III. On a new seismograph

The difficulty in earthquake measurements is to find a point which does not move during the disturbance. This condition is, in certain cases, fulfilled approximately (as regards horizontal motion) by the bob of a pendulum, and, for this reason, pendulums have been frequently used as seismometers. A long pendulum, whose period greatly exceeds the period of the earthquake waves, suspended from an exceedingly rigid frame, makes a fairly good seismograph, the earth’s motion relatively to the bob being shown by a pair of indicating levers at right angles to each other, with their short ends in contact with the bob near its centre of gravity, their fulcrums fixed to the earth, and their long ends lightly touching a plate of smoked glass which is kept revolving uniformly by clockwork. If the bob were to remain quite stationary during an earthquake, this would give two curves, showing on a magnified scale two rectangular components of the horizontal displacement of a point on the earth’s surface in conjunction with the time, from which the amount and direction of the actual horizontal movement, and its velocity and rate of acceleration at any time during the shock, could be deduced.