Supposed operating principle of the first seismoscope
 
First instrument designed to reveal the occurrence of a seism, this seismoscope conceived by the Chinese philosopher Chang-Heng was relying on balls, in unstable equilibrium. At the occurrence of an earthquake, the balls oriented according to the direction of vibration were falling in the mouth of a toad situated below, producing noise.  
The exact mechanism of this seismoscope is unknown but it is probable that falling of the balls was due to the oscillations of an internal heavy pendulum.