In 132, Zhang Heng, a famous astronomer in the Eastern Han Dynasty (25-220), invented the world's first earthquake-observation instrument, the urn-like Houfeng Seismograph that was made of fine copper with a central pendulum.

According to the principle of inertia, an Earth tremor would cause the pendulum to lose balance, activating a set of levers inside. Then, one of the eight dragons placed in eight directions outside the urn would release a bronze ball held in its mouth. The ball would fall into the mouth of a toad and give off a sound, informing people of the time and direction of an earthquake.

Author: Jessie