224 COMPARATIVE ARCHITECTURE. accomplished through the ordeal of the destructive, yet purifying dissolution of the Dark Ages, whence the true spirit of Roman construction emerged, cleared to a great extent of the extraneous elements with which it had been so long encrusted. Up to the end of the twelfth century the Proven9al architects had led the way, but at this period the lay architects of the North, seizing on the Provencal principle of the Pointed arch, soon developed from it the magnificent Gothic system of the perfected architecture of the thirteenth century. Romanesque Vaulting. The Roman system of plain cross vaulting (No. iii a), was used in Europe up to the twelfth century, when it began to be superseded by the "groin-rib" type of vaulting, in which a framework of ribs supported vaulting surfaces of thinner stone, known as " severies," or "in-filling." This method introduced a new principle in vaulting, viz., designing the profile of the groin ribs and leaving the form of the vaulting surfaces to adapt themselves to them ; whereas in Roman architecture the vaulting surface was first settled, and the profile of the groins followed as a matter of course. It was therefore necessary for the Romanesque architects to find the profile of the ribs, and especially that of the diagonal rib, which had previously been settled without design, as mentioned above, by the inter- section of the two vaulting surfaces meeting at right angles. If the vaulting surfaces were semi-cylindrical the diagonal groin was of necessity a semi-ellipse, but the use of ordinates, as shown in No. Ill E, does not appear to have been employed by the Romanesque architects, who surmounted the difficulty arising from the diflference of span of the diagonal and transverse ribs as follows : — {a.) On the Continent, especially in Germany and France, the vaulting ribs were usually portions of circular curves of similar curvature starting from the same level, thus the diagonal rib, having the longest span, rose to a greater height than the transverse and longitudinal ribs (No. 112, d^). The panelling was then filled in on the top of these ribs, and in consequence the structure was highly domical, (h.) In England, however, where the vaults were generally constructed with level ridges, this domical form was not used, the diflference in height between the diagonal and the transverse ribs being equalized by stilting the latter (No. 112 b, d g) or else by forming the diagonal rib as a segment of a circle, the longitudinal and transverse ribs becoming semicircular (No. 112 d'^). In vaulting an oblong compartment the difference between the heights of the diagonal and wall ribs was still greater and produced an awkward waving line of the groins on plan (Nos. 11 1 b and 112 c). In the vaulting of the naves of the Romanesque churches in
