This was photographed in the main stairwell of the Franklin Institute in Philadelphia. I wrote down the description so I wouldn't forget: "In 1851, Jean-Bernard-Leon Foucault proved that the Earth turns using a pendulum like this one. At 9:30 the museum starts the pendulum swinging in a North-South direction. During the day the pendulum knocks down a peg every 20-25 minutes, making it appear to change direction. By 5:00 pm, half of the pegs have been knocked down."