But much more important than these smaller chevet vaults, are those of a number of large churches, also belonging to the second quarter of the twelfth century. Of these, Saint Germer-de-Fly[360] [(Fig. 63)] has been most prominently brought to notice through Mr. Moore’s work on Gothic architecture. It is doubly of interest because it possesses chevet vaults of two distinct stages in the development of this new form. Thus in the original radiating chapels opening off the ambulatory,[361] three-part chevet vaults of the type described in the previous paragraph were employed, with this advance, namely the introduction of stilted, round-headed wall ribs. The vaults are still highly domed at the crown and it would seem very reasonable to suppose that they were completed before the vault of the great apse was begun.
Fig. 63.—Saint Germer-de-Fly, Abbey Church.
This latter shows an advance in construction beyond that hitherto seen. In the first place, the entire window is placed above the level of the impost of the radiants with a consequent raising of the vault surface above the windows and a great reduction in its domed-up character. The line of intersection of the vault cell with the apse wall, which is marked by a slightly pointed, stilted wall rib, resting upon slender shafts rising from the clerestory string-course, is almost perpendicular from the impost of the radiants to a point about at their haunch. Thus the lower portion of the masonry panel is really a flat wall resting upon the ribs. The object of the builders in thus constructing their vault panels would seem to have been twofold, first to get a large space of pleasing shape for clerestory windows and secondly to aid in overcoming the thrusts of the radiating ribs. The first is perhaps the less important of the two, for the windows in the early chevets very rarely occupy all the space beneath the wall intersection. The second, however, furnishes a much better explanation of this form of panel. And this explanation would seem to lie, not so much in the fact that the stilted wall rib concentrated the thrust along a narrow strip of exterior wall where it could be met by exterior buttresses[362] but rather in the fact that the weight of such a flat wall, rising perpendicularly above the radiating rib, practically offset all of their outward thrusts by its downward pressure while the little which remained was taken care of by the thick walls characteristic of church construction in the Transitional period. Thus it is possible to account for the almost total lack of exterior abutment in such apses as this at Saint Germer-de-Fly, where only the slenderest of shafts are found along the exterior wall serving far more for decoration than for abutment.[363] That the stilting was not done primarily to concentrate the thrusts is further shown by the fact that in many of the later Gothic churches which were built long after the flying-buttress was perfected there is no attempt to stilt the wall rib, but the masonry of the vault is actually curved outward from the very springing of the radiants, which are raised to the impost of the window heads to give the vault this form.[364]
The highly stilted wall intersection with the consequent elevation of the clerestory window and flattening of the lower part of the vault cell constitutes the great structural advance in the chevet of Saint Germer. The employment of the wall rib, however, introduces an important matter for discussion. To be sure this is not by any means the first example of its use, for formerets may be found even in groined vaults, but it is one of the early examples on a large scale and may serve to introduce the question as to the part which these ribs played in Gothic architecture.