574 FRENCH ARCHITECTURE. Part II. 428. Diagram of But- tresses. from the additional breadth of base in the direction of the thrust, much less liable to fall outwards, provided the distance of the blocks from one another is not too great, and the mass of the vault does not j^ress heavily on the inter- mediate space. This last difficulty was so much felt by the earlier French architects that, as we have seen in the South of France especially, they used the roof of the side-aisle as a continuous buttress to resist the thrust of their tunnel-vault. It was surmounted also by the introduction of intersecting vaults, inasmuch as by this exj^edient all the thrusts were collected together at a point over each pier, and a resisting mass applied on that one point was sufficient to give all the stability required. This, and the desire of raising the lights as high as possible into the roof, were the principal causes that brought this form of vaulting into general use ; still it has not yet been shown that the continuous vault is not artistically the more beautiful of the two forms, if not constructively so also. There was yet another difficulty to be mastered, which was that the principal vault to be abutted was that over the nave or central part of the church, and l)ut tresses of the requisite depth would have filled up the side-aisles entirely. The difficulty first presented itself in the build- ing of the basilica of Maxentius (Woodcut No. 202), and was there got over in some- thing like the manner jiractically adopted in the Middle Ages, except that the arch was there carried inside, whereas the Gothic architects threw the abutting arch across on the outside and above the roof. Several of the previous woodcuts^ show the system of flying buttresses in various stages of advancement. The view of one of those of the choir of St. Ouen (No. 429) exliibits the system in its greatest degree of development. Here there are two ver- tical and two flying buttresses, forming a system of great lightness, Ixit at the same time of immense construct- ive strength, and when used sparingly and with elegance as in this 429. Flying Buttress of St. Ouen (From Batissier, " Histoire del' Art.") See Woodcuts Nos. 387, 395, 407, etc.
