Zhang Heng has done many things in his life, but his most famous invention is the "seismograph". During that period, earthquakes often occurred. Sometimes several times a year. A big earthquake will bring a lot of harm to the people and the country.

At that time, the emperor and ordinary people regarded the earthquake as an ominous sign and thought it was caused by ghosts and gods. However, Zhang Heng does not believe in evil spirits. After careful investigation and experiment of recorded earthquake phenomena, he invented an instrument that can detect earthquakes, called "seismograph".

The seismograph is made of bronze and looks like a jar. There are eight dragons around it, and the faucet extends in eight directions. Every dragon has a small copper ball in its mouth. There is a toad with a big mouth under the tap. When there is an earthquake in any direction, the dragon mouth in that direction will automatically open and spit out copper balls. The copper ball fell into toad's mouth and made a loud noise, telling people that there was an earthquake there.

One day in February 138, the seismograph suddenly opened its mouth to the west and spit out a copper ball. This is a report about the earthquake in the west. However, there was no sign of an earthquake in Luoyang that day, let alone what happened nearby. As a result, there was a lot of discussion in court, saying that Zhang Heng's seismograph was a lie. A few days later, someone rode a fast horse and reported to the court that there was a big earthquake in Jincheng and Longxi, more than 1000 miles away from Luoyang, and even the mountains collapsed. Guys, this is really convincing.

Although Zhang Heng was not successful politically, his scientific inventions and experiments left brilliant achievements in the history of science in China.