110 GRAVITATIONAL METHODS [Chap. 7 



pended in the order mi, m^, rriz (see Fig. 7-17) from left to the right, and an 

 optical arrangement is provided to record two fictitious pendulums, one 

 representing 6i — 62, the second 62 — 83. In addition, pendulum 2 is 

 recorded independently with reference to a highly damped auxiliary 

 pendulum in the plane of oscillation of the other pendulums. A fourth 

 record is obtained from a second highly damped pendulum, which swings 

 in a plane at right angles to the plane of oscillation of the regular pendu- 

 lums, giving the angle of tilt j8.^^ Altogether five pendulums are contained 

 in the apparatus. The regular pendulums are as nearly isochronous as 

 IK)ssible, the differences in periods not exceeding 50 X 10~ sec. at normal 

 pressure and temperature. In the damped pendulums, one unit is mounted 

 inside the other, the outer pendulum being filled with oil. The entire 

 pendulum apparatus is suspended in a frame in which it may be leveled 

 by means of four screws. This frame, in turn, is suspended in gimbals. 



Many desirable features of the Vening Meinesz pendulum apparatus 

 have been incorporated in the Askania three-'pendulum apparatus, shown in 

 Fig. 7-18. The receiver is rigidly anchored with three leveling screws and 

 clamps (4) to the base plate (3) and consists of a roughly rectangular case 

 (1) with a hood (2), both made of duraluminum. An air-tight seal is 

 provided between them so that a pressure of about 0.1 mm may be main- 

 tained inside for 6 to 7 hours. The three pendulums are arrested and 

 released by three movements (10). During transportation from one 

 station to another an additional mechanism (8) is provided, which secures 

 the pendulum in three sockets; two of these are seen below the mirrors (14) 

 while one of them has been taken out and is shown separately in front (11). 

 Three impulse disks (7) are provided to start the pendulums at the desired 

 time with a phase difference of 180°. The pendulums are of the invariable 

 type, about 430 mm long and 26 mm around. The knife edge is located 

 about 120 mm from the center. The upper surfaces of the pendulums are 

 pohshed to act as mirrors, reflecting the light on the mirrors (14) through a 

 lens (15) into a recording apparatus shown in the center of the picture. 

 This apparatus may be used with time signals transmitted by radio from a 

 pendulum located at a central station. If the latter is adjusted to match 

 the field pendulums within 2 • 10~' seconds, concidence intervals are around 

 120 seconds and are determined with an accuracy of 0.2 second. For a 

 two-hour set, an accuracy of ±0.6 • 10~^ seconds and thus a mean error in 

 gravity of only 0.2 milligal is claimed. 



The Brown pendulum apparatus pi the U.S. Coast and Geodetic Survey 

 represents a considerable improvement over their earlier type. As in the 

 latter, only one pendulum is used, housed in an air-tight receiver (Fig. 



" See eq. (7-18«) and (T-lSj/). 



