The 20th century has witnessed increasing activity in the determination of absolute and relative values of gravity. Gravimeters have been perfected and have been widely used for rapid relative determinations, but the compound pendulums remain as indispensable instruments. Mendenhall's replacement of knives by planes attached to nonreversible pendulums has been used also for reversible ones. The Geodetic Institute at Potsdam is presently applying the Faye-Peirce method to the reversible pendulum.[115] Pendulums have been constructed of new materials, such as invar, fused silica, and fused quartz. Minimum pendulums for precise relative determinations have been constructed and used. Reversible pendulums have been made with "I" cross sections for better stiffness. With all these modifications, however, the foundations of the present designs of compound pendulum apparatus were created in the 19th century. FOOTNOTES: [1] The basic historical documents have been collected, with a bibliography of works and memoirs published from 1629 to the end of 1885, in _Collection de mémoires relatifs a la physique, publiés par la Société française de Physique_ [hereinafter referred to as _Collection de mémoires_]: vol. 4, _Mémoires sur le pendule, précédés d'une bibliographie_ (Paris: Gauthier-Villars, 1889); and vol. 5, _Mémoires sur le pendule_, part 2 (Paris: Gauthier-Villars, 1891). Important secondary sources are: C. WOLF, "Introduction historique," pp. 1-42 in vol. 4, above; and GEORGE BIDDELL AIRY, "Figure of the Earth," pp. 165-240 in vol. 5 of _Encyclopaedia metropolitana_ (London, 1845). [2] Galileo Galilei's principal statements concerning the pendulum occur in his _Discourses Concerning Two New Sciences_, transl. from Italian and Latin into English by Henry Crew and Alfonso de Salvio (Evanston: Northwestern University Press, 1939), pp. 95-97, 170-172. [3] P. MARIN MERSENNE, _Cogitata physico-mathematica_ (Paris, 1644), p. 44. [4] CHRISTIAAN HUYGENS, _Horologium oscillatorium, sive de motu pendulorum ad horologia adaptato demonstrationes geometricae_ (Paris, 1673), proposition 20. [5] The historical events reported in the present section are from AIRY, "Figure of the Earth." [6] ABBÉ JEAN PICARD, _La Mesure de la terre_ (Paris, 1671). JOHN W. OLMSTED, "The 'Application' of Telescopes to Astronomical Instruments, 1667-1669," _Isis_ (1949), vol. 40, p. 213. [7] The toise as a unit of length was 6 Paris feet or about 1,949 millimeters. [8] JEAN RICHER, _Observations astronomiques et physiques faites en l'isle de Caïenne_ (Paris, 1679). JOHN W. OLMSTED, "The Expedition of Jean Richer to Cayenne 1672-1673," _Isis_ (1942), vol. 34, pp. 117-128. [9] The Paris foot was 1.066 English feet, and there were 12 lines to the inch. [10] CHRISTIAAN HUYGENS, "De la cause de la pesanteur," _Divers ouvrages de mathématiques[mathematiques] et de physique par MM. de l'Académie Royale[Royal] des Sciences_ (Paris, 1693), p. 305.