ly
not merely put for the purpose of architectural expression, but also
because they afforded an opportunity of concealing behind the lines of
a regularly curved groin rib the irregular curves which were really
formed by the junction of the vaulting surfaces. But when the vault
become more manageable in its curves after the adoption of the pointed
arch, the groin rib became adopted in the early pointed vaulting as a
means of giving expression and carrying up the lines of the
architectural design. On its edge were stones moulded with the deep
undercut hollows of early English moulding, defining the curves of the
oblique as well as of the cross arches with strongly marked lines,
and, moreover, falling on a level with each other in architectural
importance; the oblique vault of the arch is no longer a secondary
line in the vaulting design; on the contrary, the cross arches are
usually omitted, as shown in Figs. 102 and 103 (view and plan of an
early Gothic quadripartite vault); so that the cross rib, which, in
the early Romanesque wagon vault (Fig. 90), was the one marked line on
the vaulting surface, has now been obliterated, and the line of the
oblique arch (E F, Figs. 102, 103) has taken its place.

The effect of the strongly marked lines of the groin ribs, radiating
from the cap of the shaft which was their architectural support, seems
to have been so far attractive to the mediaeval builders that they soon
endeavored to improve upon it and carry it further by multiplying the
groin ribs. One of the stages of this progress is shown in Figs. 104,
105. Here it will be seen that the cross rib is again shown, and that
intermediate ribs have been introduced between it and the oblique rib.
The richness of effect of the vault is much heightened thereby; but a
very important modification in the mode of constructing it has been
introduced. As the groin ribs become multiplied, it came to be seen
that it was easier to construct them first, and fill in the spaces
afterward; accordingly the groin, instead of being, as it was in the
early days of vaulting, merely the line formed by the meeting of two
arch surfaces, became a kind of stone scaffolding or frame work,
between which the vaulting surfaces were filled in with lighter
material. This arrangement of course made an immense difference in the
whole principle of constructing the vault, and rendered it much more
ductile in the hands of the builder, more capable of taking any form
w