Fig. 33.—Durham, Cathedral.
discussion of rectangular four-part vaulting. Those over the nave [(Fig. 33)] are especially interesting and furnish a unique variant of the standard type. It was the apparent intention of the builders to roof the nave with wood and for this purpose heavy transverse arches were constructed between the alternate piers. When vaulting was determined upon, the nave was therefore already divided into square bays each containing two clerestory windows on a side. To vault these bays the builders might naturally have been expected to adopt the Lombard system of simple four-part vaults, but here in Durham, as in Saint Étienne at Caen, the impost level of the transverse arches was so low that a four-part vault would have made impossible the retention of the windows already in position above each nave arch. As these were absolutely essential in the north of England for lighting purposes, and also most important in preserving the symmetry of the bays, a change either in their size or position would have proved impractical. The six-part vault was the Norman method of solving a similar problem. But the builders of Durham invented a new system, made up of two rectangular cross-ribbed vaults in each bay, their intermediate supports afforded by corbels, and their alternate transverse arches omitted ([Plate I-i.]). This omission of the intermediate rib gives a very unusual character to the vault but it preserves the alternate system with square nave bays so popular in Norman work, and at the same time has a great advantage over the six-part vault in that the transverse crown line of the window cells is perpendicular to the outer wall. The panels are therefore more symmetrical in elevation and the thrusts are more evenly distributed from pier to pier. The large central severy, however, afforded a difficult surface both for construction and support, and it is not surprising that the system was not repeated. As in the early ribbed vaults at Caen, wall ribs were not employed at Durham, and the abutment was provided only by flat pilasters and concealed flying buttresses, some of full and some of half arched form.[211]
Early Four-Part Ribbed Vaults in Normandy
That the rectangular four-part system of vaulting was developed in Normandy, as well as in England and the Ile-de-France, and very possibly independently of both, is proved by the early twelfth century abbey church of Lessay (Manche) (cir. 1130).[212] If the vaults of Lessay are an independent development it is hardly possible to see in them anything else than another effort to vault a church with square nave bays and yet provide the best possible vaulting to fit above the windows. A glance at the choir[213] will show that the alternate system was here employed just as in Saint Étienne at Caen, yet the builders introduced two four-part vaults instead of one of six-part type in each bay.[214] The transverse arches are still semicircular and the vault is somewhat rudimentary. The system as a whole may be considered as a fourth method[215] of the Norman builders to preserve their clerestory intact and still vault their churches. A slight advance is shown in the vaults at Pontorson (Manche) (middle of twelfth century). This is, however, a small church without side aisles and its vaults are in almost square bays with pointed transverse arches and considerably domed up at the crown. Wall ribs are still lacking as at Durham and Lessay.
Transitional Four-Part Ribbed Vaults in the Ile-de-France
The abbey church of Saint Germer-de-Fly (Oise) (cir. 1140), which still retains its original vaults in the choir and two eastern bays of the nave, presents another and perhaps more important example of rectangular four-part cross-ribbed vaulting. Its structural arches are of pointed section, and the piers and walls are strengthened by concealed flying-buttresses beneath the wooden roof of the triforium.[216] These are similar to those which have already been noted in La Trinité at Caen and in the nave of Durham,[217] but the vaults are superior in construction to those at Durham and are also provided with transverse arches between each rectangular bay. With the aid of this concealed buttress and the retention of the heavy Romanesque walls and small openings the vaults of Saint Germer were kept from falling, and it was doubtless this fact which led to the extension of the four-part system until it rivaled and at length became more popular than the six-part vaulting imported from Normandy and used at exactly the same period in the church of Saint Denis. A number of elementary features still remained at St. Germer, however. The transverse arches are but slightly pointed in section, the ribs are unusually heavy, and the diagonals of the choir bay are supported upon corbels[218] showing that the shaft arrangement was not yet in accord with the ribs to be carried.