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. He gave 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 toads 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 you could use 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 direction 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 of 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(500 meters) to the west in western Gansu and had caused landslides. By this lime, 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.