In theory this should rate as one of the biggest 24 hour clocks in the world:

It’s Foucault’s famous Pendulum, swinging away in the Pantheon, in Paris. As you can see by the markings, it can claim to be a sort of 24 hour clock. As the earth slowly rotates underneath the swinging pendulum, you can imagine the whole building – and the rest of Paris – moving slowly counterclockwise – or to the right in this picture – with the pendulum swinging freely to and fro in space.

The theory is simple, but in practice it doesn’t make a good clock. For one thing, Paris is perched on the side of the earth at latitude 50 degrees, rather than situated at the pole, and so the top of the pendulum moves around the earth’s axis as well. At this latitude, the pendulum will rotate through about 270 degrees in one day, rather than the whole 360. Also, the pendulum will gradually stop swinging due to air resistance and friction. There’s no electro-mechanical device in the ceiling that keeps it going, so it will slow down and eventually stop anyway. The attendant in the Pantheon usually grabs the bob and releases it from the right place at the start of the day.