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| Detail of linocut 'Zhang Heng' by Ele Willoughby, 2023 on 9" x 12" washi paper. |
This is a linocut print about the ancient Chinese Han Dynasty polymath and statesman Zhang Heng (78-139) who invented a device (a seismoscope, like a simplified seismometer which does not make a record of earth motions) to detect distant earthquakes and indicate their direction, 2000 years ago! I have shown him in blue with a reconstruction of his seismoscope, and a schematic of how it might have worked in bronze, as well as horizontal earthquake surface waves, and Rayleigh waves in particular, in pale pink.
A career civil servant in Nanyang, Zhang Heng (sometimes formerly written Chang Heng) was also an astronomer, mathematician, seismologist, hydraulic engineer, inventor, geographer, cartographer, ethnographer, artist, poet, philosopher, politician, and literary scholar. He was a bit of a controversial figure politically, sparing over calendar reform and with rivals amongst the palace eunuchs. But both his poetry and famous inventions are still remembered. He also improved the Chinese approximation for π and made an extensive star catalog. He understood that the Sun and Moon are spherical, and that the Moon merely reflects the light of the Sun. He also explained the nature of solar and lunar eclipses. He invented the world's first water-powered armillary sphere for astronomical observation; improved the inflow water clock by adding another tank; and, as celebrated here, he invented the world's first seismoscope, which recorded distant earthquakes and their origin (in terms of 8 cardinal directions).
China is a seismically active place, and while the cause of earthquakes remained misunderstood, in 132 Zhang Heng was able to design a device to detect seismicity from distant sources. It was named "earthquake weathervane" (hòufēng dìdòngyí 候風地動儀), and it could roughly indicate where the earthquake came from. According to the Book of Later Han (compiled by Fan Ye in the 5th century), his bronze urn-shaped device, with a swinging pendulum inside, was able to detect the direction of an earthquake hundreds of miles/kilometers away. The outside of the device was described as having 8 dragons with balls in their mouths and 8 open-mouthed frogs around the base which could catch fallen balls (and indicate direction to the source). If there was an earthquake the dragon facing its location would drop a ball into the mouth of a frog below. The Book of Later Han claims that the device was triggered by an event, which was too subtle for people to feel but that the west-facing dragon drop its ball. Officials doubted the device worked as intended, but several days later a messenger arrived from the west and reported that an earthquake had occurred in Longxi (modern Gansu Province). So, the court acknowledged it in fact worked.
Unfortunately, no ancient Chinese seismoscopes have survived and details of the mechanism are sparse. The description of the detected earthquake was written much later. So, we cannot be certain about how it worked precisely; some even doubt that it did. Later Chinese inventors were not able to reconstruct the device. However, a series of modern seismologists have put forward a series of reconstructions. There are several ways a pendulum could trigger a ball to fall. Some of the questions include: was it a regular pendulum? Was it an inverted pendulum? How was its motion transferred to the appropriate dragon and not to any other dragons? How it avoid "false positives" due to other sources of shaking?
As a geophysicist myself, I find the contemporary reconstructions of Feng Rui and others pretty convincing, so that's what I have illustrated. These scientist argue that the device would have detected horizontal motions due to surface waves which would only be due to earthquakes, and would not be set off by vertical motions (which can be caused by earthquakes or nearby shaking, say, due to people). So they built a reconstruction which they argue is consistent with the description, but detects Rayleigh surface waves. They argue by adding a second ball inside the device, it could have avoided having two opposing dragons triggered. In their model, illustrated in my print, when there is an incoming wave, for instance, from the west, the pendulum would move from west to east. They made a hollow inside, so the pendulum would drop a ball, falling on the west side as it moves off-centre. The ball follows one of 8 radiating tracks, then pushes a lever connected to the dragon mouth and the west-side ball - and no others - would fall. This would correctly identify the direction.
They also make arguments explaining that some reconstructions are not the right style of urn or dragon, arguing that Han Dynasty dragons would have been much simpler than the fancy Ming Dynasty ones shown on some reconstructions. So my illustration tries to respect the archeology of ancient Han artifacts, as well as a mechanism which apparently avoids the pitfalls of previous reconstructions. I also included a waveform, which seismologists will recognize as a horizontal Rayleigh wave (detected by a modern seismograph).
Sources
Zhang Heng, wikipedia, accessed January 2023
Feng, Rui and Yu Yan-xiang, Zhang Heng's Seismometer and Long earthquake in AD 134, Acta Seismologica Sinica, 19, 704-719 (2006)
Feng, R., Wu, Y. Research on history of Chinese seismology. Earthq Sci 23, 243-257 (2010). https://doi.org/10.1007/s11589-010-0720-z
Hong-Sen Yan, Kuo-Hung Hsiao, Reconstruction design of the lost seismoscope of ancient China, Mechanism and Machine Theory, Volume 42, Issue 12, 2007, Pages 1601-1617, ISSN 0094-114X, https://doi.org/10.1016/j.mechmachtheory.2007.01.003.
Zhang Heng Seismoscope, Atlas Obscura
Jamie Rigg, The ancient earthquake detector that puzzled modern historians, engaged, September 28, 2018
Andrew Robinson, The world's first seismometer used a toad to catch an earthquake, New Scientist, 30 November 2016
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