Seismograph

Seismograph is an instrument for recording earth's surface which, if sufficiently pronounced, become earthquakes. These motions are of two kinds - quick vibrations and slow tiltings of the surface. For measuring the quick tremors, a heavy body is suspended in such a way that it has freedom of motion in one or more directions, and is in a condition of nearly neutral equilibrium - that is to say, if it is displaced, it has hardly any tendency to return to its original position. A body so suspended would remain stationary, or nearly so, when its supports moved in consequence of earth-tremors, and the relative motions of the body and its supports would be a measure of the extent of the motion of the ground. A very long pendulum fulfils the conditions as regards horizontal motions; but a more convenient arrangement, due to Ewing, consists of two pendulums, one of which is inverted and placed vertically below the other. The two bobs are connected by a ball joint. The combination of the stability of the upper pendulum with the instability of the lower one can give the reqllired neutral equilibrium. A point, connected with the lobes by multiplying levers, records the motion on a plate of smoked glass. By allowing the pendulums to swing freely in any direction, a complete record of the horizontal tremors is obtained, while the motion is in any given direction may be observed by giving the pendulum freedom in that direction only. By the latter means the horizontal vibrations may be resolved into two components at right angles, and recorded on a disc of smoked glass rotated by clockwork; from the diagram so obtained the motion at any moment can be ascertained. An instrument for measuring vertical tremors may be constructed on the same principle. In Ewing's arrangeement a horizontal lever, pivoted at one end and weighted at the other, is supported by a spiral spring, the position of the point of attachment being so arranged that the necessary neutral equilibrium is obtained. A style moved by multiplying levers is provided to make a record on smoked glass. These instruments would not be affected by a slow alteration of level of the earth's surface, for the measurement of which other appliances are needed. The simplest arrangement is to place two spirit levels on the spot to be observed, and to watch their bubbles with microscopes. A better way is to observe the relative motion of two beams of light emanating from the same source, one being reflected from the surface of mercury, which is horizontal, and one from a mirror attached to the rock whose motion is to be observed.