Page:Great Neapolitan Earthquake of 1857.djvu/128

86 such a building, its effects are the same. The distinction of normal and abnormal wave does not exist as respects them; and unless the angle of emergence of the wave be extreme, or nearly vertical, the lines of fracture are the same in every case, viz, vertically through the axis and transverse to the line of shock. This arises from the fact that the area of fracture, and therefore the total resistance, $$\mathrm{F}$$, due to it, augment rapidly as the angle of its obliquity with the vertical, through the cylindrical walls increases. So that although the direct tendency of the wave $$ab$$ is to throw off a cylindric ungula, $$edc$$ (Fig. 59), by a fracture  perpendicular to its direction in $$c f$$, yet its direction at either side of the vertical plane of the wave transit, so obliquely through the joints, that the building always parts in the weaker line, by diametrical vertical fissures through $$k m$$. The separated masses have now each a moment of stability, the fraction $$t^2$$, in which is enormous, being equal to the radius of the cylinder or the base of the cone; and hence the fragments of such towers are seldom overturned.

Where the value of $$\mathrm{F}$$ is small, as in the very bad rubble masonry of the ancient towers, and the angle of emergence considerable, however, we have instances of the mass thrown out assuming the form of a curved ungula, obviously by the fracture commencing vertically, and following down the joints gradatim from $$k$$ to $$p$$, and thence to $$c$$; of this a remarkable example occurs at, Átena.

When the emergence is still more vertical, and the shock powerful, a number of nearly equidistant fissures