张衡和地动仪 Zhang Heng and the Seismograph
In the Exhibition Hall of the Museum of Chinese History in Beijing, there is a restored model of the first seismograph. The inventor was Zhang Heng, a famous scientist in the Eastern Han Dynasty (25-220 AD).
Zhang Heng (78-140 AD) was from Nanyang 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. It was made of fine copper, and looked like a big cup with a lid. 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 raising their heads and opening their mouths opposite the dragons'mouths. The inner side of the seismograph was ingeniously constructed: when an earthquake occurred, the dragon facing that direction would open its mouth, and the ball would fall into the toad's mouth, automatically indicating the direction of the earthquake. One day in 138 AD, the dragon in the west expelled its ball. As expected, an earthquake had occurred that day in Longxi (present-day Western Gansu Province) a thousand kilometers away. It was the first time that mankind had used an instrument to detect an earthquake. It was over 1,700 years later that a similar instrument was invented in Europe.
Zhang Heng also made the first water-driven celestial globe in the world to measure the position of celestial bodies, which was carved with known important astronomical phenomena. People could observe the movement of the sun, moon and stars. Zhang Heng was also a mechanical engineer, and made a flying "wooden eagle"and a "mileage-counting drum-cart".
People highly esteem Zhang Heng, a great scientist living more than 1,800 years ago, and they often hold commemorative activities to show respect for him. A ring of hills on the moon was named after him.