He was neither a mathematician nor a trained physicist and yet Léon Foucault always knew that a mysterious force of nature was among us. Like Newton, Galileo, Copernicus, and others before him, Foucault sensed a dramatic relationship between the rotating skies above and the seemingly motionless ground beneath our feet. But it wasn't until 1851 -- in Paris, inside the Panthéon, and in the company of fellow amateur scientist Napoleon III -- that Foucault swung a pendulum and demonstrated an extraordinary truth about the world: that it turns on its axis.