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Foucault's Pendulum lecture. This lecture, 'The Rotation of the Earth Made Visible', was delivered at the Royal Polytechnic Institution, Regent Street, London, in May 1851 by one of its founders, the British inventor George Henry Bachhoffner (1810-1879). This lecture was one of many prompted by the initial demonstration of this device, carried out in Paris, France, in February 1851, by French physicist Leon Foucault (1819-1868). The experiment involves a heavy pendulum swinging on a long arm. The vertical plane in which the pendulum swings rotates with time, tracing a circle (shown here on the floor) due to the Earth's daily rotation. (KEYSTONE/SCIENCE PHOTO LIBRARY/ROYAL ASTRONOMICAL SOCIETY/SCIENCE PHOTO LIBRARY)