where the vaults were of no great span. But though it was of slight efficiency in vaulted constructions, it yet had great value as marking the place where, in such constructions, additional strength was required by the walls. And in the later Romanesque, as vaulting became more general, the pilaster strip was developed into a true buttress (Fig. 6).
FIG. 6.A beginning was made in the direction of further progress when the Romanesque builders began to vault their naves. It was then found that the pilaster buttress against the clerestory wall was not enough to stay vaults of so much wider span than those of the aisles for which a buttress like that shown in Fig. 6 had been adequate. Expedients to augment the resistance of the clerestory buttress were accordingly resorted to which were destined ultimately to yield unforeseen results.
In the Abbaye-aux-Hommes at Caen the forms of the vaults—which date from the early part of the twelfth century, and are among the earliest that were constructed over a nave—were such as to exert powerful thrusts. That is to say, the arches of their groins were curves of low sweep, such as the Romanesque builders had derived from Roman intersecting vaults, and consequently of enormous push. To stay these vaults the expedient was adopted of constructing demi-barrel vaults, springing from the top of the aisle walls, and butting against the wall of the nave under the aisle roofs (Fig. 7). These demi-vaults were in reality concealed continuous flying buttresses. But they were flying buttresses of bad form, for only a small part of their strength met the thrusts of the vaults, the rest being exerted against the walls, between the piers, where no props were required, and where their effect would have been disastrous had not these walls been of excessive massiveness. The level of the abutment was, moreover, so low that it failed to meet the points where the thrusts were greatest. The precise chronological sequence
