Page:Great Neapolitan Earthquake of 1857.djvu/426

340 instant, to attend to many official calls, as well as domestic ones, incident to the alarm of all around, he might not have remarked any such phenomena, outside the house.

In his "salone," or drawing-room, a large clumsy pendule in an irregular hexagonal frame, with the dial of about 10 inches diameter, in the centre, like a picture, hung upon the wall parallel with $$\mathrm{B}$$, in last Figure (i. e., in a plane 20° W. of north), by a single nail and ring at top. A nail driven into the wall at N. (Fig. 202) was in contact with the frame, when the whole hung plumb, and prevented all movement of oscillation towards the S., but it was free to oscillate in the opposite direction, the nail corresponding to N. having been withdrawn from some cause. I found this pendule remaining out of plumb, and thrown to the northward, as shown by the dotted line (Fig. 202), so that the edge of the frame had moved 1.75 inch, from the nail at N. The centre of gravity of the whole, I found by trial, was in the centre of vertical figure, and the weight of the whole was 8$1⁄2$ rotuli = 16.49 lbs., the frame projecting 5$1⁄2$ inches from face of wall. By trial I also found, that the friction of the back against the wall, required a force of 1$1⁄2$ rotuli = 2.91 lbs. to set it in motion from rest. Had it been free of the nail at N., it of course would have vibrated through an are of 3.50 inches, assuming each semiphase of the ware to have equal velocity, and neglecting the effect of emergence.

From the considerable weight of the pendulum and the large proportion the friction bears to the weight (nearly 1:5), it forms by its range of motion an approximate measure of the amplitude of the wave here in an horizontal direction one too inexact, it is true, to found any