Zhang Heng presents at the Han court the Houfeng Didong Yi ('instrument for measuring seasonal winds and movements of the Earth'), a bronze urn about 2 meters in diameter with eight dragons at the cardinal points. An internal pendulum mechanism detected seismic waves and dropped a bronze ball into the mouth of a frog beneath the corresponding dragon, indicating the earthquake's direction. In 138 AD it correctly detected an earthquake in Longxi more than 500 km away. The internal design was lost; a functional replica built in 2005 by the Chinese Academy of Sciences reproduced the performance using technology contemporary with the original.