Zhang Heng was a Chinese mathematician, astronomer and geographer living in one to two centuries in eastern Han Dynasty. At that time the capital Luoyang in the present day Henan province was often haunted by earthquakes. The King levied more taxes to be used in ceremonies to show respect to heaven. But Zhang Heng the scientist believed it was a natural aspect and began research on it.

In the year 132, Zhang Heng invented the seismoscope for measuring earthquakes. A similar device did not reach Europe until sixteen hundred years later, when a seismoscope was "invented" again in France. His device was made of bronze in the shape of an urn eight feet in diameter. Inside the seismoscope is a copper pendulum shaft connected to eight copper arms.  Outside the device are eight dragon heads around the top, each with a ball in its mouth. Around the bottom, eight frogs squat with open mouths, each directly under a dragon head. When an earthquake occurred, a ball fell out of a the dragon's mouth into a frog's mouth, making a noise that was supposedly loud enough to wake the Emperor's household, alerting them of the earthquake. Then, all anyone had to do was look to see which ball had dropped to determine which direction the earthquake was coming from.

In the following four years, the instrument correctly detected every earthquakes occurred in Luoyang. Only once in 138 A.D., the dragon facing west had dropped its copper ball but nothing unusual occurred around Louyang on that particular day. People became suspicious of the effectiveness of the gadget. But four-days later, report came from Gansu, a province west of Luoyang that a violent earthquake had struck there.

This anecdote finally convinced people that the seismograph was not only very sensitive, but also accurate.