CLANG! An ornate dragon’s head suddenly spat a metal ball into the mouth of a bronze toad crouching patiently beneath it, and officials at the court of the Han emperor were astonished.

Lying in Nanyang's Shiqiao town, the Grave of Zhang Heng is the resting place of the Eastern Han Dynasty (AD 25-220) polymath who achieved wide success as an astronomer, mathematician, inventor ...