Owing to the occurrence of frequent fires in the wooden roofs of early churches, the builders aimed at carrying out the Roman practice of fireproof vaulting. This was at first done timidly and with many failures. The round barrel vault of the Romans was that generally employed; but in Provence the pointed barrel vault (Fig. 15) was adopted from an early period, being a form which exerted on the side walls a less horizontal thrust than the semi-circular arch, and was the best shape for supporting a stone roof laid upon it. In order to strengthen this vault, a projecting ring or inner arch was constructed under the main vault, and extended across the nave between each pair of the main piers. Carrying out the principle of subordination already introduced, each of these strengthening rings had a shaft with cap and base placed so as to receive it, thus indicating to the eye the principle on which it acted--viz., that of carrying the weight of the strengthening rib down to the foundation. [Illustration: FIG. 16.--Groined Vault.] By the eleventh century the principle of subordination had been carried out into every part of the structure, and some refinement was beginning to be attempted; shafts were substituted for the plain orders of the piers, and a cap and base provided for each (see Fig. 14), and the sharp angles of the orders of the arch were softened by having large roll mouldings wrought upon them. The next step in the progress of development was the subdividing or groining of the vault, so as to admit of windows for the purpose of lighting the upper part of the building, which, with the barrel vault, was found to be very dark. (Fig. 16). Many different plans were tried to improve the lighting of the vault, but without satisfactory result, till recourse was had to the intersecting vault of the Romans, still visible in many structures then surviving. This mode of vaulting consisted in the introduction of a vault crossing the structure between each set of piers at right angles to the main vault, and it had at once the desired effect, as it enabled the side walls containing windows to be carried up as high as the top of the vault, thus giving ample light. Other very important consequences followed from this improvement of the vaulting. Hitherto, with the barrel vault, the pressure had been uniform over the side walls, which were made very solid to resist it; but with the introduction of the groined vaulting, the weight of the vault was concentrated on the points where the groins rested. This necessitated the strengthening of these points, which was managed by means of buttresses. These are simply portions of wall set at right angles to the building in order to receive the weight and thrust of the arches and carry them down to the ground. The portion of the side wall between the buttresses was thus relieved from any pressure beyond its own weight. It is evident that an entire