[11] ISAAC NEWTON, _Philosophiae naturalis principia mathematica_ (London, 1687), vol. 3, propositions 18-20. [12] PIERRE BOUGUER, _La figure de la terre, déterminée par les observations de Messieurs Bouguer et de La Condamine, envoyés par ordre du Roy au Pérou, pour observer aux environs de l'equateur_ (Paris, 1749). [13] P. L. MOREAU DE MAUPERTUIS, _La figure de la terre déterminée par les observations de Messieurs de Maupertuis, Clairaut, Camus, Le Monnier, l'Abbé Outhier et Celsius, faites par ordre du Roy au cercle polaire_ (Paris, 1738). [14] Paris, 1743. [15] GEORGE GABRIEL STOKES, "On Attraction and on Clairaut's Theorem," _Cambridge and Dublin Mathematical Journal_ (1849), vol. 4, p. 194. [16] See _Collection de mémoires_, vol. 4, p. B-34, and J. H. POYNTING and SIR J. J. THOMSON, _Properties of Matter_ (London, 1927), p. 24. [17] POYNTING and THOMSON, ibid., p. 22. [18] CHARLES M. DE LA CONDAMINE, "De la mesure du pendule à Saint Domingue," _Collection de mémoires_, vol. 4, pp. 3-16. [19] PÈRE R. J. BOSCOVICH, _Opera pertinentia ad Opticam et Astronomiam_ (Bassani, 1785), vol. 5, no. 3. [20] J. C. BORDA and J. D. CASSINI DE THURY, "Expériences pour connaître la longueur du pendule qui bat les secondes à Paris," _Collection de mémoires_, vol. 4, pp. 17-64. [21] F. W. BESSEL, "Untersuchungen über die Länge des einfachen Secundenpendels," _Abhandlungen der Königlichen Akademie der Wissenschaften zu Berlin, 1826_ (Berlin, 1828). [22] Bessel used as a standard of length a toise which had been made by Fortin in Paris and had been compared with the original of the "toise de Peru" by Arago. [23] L. G. DU BUAT, _Principes d'hydraulique_ (Paris, 1786). See excerpts in _Collection de mémoires_, pp. B-64 to B-67. [24] CAPT. HENRY KATER, "An Account of Experiments for Determining the Length of the Pendulum Vibrating Seconds in the Latitude of London," _Philosophical Transactions of the Royal Society of London_ (1818), vol. 108, p. 33. [Hereinafter abbreviated _Phil. Trans._] [25] M. G. DE PRONY, "Méthode pour déterminer la longueur du pendule simple qui bat les secondes," _Collection de mémoires_, vol. 4, pp. 65-76. [26] _Collection de mémoires_, vol. 4, p. B-74. [27] _Phil. Trans._ (1819), vol. 109, p. 337. [28] JOHN HERSCHEL, "Notes for a History of the Use of Invariable Pendulums," _The Great Trigonometrical Survey of India_ (Calcutta, 1879), vol. 5. [29] CAPT. EDWARD SABINE, "An Account of Experiments to Determine the Figure of the Earth," _Phil. Trans._ (1828), vol. 118, p. 76. [30] JOHN GOLDINGHAM, "Observations for Ascertaining the Length of the Pendulum at Madras in the East Indies," _Phil. Trans._ (1822), vol. 112, p. 127. [31] BASIL HALL, "Letter to Captain Kater Communicating the Details of Experiments made by him and Mr. Henry Foster with an Invariable Pendulum," _Phil. Trans._ (1823), vol. 113, p. 211. [32] See _Collection de mémoires_, vol. 4, p. B-103. [33] Ibid., p. B-88. [34] Ibid., p. B-94.