PLATE XII
Plates XI and XII
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.