Zhang Heng and his
Earthquake-Detector
During
the Eastern Han Dynasty, earthquakes were frequent in China. Sometimes
there was one each year, sometimes there might even be two, wreaking great
havoc on the country. Whenever a big earthquake occurred, it would take
a heavy toll of human and animal lives and destroy numerous houses, inflicting
heavy losses on the people. The feudal rulers of those days attributed
the disasters to the fury of gods and demons who had taken offence. In
pacifying them, they spread superstitious beliefs.
At that time, there was a man of science named Zhang Heng who was not
only a fine writer but also well versed in astronomy, calendar-making
and mathematics. To him we owe many scientific propositions, and the first
apparatus in the world for observing astronomical phenomena, the "Celestial
Globe".
Refusing to accept superstitious explanations of the earthquake , Zhang
Heng made careful observations and recorded the symptoms of disasters
in an attempt to diagnose the causes of earthquake by scientific means.
And he made up his mind to make a device with which to forecast earthquakes
so that he could refute superstitious nonsense with scientific findings.
Thanks to his painstaking research and repeated experiments, Zhang Heng
eventually succeeded in A. D. 132 in the invention of a seismograph with
which to determine the direction of an impending earthquake. He called
this the Di Dong Yi (Earth Motion Instrument).
The
urn-shaped, bronze seismograph which is eight Chinese feet in diameter
has a thick, copper-made pendulum shaft in the center to which are connected
eight thin copper arms. On the outside of the urn-shaped thing are eight
dragons facing in the eight directions of north, south, east, west, northeast,
southeast, northwest and southwest. In the slightly protruding mouths
of the dragons, which are connected with the eight arms, there is a small
copper ball, and under each dragon is a squatting copper toad with head
uplifted and mouth wide open, ready at any moment to catch the ball from
the dragon's mouth. The heads of the dragons and the toads have such an
animated look that it seems as if they were frolicking with each other.
Hence the use of the phrase "toads frolicking with dragons"
to describe the appearance of the seismograph. When a tremor occurs in
a certain direction, the copper arm pointing in that direc tion will tip
and the dragon head connected to it will open its mouth and drop the small
copper ball into that of the toad underneath, creating a clear ring and
thus letting people know the direction from which trouble comes.
One day in February, AD 138, Zhang Heng and several others found that
the dragon facing west had dropped its copper ball into the mouth of the
toad below. At this Zhang Heng made a report to the emperor saying that
an earthquake had happened somewhere to the west of the capital, Luoyang.
However, on that particular day, nothing unusual occurred in or around
Luoyang , nor was there any news about an earthquake elsewhere. This led
to various suspicions and accusations to the effect that Zhang Heng's
seismograph was but a fraud and that he himself was a liar.
However, within two days, several men riding on horseback were seen galloping
in the direction of Luoyang along the road that led west from the capital.
The message they brought to the emperor was that a violent earthquake
had taken place more than a thousand li to the west in western Gansu and
had caused landslides. By this time, people were fully convinced of the
effectiveness of the seismograph and spoke highly of Zhang Heng for his
expertise in science.
The invention of the seismograph by Zhang Heng made possible an accurate
recording of earthquakes in Chinese history. This invention, which pre-dated
that made in Europe by about 1700 years, was the first seismograph in
the world.