Ancient Seismometer

A seismometer -- an earthquake detector -- was invented in 132 A.D., by Chang Heng, a brilliant mathematician and scientist in the Han Dynasty.

The ancient Chinese believed seismic events, with their death and destruction, were important signs from heaven. So it was important to the Han Court be alerted to earthquakes occurring anywhere in the kingdom. A correct call wins one immortal fame, as in Chang Heng’s case, while disgrace and punishment awaits a wrong call.

What did the original seismometer look like?

Cheng Heng’s original seismometer was lost. Luckily, its description survived. It was a cast bronze vessel with a domed lid, resembling a wine jar. The working innards, of "toothed machinery and ingenious constructions," were hidden. The surface of the vessel was decorated with motifs of mountains, tortoises, birds, animals and antique writing. Around the vessel were eight dragons whose mouths held a ball. Round the base of the vessel sat eight corresponding toads with their mouths open, looking upwards.

The sign for an earthquake was the falling of a ball from the dragon into the mouth of a toad. The direction of the earthquake is indicated by the position of the toad which swallowed the ball. This instrument was most remarkable:

On one occasion one of the dragons let fall a ball from its mouth though no perceptible shock could be felt. All the scholars at the capital were astonished at this strange effect occurring without any evidence of an earthquake to cause it. But several days later a messenger arrived bringing news of an earthquake in Lung-Hsi (400 miles away). Upon this everyone admitted the mysterious power of the instrument.

The workings of Chang Heng’s seismometer were never revealed. Most experts agree however, that it worked on the principle of inertia. A mass is suspended. An earthquake shakes the vessel, causing a slight displacement between the mass and the vessel. This movement is transmitted via levers and gears to push out a ball.

In the design of any instrument, the goal is to make the instrument sensitive to the desired signal while simultaneously rejecting false signals. For a pendulum, this optimal point is at the resonance.

Our seismometer is intended as a demonstrator. The visitor shakes the table to simulate an earthquake. So our seismometer’s pendulum is "tuned" to this input. The crust of the earth absorbs the high frequency content of a quake. The signal from a distant earthquake is in the sub-audio range. So to detect actual earthquakes the pendulum would need to be several feet in length.

Indeed, we read that Chang Heng’s seismometer was described as having a diameter of "eight chhih"! (A little more than 6 ft.) His device appears to be -- incredibly -- "scientifically designed!" In 132 A.D., did he "really know" what we know today? Or did he hit on it by pure luck? Earthquakes occur so infrequently, how does one study it? Consider also the expense of making bronze castings the size of a horse in that age.

Another question you may want to ponder is why did Chang Heng take such pains to decorate his instrument with celestial motifs. What was the real purpose of these decorations? Do you think this helps the instrument to function better? Or did he hope this would make his device more believable?

Acknowledgment: The materials for this model were donated by and the model fabricated in the workshop of The Tech Museum of Innovation, San Jose.

Further Information: The following are available at the San Jose Library:

• Science and Civilization in China, by Joseph Needham and others, several volumes;
• The Shorter Science & Civilization in China, edited by Colin A. Ronan, two volumes;
• The Genius of China, by Robert Temple;
• Everyday Life in Early Imperial China by Michael Loewe.

Submitted by Stanley Hoo: Based on his exhibit at the 1999 Chinese Summer Festival.