Chap. 7] GRAVITATIONAL METHODS 165 



milligals. Therefore, the maximum possible variation between the maxi- 

 mum and minimum of the curves would have been approximately 0.34 

 milligals. The theoretical values agreed well with the records of a sta- 

 tionary vertical seismo-gravimeter. 



B. Secular (Geologic) Varl\.tions 



In the course of geologic time, considerable changes in mass distribution 

 occur near the earth's surface because of erosion, deposition of sediments, 

 glaciation, removal of ice caps, orogenic and epeirogenic movements, fault- 

 ing, volcanism, magma migrations, and the like. That some of these 

 factors bring about changes in elevation has been definitely estabUshed. 

 A well-known example is the uplift of Fennoscandia, whose postglacial 

 rise is assumed to amount to a maximum of 275 m. Areas in which such 

 movements occur are generally characterized by strong gravity anomalies, 

 indicating incomplete isostatic compensation. Since the earth's crust 

 tends to re-establish the isostatic equilibrium disturbed by geologic factors, 

 it must be expected that the greatest gravity changes occur in areas of 

 large gravity anomaUes. However, definite proof of this appears to be 

 scarce. Virtually the only corroborative material has been supplied by 

 measurements in India in 1865-1873 and their repetition in 1903 and 1904. 

 The observed differences average 54 milligals, which is equivalent to an 

 increase in gravity by about 1| milligals per year. Some authors have 

 expressed doubt as to the accuracy of the earlier measurements, and 

 confirmation of such gravity variations in other regions is urgently needed. 



Subsurface mass displacements brought about by faulting, volcanic 

 phenomena, and the like may also be expected to cause gravity variations. 

 With an increase in the accuracy of recording gravimeters it is probable 

 that useful information may be accumulated over a period of years m 

 active earthquake regions and may lead to a solution of the problem of 

 earthquake prediction. Combination of recording gravimeters with 

 recording magnetometers and seismographs would be particularly useful. 

 As is shown below, subsurface mass displacements can likewise be recorded 

 by the torsion balance. 



C. Changes in Water Level (Tides and the Like) 



That changes in water level have a definite effect on the torsion balance 

 has been shown both theoretically and by actual measurements.^^ If 

 a torsion balance is set up on the edge of a quay having vertical walls, 

 80 that the center of gravity of the balance is 13 feet above water level 



•«K. Jung, Handb. Exper. Phys., 25(3), 168 (1930). A. Schleusener, Beitr. 

 angew. Geophys., 6(4), 480-518 (1936). 



