87. The term mortar is usually applied to the mixture of sand and cementing material which is placed between the large stones of a stone structure, although the term might also be properly applied to the matrix of the concrete in which broken stone is embedded. The object of the mortar is to furnish a cushion for the stones above it, which, as far as possible, distributes the pressure uniformly and relieves the stones of transverse stresses and also from the concentrated crushing pressures to which the projecting points of the stone would be subjected.