CHAPTER II

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.

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.

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.

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).

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).

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.