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The seismograph was made of fine copper, and was an urn-like instrument with a central pendulum. The instrument was cast with eight dragons on the surface which heads pointed in eight directions -east, south, west, north, southeast, northeast, southwest, and northwest, each one holding a copper ball in its mouth. Below the dragons were eight copper toads with raised heads and open mouths opposite the dragons' mouths.
When an earthquake occurred, an earth tremor would cause the pendulum to lose balance and activate a set of levers inside. Then, one of the eight dragons outside the urn would release the bronze ball held in its mouth. The ball would fall into the mouth of the toad and give off a sound, letting people know when and in which direction an earthquake had occurred.
In 132 AD, Zhang Heng, a great scientist in the Eastern Han Dynasty, invented the seismograph - the earliest instrument in the world for forecasting and reporting the movement of an earthquake. It is one of the most brilliant achievements in ancient China. The seismograph is decorated with tortoises, birds, dragons, toads and other animal images, which makes it unique and highly enjoyable.
In ancient Chinese philosophy, the dragon symbolizes
Yang, while the toad symbolizes Yin. Thus, it constitutes the dialectic relationship between Yin and Yang, upwards and downwards, and movement and stillness. Meanwhile, it also reflects the dialectic thoughts in Book of Changes. Thus it is endowed with profound connotations.
The ancient Chinese believed seismic events, with their death and destruction, were
important signs from heaven. So it was important to the Chinese leaders to be alerted to
earthquakes occurring anywhere in the kingdom. A correct call about an earthquake
occurring won a person immortal fame, as in Chang Heng’s case. A wrong call about an
earthquake resulted in disgrace and punishment.