SEISMOGRAPH   

The science of earthquakes is known as seismology and the instruments which records earthquakes waves is called seismograph. The seismograph is a very sensitive instrument which can record earthquakes which occur thousands of kilometers away. It can also records vibrations so feable which are not usually felts by human sense. The construction of a seismograph is based on the principle of inertia—the tendency of any mass to resist a change in a state of rest or of uniform motion in a straight line is greater, the greater the mass of the object. It consists of a heavy solid object suspended from a vertical stand with the help of wire. Boom is used to keep it horizontal and stop its vertical motion. The heavy object is fitted with a mirror which receives an intensely bright ray of light which is reflected from it and falls on a revolving drum. A film is attached to the drum. If there is no earthquake the reflected rays of light make a straight line. When there is an earthquake, the rays make a zigzags line.

 The vertical stand is fixed on the base rock deep into the earth. The record of earthquakes waves by a seismograph indicates that three conspicuous pulses known as P, S and L-waves can be picked up. The first to arrive at the recording station are the primary or p-waves. These minutes. After this, a second burst of activity sets in with the arrival of secondary or S-waves. These have somewhat longer amplitude then the P-waves.