Used with
permission of Maiken Naylor, University at Buffalo, Buffalo, NY, USA,
http://ublib.buffalo.edu/libraries/units/sel/exhibits/stamps
Leon Foucault (1819-68), the French physicist, is remembered for his pendulum experiment which proved the rotation of the earth. A very long pendulum is suspended and left to swing in the vertical plane containing the pendulum bob and the point of suspension. As the earth rotates beneath the bob, the plane of oscillation appears to rotate uniformly 2pi cos theta radians a day, where theta is the colatitude of that point on earth. Visitors to the United Nations in New York may observe just such a pendulum in the lobby. Foucault also successfully measured the speed of light by means of a system of rotating mirrors, and observed a bright yellow spectral line in an electric arc light at the exact position of the yellow lines observed by Fraunhofer.