Page:Great Neapolitan Earthquake of 1857.djvu/141

 Rh difficulties do not arise with horizontal transverse fractures.

The strain is here applied almost with the rapidity of a blow. Almost the whole stress falls instantly upon the less elastic mortar joints, at their surfaces of contact with the stone, when exposed to direct pull, and the mortar joint parts off from the stone with a resistance of only one-half that due to its statical adherence, or to its statical coherence. This fact is rendered familiar to the senses, by the facility with which two bricks from an indurated building, that would require a slowly applied load of perhaps half a ton to tear them directly asunder, may be caused to part and drop asunder, by a slight blow from a hammer upon one of the bricks while the other is held in the hand. As applied to our subject, this sufficiently indicates, that the portion of the total force of shock required to produce fissure, or horizontal fracture of the base, of the severed masses, to permit overthrow, is, when these are large, relatively very small; so much so, where the masses, are large in relation to the surfaces of fracture, and the co-efficient $$f$$, of adherence very low as in the case of the Neapolitan provincial mortar, that it may be frequently neglected in calculations of seismic statistity. At p. 139, et seq., will be found the method of calculating the velocity due to fracture of the horizontal mortar joints, at the base of walls overthrown, which is the most important and frequent case of fracture that occurs, in seismometric observation.