Page:Great Neapolitan Earthquake of 1857.djvu/198

150 When the question relates to the fracture of a homogeneous body—such as a column shaft, of one block of stone for example then the force $$\mathrm{P}$$ to be taken, is that which applies to the material, and $$\delta$$ its sp. gr. But when the fracture occurs in walls of whatever sort, it takes place by the giving way, by loss of adhesion (generally), or sometimes of its own cohesion, of the mortar or other cement, as being the weakest part of the heterogeneous mass: in which case, $$\mathrm{P}$$ is to be taken for the rupturing force of either the adhesion or cohesion (as the case may be) of the mortar or cement, and $$\delta$$ the specific gravity due to the whole mass of masonry.

Fracture seldom or never occurs through the solid stone, in masonry, but always at the mortar joints, and generally by their loss of adhesion, to the stone at the faces of the joint. It rarely occurs through the brick, in brickwork, and only when the cohesion of the brick itself, is less than that of the cement.

To enable these equations to be applied generally, in earthquake countries, I have arranged the two following tables, I. and II., which embrace almost all the reliable information we as yet have, applicable to the matter, and from which, the value of $$\mathrm{L}$$ may be deduced, for a great variety of cases.

Many of the numbers, for want of better experimental data, can only be viewed as approximative.

The most important numbers by far, are those relating to the adhesion and cohesion, of the varieties of common mortar; and, fortunately, these have been ascertained by Boistard, Gauthey, Treussart, and Colonel Totten, with considerable accuracy.