Seismograph
a seismometer; an instrument for recording the period, extent, and direction of each of the vibrations which constitute an earthquake. For a complete seismography, three distinct sets of apparatus are required: (1) To record horizontal motion; (2) to record vertical motion; and (3) to record time. The horizontal and vertical motions must be written on the same receiver, and if possible side by side, while at the instant at which the time is recorded a mark must be made on the diagram which is being drawn by the seismograph. The first instruments were merely modifications of the seismoscope, but successive improvements have been introduced, and the seismograph has been brought to a high pitch of perfection.
Some of the best, if not the best forms known are in use in the Imperial Observatory at Tokio, Japan.