Fig. 1.
In the Baths of Diocletian, the Basilica of Maxentius, and other great Roman halls, this was met by cross walls pierced only by small archways, and placed at intervals, dividing the aisles into chambers, each of which was covered by a short barrel vault at right angles to that over the central space ([Fig. 1]). This, however, would be inconsistent with the uses of a church, and, indeed, applies to a groined rather than a barrel vault, though a very similar expedient was sometimes used by the Romanesque builders, by covering the aisles with cross barrel vaults, as those above described, supported by arches across the aisles, instead of by cross walls ([Fig. 2]). Another system was to cover the aisles by a half or little more than a half longitudinal barrel roof, forming a continuous arched buttress to the continuous central vault ([Fig. 3]). This gave them a perfectly vaulted building of trustworthy construction, provided only that the aisle walls were of sufficient strength. The barrel vaults were often both strengthened and their monotony relieved by arched ribs added to their thickness over each pillar of the nave, and repeated over the aisles, while these planes of extra strength were carried through to the exterior in the form of buttresses of small projection against the aisle walls ([Fig. 4]).
| Fig. 2. | Fig. 3. |
Fig. 4.
The builders of such churches were not, however, ignorant of the principles of the groined or intersecting vault formed by the inter-penetration of two demi-cylinders, and so largely used by the Romans. They did not use them in such buildings, because their main vault rising into the roof, they could not, under the same roof-plane, introduce the intersecting vaults,—though this had been effected in Roman structures by a series of cross gables over the cross vaults. In churches of the same kind, however, we find the groined vault used to carry a gallery in the aisles, all the rest remaining as before ([Fig. 5]).