1430

THEORY AND PRACTICE OF ENGINEERING.

Rook II.

Diagonals drawn from one angle of the hexagon to the other, and passing through thecentre, form salient angles in the voute darete , and re-entering angles in the voute de cloitre.

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Fig. 244-1

Fig. 2443. ARC DB CLOITRB.

Fig. 2442. VOUTE D’ARETE.

ARC DE CLOITRB.

£ 1 /

In the Voute dardte, composed of triangular voussoirs, these parts have no other supportsthan the angles of their plan: whilst in the vaults termed arc de cloitre they are sup-ported on their side, which is carried on the wall throughout its whole length : hence thelatter are much more solid, and have less pressure than the voute d'ardte.

Flat Arches are constructed by making each course of voussoirs run parallel with thefaces of the walls which support them.

In all arches stability beingthe object, cohesion andfriction are necessary: thelarger the voussoirs, or, inother words, the fewer thejoints introduced, the lessliability there is to derange-ment, as the cohesion is in-creased : but we must alwaysregard the nature of the ma-terial employed, and its dis-position to fracture. Themason as well as the brick-layer should avoid the im-propriety of making his skewbacks to flat arches too great;for an arch composed of stoneswith parallel beds and slightlyinclined joints cannot yielduntil some one of them slidesout, or becomes crushed. Atpage 410. (fig. 410.)isshowna flat arch at ConingsburgCastle, where the voussoirsare held up by an indentedjoint: similar arches arefound among the buildings ofthe Romans, over the greatwestern door of the cathedralat Rochester, and elsewhere.

Fig. 2445. FLAT ARCHES.

Fig. 244G. FLAT ARCHES COM-PRISED BETWEEN FOUR WALLS.

Flat Vaults over a square plan are comprised between four walls which support them:the rows of voussoirs form concentric squares; those of the angles are common to two ofthe sides; the key is square, and locks the whole together. The lines traced on theplan show the position of each stone; the dark line is the lower joint, and the lighter linethe upper. Vaults of such a description are rarely to be met with in England, and requiregreat skill for their execution: wherever they are introduced, an iron tic or a strong abut-ment is requisite to prevent the yielding which takes place at the joints of the stone.