286 COMPARATIVE ARCHITECTURE. consisted entirely in the design of tlie vaulting planes or surfaces without reference to their meeting lines or groins, whereas mediaeval vaulting consisted in profiling the groins which were erected first and supporting the vaulting surfaces which were made to adapt themselves to them. The problem for the mediaeval architects was to vault, in stone, the nave of a church of the basilican type, and at the same time to provide for the lighting of the building by means of clerestory windows in the nave walls above the aisle roofs. The church was thus crowned with a fire-resisting covering over which a wooden roof was placed in order to protect it from the weather. The evolution of vaulting in England, as on the Continent, involved the solution of a group of constructive problems which have been already hinted at on page 272. Thus it was in con- nection with the necessity for counteracting the thrust of the nave vaults brought down on piers that the greater part of the evolution of the constructive side of the style took place. The following may be taken as the main features of vaulting in each period, and are indicated in Nos. iii and 112. Norman. — The Roman system was in vogue up to the twelfth century, but the introduction of transverse and diagonal ribs in this period rendered temporary centering necessary for these. In England the raising of the diagonal rib, which produced the domical vault employed on the Continent, seems to have been but little used, and the method was either (a) to make diagonal ribs segmental, as in the aisles at Peterborough Cathedral (No. 112 D, g); or (b) to make the diagonal ribs semicircular and stilt the springing of the transverse and longitudinal ribs. A great advance was made by the introduction of the pointed arch, which was used firstly for the transverse and wall ribs only, the diagonal ribs (i.e., those with the longest span) remaining semi- circular. Norman vaulting was either (a) cylindrical or barrel vaultmg, as at the Tower of London (No. 135) ; (b) groined cross vaulting in square bays (No. 112 a) ; (c) other shapes in which the narrower vaulting arches were stilted (No. 112 b, c), or, in the later period, were pointed ; (d) Sexpartite (six part) vaulting as in the choir at Canterbury Cathedral, rebuilt by William of Sens in a.d. 1174. Two views of this type of vaulting at the Abbaye-aux-Hommes at Caen are shown in No. 112 e, f. Early English (Thirteenth Century). — The pointed arch became permanently established, surmounting all the difficulties of difference in span, and enabling vaults of varying sizes to intersect without stilting or other contrivances, as shown in Nos. hid and 112 J, L. The cells, also known as " severies " or " infilling " were quite subordinate to the ribs and were of clunch or light stone in thin beds, resting upon the back of the ribs. These severies were of arched
