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
Such a study will, however, show a decided difference in the elevation of the transverse ribs and consequent shape of the vaults, which is worthy of some notice. If, for example, a triangle be inscribed beneath a number of these transverse arches, it will be found that the angles inside its base vary from about fifty degrees in Saint Germer-de-Fly, Rouen cathedral and Beverley Minster;[220] to fifty-five degrees in Soissons, Amiens, Salisbury, and Milan cathedrals, and Westminster Abbey; and even to sixty degrees in the cathedrals of Cologne and Reims. Moreover there is a great difference in the curve of these same transverse ribs. Those in Saint Germer, Beverley, and Rouen closely approach a semicircle, those in Amiens and Salisbury are much more pointed, but made up of two arcs without, however, a long radius with the resulting flattened appearance to be noted at Cologne and Reims and more decidedly at Milan. All this would seem to indicate that the elevation of these ribbed vaults,—and this is true of six-part and complex vaults as well,—was largely a matter of individual taste with a tendency to favor the form used at Amiens. The reason for the employment of very sharp curves like those of Reims, Cologne and Milan, was doubtless due to the appreciation on the part of the builders of the fact that such curves greatly reduced the outward thrusts, rather than to any idea of beauty of appearance to be gained, for in this they are perhaps inferior to the less pointed examples.
Rectangular Four-Part Ribbed Vaults in Churches without Side Aisles
