Figure 30. — The accumulated data of gravity observations over the earth's 

 surface have indicated that irregularities such as mountains do not have 

 the effect which would be expected in modifying gravity, but are somehow 

 compensated for. The most satisfactory solution to this still unanswered 

 question has been the theory of isostasy, according to which variations in 

 the density of the material in the earth's crust produce a kind of hvdrostatic 

 equilibrium between its higher and lower parts, as they "float" on the earth's 

 fluid core. The metals of different density floating in mercury in this 

 diagram illustrate isostasy according to the theory of Pratt and Hayford. 



application — was rapidly followed for pendulum ap- 

 paratus for relative determinations by Potsdam, 91 

 Cambridge (England), 92 Gulf Oil and Development 

 Company, 93 and the Dominion Observatory at 

 Ottawa. 94 Heiskanen and Vening Meinesz state: 



The best way to eliminate the effect of flexure is to use 

 two synchronized pendulums of the same length swing- 

 ing on the same apparatus in the same plane and with 

 the same amplitudes but in opposite phases; it is clear 

 then the flexure is zero. 95 



In view of the fact that the symmetrical reversible 

 pendulum is named for Bessel, who created the theory 

 and a design for its application by Repsold, it appears 

 appropriate to call the method of eliminating flexure 

 by swinging two pendulums on the same support the 

 Faye-Peirce method. Its successful application was 

 made possible by Maj. von Sterneck's invention of the 

 short, %-metCT pendulum. 



81 A. BERROTH, "Schweremessungen mit zwei und vicr 

 gleichzeitig auf demselbcn Statu schwingenden Pendeln," 

 Zeitsckrifl fur Geophysik, vol. 1 (1924-25), no. 3, p. 93. 



""Pendulum Apparatus for Gravity Determinations," 

 Engineering (1926), vol. 122, pp. 271-272. 



''Malcolm W. Gay, "Relative Gravity Measurements 

 Using Precision Pendulum Equipment," Geophysics (1940), vol. 

 5, pp. 176-191. 



04 L. G. D. Thompson, "An Improved Bronze Pendulum 

 Apparatus for Relative Gravity Determinations," [published 

 by] Dominion Observatory (Ottawa, 1959), vol. 21, no. 3, pp. 

 145-176. 



• 5 W. A. Heiskanen and F. A. Vening Meinesz, The Earth 

 and its (, 1 i McGraw: New York, 1958). 



Absolute Value of Gravity at Potsdam 



The development of the reversible pendulum in the 

 19th century culminated in the absolute determina- 

 tion of the intensity of gravity at Potsdam by Kidmen 

 and Furtwangler of the Royal Prussian Geodetic In- 

 stitute, which then became the world base for gravity 

 surveys. 96 



We have previously seen that in 1869 the Geodetic 

 Institute — founded by Lt. Gen. Baeyer — had ac- 

 quired a Repsold-Bessel reversible pendulum which 

 was swung by Dr. Albrecht under the direction of 

 Dr. Bruhns. Dissatisfaction with this instrument was 

 expressed by Baeyer in 1875 to Charles S. Peirce, 

 who then, by experiment and mathematical analysis 

 of the flexure of the stand under oscillations of the 

 pendulum, determined that previously reported re- 

 sults with the Repsold apparatus required correction. 

 Dr. F. R. Helmcrt, who in 1887 succeeded Baeyer as 

 director of the Institute, secured construction of a 

 building for the Institute in Potsdam, and under his 

 direction the scientific study of the intensity of gravity 

 was pursued with vigor. In 1894, it was discovered 

 in Potsdam that a pendulum constructed of very 

 flexible material yielded results which differed 

 markedly from those obtained with pendulums of 



" F. Kuhnen and P. Furtwangler, Besiimmung der Absoluten 



Grbszt der Schverkrajt zu Potsdam mil Reversionspendeln (Berlin: 

 Veroffentlichungen des Koniglichen I'm t 1 . !• 11 Geodatischen 

 Instituts, 1906), new ser., no. 27. 



338 



II I I.ETIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



