Amir Aczel talks about his book, Pendulum: Leon Foucault and the Triumph of Science, published by Atria Books, which is a biography of 19th century French physicist Leon Foucault (1819-1868). In 1851, Foucault, who had little formal scientific education, used a pendulum to prove to the skeptical Roman Catholic clergy that the Earth was not stationary, but rotated on its axis. Foucault's inventions included the first gyroscope, an electric compass, an electric microscope, and a telescope that is still in use. In addition, he drew important conclusions regarding color theory, heat waves, and the speed of light. The book follows Foucault from his beginnings as a medical student and science journalist to his installation by Napoleon III as the official physicist of the Imperial Observatory in Paris and his belated election to the Academy of Sciences. After his presentation the author answered audience members' questions.