An attempt at a working replica (Science & Society Picture Library via Getty)

Zhang Heng's invention was initially met with skepticism. His views on certain topics made him unpopular with some of his peers, and worse yet, his seismoscope was unproven. But as the story goes, several years later, a ball finally dropped. This in and of itself wasn't evidence it worked, of course -- no earthquake had been felt in the capital -- but days later a messenger arrived to report of a serious earthquake that originated hundreds of miles away, off to the west of Luoyang in the direction the now empty dragon mouth faced.

Zhang Heng called his seismoscope Houfeng Didong Yi, meaning an "instrument for measuring the seasonal winds and the movements of the Earth." While many people of his time believed earthquakes had spiritual catalysts, he and a collection of other scholars were of the opinion the events were caused by winds and changes in air pressure -- bearing in mind this was way, way before the seed of plate tectonic theory sprouted in the early 20th century.

In the centuries after Zhang Heng's death, other Chinese intellectuals were said have created successor seismoscopes of his design. Since nothing tangible survived the passage of time, however, historians of our era have struggled to reconcile these centuries-old accounts with a working replica of Zhang's device. Some even speculated it never existed. While the ornate nature of the seismoscope was well described, the exact mechanisms driving it weren't. Attempts to reinvent it in the 19th and 20th centuries proved unsuccessful. It remained unclear, for example, how an ancient pendulum design could be sensitive enough to detect earthquakes hundreds of miles away. Furthermore, how could the movement trigger just one mechanism and spare the others?

In 2005, a group of seismologists and archaeologists from the Chinese Academy of Sciences announced they had created a proven, functioning replica. In their version, the pendulum itself doesn't interact with any levers; instead, it's suspended above another ball perched atop a thin pedestal. When the pendulum swings, it nudges that central ball down one of eight channels, where it hits a trigger system that animates the external dragon mouth. We don't know this is exactly how Zhang's model worked, of course, but it shows that only a minor reinterpretation of how the seismoscope is described in historical texts can lead to the creation of a sensitive, direction-aware device.

Even though Zhang's device is nearly two millennia old, the working principle behind it is still commonly used today. A popular form of modern seismograph uses exactly the same properties of inertia, whereby a static base and hanging pendulum move independently of each other when the ground shakes. Only nowadays the pendulum is a magnet, and the induced current its swinging produces in the conductive base is the record. Converting the fluctuations into a digital signal allows such seismographs to document tremor intensity, their timespan, etc. Zhang's invention noted the simple occurrence of an earthquake, hence the name seismoscope.