English: A replica of an ancient Chinese Seismograph from Eastern Han Dynasty (25-220 CE).
This picture is taken from an exhibition at Chabot Space & Science Center in Oakland, California.
The plaque of the exhibit said:
Detecting a Quake
In 132 CE, after several earthquakes in China, astronomer Zhang Heng invented this instrument to warn people of the next one. When the ground shook, it moved a pendulum inside the jug. The pendulum pushed a lever that opened one dragon's mouth. A ball rolled out and into the toad's mouth below, sounding an alarm. The open dragon mouth pointed in the direction of the earthquake, notifying the Emperor.