The History of China : Ancient Seismograph

This is the world's first seismograph. It was invented by Zhang Heng and its name is Didong Yi. Today only replica of it remains.

First, I want to tell you about the principle of the seismograph. We have to fix the pen on the pendulum which hangs in the air. Then we should put the paper(rotating drum) under the pen. If the earthquake occurs, the pen doesn't move but rotating drum moves. So the pen can record a seismic wave.

Second, I want to talk about its principle. It was an urn-like instrument with a central pendulum. An earth tremor would cause the pendulum to loose balance and activate a set of levers inside. Then, one of the eight dragons which are placed in eight directions outside the urn would release a bronze ball in its mouth. The ball would fall into the mouth of a toad and give off a sound. So people can notice that the earthquake will happen soon.

Third, why did Zhang Heng invent this seismography? At that time, people believed earthquake as signs of heaven. So, person who expect the earthquake rightly, he can get immortal fame.

Sources : http://100.naver.com/100.nhn?docid=142932

http://www.china.org.cn/english/scitech/131762.htm

http://earthquake.usgs.gov/learn/glossary/?termID=167