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Foucault's Pendulum. Illustration depicting the demonstration of Foucault's Pendulum at the Pantheon, Paris, in 1852. Jean Foucault (1819-68) noticed that a large free-mounted pendulum will continue to swing in the same plane irrespective of the rotation of its mounting. In order to test this hypothesis, Foucault fitted a 28- kilogramme iron ball to the end of a 67-metre steel wire, suspended from the dome of the Pantheon. Once in motion, the pendulum did apparently change its plane of oscillation with time, but it was the Earth which was moving under the pendulum. (KEYSTONE/SCIENCE PHOTO LIBRARY/SCIENCE PHOTO LIBRARY)