GRAVITATIONAL METHODS 269 



entation indicates the direction in which the horizontal rate of change of 

 gravity is a maximum. ( Figure 131). 



Differences in gravity at two points can be determined by taking the 

 average gradient between these points and multiplying it by the distance 

 separating them, as will be shown in detail in a later section. 



Absolute Gravity Measurements 



The pendulum is essentially an instrument for the measurement of the 

 absolute value of gravity. It has been employed chiefly by the various gov- 

 ernmental and other agencies engaged in estabhshing absolute values of 

 gravity for use in certain scientific and geodetic investigations. However, 

 the pendulum has been used in oil exploration, especially by the Gulf Oil 

 Corporation, which made many thousand observations during the period 

 from 1930 to 1935. After that date their pendulum equipment was replaced 

 by the more rapid and more accurate gravity-meter. 



The Place of the Pendulum in Exploration. — The gravity pendulum 

 has certain unique advantages in exploration. For sub-sea work it was first 

 mounted in a submarine by the Dutch scientist, Dr. F. A. Vening Meinesz, 

 in 1923.t Gravity observations at sea are of value in geophysical pros- 

 pecting, on a reconnaissance basis. They also aid in elucidating the broad 

 geological theories of continents and ocean basins, of mountain building and 

 of the structure of island arcs. 



The Vening Meinesz pendulum gravity apparatus was initially designed 

 for observations on unstable bases or on board ship where the roll and pitch 

 were less than 5 degrees. It was later adapted for use in underwater craft. 

 Readings are taken with the submarine submerged to 50 feet or more, 

 where stability may actually be greater than on a surface vessel. The equip- 

 ment developed by Meinesz carries 3 working pendulums 0.25 meter long 

 and is mounted on a specially-designed frame suspended in gimbals. Oscilla- 

 tions of the pendulum supports are transmitted to two other pendulums 

 and thus measured. All recording is photographic. The use of a portable 

 crystal chronograph will give the time of a J^-hour swing of the pendulums 

 to 0.005 second. The accuracy of sea gravity measurements (in 1936) was 

 from 1 to 5 milligals. 



From 1923 to 1938 some 21 marine pendulum gravity surveys were 

 made which involved a total of 1200 stations. The first such work relating 

 to this country was conducted in 1928 by Dr. Meinesz in the submarine 

 S-21, under the auspices of the Navy Department and the Carnegie 

 Institution.ff 



Hess| brings out, in connection with such marine gravity measurements, 



t M. Ewing, "Marine Gravimetric Methods and Surveys," Am. Phil. Soc, Proc. Vol. 79, No. 1: 

 pp. 47-70, April 21, 1938. 



tt See J. J. Hoskinson, "Gravity at Sea by Pendulum Observations," A. I. M. E. Tech. Pub. 

 No. 955, 7 pages, February, 1938. 



t H. H. Hess, "Gravity Anomalies and Island Arc Structure with Particular Reference to the 

 West Indies," Am. Phil. Soc. Proc, Vol. 79, No. 1, pp. 71-96, April 21, 1938. 



