Fig. 349. | ||||||
A. Transverse ribs. | B. Diagonal ribs. | C. Wall ribs. | D. Ridge ribs. | |||
Before going further, I will, to prevent mistake, give the names of the parts of a groined compartment ([Fig. 349]). The main ribs from wall to wall are called by us transverse ribs; by the French, arcs doubleaux. Those which pass from angle to angle, intersecting in the middle, we call diagonal ribs; the French, arcs ogives. Those which adhere to the wall, we call wall ribs; the French, formerets. If there is a rib or moulding along the apex, we call it a ridge rib; the French, a lierne. The latter, however, does not exist in early examples. Other features appear as we proceed, but I limit my first list to the simpler forms of vaulting. The French names are found in the treatise of Philibert de l’Orme, a work of the sixteenth century; whether they have been traditionally kept up I do not know, but they are now universally adopted by French writers on the subject.
I will just go over our leading cases, already treated of, showing the changes effected in them by the use of the pointed arch.
In the square groined space with level ridges there was no alteration excepting in the form of the arch, and in the more finished mouldings made use of. The diagonal ribs often took the form of a round arch, but this depended wholly on the proportions of the surrounding pointed arches.
As the diagonals were not formed by elliptical curves, it followed that the vaulting surfaces were not portions of cylinders, and that an error had to be thrown into them. In fact, they were filled in from rib to rib without any view to purely geometrical forms.
When the pointed arch is applied to an oblong compartment, or to the sides of a polygonal apse, its advantage becomes more manifest; for the power of making the narrow arches against the walls as high as we please wholly removes the difficulty which we encountered while limited to the round arch, and that without the necessity of stilting, though the convenience which the last-named method offered for the introduction of windows still led to its frequent use.
The irregular compartments of an apsidal aisle ceased now to present difficulties, as all their arches could be made of equal height.
It is curious that, while we have in London two specimens of such aisles in the round-arched style (those in the Tower of London and St. Bartholomew’s), so have we also two in the pointed arched style, and those very different indeed in their treatment. The aisle round the apse of Westminster Abbey has compartments enormously wider on one side than on the other, and this is met simply by the varied proportions of the arches ([Fig. 350]); while that surrounding the round portion of the Temple Church[51] has double as many compartments as there are pillars in the arcade, and, consequently, behind every arch of the great arcade is a groined compartment which is nearly square, while behind every pillar is one of a triangular plan, vaulted in a peculiar manner from its corners without any ribs between the transverse ribs.