Revisiting the Ancient Chinese Seismograph
Zhang Heng’s Didong Yi
Zhang Heng, from Nanyang of Henan Province, was a studious inventor who was especially fond of astronomy, the 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. Contrary to popular belief at that time, Zhang Heng maintained that earthquakes were not signs of Heaven's anger but natural disasters.
The seismograph was made of fine copper, and was an urn-like instrument with a central pendulum. 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), each one holding a copper ball in its mouth. Below the dragons 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, an earth tremor would cause the pendulum to lose balance and activate a set of levers inside. Then, one of the eight dragons outside the urn would release the bronze ball held in its mouth. The ball would fall into the mouth of the toad and give off a sound, letting people know when and in which direction an earthquake had occurred.
Zhang Heng also made the first water-driven celestial globe to measure the position of celestial bodies, which was carved with 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 regard the great scientist Zhang Heng, often holding commemorative activities to show respect for him. A ring of hills on the moon was even named after him. Didong Yi replicas
The Didong Yi was first reconstructed by a Japanese scholar in 1875 based on the description of the device in Zhang's biography History of the Latter Han and archaeological research findings. The current well-known model was redesigned by noted Chinese museum researcher Wang Zhenduo in 1951. However, none of the replicas can detect an earthquake.
In 2005, Chinese seismologists and archeologists announced they created a new replica of Didong Yi, the world's first seismograph.
Seven scientists in seismology, archeology and mechanical engineering from the Chinese Academy of Sciences, the National Museum and the China Earthquake Administration confirmed that the replica was a "historic step" towards complete reconstruction.
"What we are exhibiting is a scientific device, not a toy," said Tian Kai, deputy curator of Henan Museum. "If we put a seismograph that is unable to move or detect on exhibition," Tian said, "we will not only deceive our audience, but also show our apathy and irresponsibility towards our nation's splendid cultural legacy."
"As a treasure of our Chinese nation, Didong Yi is an attractive goal for reconstruction to scientists around the globe," Teng said. "If we can't get the job done, it will be our fault."
However, there has been some scholarly disagreement about the exact scientific principles applied on the seismograph and how precisely the instrument originally worked.
Some foreign seismologists argue that if Zhang Heng's seismograph worked on principles of inertia, then two (not one) "pearls" should fall out from the dragon’s mouths situated on opposite sides of the device.
Others hold that all the replicas are just reconstructed from guesses and imagination rather than from actual knowledge as to how the real device looked like. A few Western scholars even contend Zhang Heng's device was lost because it was never a reality.
Feng Rui, a China Seismological Networks Center research fellow who heads the restoration team, believes he and his colleagues can testify the existence of Zhang Heng's seismograph through collection of historical data and simulated analysis.