The earthquake sensor was cast in bronze, in the shape of a barrel about 2.5 meters in diameter. Eight bronze dragons circled around the barrel, their heads aligned respectively to the east, south, west, north, northeast, northwest, southeast and southwest. Each of the dragons' mouths had a ball, which dropped into the yawning mouth of a bronze frog when an earthquake occurred, somewhere in the direction the dragon's head was aligned, so that people observing the machine knew where the seismic waves were originating from. This allowed ancient earthquake scientists to register earthquakes, as well as helping the government to locate the area of the quake so that relief could be delivered there.
Zhang Heng's earthquake sensor was put into use and proved reliable. In 138, for instance, when an earthquake shook the southeastern part of Gansu Province, the sensor, placed over 500 kilometers away in Luoyang, responded, and sent the dragon facing west to drop its ball into the corresponding frog. The researchers were skeptical because they themselves had felt nothing. Everybody was most surprised when reports of an earthquake came in from Gansu a few days later.
Zhang Heng was promoted for having built the earthquake sensor. Yet despite an increased opportunities to be heard by Emperor Shun Di (r. 126- 44), a Taoist worldview made Zhang see potential angers in what was usually seen as success and favor; thus he shied away from such worldly temptations. However, his upwardly mobile career ad already made the eunuchs jealous and desirous f removing him from the emperor's favor. As an old man, he asked the emperor, but without success, for permission to retire so that he could spend his remaining years in his hometown. In the end, Zhang Heng was not allowed to return home until after he died, and was buried in the land of his birth.
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