Page:Great Neapolitan Earthquake of 1857.djvu/255

Rh instruments), are two clocks, $$c c$$, whose pendulums vibrated in the plane of the prime vertical, and showing sidereal time. These were both stopped, but, according to Signor de Gasparis, at different moments, and each at an unequal period after the shock, owing to their structure, so that nothing could be concluded as to the precise moment of the first shock from them. I could not ascertain upon what precise data the moment stated in the Giornale as above, for the occurrence of the shock was based, and from other facts entertain some doubts as to its precision.

I found by measurements that a moment in the line of the meridian, and therefore transverse to the plane of vibration of less than 0.5 inch would have been sufficient to have stopped either of these clocks, unless the contact with the case and pendulum so produced, had been instantly removed by a movement in the opposite direction, and before time were given to destroy by friction the momentum of the pendulum.

In the Salle Centrale, which is also the library of the Observatory, and leads by a winding stone staircase at one end, to the top of the tower where the equatorial is fixed, is a third clock, showing Naples mean time, whose pendulum, an extremely heavy one, oscillates in the plane of the meridian, which was not stopped. A movement of the pendulum bob of 0.625 inch transverse to the plane of oscillation would have stopped this clock.

These clocks are not screwed to the walls, and neither they nor any of the other instruments had suffered damage or derangement.

Two chronometers lent by the Lords of the Admiralty, and brought with me from England, going Greenwich mean