In 132 AD he built the first seismoscope in history — a bronze vessel ringed with dragons that caught earthquakes a thousand kilometers away.
One day in 138 AD, a ball dropped from a dragon's mouth in the Han capital — and nobody in the city had felt a thing. The court laughed at Zhang Heng's bronze contraption. Days later, riders arrived from the west: an earthquake had struck a thousand kilometers away, in exactly the direction the dragon had pointed. Eighteen centuries before the seismograph, a Chinese scholar-poet had taught metal to listen to the earth.
