Educated in the capital cities of Luoyang and Chang’an, he achieved success as an astronomer, mathematician, seismologist, hydraulic engineer, inventor, geographer, cartographer, ethnographer, artist, poet, philosopher, politician, and literary scholar.
Born in the town of Xi’s in Nanyang Commandery (north of the modern Nanyang City in Henan province), Zhang Heng came from a distinguished but not very affluent family. In 112, Zhang was summoned to the court of Emperor An (r. 106–125), who had heard of his expertise in mathematics.
Structure of the Ancient Seismoscope
The seismoscope invented by Zhang Heng was cylindrical in shape. It had eight dragons’ heads arranged around its upper circumference, each with a ball in its mouth.
Below there were eight frogs, each directly under a dragon’s head. When an earthquake occurred, a ball dropped and was caught by a frog’s mouth, generating a sound.
How did the Seismoscope function?
With the incoming of a seismic wave, the ancient seismoscope would sense it and one of the balls would drop. Along with it, the sound generated by the falling of the ball would alert observers to the earthquake, giving a rough indication of the earthquake’s direction of origin.
Ancient texts describe the great accuracy of this device, however, the exact design of the instrument has remained a mystery. The physical appearance of the device on the outer part and its purpose is precisely known, but its inner workings remain open to speculation. Certain replicas have been erected from time to time on the best guesses from such speculation.
The Chinese were concerned about the destructive force of earthquakes since the earliest. Similar accounts have been mentioned in Sima Qian’s Records of the Grand Historian of 91 BC. The account narrates the destruction caused by a catastrophic earthquake that happened in 780 BC. The Earthquake was powerful enough to divert the courses of three rivers.
The scientific reason that causes an Earthquake which is the movements of tectonic plates in the Earth’s crust was not known to human civilization. The people of the ancient Zhou Dynasty explained them as disturbances with cosmic yin and yang, along with the heavens’ displeasure with acts committed (or the common peoples’ grievances ignored) by the currently ruling dynasty.
The first seismoscope was presented by Zhang Heng at the Han court in 132 AD. The invention of the seismograph was believed to be one of his most impressive inventions by historians.
It was named “earthquake weathervane” (houfeng didongyi) instrument for measuring the seasonal winds and the movements of the Earth), and it was able to roughly determine the direction (out of eight directions) from where the earthquake actually originated.
A famous historian, Fan Ye in the 5th century compiled the works of the Later Han dynasty. According to the book, the bronze urn-shaped device, with a swinging pendulum inside, was capable enough to determine the direction of an earthquake from hundreds of miles/kilometers away.
The device recorded its first-ever seismoscope that supposedly occurred somewhere in the east. A few days after the occurrence, a rider from the east reported this earthquake.
Scientists in Zhengzhou district, China built a replica of Zhang’s seismoscope in 2005. Using the replica, the scientists estimated the content of the inner mechanism by using technology that was available during the great inventor’s time.
The discovered seismoscope was used to detect simulated earthquakes based on waves from four different real-life earthquakes in China and Vietnam. Interestingly, the data gathered from the tests corresponded accurately with that gathered by modern-day seismoscopes.
However, there has been some scholarly disagreement about the exact scientific principles applied to the seismograph and how precisely the instrument originally worked.
