SINCE this chapter must be somewhat technical, the following abstract may be of use.
The origin of Gothic lies in the attempt to vault a basilica, and in the consequent use of ribbed vaults. These sprang from the groined or intersecting vault derived from the Romans. The groined vault was a solid mass consisting of two equal semi-cylindrical tunnels crossing at right angles. The Gothic vault consisted of arched ribs placed at right angles to each other and diagonally, forming a skeleton of stone work above all oblong or square space, and supporting thin independent stone panels, which in France are made up of a number of flat arches of small stones thrown from rib to rib, and which are therefore self-supporting. In England they are frequentlt, not arched, but flat, is if made of planks, and consequently weak and in need of intermediate ribs (tiercerons) to support them. The pointed arch was used as a means of getting the summits of the arches approximately level.
The diagonals and interniediate tiercerons being of different lengths would rise to different heights; their curve therefore was altered in the upper part to keep all the crowns level. Hence arose the fourcentred arch. To hide the breaks of curve, cross-pieces -- liernes -- were inserted with bosses at the intersections (lierne vaulting). If the ribs are very numerous and have all the same curve, an inverted cone or fan results from this construction (fan vaulting).
THE development of Gothic architecture springs from the attempt to put a stone roof on the thin walls of the central aisle of a basilica, a thing which the Romans had never tried to do. They had put concrete vaults over the great halls of their baths, and had built domes of horizontally1 coursed brickwork as at the Pantheon, in both of which cases the thrust became very small as soon as the wonderful pozzolana mortar had set; but when the boundless resources of the Empire had failed such work became____________________