參考答案:

During the Eastern Han period，earthquakes occurred frequently. Through a series of careful observations of earthquakes，Zhang Heng invented the seismograph ,in 132 AD, which was the world's first instrument to identify and ascertain the direction of earthquakes. The seismograph is cast with eight dragons on the surface,and each dragon has a copper ball in the mouth. On the ground below the dragons there are eight copper toads raising their heads and opening their mouths opposite to the dragons'mouths. When an earthquake occurred, the dragon facing the direction of the earthquake would open its mouth，and the ball in its mouth would fall into the toad's mouth, automatically indicating the direction of the earthquake.

來源:可可英語 http://www.ccdyzl.cn/menu/201405/302144.shtml