In St. Front and kindred structures we have seen the problem solved in one way by the introduction of Byzantine domes; but these churches were confined to a province of Southern France, and had but little influence in other districts. In St. Etienne de Nevers, St. Sernin de Toulouse, and in Notre Dame du Port at Clermont in Auvergne, and others, this difficulty is partially overcome by the building of a half vault over the upper galleries connecting the tunnel-vault of the nave with the outer main walls, and taking the strain continuously, the thickness of the outer wall not being considered of consequence. This system permitted the placing of roofing-tiles directly upon the extrados of the vaults, and the entire suppression of wooden rafters, which was advantageous in diminishing the risk of fire, although the pitch was scarcely sufficient to prevent leakage. The great disadvantage, however, was that the nave had only borrowed light, and in large churches it was inconveniently dark. Another method adopted was that of suppressing the upper gallery, and bringing the arches of the aisles up to the level of the springing of the main vault, so that the summits of the side vaults and the walls erected between them, which were at right angles to the nave, served to counteract the strain of the upper vault. We have examples of this in the cathedral of Limoges and at Fontenay, but it is open to the same objection, that of darkening the nave. Still another system consisted in binding the vault over the nave by ribs or arches thrown across to opposite piers, which were strengthened by buttresses. These buttresses, however, were built upon the top of the arches, thrown across the aisles, and did more harm than good. [Illustration: ELEVATION. ROMANESQUE CONSTRUCTION] [Illustration: SECTION.] There is an example of unusual construction at Tournus, in Burgundy, where the difficulty is effectually surmounted by the building of a number of arches at right angles to the axis of the nave, between each set of piers; but the effect is far from satisfactory. Finally at Vezelay, in France, the tunnel-vault was abandoned and diagonal intersecting vaults were thrown across the nave, framed in between semicircular arch ribs carried upon piers spaced at equal intervals, the weight being thus wholly transferred to the four points at the angles of each compartment. It was found, however, that these piers needed strengthening, as the strain upon them was excessive, and it was thus that external buttresses were resorted to, which were connected with the piers by arches, called flying buttresses, bridging the side aisles and conveying the pressure to the outer wall. A weight was placed over each buttress, generally taking the form of a pinnacle, which stiffened it and counteracted the pressure of the arch.