How it began
Zhang Heng served the Eastern Han court as Chief Astronomer, in a first term from 115 to 120 and a second from 126 to 132. In 132 he presented to the court what many historians consider his most impressive invention, the first seismoscope. It was named the earthquake weathervane, and he claimed that it could detect the cardinal direction of a distant earthquake. He most likely worked with the professional casters and mold makers of the imperial workshops.
How it works
Britannica describes the instrument as apparently cylindrical, with eight dragons' heads arranged around its upper circumference, each holding a ball in its mouth. Below were eight frogs, each directly under a dragon's head. When an earthquake occurred, a ball dropped and was caught by a frog, showing the direction of the shock. Inside, eight mobile arms were connected with cranks and catch mechanisms. When tripped, a crank and right-angle lever raised a dragon head and released the ball. Reconstructions have used a pendulum as the central motion sensor.
What changed first
The device could roughly determine, out of eight directions, where an earthquake had come from. Earthquakes mattered politically in Han China. A year after Zhang presented the instrument, officials and candidates were asked to comment on a series of recent earthquakes, which could be interpreted as signs of displeasure from Heaven.
What lasted
A seismoscope makes no time record of ground oscillations but simply indicates that shaking has occurred, and Britannica's account of the development of the seismograph begins with Zhang Heng's instrument of 132. Later researchers repeatedly tried to rebuild it: a reconstruction of 1936 used a suspended pendulum, Akitsune Imamura and Hagiwara built one at Tokyo University in 1939, a 1963 model used an inverted pendulum, and in 2005 Chinese seismologists announced a working replica.
Where it stands today
The original instrument does not survive, and its internal mechanism is known only from descriptions and modern reconstructions. A model of the Zhang Heng seismoscope is held in the collection of the Science Museum Group in the United Kingdom, where its record notes that a ball falls from the dragon's teeth into the mouth of a toad below to record the event.
