.1sIdar masonry is used for works in which great strength and stability are required. The stAmes for ashlar masonry usually have a length of from three to five time the depth. and a breadth of from one and a half to two times the depth. The thickness of the mortar joints in the very best class of ashlar masonry for building purposes is about inch; for railway and bridge masonry about ti inch to inch. The stones are laid so that the vertical joints of one course come approximately over the middle of the stones below, or, technically. the stones 'break joints.' The arrangement of headers and stretchers varies: the strongest arrangement is where a header and a stretcher are used alter nately. Dowels and cramps arc used where ex ceptional strength is required. Pointing is done by scraping out the mortar to a depth of at least inch from the face of each joint and tilling the void with a high-quality mortar thoroughly rammed, and sometimes finishing the edge with a bead. Ashlar masonry i; usually backed with rubble masonry, the backing being built simultaneously with the facing. Squared masonry is built like ashlar masonry ex cept that dressed stones are not used and range work is seldom employed; the backing 111(1 point ing are the saine as for ashlar masonry.
Rubble masonrg is employed for backing ash and squared stone masonry is used for small sized abutments. culverts, small building founda tion walls, erne. The stones are prepared for laying
by simply knocking elf the weak corners and loose pieces. All interstices are filled with small pieces of stone and mortar, and the mortar joints are made thick enough to prevent adjacent stone; front touching. Very often rubble masonry is laid without mortar, and is then called dry rubble masonry. The strength of stone masonry varies with the strength of the stone, the size of the blocks. the accuracy of the dressing of the joint faces, the proportion of headers to st retehers, ;Ind the kind and quality of the mortar used. Prof. 1. I I. Baker. in .I. Treutise on Masonry Construe (New Vork, 1!10u1, gives the following as a safe 1.,nol per square loot on dilierellt kinds of ',tone Ilnsonty: Ill to 15 tons; squared stone, 15 to 20 tons; limestone ashlar, 20 to 2.5 tons: granite ashlar. 30 tons. In lain classes of stone masonry, such as arch bridges and lighthouses, the stones are emit to exact dimension; and to special forms. In light house constriletion special folios are some timt•-. finite intricate. (See 1,mterit1esE.) In building masonry arches a framework of timber hose top surface i; floored over on a curve cor responding •.xactly to the curve of the arch i; used on Which to set the wedge-shaped stones of the arch ring. See CENTRING, and illustrations in article Ilt•tt.ot?“.