di dong yi 地动仪 Earthquake Detector
Mikayla Lewis, Erika Rodriguez P.5
Seismograph Background Information:
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.
One day in 138 AD, the dragon facing 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 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.
In comparison, it wasn’t until the 18th century (AD), about seventeen hundred years later, that there was any record that Western scientists even worked on developing a seismograph.