Zhang Heng: the inventor of the seismoscope, detecting earthquakes in first-century China

It’s been a while since we looked at a Chinese engineer and longer since we looked at one only paid in bread, so I thought we’d start the New Year with one born nearly 2,000 years ago, in the Eastern Han dynasty.

Being an engineer in Han China wasn’t particularly easy and Zhang Heng’s firm scientific principles made it even harder.

Firstly, climbing the Imperial greasy pole in first-century China required many more skills than simply engineering. Potential members of the Imperial secretariat were expected to be cultured and artistic. Before his summons to serve the Emperor An, Zhang had done a stint composing official rhapsodies on capital cities and dirges for the governor. When, much later in life, he was slandered by the court eunuchs, who he thought were corrupt, he responded not with a writ but with an exemplary fu rhapsody called ‘Fu on Pondering the Mystery’. How many modern engineers can turn their hands to that?

Yet Zhang’s real genius was mechanical and his mathematical brilliance is what brought him to the emperor’s attention. He soon rose to the office of Chief Astronomer, but this simply got him into trouble.

When the somewhat better-placed court official Dan Song suggested reforming the Chinese calendar based on what Zhang considered mathematically unsound teachings, he complained. In fact he demanded that the apocryphal teachings they were based on should be banned. This was bit too much for the Emperor, who rejected the calendrical change, but refused to ban the writings.

With few allies at court, it soon became clear that Zhang’s career was likely to stall. He did manage to get himself reappointed as Chief Astronomer under the Emperor Shun, but was paid just 600 bushels of grain, which to be fair he could also take in coin or silk. This might sound like a lot of bread – it’s over 16 litres of grain a day – but it was in truth the very lowest pay an emperor could directly award to one of his own staff.

Clearly, Zhang had to do something to demonstrate his value. The opportunity came with earthquakes. Earthquakes were and remain a major problem in China, and in case of disaster, the Han dynasty needed to know where they had struck in their vast empire so resources could be directed to the right place. This was made even more important by the fact that earthquakes themselves were considered to show the gods were displeased with the administration of the country and perhaps even the emperor himself.

Zhang’s answer to this problem was a magnificent piece of instrumentation – the very first seismoscope. Exactly how this worked remains somewhat contentious as the only description we have is in the 196-character-long ‘Zhang Heng’s biography’ in the ‘Book of Later Han’, which interestingly also contains the earliest Chinese mention of the Roman empire.

Reconstructions suggest the device consisted of a central pendulum in a bronze vessel gently resting on a bronze ball from which eight tracks emerged to the eight cardinal points. When the horizontal movement of an earthquake was registered the pendulum would swing, pushing the bronze ball down the channel in the direction closest to the direction of origin of the tremor. This alone would have been sufficient but the extant description suggests that the rolling ball then triggered a lever leading to one of eight dragon heads moulded on the sides of the vessel at the cardinal points. Depressing the lever caused the mouth of the dragon to open, making it drop another ball held in its jaws. This then fell into a the mouth of a bronze toad placed beneath the head. So the appearance of a ball in a toad’s mouth indicated the direction of an earthquake, making it both a delicate scientific instrument and a thing of beauty. It’s a shame modern seismological equipment has so few dragons and toads on it.

Of course making the device was one thing but proving it worked was quite another. According to the ‘Book of Later Han’, when the device was triggered court officials ridiculed Zhang as not a single tremor had been felt. But Zhang Heng was a patient man and, sure enough, several days later an Imperial messenger arrived to say there had been a major earthquake around 450km away in Gansu province – exactly the direction the seismoscope had indicated. A year later Zhang was promoted to Palace Attendant and his salary nearly quadrupled. He’d finally earned his bread.