Chap. 7] 



GRAVITATIONAL METHODS 



127 



follows from eq. (7-29) that a deflection of 2.5 10~V is produced bj^ a 

 gravity anomaly of one milligal, since the natural frequency is 1 sec. This 

 displacement, with a magnifica- 

 tion of 4 10*, gives a galva- 

 nometer deflection of 10 mm. 



4. Astaiization of gravimeters is 

 equivalent to lowering their nat- 

 ural frequency. It involves the 

 application of a negative restor- 

 ing force in such a manner as to 

 drive the mass away from its rest 

 position and to aid any deflect- 

 ing force. 



While unastatized gravimeters 

 are invariably vertical seismo- 

 graphs, the process of astatization 

 makes it possible to utilize hori- 

 zontal seismographs for gravity 

 measurements. Virtually all 

 horizontal seismo-gravimeters are 

 inverted pendulums, analogous 

 to the Wiechert astatic seismo- 

 graph (Lejay-Holweck and the 

 Ising gravimeters) . Vertical seis- 

 mographs may be astatized by 

 attaching the suspension spring 

 below the horizontal axis of the 

 lever arm (Ewing), by using the 

 spring at an angle of less than 90° 

 with the beam (Berlage, La- 

 Coste*^), by combining a hori- 

 zontal pendulum with a vertical 

 balance (Schmerwitz*^), by fas- 

 tening an inverted pendulum per- 

 manently to the center of the 

 beam (Tanakadate, Thyssen), 

 or by providing an additional 



Fig. 7-286. Gulf gravimeter (after Hoyt). 



" H. P. Berlage, Jr., Handbuch der Geophysik, IV (2), 385. 



*5L. J. B. LaCoste, Physics, 5(3), 174-176 (1934); Seis. Soc. Amer. Bull., 25(2), 

 176-179 (April, 1935). 



"Zeit. Geophys., 7(1/2), 95-103 (1931); Beitr. angew. Geophys., 4(3), 274-295 

 (1934). 



