Hou Feng Di Dong Yi 候风地动仪

Seismograph(地震仪) is an instrument that makes a record of seismic waves caused by an earthquake, explosion, or other Earth-shaking phenomena. An early seismic instrument called Hou feng Di dong Yi ,was invented by Zhang Heng①, a famous scientist in the Eastern Han Dynasty. Zhang Heng (78-140 AD) was from Nan Yang of Henan Province. He studied diligently and was especially fond of astronomy, calendar, and mathematics.

In 132 AD, in the then national capital of Luoyang, Zhang Heng made the ancient seismograph to determine the direction of an earthquake. According to the History of the Later Han Dynasty, Zhang Heng’s seismograph, made of fine copper, was an urn-like instrument with a central pendulum. The instrument was cast with eight dragons on the surface (whose heads pointed in eight directions -- east, south, west, north, southeast, northeast, southwest, and northwest), and each dragon had a copper ball in the mouth. On the ground below the dragons, there were eight copper toads(蟾蜍) raising their heads and opening their mouths opposite to the dragons’ mouths. An earth tremor would cause the pendulum to lose balance and activate a set of levers inside. Then, one of the eight dragons would release a bronze ball held in its mouth. The ball would fall into the mouth of a toad and give off a sound in the meantime, letting people know when and in which direction an earthquake had occurred. 

It is recorded as detecting an earthquake in February in the year 138 about 600 kilometers away from Luoyang, then China’s capital, according to the History of the Later Han. It was over 1,700 years later that a similar instrument was invented in Europe.

People highly esteem Zhang Heng, and they often hold commemorative activities to show respect for him. A ring of hills on the moon was named after him.

 However, the original seismograph did not survive history. It was first reconstructed by a Japanese scholar in 1875 based on the description of the device in Zhang’s biography in the History of the Later Han and archaeological research findings. The currently well-known model was redesigned by noted Chinese museum researcher Wang Zhenduo in 1951. 

Abridged and revised from

http://www.britannica.com/EBchecked/topic/532934/seismogram

Notes:

① Zhang Heng: 张衡（南阳五圣之一，与司马相如、扬雄、班固并称汉赋四大家。中国东汉时期伟大的天文学家、数学家、发明家、地理学家、文学家，为中国天文学、机械技术、地震学的发展作出了杰出的贡献，发明了浑天仪、地动仪，是东汉中期浑天说的代表人物之一。