Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/377

312 Taking the specific gravity of Apennine limestone, forming, the walls of the cavity, and the whole of its superincumbent mass, = 2·700, and 29·92 inches of mercury, specific gravity 13·57, equaling one atmosphere; it follows that a column of 150·4 inches of the limestone, or of nearly 12·5 feet, will also balance an atmosphere of steam.

Hence, in round numbers, we have the following depths of limestone, necessary to balance the corresponding tensions—

We therefore remark, that although the steam pressure, was enough, to lift the enormous column of 8550 feet of limestone, it was not sufficient, by about one-half, to blow up, the weakest portion, of the covering of the cavity, whose least thickness was 16,700 feet below the surface.

The steam tension, of 684 atmospheres, is about one-third that of fired gunpowder, and hence amply sufficient, if either suddenly generated, or suddenly admitted, to the cavity, to produce the wave of impulse, by the sudden compression, of the walls of the cavity; to which, even the mean tension, would be sufficient.

The pressures upon the walls, due to the preceding tension, are

so that at the latter pressure—that due to the highest