The direction of the earthquake is indicated by the position of the toad which swallowed the ball. All the scholars at the capital were astonished at this strange effect occurring without any evidence of an earthquake to cause it. Upon this everyone admitted the mysterious power of the instrument. The modern model in the photograph above is in the shape of a watchtower in a Han period frontier village. The workings of Chang Heng's seismometer were never revealed.
Most experts agree however, that it worked on the principle of inertia.
A mass is suspended. An earthquake shakes the vessel, causing a slight displacement between the mass and the vessel. This movement is transmitted via levers and gears to push out a ball. In the design of any instrument, the goal is to make the instrument sensitive to the desired signal while simultaneously rejecting false signals. For a pendulum, this optimal point is at the resonance.