=Rib-vaulting.=--While this was possible in the actual operation of placing the stones, it would have needed exceedingly delicate calculation to build timber centering adjusted in advance to these domelike surfaces. Moreover, the ponderousness of the dome nave vaulting had made the use of timber centering extremely costly, even where timber was plentiful; while in districts sparsely supplied the cost had been prohibitive. Consequently, the ingenuity of the builders devised a system of construction that reduced the need of timber centering to a minimum. This was the system known as rib-vaulting. Briefly, it consisted in spanning the space--longitudinally, transversely, and diagonally--with preliminary arches of masonry, thus forming a skeleton frame composed of what are known as ribs. Each of these ribs, being comparatively light, could be constructed on a single moveable and expansible piece of centering, called a _cerce_. When the ribs had set, they offered sufficient support to hold up the doming of the compartments while it was being laid. To some extent this method of construction had been anticipated by the Romans who in certain instances built preliminary transverse ribs to act as permanent centerings of the vault, in the masonry of which the ribs were buried from sight. The reintroduction of this device and its further development, as above described, originated with the Lombard architects. This has been definitely determined by the English architect, Arthur Kingsley Porter, who has proved that the adoption of the system was prompted by the scarcity of wood in this locality. From Italy it spread to France, where it made its appearance in the Ile de France about 1100 or some 60 years after its adoption in Lombardy. It was at first employed purely as a necessary constructive expedient. Later its æsthetic possibilities came to be recognised, and the rib was developed by the Gothic architects into an element of great beauty, one of the characteristic features of the Gothic style. Meanwhile, the use of vaulting by the Romanesque architects affected the character of the exterior. Mention has already been made of the masonry piers and the massive outside walls, pierced with small windows. For the further support of the vaulting-thrust towers were freely used. While in Italy the campanile was frequently detached from the main edifice, the towers in western and northern Romanesque churches became elements of prominence in the design. A pair frequently flanked the apse or four rose in the angles of the transepts and choir, while another pair, sometimes connected by a gallery, flanked the west end. A tower or dome might also surmount the _crossing_ of the nave and transepts. The towers were square, polygonal, or circular, divided into stories which were pierced with windows or embellished with arcades. They were crowned, like the nave and aisles, with an exterior sloping roof.