 desired amplitude. 



Brown Pendulum 



The current model Brown 

 pendulum apparatus (Figure 

 139) consists of a standard invar 

 quarter-meter pendulum housed 

 in an evacuated chamber. The 



Fig. 139. — Brown gravity apparatus, with 

 front cover removed. A, base chamber or vac- 

 uum case; B, photoelectric recording device; C, 

 control case with cover removed. (Courtesy of 

 U. S. Coast and Geodetic Survey.) 



