Introduction to the seismometer: The seismograph is a masterpiece handed down from generation to generation by Zhang Heng, a scientist from the Han Dynasty. It has eight directions, and in each direction there is a dragon with a copper bead in its mouth. Under each dragon there is a toad corresponding to it. If an earthquake occurs in any direction, the copper beads contained in the dragon's mouth in that direction will fall into the toad's mouth, and the direction of the earthquake can be determined.