How does the seismograph measure an earthquake?

Seismographs record the vibrations of the earth. It is a delicately hung weight which remains still when an earthquake occurs. But the post which holds it, moves. There is a chart which is attached to the post under the weight. When the chart moves, the weight makes a record on it. From this, the time of the wave, force of the motion, and direction from which the wave comes, can be grasped.