In 132 CE, the Chinese polymath Zhang Heng invented the world's first seismoscope, a device designed to detect distant earthquakes. The device was a large bronze urn with eight dragon heads pointing in different cardinal directions. Each dragon held a bronze ball in its mouth. Directly below each dragon sat a bronze toad with an open mouth. Inside the urn, a sensitive pendulum mechanism was designed so that when a tremor occurred, it would trigger a lever, causing the ball to drop from the dragon's mouth into the toad's mouth. The direction of the earthquake was indicated by which ball fell. On one occasion, a ball fell even though no tremor was felt in the capital; days later, a messenger arrived to report an earthquake in the exact direction indicated. It was a masterpiece of ancient mechanical engineering.