Page:Great Neapolitan Earthquake of 1857.djvu/136

92 before it fell, in a storm of wind; and that not until after many such oscillations had disintegrated many of the horizontal joints, and produced several vertical fractures. The point of greatest flexion traversed along the length of the wall, as each oblique gust of wind impinged upon it, like the waves of a rope suspended from one end, and jerked transversely at the other.

An octagonal brick chimney stalk, with a heavy granite capping 160 feet in height above the ground, and 15 feet diameter at the base, was observed by me, instrumentally, to vibrate in a moderate gale of wind, when a few months built, nearly 5 inches at the top.

These are illustrations of the extent of flexibility in good brickwork, which possesses it in a far higher degree than stone masonry, the bond of the mortar being better, the flexibility greater, both in the brick and thick mortar joints, these very numerous, and the elasticity more nearly alike in both, than in stone masonry. When the joints are much fewer in proportion, the stone relatively to the mortar, highly elastic and rigid, and the bond, so far as adhesion of the mortar is concerned, small, (indeed, in the case of many hard, siliceous stones, such as granite, almost nil,) the result of this difference is, that a well-built and indurated brick wall, when fractured, breaks indifferently nearly, through joints and bricks; but in stone walls, the line of fracture is confined to the mortar joints, with rare exceptions, the rigidity of the several blocks, transferring the whole of the compressions and extensions due to the strains to the mortar alone. From this cause, it was observed very uniformly throughout this earthquake region, that when brick construction was superimposed upon stone