BUILDING. 640 BUILDING. present the neatest possible appearance. All void spaces are packed with spalls or stone chips and flushed full of mortar, llortar is also placed between the stones, so that tliov do not touch them, and as fast as the facing is carried up it i.i followed by the supporting rubble or brickwork backing. At intervals a facing-stone of unusual depth is inserted, which extends back into the hacking and ties the facing to the backing. Sometimes metal ties are used in place of deeper facing-stones to insure this bonding together of the wall. The arranjtement of the stones com- jKising the ashlar facing, or, so to speak, the FIG. 1. RUBBLE-STOSE VALL. each other, but are held together by the hardened matrix. All rubble stonework is built substan- tially as described, except in some cases the face- stones have their joints dressed (Fig. 2) so that FIG. 2. bubble-sto.m: avall, itii dres.sed joint. when laid up the face of the wall lias a mosaic- like appearance, while in other cases round field- stones or cobblestones are used for the facing. Ashlar stonework, or stonework of rectangular cut stone (Figs. 3 and 4). is used for walls in which the solidity and regularity is desired that is given only by stones cut and dressed to perfect parallelopipedons. Walls are seldom built of ashlar cut stone for their whole thickness, but are composed of an ashlar facing, backed by rubble stonework or bv lirickwork. V FIG. 3. RANDOM ASHLAR 8TO.NK WALL. In laying ashlar facing, the slonelayer super- imposes one stone upon another side by side, or end to end, with thin mortar joints between FIG. 4. COrRSED ASHLAR STONE WALL. pattern of the facing, is specified by the architect so that the stonelayer has only to follow these instructions. After the wall, including facing and backing, has been completed, the stonelayer scrapes out and removes the mortar joints to the depth of about one inch, and fills the space thus secured with finer quality of mortar, giving it a rounded edge or beading. This work is termed pointing. After the pointing has been completed long enough for the wall to become hard, the face of the wall is cleaned orscrubbed with water and mu- riatic acid, which completes the work. This work is termed cleaning. Arches are almost always built of cut and dressed stones (Fig. 5), having £x-trado3 FIG. 5. ASHLAR STONE ARCH. the shape of the frustum of a wedge, and called voussoirs. (See Arch.) Kubble arches are sometimes used, but they require the selection of suitably shaped stones for voussoirs, and the use of wedge-shaped mortar joints. In cut-stone arches nearly always an odd number of voussoirs are used, so that one of them, called a keystone, comes exactly at the centre of the top of the arch. In laying stone arches, the stonelayer first erects a centre (Fig. G), which is a structure of wood or iron whose to]) surface constitutes a
