The ribs of all vaulting of early date are square and flat at the back; the vaulting, which is often very thick, passing over and resting upon their backs. In later works the ribs were usually deeper from intrados to extrados, and were notched, or as it is technically called, “rebated,” to receive the vaulting, or at least the lower part of its thickness ([Fig. 362]); for where the surface was not intended to be plastered, the wrought stonework was often a thin casing covered over above by a thicker mass of rough work. The curvature of the courses of wrought stone enabled them to be set without the use of continuous timber centering, and this inner facing, once finished, would itself form a substantial centering for the outer rough vault.
At a later period this outer thickness was dispensed with as a superfluous load. In all cases the hollow space against the wall behind was filled in solid to a certain height to strengthen the haunches of the vault.
The ribs now became beautifully moulded, and sometimes decorated with carving. In early works, as at St. Cross;[52] St. Peter’s, Oxford; St. Joseph’s Chapel at Glastonbury;[53] and in the aisles at Canterbury, the old Norman chevron was continued in the ribs. The meetings and intersections of the ribs at their apex were usually ornamented with bosses, and beautifully carved. These bosses assumed many varieties of form—sometimes a small rosette, or a little tuft of foliage, merely to decorate the centre of the intersection without covering the mouldings; sometimes the mouldings themselves return round a central opening, with or without foliage; sometimes a head of part of a figure was added to the last-named form in each angle, nearly at the plane of vaulting; sometimes beneath such moulded boss a disk was attached with or without foliage, as if to form a cover to the central opening; indeed, it was occasionally actually the moveable cover of such an opening. In England the usual form is a group of foliage covering the intersection, and frequently containing figure sculpture. Westminster Abbey furnishes admirable examples both of the foliated and sculptured bosses.
As regards the intermediate surfaces of the vaulting, a curious difference is found to obtain between the methods adopted in France and in England.
In France the courses of stone run parallel to the ridges, as would naturally suggest itself from the original intersecting vaults (Fig. [363]); while in England they often take an irregular direction, as if suggested by placing them at right angles to an imaginary centre line of each triangular space, though really deviating much and irregularly from such a rule.
Fig. 363.
The French seem much offended by the appearance of the English system; and I remember feeling in the same way when I first saw the French method. The latter seems to throw undue pressure on the diagonal ribs, while the English mode appears to throw it more equally on all the ribs; throwing it, in fact, down into the direction of their meeting-point.[54]
I will now describe a form of vaulting which, though it originated during the round-arched period, seems more properly to belong to that now under consideration. We have seen that the arches of churches were frequently arranged in pairs; the piers alternating in size and design. Supposing each arch to be about half the width of the nave, each pair of arches would form a square on the plan; and, though such a square space may be, and often was, divided into two oblongs in the vaulting, it is equally natural to vault it as a single square. As, however, this leaves the alternate piers unrepresented in the vaulting, it became frequent to carry across from this intermediate pier a single transverse rib crossing the diagonals at their point of intersection, and between it and those diagonals to introduce oblique vaulting cells, whose ridges strike from the centres of the half-bays to the point of intersection.