their original vaulting. FIG. 32.--BEAUVAIS, SAINT ÉTIENNE. IRREGULAR FOUR-PART VAULTING OF DURHAM CATHEDRAL The first of these is the English cathedral of Durham. The date of its vaults is still the subject of a decided controversy, but whether they were built between 1093 and 1133 as Bond,[207] Rivoira,[208] and Moore[209] believe, or are later than those of Saint Denis, which is the claim of Lasteyrie,[210] they are of sufficiently early date to be important in a [Illustration: 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