James Forbes Seismometer 1844

The most significant instrument resulting from the Association's work was an inverted-pendulum "seismometer", designed by James Forbes in 1844. It consisted of a vertical metal rod having a mass (C) moveable upon it. The rod was supported on a vertical cylindrical steel wire. The wire could be made more or less stiff by pinching it at a greater or lesser height by means of a screw (S). By adjusting the stiffness of the wire, or the height of the ball, the free period of the pendulum might be altered. A pencil L placed on the prolongation of the metal rod wrote a record on a stationary, paper-lined, spherical dome. By placing the pencil sufficiently far above the mass, a magnification of the motion of the mass by a factor of two or three could be obtained.

James Forbes used the inverted pendulum, mounted on a stiff wire, to provide a sensing element in weakly-stable equilibrium, without having to use a very long common pendulum. An upright inverted pendulum, by itself, is in unstable equilibrium. A slight motion topples it over. When the inverted pendulum is mounted on a suitably stiff wire, the apparatus may be rendered stable, so that the inverted pendulum returns to an upright position after having been disturbed.

James Forbes was probably the first inventor to attempt to give a seismological instrument a longer recording period. Forbes clearly wanted to measure ground displacement in an earthquake. Forbes was also the first person to describe mathematically the behavior of a seismic instrument in an "earthquake".

Six of the inverted-pendulum seismometers that Forbed invented were set up at Comrie, Scotland. Their sizes varied form thirty-nine inches long to ten feet long. However, the Forbes' seismometers gave disappointing performances. In one year, for example, the seismometers recorded only three of sixty earthquakes felt at Comrie.