Zhang Heng seismoscope, 132 AD.
© Science Museum / Science & Society Picture Library
Model, cut away to show internal mechanism. The Chinese astronomer, mathematician and seismologist, Zhang Heng (78-139 AD) described the earliest seismoscope known in about 132 AD. Arriving shock waves displace a pendulum linked to a mechanism which opens the jaws of the dragon facing the direction of the earthquake. A ball falls from the dragon's teeth into the mouth of a toad below to record the event.