56

OF GROINED IUBBED VAULTS.

of the vault (Fig. 18). To obviate this objection,other ribs were introduced at intervals, which maybeconceived to be groined ribs over various oblongs,one side continually decreasing, (Fig. 19). * This prac-tice had a further advantage, as the pannels or vaultsbetween the ribs might become proportionally thinneras the principal supports increased. It is now thatthe apparent magic hardiness of pointed vaulting, andthe high embowered roof, began to display itself jfrom slender columns to stretch shades as broad asthose of the oak’s thick branches, and, in the levity ofthe pannel to the rib, to imitate that of the leaf to thebranch.

On comparing rib pointed vaulting with Romanvaulting, it will be invariably found, that the rib itselfis thinner than the uniform thickness of the Romanvault under similar circumstances; and, that thepannel, which is the principal part of the vault insuperficial quantity, sometimes does not exceed one-ninth part of the rib in thickness. The Gothicarchitects, it has been expressively said, have givento stone an apparent flexibility equal to the most ductilemetals, and have made it forget its nature, weaning itfrom its fondness to descend to the centre.

* Fig. 19. The French call the “arcs doubleaux” over thelines ad, be, “ nervures.” Those over the lines ae, d c, whenthey abut against the walls, “ formerets.” Those over a e, a i, &c.

“ tiercerons.” Those over the diagonals a c, b d, “ ogives." Theridges over om, nr, &c. “ liernes.”