Houfeng Didong Yi

The Chinese have always had a concern for earthquakes but did not understand that it was caused by the shifting of tectonic plates. Instead, popular theory at the time suggested that they were caused by disturbances with cosmic yin and yang, and as punishment given by the heavens. There was a group of scholars that also believed earthquakes were caused by the wind and air. They theorized that earthquakes would occur when wind and air becomes trapped in a mountain or cave.

Among this group of scholars there was Zhang Heng, an astronomer, mathematician, inventor, geographer, cartographer, artist, poet, and literary scholar. His most impressive invention was the Houfeng Didong Yi, the world’s first seismometer. This powerful tool could accurately tell from which direction earthquakes were occurring from up to 700 kilometers away which was vital for quickly sending aid to devastated towns. The seismometer would indicated the direction from which an earthquake was happening by dropping a ball from one of the eight dragon shaped arms, into the mouth of one of the toads, each representing a direction. Within the device there is a pendulum that acts as the motion sensor, when activated it in turn trips a crank that opens the upper jaw of the dragon head and releases the ball in its mouth.

In later dynasties the Chinese were able to reinvent the seismometer with the help of mathematician and surveyor, Xindu Fang. Unfortunately during the Yuan Dynasty they decided not to preserve the technology for this device, claiming that all detailing and instruction were no longer to be found. Similar devices were later seen in Japan. These devices were also pendulum activated but used pins to poke holes in paper to indicate direction.

Nick Choi