The Invention of the Seismoscope

There are few things more disconcerting than the sensation of the seemingly-solid Earth suddenly rolling and pitching beneath one's feet. As a result, humans have sought ways to measure or even predict earthquakes for thousands of years.

Although we still can't accurately predict earthquakes, humans have come a long way in detecting, recording, and measuring seismic shocks. This process began nearly 2000 years ago, with the invention of the first seismoscope in China.

The First Seismoscope

In 132 CE, inventor, Imperial Historian, and Royal Astronomer Zhang Heng displayed his amazing earthquake-detection machine, or seismoscope, at the court of the Han Dynasty. Zhang's seismoscope was a giant bronze vessel, resembling a barrel almost 6 feet in diameter. Eight dragons snaked face-down along the outside of the barrel, marking the primary compass directions. In each dragon's mouth was a small bronze ball. Beneath the dragons sat eight bronze toads, with their broad mouths gaping to receive the balls.

We don't know exactly what the first seismoscope looked like. Descriptions from the time give us an idea about the size of the instrument and the mechanisms that made it work. Some sources also note that the outside of the seismoscope's body was beautifully engraved with mountains, birds, tortoises, and other animals, but the original source of this information is difficult to trace.

The exact mechanism that caused a ball to drop in the event of an earthquake also is not known. One theory is that a thin stick was set loosely down the center of the barrel. An earthquake would cause the stick to topple over in the direction of the seismic shock, triggering one of the dragons to open its mouth and release the bronze ball.

Another theory posits that a baton was suspended from the lid of the instrument as a free-swinging pendulum. When the pendulum swung widely enough to strike the side of the barrel, it would cause the closest dragon to release its ball. The sound of the ball striking the toad's mouth would alert observers to the earthquake. This would give a rough indication of the earthquake's direction of origin, but it did not provide any information about the intensity of the tremors.

Proof of Concept

Zhang's wonderful machine was called houfeng didong yi, meaning "an instrument for measuring the winds and the movements of the Earth." In earthquake-prone China, this was an important invention. 

In one instance, just six years after the device was invented, a large quake estimated at a magnitude seven struck what is now Gansu Province. People in the Han Dynasty's capital city of Luoyang, 1,000 miles away, did not feel the shock. However, the seismoscope alerted the emperor's government to the fact that a quake had struck somewhere to the west. This is the first known instance of scientific equipment detecting an earthquake that had not been felt by humans in the area. The seismoscope's findings were confirmed several days later when messengers arrived in Luoyang to report a major earthquake in Gansu.

Chinese Seismoscopes on the Silk Road?

Chinese records indicate that other inventors and tinkerers in the court improved upon Zhang Heng's design for the seismoscope over the centuries that followed. The idea seems to have spread westward across Asia, probably carried along the Silk Road. 

By the 13th century, a similar seismoscope was in use in Persia, although the historical record does not provide a clear link between the Chinese and Persian devices. It is possible that the great thinkers of Persia hit upon a similar idea independently.