In England, we have not, as far as I am aware, any instance of a glazed triforium, but it is one of the most fascinating features in the later styles of the French architects, and where it retains its coloured glass, which is indispensable, produces the most fairy-like effect. It is however, questionable whether the deep shadow and constructive propriety of the English practice is not on the whole more satisfactory. In a structure of glass and iron nothing could be more appropriate than the French practice; but in a building of stone and wood more solidity is required to produce an effect which shall be permanently pleasing. VAULTS. It has already been explained how essential a part of a Gothic church the vault was, and how completely it was the governing power that gave form to the art. We have also seen the various steps by which the architects arrived at the intersecting vault, which became the typical form in the best age. In France especially the stone vault was retained throughout as a really essential feature, for though the English were so successful in the art of constructing ornamental wooden roofs, the practice never prevailed in France. [Illustration: 660. Diagram of Vaulting.] In the best age the arrangement of the French vaults was extremely simple. The aisles were generally built in square compartments, the vaults of which were first circumscribed, each by four equal arches (Woodcut No. 660), of which A A were transverse ribs or _arcs doubleaux_ as the French called them, and were used, as we have seen, in the old tunnel-vaults. These arches, as springing from the main points of support, were the principal strengtheners of the vault, and served as permanent centres for the superstructure. B was called the _formeret_, and was a rib built into the wall, of the same form as the transverse ribs, and so called because, being the first constructed, it gave the form to the vault. Lastly, there were two more ribs springing from angle to angle, and intersecting one another at C. These were called _ogives_, from the Latin word _augere_, to strengthen,[45] the chief object of their employment however being to serve as centering. In Roman vaulting similar ribs were employed, but the spaces between were subsequently filled in flush with concrete. In Renaissance and in modern work (such as in cellar or dock-vaults, for instance), when built in brick, stone voussoirs are used for the groins, because the brickwork used there would be liable to be crushed or fall out; here also the stone is flush with the brickwork, but the Mediæval architects recognised the value of the rib, not only as a permanent centre, but as suggesting the appearance as well as the reality of strength.