GOTHIC VAULTS

STONE-VAULTED roofs became necessary in church building early in the Norman period for security against fire. They were made after the Roman manner, semicircular, with similar vaults intersecting at right angles. The lines of their intersections are the groins. When two intersecting vaults are of equal semicircles, each groin is a semi-ellipse. This groin is the weakest part of the vault; in order to strengthen it the Normans built an arch called the groin rib, underneath the groin, to support it. Difficulties were met in forming intersecting vaults of unequal span, clumsy contrivances were resorted to, until, in the Transition period, 1145-1190, the introduction of the pointed arch solved the problem, and led the way to the development of Gothic vaulting. The Roman and Norman vaults were built upon temporary centering. A centre is a timber frame made like a roof-truss shaped to the form of the arch; a series of centres were placed at convenient distances apart and covered with strong boarding upon which the vault was built. After the masonry was completely set, the temporary centering was removed, leaving the vault to carry itself.

As the system of vaulting developed, cross ribs and wall ribs (Plate XI., Fig. 3) were added, and much of the temporary centering was dispensed with; ornamental arrangements were designed by introducing more ribs, and the web—the covering surface of masonry—was reduced to small panels in the Decorated and Perpendicular periods. In the Tudor period, 1485-1558, the web became the principal part, the ribs being mere mouldings worked upon its surface in the form of fan-vaulting, a simple example of which can be studied in the south porch of Chester Cathedral, and the most elaborate in the roof of the Chapel of Henry VII., Westminster Abbey.

A bay of a cathedral is one of the spaces into which its length is divided by the supports of the roof as piers, arches, or principals. The bays of the aisles are usually square—those of the nave, choir, or transepts rectangular on account of their greater width.

Vertically each of the bays of the nave, etc., is divided into three stories (Plate XI., Fig. 2), the groundstory rising from the floor; the triforium, or blindstory, having no windows, is over the aisles, and the clerestory over the triforium. The prefix clere—bright—indicates the brilliancy of its light.

In the Norman period these three divisions were nearly equal in height. In the succeeding periods the groundstory attained about half of the total height of the bay, the clerestory was extended downwards, and the triforium reduced, until, in the Perpendicular period, it entirely disappeared.

Plate XI., Figs. 3 and 4, show a few square bays of Gothic vaulting in skeleton diagrams with the forms of plan indicated by dotted lines upon their base-planes. All the lines represent ribs.

Plate XI., Figs. 2 and 3, show quadripartite vaulting—i.e., having four compartments in one bay. This is the simplest form of Gothic vault, and belongs chiefly to the Early English period. The ribs ah, bh, ci, di are wall ribs; bg, cg, ag, dg, are cross ribs; ae, ce, be, de, are the diagonal ribs. The ridge-ribs eg and h, e, i, are horizontal, and intersect the summits of the cross ribs and diagonals. At every intersection there is generally a carved keystone or boss.

In the vaulting of a nave the breadth across is about twice the breadth of the aisles, so that the nave bays are not square, but rectangular.

Plate XI., Fig. 4, and Plate XII., Fig. 2, show lierne-vaulting, having lierne-ribs, the short ribs joining and supporting all the ascending ribs as h, k, l, m, g.

Plate XII., Figs. 2 and 3, are lierne vaults. That shown in Fig. 3 is under the belfry of a church tower at Coventry, with circular opening for hoisting the bells.

The detail at B shows the method of collecting the three ribs into one at the springers in the corners A, B, C, D, by small arches in the tracery of ribs.

Plate XII., Fig. 1, shows fan-vaulting having no ribs. The lines shown indicate mouldings on the masonry imitating ribs. The structure is built up of slabs of stone, accurately joined together forming concave half-cones, their vertices being the springers of the vault. The dotted lines show some of the jointing; the other lines represent the imitation ribs. The crown of the vault is the flat surface gh, gi, generally richly ornamented.