The plan of this instrument was submitted amongst others to a Committee of the British Association appointed to devise means for registering earthquake shocks. A heavy pendulum, suspended from a frame, will evidently have its bob left behind by its inertia when the frame is moved forwards by any concussion. To render such an instrument very sensible, however, the pendulum must be of great length, which presents many inconveniences in practice. The author, therefore, proposed an inverted pendulum, sustained by a steel wire, on the principle of the noddy invented by Mr Hardy, for ascertaining the stability of clock cases, The balance of gravity and elasticity (which act, the former to displace, the latter to redress, the pendulum) may be rendered as nice as we choose, and hence the sensibility of the instrument is wholly independent of its dimensions.