1452

THEORY AND PRACTICE OF ENGINEERING.

Book II.

Fig. 2581.

Fig. 2581.

Fig. 2585.

Fig. 2583.

Fig. 2587.

ARC DE CLOITRE WITH A FLAT TOP.

Vault of Arc tie Cloitre , with aflat top , called by the Italians Volta a Concha , is used in largerooms; the form which has the best effect is that which results from dividing the breadthinto three equal parts, giving two to the vaulting, and the other to the middle platform : togive this vault all the strength possible, it is necessary to pay particular attention to the di-rection of the joints of the level part in the centre ; setting out the width of the platform onthe base of the section, and then forming upon it an equilateral triangle, we obtain the pointG, to which all the joints of the centre compartment tend.

All that has been previously described will apply to this vault; the re-entering anglesbeing all similar, it is sufficient to make the mould for one, as indicated by H, I, K, L, M,which show the form of the intrados of the several stones marked on the plan. H is thespringing voussoir, and I the one resting on it.

Where vaults are constructed entirely for ornamental purposes, the thickness of thevoussoirs may be very small in some instances: at the Chapter House at Wells, they arenot more than 3 or 4 inches; particularly in the panels between the several ribs, tufa,chalk, and stone of light weight is selected, to prevent any pressure or thrust against theouter walls or their abutments, which in several buildings is entirely resisted by the .weightthrown vertically upon them. The masons of the middle ages knew well how to calculatethese vertical and lateral thrusts in all their constructions, and seldom used more materialthan was absolutely necessary to produce the effect they aimed at; the spandrills of theirarches were kept from springing by a mass of rubble; in some cases we find the wholespandrill formed of one large stone tailed into the wall, and maintained in its positionby the weight of several courses of masonry placed upon it; in the construction thereforeof vaults like that of the arc de cloitre, centering is only required for the voussoirs markedC, D, and E, in fig. 2567., or N O D, in fig. 2580., where the plan is an oblong : by a