INTRODUCTION 7 



Ising in 1918 was probably the first investigator to propose a practical 

 astatic gravity-meter design. The intensive development of gravity meters 

 began during the years 1928-1930, when various American oil companies 

 became interested in new geophysical instruments. At present there are 

 several different types of gravity meters having an accuracy greater than 

 1 millidyne per gram, including a few with accuracy greater than 0.1 

 millidyne per gram. (Measurements to within 0.1 millidyne correspond to 

 an accuracy of one ten-millionth of the total value of gravity.) 



The attempt to determine how much the earth weighs led to a study of 

 the force of gravity by Henry Cavendish, an 18th-century English chemist. 

 Cavendish used the torsion balance, previously employed by Coulomb for 

 studies of magnetic and electrical attraction, to measure the force of 

 attraction between two masses. Further studies were pursued by Baron 

 Roland von Eotvos, (1848-1919), who refined and applied the principles of 

 the original Coulomb balance, primarily to the field of geodetic research. 

 With the Eotvos instrument, geophysicists have been able to obtain con- 

 crete evidence of the variation of density with depth in the outer crust of 

 the earth. 



The characteristic feature of the Eotvos balance which renders it a 

 practical and useful instrument for geophysical exploration is that, by 

 placing two equal masses at different levels and by proper choice of instru- 

 mental constants, the balance may be made extremely sensitive to variations 

 in the rate of change of gravity in a horizontal plane and to the curva- 

 ture values. The double beam balance introduced by Eotvos in 1902 is 

 essentially the same as the torsion balance employed at present. In addition 

 to designing a practical field instrument, Eotvos investigated various 

 fundamental principles of gravitational prospecting and applied his method 

 to certain regions in Hungary. Schweydar modified the Eotvos balance 

 by using a "Z-beam." Following him, Shaw and Lancaster Jones introduced 

 a gradiometer which measured the gradient of gravity in a horizontal plane. 



The application of gravity surveys as an aid in prospecting for petro- 

 leum was investigated in 1914 by E. de Golyer, who took an active part in 

 subsequent exploration. The correlation of gravity variations and geology 

 was due largely to Hugo V. Boeckh who, in 1917, gave geological reasons 

 explaining why anticlines and domes, with cores of different density, pro- 

 duce gravity anomalies of magnitudes which are measurable by the Eotvos 

 balance. He was probably the first to realize the general possibilities of 

 such measurements in prospecting for oil. E. W. Shaw, about the same 

 time, suggested the possibility of locating salt domes by gravity field 

 studies. In 1918, Schweydar under the guidance of Boeckh made success- 

 ful measurements over a German salt deposit. In 1920, de Golyer con- 

 tracted for two balances to be built by F. Siiss of Budapest, and in 1922, 

 D. C. Barton went to Budapest to receive the instruments and instructions 

 for their use. Barton occupied an important position in subsequent gravity 

 work in the United States. The first commercial application of pendulum 



