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The Chinese philosopher Chang Hêng [Chang Hêng is also referred to as Choko and Tyoko, modifications of the Japanese form of his name.] invented the earliest known seismoscope in 132 A.D. The instrument was said to resemble a wine jar of diameter six feet (Figure below). On the outside of the vessel there were eight dragon heads, facing the eight principal directions of the compass. Below each of the dragon heads was a toad, with its mouth opened toward the dragon. The mouth of each dragon held a ball. At the occurrence of an earthquake, one of the eight dragon-mouths would release a ball into the open mouth of the toad situated below. The direction of the shaking determined which of the dragons released its ball. The instrument is reported to have detected a four-hundred-mile distant earthquake which was not felt at the location of the seismoscope.
The inside of the Chinese seismoscope is unknown. Seismologists of the nineteenth and twentieth centuries have speculated on mechanisms which would duplicate the behavior of Chang Hêng's seismoscope, but would not be beyond the Chinese technology of Chang Hêng's time. All assume the use of some kind of pendulum as the primary sensing element, the motion of which would activate one of the dragons. In his translation of the original Chinese description of Chang Hêng's seismoscope. English seismologist, Milne implied that the pendulum was a suspended mass, a common pendulum. Seismologist, Imamura thought an inverted pendulum was more probable. Hagiwara constructed an inverted-pendulum seismoscope which behaved nearly as Chang Hêng's was reported to have behaved. The model designed by Hagiwara, however, responded most frequently to transverse motion, and indicated a direction normal to the azimuth between observer and epicenter, whereas the Chinese seismoscope was reported to have indicated the azimuth of the earthquake. It has been suggested that Chang Hêng's "earthquake weathercock" was calibrated empirically for its direction-determining properties.
Chang Heng's
seismoscope
Needham reports that knowledge of Chang Hêng's instrument remained for over four centuries. Books describing the working of "earthquake weathercocks" were written as late as the end of the sixth century. In later years, however, the seismoscope seemed to disappear from Chinese science; later Chinese writers questioned that such a machine was possible.
