into an octagon and furnishing the impost for the domes.[26] These are octagonal in elevation as well as plan and are laid up in flat panels, or gores, which meet at the crown (Fig. 6). It is a type of dome admirably suited to its impost since it presents none of the awkward appearances of a circular dome on an octagonal base.[27] It is also very practical from a structural standpoint. Since the gores are flat, the stone cutting is far less elaborate than in a hemispherical dome, and the gored dome has the further advantage of great flexibility since it may be flattened or raised at the crown, placed over a square bay or one with any number of sides, and made equilateral or with gores of different widths, all with great facility. Furthermore, when the naves are of reasonable width, as in most churches with side aisles, the thrust of the dome is very slight and its downward pressure is not excessive. [Illustration: FIG. 5.--LE PUY, CATHEDRAL.] [Illustration: FIG. 6.--LE PUY, CATHEDRAL.] But with all its structural advantages, a system like that at Le Puy was not a satisfactory solution of nave vaulting. The transverse arches were necessarily so far below the surface of the dome that the continuity of the nave as a whole was destroyed, and the appearance was rather that of a series of lantern towers or crossings juxtaposed than of a single homogeneous vault. The side aisles of Le Puy are of less importance than the nave, though the fact that some of their bays were vaulted, or revaulted, at nearly every period of mediaeval architecture makes them interesting for a study of consecutive methods. In the bays to the east the vaults are groined on stilted, round headed transverse arches in the early Romanesque manner, while the succeeding bays have pointed transverse arches with groined vaults closely resembling those of the school of Bourgogne, and the bays nearest the west end have ribbed vaults, in one case with the early heavy-torus rib, in another with the light rib of pointed section of a late Gothic rebuilding. [Illustration: FIG. 7.--POITIERS, SAINT HILAIRE.]