In Provence the system of vaulting generally adopted was of a more complete character, derived in all probability, as already mentioned, from the Roman system (as used in the Nymphæum at Nimes), and perhaps also aided by the examples of the vaulted churches seen by the Crusaders in Syria. When the revival of the eleventh century took place, the Provençal churches were usually erected on the basilican plan, which doubtless was the traditional one. These churches are small, but they generally embrace a central nave with two side aisles, each terminated to the eastward with an apse. The roof is almost invariably composed of a pointed barrel or tunnel vault, with strengthening transverse ribs springing from the caps of pilasters carried up from the nave piers, as for instance in St Trophime at Arles. The side aisles are also arched, each with one half of a pointed vault thrown against the upper part of the nave wall, so as to abut the central vault. The roof consists of tiles laid directly on the extrados of the arches, after the Roman manner, so that there is here nothing liable to suffer from fire. There is, however, it will be noticed, one remarkable divergence from the Roman model, in which the vaults and arches are always round. In Provence they are invariably pointed. This form of vault, as mentioned by Mérimée, Fergusson, and others, was adopted, not from choice but as a necessity, or at least a convenience of construction. The pointed form was found to have several advantages over the round. It was easier of construction, a matter of great consequence in those rude times; it exerted less thrust on the side walls, and was therefore more stable; and it fitted better the slope of the tiled roof covering. It is evident that the roof of the side aisles, in order properly to abut the central vault, had to be carried up to a considerable height. This height being more than was necessary in the aisles, is sometimes divided into two storys, the upper one forming a gallery--an arrangement which was frequently adopted in Lombardy and the Rhineland, and also, as we have seen, in the Syrian churches above referred to. One great objection to the Provençal system of vaulting is that the churches are very dark--a clerestory being obviously impossible consistently with safety. Numerous expedients were adopted to provide more light, such as by introducing windows in the gables, and by heightening the side walls so as to admit of a small clerestory over the roof of the aisles. But the latter was found to be a very unsafe course, and at the best only clerestory windows of very small size could be introduced, so that the long barrel vaults still remained dark and gloomy.