Chap. XXIV.

MASONRY.

1427

arc made in cement or plaster,for the sake of economy, theyarc run either upon projectionsof rough stone or tile ; the toolsrequired are trowels of differentforms, and chisels and hammersto cut away the masses whichmay obstruct the working of therunning mould, which is cut outto the form intended, and passedalong the face of the cement,spread over the cornice.

Fig. 2414. Fig. 2415. Fig. 2416.

Fig. 2421.

Fig. 2424.

Stone Pipes are in many parts of the continent madeuse of for conducting water, and are sometimes laid inthe thickness of the walls, to bring down that whichfalls upon the roofs: they are bored by simple means,setting the block of stone upright, and in the centreof the intended pipe or cylinder placing a wooden plughaving a hole through which the axis of the spindlepasses; the cutter is attached to this, and rotary mo-tion is obtained by means of a pulley; the action ofthis borer is that of the common drill.

Arches composed of four voussoirs cannot sustainthemselves, whatever be the resistance of the piers, if

their thickness be less than the seventeenth part of their Fig. 2425. boring stone pipes.span ; but they sustain themselves with a less thickness

when the arch is extradossedequally over § of its extent, the surplus being comprised in thepiers, as shown in fig. 2427 . : when the thickness of the arch is increased, it may be madeonly gtj part of the span.

Fig. 2420.

Arches of Equilibrium , being calculated to stand by themselves, must necessarily be better

4 y 2