Very practical but lost ancient crafts include seismograph invented by Zhang Heng, tenon and mortise craft, wooden ox and flowing horse invented by Zhuge Liang, etc. Among them, the seismograph invented by Zhang Heng showed brilliant achievements in the history of China, but now it has been lost. His invention was gradually replaced by new science. In the history of China, there are not only literati, but also military commanders. There have also been many scientists, and Zhang Heng is one of them. Zhang Heng invented the seismograph. Earthquakes often happened at that time, sometimes several times a year. Under the limited medical and economic conditions, earthquakes will bring great harm to the country and people. At that time, the rulers and people regarded the earthquake as an ominous sign and regarded it as a ghost.

Unfortunately, Zhang Heng is a ghost. After careful investigation and experiments, he invented an instrument that can detect earthquakes, called seismograph. Seismographs are made of bronze. It is shaped like a wine jar with eight faucets cast around it. The faucets face in eight directions respectively. Every dragon has a small copper ball in its mouth, and a toad with a big mouth is squatting under the faucet. No matter which direction an earthquake occurs, the dragon mouth in that direction will spit it out.

In AD 138, the seismograph suddenly spit out a copper ball facing the west Longkou, indicating that there was an earthquake in the west. However, there was no sign of an earthquake in Luoyang that day, let alone an earthquake nearby. Therefore, everyone in the imperial court thinks that Zhang Heng's seismograph is a deceptive fake. But a few days later, the news of the earthquake came. A great earthquake occurred in Jincheng and Longxi, more than 1000 miles away from Luoyang, and even the mountains collapsed. Later, some people thought that seismographs were widely used. Although Zhang Heng was not successful in politics, his invention left a glorious mark in the history of science in China.