THE ORIGIN OF THE INTERSECTING VAULT
So early as the eleventh century churches were built with one or
several aisles, and in this latter case the side aisles only had
ribbed vaults, the nave being covered by a timber roof. The next step
was to vault all three aisles, buttressing the barrel-vaulted nave by
continuous half-barrel vaults or ribbed vaults over the aisles, and
further strengthening it by projecting transverse arches, or _arcs
doubleaux_, the whole being crowned by a roof which embraced the side
aisles. These cumbrous and timidly constructed buildings were merely
imitations of the Roman basilicas. To ensure their solidity they had
perforce to be narrow; and the necessary abolition of top lighting
made them gloomy. We find then that, before the appearance of the
cupola, mediæval architects were perfectly acquainted both with the
barrel vault and the ribbed vault, the latter formed, on traditional
principles, by the interpenetration of two demi-cylinders. They had
even attempted to improve upon the construction by strengthening the
line of penetration with a salient rib, giving an elliptic arch. But
this rib was purely decorative, for in the Roman vault the stones
at the line of intersection, whether ribbed or not, were in complete
solidarity with the filling on either side in which they were buried.
It follows that we shall seek in vain in the Roman ribbed vault the
germ of the intersecting arch, with its essentially active functions.
For the origin of the intersecting arch we must turn to the eleventh
century. We shall find it in the dressed stone cupola of St. Front,
and more especially in its pendentives.
Fig. 1 gives the plan of one of the cupolas of St. Front. It is
composed of four massive transverse arches, the thrusts of which are
received upon four piers united by pendentives (Figs. 2 and 3) passing
from the re-entering angles at the spring of the arches to the base
of the circular dome itself, each of the concentric courses bearing
upon the keys of the _arcs-doubleaux_, and transmitting to them, and
therefore to the piers by which they are supported, the weight of the
cupola itself.
[Illustration: 1. PLAN OF A CUPOLA OF THE ABBEY CHURCH OF ST. FRONT AT
PÉRIGUEUX]
[Illustration: 2. PENDENTIVE (MARKED A) OF A CUPOLA OF THE ABBEY
CHURCH OF ST. FRONT]
[Illustration: 3. SECTION OF A PENDENTIVE ON THE DIAGONAL A TO B IN
PLAN, FIG. 1]
Fig. 3 is a section through one of the pendentives of St. Front,
following the line A B in Fig. 1. It shows that the first six courses
are cut so as to make what is called a _tas de chargé_; the upper
surfaces are horizontal, the faces curved to the radius of the dome
itself. After the sixth course the voussoirs are cut normally to the
curve of the arch. The vaulting of religious buildings having long
been the crux of mediæval architects, the construction of the St.
Front cupolas must have been an event much noised abroad, for towards
the close of the eleventh century a large number of churches with
cupolas were built in imitation of the mother church at Périgueux.
The construction of the churches of Angoulême and Fontevrault in the
first years of the twelfth century shows that the architects were
attempting to cover spaces of ever-increasing span on the Aquitainian
model, while at the same time they set themselves to lighten their
vaults, and consequently to reduce their points of support.
Fig. 4 gives the plan of one of the cupolas of Angoulême or of
Fontevrault, both being built on precisely similar plan, with the
exception of the number of bays to the nave.
[Illustration: 4. PLAN OF A CUPOLA OF ANGOULÊME OR FONTEVRAULT]
Fig. 5 gives the section of a bay in one of these churches, and
illustrates the considerable difference already existing between
the mother cupola of St. Front and its offspring. The cupola on
pendentives begins to show a certain attenuation, and we shall
presently note a fresh step forward towards the solution of that
problem so persistently grappled with by the mediæval architect--how
to reduce the weight of the vault.
[Illustration: 5. SECTION OF A BAY OF THE CUPOLAS OF ANGOULÊME]
The Church of St. Avit-Sénieur furnishes a most instructive example.
The cupola of this building is strengthened by stiffening ribs. It
becomes an annular vault, formed of almost horizontal keyed courses,
sustained by transverse and diagonal ribs, which act the part of a
permanent centering.
The Church of St. Pierre at Saumur marks a further step onwards in the
construction of vaults derived from the cupola.[5]
[5] _L'Architecture Romane_, by Ed. Corroyer.
Finally, the architects of Maine and Anjou achieved the long-desired
consummation. Under their treatment the pendentives resolved
themselves into their actively useful elements, the visible signs of
which were diagonal or intersecting arches, salient and independent,
set in precisely the same manner as the pendentives of the cupola
(Fig. 3), and performing identical functions (Fig. 8).
[Illustration: 6. SECTION OF A BAY IN THE CHURCH OF ST. AVIT-SÉNIEUR]
[Illustration: 7. PLAN OF VAULT ON INTERSECTING ARCHES]
The vault proper is no longer formed of concentric courses, as in the
mother cupola. It consists thenceforward of voussoirs cut normally to
the curve, and filling the triangles (A, B, C, D, Fig. 7) determined
by the longitudinal, the diagonal or intersecting, and the transverse
arches. These arches form a stone skeleton, no less solid though far
less ponderous than the cupola pendentives, and sustain the vault by
distributing its thrusts over four points of support.
The triangular fillings no longer imprison the ribs, or, more exactly
speaking, the intersecting arches, nor do they any longer neutralise
their active functions. These fillings, on the other hand, have, like
the intersecting arch, gained a new independence. They now contribute
to the elasticity of the divers organs of the vault, a most essential
element in its solidity. The peculiar arrangement of the intersecting
arches in the nave of Angers gives incontrovertible proof of the
direct filiation of this building to the Aquitainian cupola. The
voussoirs of the intersecting arches are about equal in horizontal
section to those of the transverse arches, while their vertical
section equals the thickness of the filling plus the internal salience
which marks their function. They look in fact like slices cut from
the pendentives of a cupola (A, Fig. 8). It must be remarked, too,
that at Angers the stones of the filling do not yet rest upon the
extrados of the ribs, in the fashion adopted some years later in the
Ile-de-France and elsewhere (see B, Fig. 8), but embrace them (as at
A).
[Illustration: 8. SECTION OF AN INTERSECTING ARCH]
The identity of function in the pendentive and in the Gothic
intersecting arch, both constructed, as they are, of stones dressed
normally to their curves, shows that they sprang from a common origin,
which is as much as to say that the Aquitainian cupola begat the
intersecting vault.