[89] P. FURTWÄNGLER, "Über die Schwingungen zweier Pendel mit annähernd gleicher Schwingungsdauer auf gemeinsamer Unterlage," _Sitzungsberichte der Königlicher Preussischen Akademie der Wissenschaften zu Berlin_ (Berlin, 1902) pp. 245-253. Peirce investigated the plan of swinging two pendulums on the same stand (_Report of the Superintendent of the U.S. Coast and Geodetic Survey for 1880-81_, Washington, 1883, p. 26; also in CHARLES SANDERS PEIRCE, _Collected Papers_, 6.273). At a conference on gravity held in Washington during May 1882, Peirce again advanced the method of eliminating flexure by hanging two pendulums on one support and oscillating them in antiphase ("Report of a conference on gravity determinations held in Washington, D.C., in May, 1882," _Report of the Superintendent of the U.S. Coast and Geodetic Survey for 1881-82_, Washington, 1883, app. no. 22, pp. 503-516). [90] F. A. VENING MEINESZ, _Observations de pendule dans les Pays-Bas_ (Delft, 1923). [91] A. BERROTH, "Schweremessungen mit zwei und vier gleichzeitig auf demselben Stativ schwingenden Pendeln," _Zeitschrift für Geophysik_, vol. 1 (1924-25), no. 3, p. 93. [92] "Pendulum Apparatus for Gravity Determinations," _Engineering_ (1926), vol. 122, pp. 271-272. [93] MALCOLM W. GAY, "Relative Gravity Measurements Using Precision Pendulum Equipment," _Geophysics_ (1940), vol. 5, pp. 176-191. [94] 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. [95] W. A. HEISKANEN and F. A. VENING MEINESZ, _The Earth and its Gravity Field_ (McGraw: New York, 1958). [96] F. KÜHNEN and P. FURTWÄNGLER, _Bestimmung der Absoluten Grösze der Schwerkraft zu Potsdam mit Reversionspendeln_ (Berlin: Veröffentlichungen des Königlichen Preussischen Geodätischen Instituts, 1906), new ser., no. 27. [97] Reported by Dr. F. Kühnen to the fifth session, October 9, 1895, of the Eleventh General Conference, _Die Internationale Erdmessung_, held in Berlin from September 25 to October 12, 1895. A footnote states that Assistant O. H. Tittmann, who represented the United States, subsequently reported Peirce's prior discovery of the influence of the flexure of the pendulum itself upon the period (_Report of the Superintendent of the U.S. Coast and Geodetic Survey for 1883-84_, Washington, 1885, app. 16, pp. 483-485). [98] _Assistants' Reports, U.S. Coast and Geodetic Survey, 1883-84_ (MS, National Archives, Washington). [99] C. S. PEIRCE, "Effect of the Flexure of a Pendulum Upon its Period of Oscillation," _Report of the Superintendent of the U.S. Coast and Geodetic Survey for 1883-84_ (Washington, 1885), app. no. 16. [100] F. R. HELMERT, _Beiträge zur Theorie des Reversionspendels_ (Potsdam: Veröffentlichungen des Königlichen Preussischen Geodätischen Instituts, 1898).