Nearly 1,900 years ago, a Chinese scientist built a device capable of detecting earthquakes that people nearby couldn't necessarily feel.

In 132 CE, during the Eastern Han dynasty, astronomer and engineer Zhang Heng created the first known seismoscope, an instrument designed to detect earthquakes and indicate the direction from which the seismic disturbance had come.

His device was a large bronze vessel surrounded by eight dragons, each holding a bronze ball in its mouth. Below each dragon sat a bronze toad. According to ancient Chinese accounts, when the instrument detected an earthquake, a ball dropped from one of the dragons into the mouth of the toad below, indicating the direction of the disturbance.

One account describes an especially impressive test. Zhang Heng's instrument released a ball even though no earthquake had been felt in the capital, Luoyang. Several days later, a messenger reportedly arrived with news of an earthquake to the west, in the direction the instrument had indicated.

Zhang Heng's original device has not survived, and ancient descriptions don't reveal enough about its internal workings for researchers to know exactly how it functioned. Modern versions are reconstructions.

What is clear is that by the second century CE, Chinese scientists understood that earthquakes produced physical motion that could be detected mechanically, and Zhang Heng had devised an instrument to do it more than 1,700 years before the development of modern seismographs.