GRAVITATIONAL METHODS 



389 



that in the Gulf Coast area the ocean bottom is rarely still, and there is 

 usually some motion of the scale indicator, which may make reading diffi- 

 cult. The electronic averaging circuits are of considerable value under 

 these conditions. 



Fig. 235. — Diving bell in elevated position, when under way be 

 stations. A, "A" frame hoist; B, diving bell, with large extended base C 

 for better stability in areas with soft bottoms. (Courtesy of Union Oil 

 Company of California.) 



Calibration of Gravimeters 



A gravimeter must be calibrated so that the change in the acceleration 

 of gravity can be determined from the measured displacement of the mass. 

 This may be done by several methods. One method, which is applicable 

 for practically all types of gravity meters, consists in measuring the varia- 

 tion of gravity as a function of height or elevation. Such calibration 

 measurements may be carried out conveniently in an}' tall building.* 



Less simple is the calibration of an instrument by observations made at 

 stations whose gravity differences have previously been measured ac- 

 curately with pendulums or torsion balance. 



Certain meters, for example, the Thyssen instrument described on page 

 376, may be calibrated conveniently by adding small weights. The sensi- 

 tivity, or number of milligrams required to produce one scale deflection 

 is obtained directly from a graph of scale divisions versus load (in milli- 



* See, for example, "Gulf gravimeter tests in Empire State Building, Washington 

 Monument, and Pittsburgh Cathedral of Learning," Report in National Research 

 Council, Transactions of American Geophysical Union, Vol. I, Washington, July, 1938 



