the substructure a, including that portion of it which is beneath the aisle roof, would form a sufficient abutment to these lower pressures, but this did not prove to be the case. 
FIG. 43. The piers subsequently yielded at the springing, and it was found necessary to add a second arch beneath the first. Experience, in fact, showed that the pressures of a vault cannot be concentrated upon any single point, but only upon a line which extends over a considerable portion of the pier from the springing point upwards.
In the buttress system of the nave of Noyon (Fig. 43), which dates from the time of the reconstruction of the vaults early in the thirteenth century, the flying buttress assumes an improved form, in being both narrower and deeper, thus covering at once a greater vertical and a diminished lateral extent upon the pier,—a form more in accordance with the exigencies of the vault pressures. The intrados of the flying buttress, which in St. Leu is on a level with the impost of the longitudinal vault rib, is here in Noyon considerably below this level, while its upper part reaches as high as that of St. Leu; and instead of a shallow clerestory buttress terminating where the arch abuts, there is a vigorously salient one reaching to the top of the wall. The flying buttress is thus brought to bear upon a line (which is already considerably fortified by a pier buttress) rather than upon a point. Just what the form of the structure may be under the roof I am not aware; but as this nave has a high vaulted triforium gallery, there is doubt-
