An Act for ascertaining and establishing Uniformity of Weights and Measures, British Parliament, 17 June 1824, reprinted in Raithby, John (1824). The Statutes of the United Kingdom of Great Britain and Ireland, Volume 27. London: Andrew Strahan. p. 759. The wording of the Act indicates that the pendulum definition is to be used to restore the yard if the prototype is destroyed.
Nave, C. R. (2005). "Simple Pendulum". Hyperphysics. Dept. of Physics and Astronomy, Georgia State Univ. Retrieved 2009-02-20.
gutenberg.org
Victor F., Lenzen; Robert P. Multauf (1964). "Paper 44: Development of gravity pendulums in the 19th century". United States National Museum Bulletin 240: Contributions from the Museum of History and Technology reprinted in Bulletin of the Smithsonian Institution. Washington: Smithsonian Institution Press. p. 307. Retrieved 2009-01-28.