Page:Charles Moore--Development and Character of Gothic Architecture.djvu/40

16 semicircular arch is determined by its span. In vaults over oblong compartments the crowns of the round arches which spanned the narrow sides would not reach the level of those  which spanned the longer sides. While if full semi-circular arches were used for the diagonals of such vaults their crowns would reach highest of all. Thus in Fig. 9 the height c d of the arch a c b is less than f e, the height of the arch a f g, which again is less than i h, the height of the arch a i j. A vault constructed upon such a system of arches must have an excessively domed form. To obviate this, in part, the expedient was adopted of stilting the narrow arches. The point of their springing was raised to a level considerably above the springing of the greater arches, so as to bring all the crowns nearer to the same height, and thus to reduce the amount of doming required. But even with this modification a vault over an oblong compartment, upon a system of round arched ribs, is excessively heavy, exerts powerful thrusts, and presents an awkward appearance. Oblong groined vaults, though sometimes constructed, were therefore usually avoided by the Romanesque builders, who, indeed, had rarely vaulted their naves, the portion of the building where, in North-Western Europe, oblong compartments most frequently occur. They contented themselves with vaulting the aisles whose compartments were commonly square, and where groined vaults, on round arches, were easily constructed and easily rendered secure. The introduction of the pointed arch, however, obviated these difficulties. It now became possible to construct