Zhang Heng and His Seismograph
Astronomers in ancient China created a large number of devices for astronomical observations, and many of them could truly and vividly present the changes in the positions of celestial bodies.
The celestial equator is a great circle on the celestial sphere in the same plane as the earth's
equator is, and the ecliptic is the apparent great-circle annual path of the sun in the celestial sphere, as seen from the earth. The ecliptic plane intersects with the celestial equator at a deflection of 23.26~. The sun meets the celestial equator on the two solar terms of vernal equinox and autumnal equinox. The devices made by ancient Chinese astronomers were mainly used to show the two coordinate systems: the equatorial system and the ecliptic system, and their conversion. Zhang Heng (78-139), a noted astronomer of the Eastern Han Dynasty, was an expert at making such devices. One of the devices made by Zhang Heng is the hun xiang, a kind of celestial globe. In the hun xiang, Water from a clepsydra drives a number of gears so that the globe turns on its axis in tempo with the movement of the earth. The changes in the celestial sphere are thus showed accurately and vividly. Before that astronomers in ancient China had created other similar devices, such as hun tian yi, jian yi, yang yi, and gui yi. But all these devices including the hun xiang are quite complicated to explain in simple words. In comparison, the principle of the di dong yi or seismograph invented by Zhang Heng might be easier to understand.
Shocked by the catastrophes caused by earthquakes, Zhang Heng intended to create such a device that it could both report earthquakes that occurred in remote places and forecast major earthquakes by detecting preceding minor tremors. As a legend goes, Zhang Heng was once riding in a wagon, and his wagoner stopped the horses abruptly in an emergency. Due to the force of inertia, Zhang was almost thrown out of the wagon. This experience gave him an inspiration: As an earthquake took place in an instant, an object would be shocked by its inertia; if this force was recorded, wasn't it possible to report and forecast earthquakes?
The seismograph invented by Zhang Heng had a huge copper body - about eight feet in diameter - roughly the shape of a barrel, and on the barred were eight dragons with their heads downward, pointing to eight directions: east, west, north, south, northeast, southeast, northwest, and southwest. In the mouth of each dragon was a copper ball, and right under the dragon head was a copper toad, with its mouth wide open. Inside the barrel was a complicated mechanism, of which historical records gave little details. Researchers believe there must be a suspended pendulum connected with eight levers, which controlled the mouths of the dragons. At normal times the whole mechanism was in a state of unstable equilibrium. When a quake took place, the tremor wave would break the balance, and a lever in the direction of the quake would be activated to open the mouth of a dragon. The copper ball inside the dragon mouth would drop into the mouth of the toad right under it, and observers would then hear the sound and know that a quake had occurred. The device was so smartly designed that only one dragon mouth would open when a quake took place, while the rest seven would remain shut.
The seismograph was set up in Luoyang, the capital of Eastern Han Empire. It had reported accurately several earthquakes, but in 138, the eighth year of the reign of Emperor Shundi, something strange took place. One day, the dragon that faced west opened its mouth, and the copper ball dropped into the toad mouth, but no one in the capital felt any tremor. However, several days later reports came from Longxi, several hundred miles west of the capital, saying there had been a quake. This event amplified people's admiration of the magic device. From then on, historians were ordered to record earthquakes in accordance with the reactions of the seismograph. Outside China, a similar device was made in Persia in the 13th century, or more than 1,000 years after Zhang Heng invented his seismograph. The principle of Zhang's invention is still in use today in modern seismographs.
The di dong yi was so smart and impressive that it has become a symbol of the achievements of science and technology in ancient China. Its inventor Zhang Heng, as a great astronomer and writer, has been remembered from generation to generation. |
