GRAVITATIONAL METHODS 281 



Pendulum gravity apparatus, as indicated in Figures 135 to 140 inclus- 

 ive, usually consists of the following essential parts : 



1. One or more pendulums. 



2. An air-tight receiver surrounding a suitably constructed base on 

 which the pendulums are swung, under reduced air pressure. 



3. A flash box and/or light source and telescope for observing pendu- 

 lum vibration in measuring the period of oscillation. 



4. One or more chronometers of high accuracy, and the necessary radio 

 equipment for receiving standard time signals. 



5. One or more chronographs. These consist of a cylindrical drum, 

 rotated by a clock-work mechanism carrying a roll of paper on 

 which a stylus marks the beats of a chronometer. They are used 

 for checking rates and comparing the accuracy of chronometers. 



6. An interferometer, or other device, for measuring the movement of 

 the receiver caused by the oscillation of the pendulum. 



7. Thermometers, installed in the receiver. 



8. A manometer to measure air pressure in the pendulum housing. 



9. An air pump for evacuating the receiver. 



Field Measurements 



The basic problem in pendulum gravity measurements is to determine, 

 with the necessary accuracy, the period of vibration of a pendulum set up 

 at each field station. The descriptions given relate, more particularly, to 

 the Mendenhall apparatus (Figure 135), and serve to outline the general 

 field technique and the necessary corrections. Instrumental developments 

 in the years just prior to World War II (particularly the use of two-way 

 radio between base and field stations) have simplified field operations and 

 improved the accuracy of pendulum gravity measurements. 



Timing. — Accuracy in gravity observations requires precise measure- 

 ment of the period of the pendulum. Hourly time signals are sent out by 

 the U.S. Navy stations at Annapolis on 4390, 9425, and 12630 kilocycles. 

 Any sensitive short-wave receiver may be employed for receiving the time 

 signals. 



The Method of Coincidences. — In pendulum equipment where a flash 

 box is used, two small mirrors catch the beam of light from the flash box 

 and reflect it. One of the mirrors is attached to the support on which the 

 pendulum swings and the other to the head of the pendulum itself. 



With the pendulum at rest and the shutter of the flash box open, the 

 beam of light, as seen through the telescope on top of the flash box, appears 

 as two bright lines which coincide. When the pendulum is set in motion 

 the reflected image or line from one mirror moves, while that from the 

 fixed mirror remains stationary. 



The shutter of the flash box is opened for a brief instant by an electro- 

 magnet actuated by the seconds beats of a chronometer. An observer look- 



