An oscillating system suitable for use in a long-period horizontal-motion seismometer is described. The system consists of a short vertical pendulum whose gravitational restoring torque is opposed by a vertical zero-length spring. By adjusting the spring constant with respect to the mass of the pendulum the net restoring torque can be reduced to almost zero, thus giving the system a long period. The period is less sensitive to tilt than that of the horizontal pendulums commonly used in seismometers for horizontal motion. The lateral dimensions of the system are small so that it is adaptable for use in a bore hole and for easy transportation and setting up. A stable period of 30 seconds has been observed in a prototype model.

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