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Zhang Heng  |
| CCTV.COM 2002-03-20 14:03:24 |
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 "Bamboo Book Annals", written in ancient times, recorded a strong earthquake happened in the 12 century BC in China. The Chinese people always wanted to observe this disastrous phenomenon of nature. Zhang Heng, a scientist of the Eastern Han period （25－220）, carried out scientific experiments and in 123 invented a seismograph, the first of its kind in the world, that was able to record earthquakes happened hundreds of kilometers away.
According to "Biography of Zhang Heng" in "History of Later Han", this seismograph had a diameter of 1.8 meters. Made of bronze it had 8 dragon figures facing 8 directions. There was mechanism in the head of each dragon. When seismic waves were felt, a rod in the center of the seismograph called "duzhu" would incline to the direction of the earthquake and trigger the lever in the dragon head, opening its mouth and releasing a bronze ball. The ball fell into the mouth of a toad, thus enabling people to know the time and direction of the quake.
Zhang Heng's invention came into being 1,700 years before that in Europe. Joseph Needham of Britain claimed that it was a doubtless fact that China was the earliest inventor of seismograph； and that it was a contribution of the distinguished mathematician, astrologer and geographer Zhang Heng.
 Zhang Heng grew up in a scholarly family. He was a literati and a man with rare interdisciplinary talents. He constructed the south－pointing carriage and the odometer. No matter which direction the south－pointing carriage moved, the wooden human figure standing in the carriage would always point to the south due to the gear system. As for the odometer, after each li it covered, the wooden human figure on it will strike a drum. It is quite similar to present－day taximeter.
This instrument in Beijing Ancient Observatory was constructed in the Qing Dynasty （1644－l9ll） in imitation of Zhang Heng's celestial globe.
According to the theory of sphere－heavens Zhang Heng constructed a celestial globe, which was a hollow ball covered with stars. His device was operated by water whose flow was regulated by a clepsydra. The daily revolution of the device corresponded to the position of stars in the celestial. Zhang Heng also discovered that the moon reflected the light of the sun, and that it eclipsed when sunlight was blocked. This was man's first explanation of lunar eclipse. Owing to his great astronomical achievement, the international astronomy organization decided to name a lunar crater at the back of the moon and a small planet of the No. 1802 in his name. This great scientist will live forever with the sun, the moon and the stars.
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