s an ellipse with a very flat crown, and very liable to fail.
If we attempt to make the oblique arch a segment only of a large
circle, as in the dotted line at 94, so as to keep it the same level
as the other without being so flat at the top, the crown of the arch
is safer, but this can only be done at the cost of getting a queer
twist in the line of the oblique arch, as shown at D, Fig. 93. The
like result of a twist of the line of the oblique arch would occur if
the two sides of the space we are vaulting over were of different
lengths, i.e. if the vaulting space were otherwise than a square, as
long as we are using circular arches. If we attempt to make the
oblique arches complete circles, as at Fig. 96, we see that they must
necessarily rise higher than the cross and side arches, so that the
roof would be in a succession of domical forms, as at Fig. 97. There
is the further expedient of "stilting" the cross arches, that is,
making the real arch spring from a point above the impost and building
the lower portion of it vertical, as shown in Fig. 98. This device of
stilting the smaller arches to raise their crowns to the level of
those of the larger arches was in constant use in Byzantine and early
Romanesque architecture, in the kind of manner shown in the sketch,
Fig. 99; and a very clumsy and makeshift method of dealing with the
problem it is; but something of the kind was inevitable as long as
nothing but the round arch was available for covering contiguous
spaces of different widths. The whole of these difficulties were
approximately got over in theory, and almost entirely in practice, by
the adoption of the pointed arch. By its means, as will be seen in
Fig. 100, arches over spaces of different widths could be carried to
the same height, yet with little difference in their curves at the
springing, and without the necessity of employing a dangerously flat
elliptical form in the oblique arch. A sketch of the Gothic vault in
this form, and as the intersection of the surfaces of pointed vaults,
is shown in Fig. 101.

But now another and most important change was to come over the vault.
The mediaeval architects were not satisfied with the mere edge left by
the Romans in their vaults, and even before the full Gothic period the
Roman builders had emphasized their oblique arches in many cases by
ponderous courses of moulded or unmoulded stone in the form of
vaulting ribs. These, in the case of Norman building, were probab