The head of an intricately carved dragon atop a vessel resembling a vase suddenly released a metallic ball into the mouth of a bronze toad waiting below. This astonished the courtiers of the Han Emperor, as it appeared to be a warning of an impending earthquake somewhere in China. In the capital city of Luoyang, no tremors were recorded. However, just a few days later, a horse rider galloped in with important news: an earthquake had just devastated Longxi, a region located 700 kilometers from the capital. “That was the moment everyone had to acknowledge the latent power of this device,” they said.
From then on, officials working at the National Office of Astronomy and Calendar were instructed to pay attention to any movements of the aforementioned strange device – the world’s first seismograph. China had experienced earthquakes at least since 780 BCE, earlier than any other country in the world, and it was also the nation that had suffered the two most devastating earthquakes in history. Therefore, it is not surprising that a Chinese official, – Zhang Heng, was likely the first to create an earthquake forecasting device as early as 132 CE. The remarkable story of Zhang Heng is meticulously recounted in the Book of the Later Han, one of the most famous historical texts in China.
Now, perhaps it’s time to discuss this strange device. Shaped like a vase over 2 meters tall, the instrument was adorned with eight dragon heads facing the cardinal directions, with eight toads waiting below. If the dragon released the metallic ball it held in its mouth into the belly of any toad, an earthquake would occur in that direction. Zhang Heng referred to it as “Dìdòngyí”, meaning “the instrument for monitoring the winds and movements of the Earth.” The creation of this device stemmed from his belief that earthquakes were related to air movements, especially when storms encountered obstacles in their path, such as mountains.
However, the reason he decided to invent the earthquake forecasting device was that, at that time, people believed earthquakes only occurred when a certain official in the court committed a wrongdoing. “The heavens crack and the Earth trembles,” was one of many phrases Chinese people used when discussing politics and daily life. After an earthquake shook the capital, Zhang Heng reported to the emperor, describing the disaster as a divine reprimand for the failure of a new policy aimed at recruiting talented and virtuous individuals for various departments of the court. If the movements occurring beneath the Earth were omens of corruption, a tool to detect earthquakes would greatly assist wise emperors. However, the eunuchs in the court did not feel the same way.
Once, a young emperor reportedly summoned Zhang Heng and the eunuchs into a room, demanding that he name the hypocrites present. They glared at Zhang Heng in a threatening manner, forcing him to speak favorably about them. However, later on, due to multiple instances of exposing the eunuchs’ incompetence, they grew to hate and criticize Zhang Heng, leading to a failure in his career at court. Reflecting his struggles in palace life through poetry also helped him become one of the most famous poets in China.
About Zhang Heng
Zhang Heng (78 – 139 CE) was an astronomer, mathematician, geographer, and inventor who lived during the Han Dynasty. Thanks to his talent and diligent study and research, he became the most renowned astronomer in China at that time. In 116, when Zhang Heng was 38 years old, he was appointed Grand Historian, responsible for celestial observations at the court.
Zhang Heng is famous for inventing the first water-powered armillary sphere in the world (a working model to describe the movements of celestial bodies in the universe). He also achieved many accomplishments in improving water clocks and recorded 2,500 stars in a detailed star catalog.
Zhang Heng.
Before venturing into the officialdom, Zhang Heng spent at least several decades of his life living far from the capital. He was born in 78 to an educated but not wealthy family. In 112, after a period of relentless study, he was invited to the capital to hold an official position there. Due to his talent, Zhang Heng was subsequently promoted, during which he held a role akin to that of an astrologer. Inspired by the sky, he was the first Chinese person to clearly describe the Earth as spherical with an equator, and pointed out the differences in the Sun throughout the year.
In 134, Zhang Heng was granted the privilege of residing in the palace, allowing him to offer advice to the emperor. Although he was a rare talent, Zhang Heng’s attitude toward the eunuchs at that time caused his name and great inventions to never be recorded in history. He opposed the idea of reforming the Chinese calendar and compiling the history of the Han Dynasty based on unfounded teachings, similar to the prophecies of Nostradamus.
However, this idea was very popular with the king and courtiers. In 136, perhaps due to excessive political pressure, Zhang Heng left the capital, took on a minor official position, and by 138, he retired to live a peaceful life at home. Not long after, he was summoned back to the capital but passed away shortly thereafter – in 139.
Dìdòngyí
To this day, the seismograph invented by Zhang Heng remains a significant mystery, partly due to its disappearance without a trace, and partly because seismologists still do not agree on the internal mechanism of this device. The Book of the Later Han contains a passage about the dìdòngyí but describes very little about what is inside: “A cylinder located at the center capable of moving horizontally in eight directions to open or close the mouth of the dragon.”
According to historian Christopher Cullen, who studies ancient China, “Descriptions of the device provide us with enough details to lead to many reconstruction attempts, but they are still not clear enough to perform one accurately.” He believes that the cylinder described in Chinese historical texts must have been an extremely sensitive pendulum. However, there is not enough evidence to know how this mechanism could maintain sufficiently low mechanical friction to detect the extremely slight movements of the ground, beyond human perception.
Dìdòngyí.
Moreover, what we may never know is whether the earthquake detection abilities of the seismograph created by Zhang Heng were as accurate as recorded in the Book of the Later Han, which was completed around 440, a century after his death. Additionally, this discovery was not mentioned in earlier historical works of China. Cullen suspects the capabilities of the dìdòngyí were embellished by “fans” who were overly impressed by Zhang Heng’s mechanical talents.
Despite the numerous questions, this does not prevent experts from China, Japan, and several other countries from attempting to recreate the dìdòngyí. Wang Zhenduo – the head of the National Museum of China, has reconstructed the dìdòngyí twice: one completed in 1936 with a pendulum installed inside, and the other made in 1950 equipped with an inverted pendulum. Unfortunately, neither model was able to respond to real earthquakes, including the 1976 Tangshan earthquake that killed hundreds of thousands of people.
This earthquake also shocked Beijing, where the second reconstructed dìdòngyí by Wang was kept. About a decade ago, a group of scientists from the Chinese Academy of Sciences led by geophysicist Feng Rui developed and tested a seismograph model based on the movement of a pendulum. This product is currently preserved at the Beijing Museum of Science and Technology; however, it has not detected any earthquakes, even when a devastating earthquake struck Sichuan in 2008.
At the Natural History Museum in London (UK), there is also another dìdòngyí model, reconstructed based on Wang’s design, which was also used by the BBC for a broadcast in the 1970s. Currently, this model has yet to have the opportunity to “test its skills” due to the seismic stability in the UK. However, its popularity shows that Zhang Heng’s invention not only captivates the Chinese scientific community but also the global scientific community.
