approximately 1 second. Their result, published in 

 1936, was interpreted to indicate that the value at 

 Potsdam was too high by 20 parts in 1 million. 103 

 This estimate was lowered slightly by Sir Harold 

 Jeffreys of Cambridge, England, who recomputed 

 the results of Heyl and Cook by different methods. 104 

 In 1939, J. S. Clark published the results of a 

 determination of gravity with pendulums of a non- 

 ferrous Y-alloy 105 at the National Physical Labora- 

 tory at Teddington, England, and, after recomputa- 

 tion of results by Jeffreys, the value was found to be 

 12.8 parts in 1 millionyess than the value obtained by 

 transfer from Potsdam.N Dr. Hugh L. Dryden of the 

 Xational Bureau of Standards, and Dr. A. Berroth 

 of the Geodetic Institute at Postdam, have recomputed 

 the Potsdam data by different methods of adjustment 

 and concluded that the Potsdam value was too high 

 by about 12 parts in a million. 106 Determination of 

 gravity at Leningrad by Russian scientists likewise 

 has indicated that the 1906 Potsdam value is too 

 high. In the light of present information, it 

 appears justifiable to reduce the Potsdam value of 

 981.274 by .013 cm sec 2 for purposes of comparison. 

 If the Brown transfer from Potsdam in 1933 was 

 taken as accurate, the value for the Washington base 

 would be 980.105 cm/sec 2 . In this connection, it is 

 of interest to note that the value given by Charles 

 S. Peirce for the comparable Smithsonian base in 

 Washington, as determined by him from comparative 

 methods in the 1880's and reported in the Annua! 

 Report cf the Superintendent of the Coast and Geodetic Survey 

 fur the year 1890-7891, was 980.1017 cm/sec 2 . 107 This 

 value would appear to indicate that Peirce's pendulums, 

 observations, and methods of reduction of data 

 were not inferior to those of the scientists of the 

 Royal Prussian Geodetic Institute at Potsdam. 



103 Paul R. Heyl and Guy S. Cook, "The Value of Gravity 

 at Washington," Journal of Research, National Bureau oj Standards 

 (1936), vol. 17, p. 805. 



I0 * Sir Harold Jeffreys, "The Absolute Value of Gravity," 

 Monthly Notices of the Royal Astronomical Society, Geophysical Supple- 

 ment (London, 1949), vol. 5, p. 398. 



105 J. S. Clark, "The Acceleration Due to Gravity," Phil. 

 Trans. (1939), vol. 238, p. 65. 



1 " Hugh L. Dryden, "A Reexamination of the Potsdam 

 Absolute Determination of Gravity," Journal of Research, 

 National Bureau of Standards (1942), vol. 29, p. 303; and A. 

 Berroth, "Das Fundarncntalsystcm der Schwere im Lichte 

 neuer Reversionspcndclmessungen," Bulletin G'eodesique (1949), 

 no. 12, pp. 183-204. 



107 T. C. Mendenhall, op. cit. (footnote 83), p. 522. 



Doubts concerning the accuracy of the Potsdam 

 value of gravity have stimulated many new determina- 

 tions of the intensity of gravity since the end of World 

 War II. In a paper published in June 1957, A. H. 

 Cook, Metrology Division, Xational Physical Labora- 

 tory, Teddington, England, stated: 



At present about a dozen new absolute determinations 

 are in progress or are being planned. Heyl and Cook's 

 reversible pendulum apparatus is in use in Buenos Aires 

 and further reversible pendulum experiments have been 

 made in the All Union Scientific Research Institute of 

 Metrology, Leningrad (V N II M) and are planned at 

 Potsdam. A method using a very long pendulum was 

 tried out in Russia about tgio and again more recently 

 and there are plans for similar work in Finland. The 

 first experiment with a freely falling body was that 

 carried out by Volet who photographed a graduated 

 scale falling in an enclosure at low air pressure. Similar 

 experiments have been completed in Leningrad and 

 are in progress at the Physikalisch-Technische Bundes- 

 anstalt (Brunswick) and at the National Research 

 Council (Ottawa), and analogous experiments are being 

 prepared at the National Physical Laboratory and at 

 the Xational Bureau of Standards. Finally, Professor 

 Medi, Director of the Istituto Nazionale di Geofisica 

 (Rome), is attempting to measure the focal length of 

 the paraboloidal surface of a liquid in a rotating dish. 10s 



Application of Gravity Surveys 



We have noted previously that in the ancient and 

 early modern periods, the earth was presupposed to 

 be spherical in form. Determination of the figure of 

 the earth consisted in the measurement of the radius 

 by the astronomical-geodetic method invented by 

 Eratosthenes. Since the earth was assumed to be 

 spherical, gravity was inferred to be constant over 

 the surface of the earth. This conclusion appeared 

 to be confirmed by the determination of the length 

 of the seconds pendulum at various stations in Europe 

 by Picard and others. The observations of Richer in 

 South America, the theoretical discussions of Newton 

 and Huygens, and the measurements of degrees of 

 latitude in Peru and Sweden demonstrated that the 

 earth is an oblate spheroid. 



The theory of gravitation and the theory of central 

 forces led to the result that the intensity of gravity 

 is variable over the surface of the earth. Accordingly, 



108 A. H. Cook, "Recent Developments in the Absolute 

 Measurement of Gravity," Bulletin Geodesique (June 1, 1957), 

 no. 44, pp. 34-59. 



342 



BULLETIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



