In 132,Zhang Heng, a famous astronomer in the EasternHan 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 eightdragons placed in eight directions outside the urn would release abronzeball 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
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