good proportion was secured. At Laon the ill proportion exhibited by St. Denis was partially avoided by the employment of the profile b, Fig. 133, for the transverse rib, and the profile d, Fig. 134, for the diagonal; but the result is very imperfect. The diagonals are 
FIG. 135.still disproportionately heavy. In the apsidal chapels of Senlis the profile c, Fig. 134, of the diagonals is associated with the profile, Fig. 135, for the transverse ribs. But though this tranverse rib, by its great lower round, secures proportion, it is a comparatively clumsy and inelegant form, and was not much employed after the middle of the twelfth century.
At Amiens the rib loses the square section by the simple addition of a larger round member to its under side, as at a, Fig. 136; and this may be taken as the perfected Gothic vault rib. The profile is admirable both functionally and artistically. The added member augments the strength to bear downward pressure, which is the chief pressure that the rib has to sustain, and makes it safe materially to reduce the width. From this results great lightness of effect, which
FIG. 136.
is further increased by the deep hollows that separate the round members. A fine harmony of parts is secured by the similarity of rounds and hollows, a subtle proportionate relationship by their varied magnitudes, and effective contrasts by the sharp edges of the fillet on the face of the lower round and the projecting and re-entering angles above. On such a moulding the light brings out an
