In Exhibition Hall of Museum of Chinese History in Beijing, there is a restored model of the first seismograph of the world, whose inventor is Zhang Heng, a famous scientist in Eastern Han Dynasty (25 - 220).

Zhang Heng (78 - 139) was from Nanyang in Henan province, He studied diligently, and was especially keen on astronomy, calendars and mathematics. As a whole, Zhang Heng can be regarded as an erudite and talented scientist.

In 132 AD, Zhang Heng made a seismograph named Houfeng used for measuring the direction of a certain earthquake in the capital city of Luoyang, It was made of fine copper, and looked like a big cup with a lip. The surface of the instrument was cast with eight dragons, whose bodies were downward vertically and heads pointed to eight directions (east, south, west, north, southeast, northeast, southwest and northwest), and each dragon had a copper ball in the mouth. On the ground vertically below the mouths of each dragons, there were eight copper toads raising their heads and opening their mouths. The inner side of seismograph was elaborately constructed. When an earthquake occurred in a place at a certain direction, to which the dragon pointed would open its mouth, and the ball would fall into the toad's mouth, automatically indicationg the earthquake occurring at that direction. One day in 138 AD, the dragon pointing to the west expelled its ball. As expected, an earthquake had occurred on that day in Longxi (now is Gansu province) a thousand miles away from Luoyang. It was the first time that mankind had used the instrument to sense an earthquake.

Zhang Heng also made the first water-driven armillary sphere in the world to measure the position of celetial bodies. All the important astronomical phenomena known to all had been carved on his instrument, which can be used for observation of the movement of the sun, the moon and stars. Meanwhile, Zhang Heng was also a mechanical engineer, who had made a mileage counting drum car.

Most people highly esteem Zhang Heng, and therefore they often hold commemorative activities to show respect for him. A crater on the moon was named after him.