In 1856, Luigi Palmieri installed his "sismografo elettro-magnetico" in the volcanic observatory on Mount Vesuvius. This instrument was intended to give the direction, intensity, and duration of an earthquake, and was capable of responding to both horizontal and vertical motions. It was not one "seismograph" but rather a collection of seismoscopes, each intended to record particular parameters of an earthquake.
The seismoscope for detecting vertical motion consisted of a conical mass on a spiral spring. The mass was suspended just over a basin of mercury. When a slight motion caused the tip of the cone to touch the mercury, an electric circuit was completed, which caused a clock to stop, indicating the time of the shock. The spiral spring was constructed so that thermal changes in the length of the spring were balanced by thermal changes in the length of the frame to which the spring was attached.
Horizontal motion was detected with common pendulums, whose swinging completed the same circuit as that completed by the mass-spring seismoscope. In addition, U-tubes filled with mercury were used to detect horizontal motion.
The closing of the above-mentioned electric circuit, besides stopping the clock, started a paper recording surface and caused a pencil to be pressed against the surface. The recorder, once started, continued running until the paper was used up. Every time the circuit was completed, a pencil dash would be left on the moving paper. The duration of the quake was thereby recorded. The size of the earthquake was indicated by the amplitude of oscillations suffered by a mass on a spring and by the amplitudes of the oscillations of the mercury in the U-tubes. The size of the earthquake was measured in "degrees".
