A gradual development of the flying-buttress, and of the compound pier, a reduction in the size of the ribs,[219] and many other structural refinements rapidly followed one another in the period subsequent to the construction of Saint Germer and led to the perfection of rectangular four-part vaulting. The cathedral of Soissons (Aisne) (cir. 1212 on) (Fig. 67), for example, shows a considerable structural advance over Saint Germer. Its ribs are more decidedly pointed though still somewhat heavy and there is no hesitation in raising the impost of the vault far above the clerestory string-course, since its thrusts are easily met by exterior flying-buttresses. DEVELOPED FOUR-PART RIBBED VAULTS It is in the cathedral of Amiens (beg. 1218) (Fig. 69), however, that the four-part vault reaches its most daring if not its most perfect form. Here the builders constructed a vault similar to that of Soissons, but rising over one hundred and forty feet from the pavement. Its ribs are perfectly proportioned and finely moulded and the buttress system is completely developed. One awkward feature does, however, appear in the fact that the builders, perhaps, in order to concentrate the thrusts of the vaults upon the narrowest possible strip of outer wall, have made the wall intersection of the window severies follow an irregular curve which does not correspond to that of the wall rib in the portion from the impost to a point near the haunch. In spite of this defect, the cathedral of Amiens may well be considered as marking the highest development of rectangular ribbed vaulting. A study of other Gothic churches will disclose few, if any, improvements, either in appearance or construction, and many of the finest closely resemble this masterpiece. THE CURVE OF VAULT RIBS