Lectures on the rise and development of medieval architecture; vol. 1

by Sir George Gilbert Scott

Fig. 7.

This difficulty led to the somewhat unpalatable measure of lowering the springing of the main vault so much as to bring its crown below the level of the walls, and to convert it from a barrel into a groined vault. The springing being then level with the impinging line of the aisle roofs, a good abutment was obtained, while the cross vaults afforded ample space for clerestory windows ([Fig. 7]). I called this an unpalatable expedient for two reasons:—Ist, Because it involved the loss of the entire height of the roof as a part of the interior; and, secondly, because it led to the relinquishment of the incombustible construction, by rendering it impossible to make the vaulting to form the actual roof, and the consequent necessity for a timber roof above it. In a Northern climate, however, this was not an unmitigated loss, for a vault immediately under the roof-covering is always damp, and extremely difficult of repair; and we shall see that the loss of height was soon compensated for by a subsequent invention, while the substitution of a groined for a barrel vault not only introduced a beautiful in place of a comparatively dull form, but did away with the illogical characteristic of a continuous vault supported by detached pillars; the load being now collected together into points immediately over its supports. The same cause would naturally lead to the abandonment of the half-barrel vaulting of the aisles, the need of abutment being now not continuous, but in detached points. The aisles were consequently covered with groined vaults, a cross wall being raised upon their transverse arches, or arcs-doubleaux, which served as buttresses to the main vault, or would even carry external buttresses against the clerestory wall. The blank wall in the nave, caused by the space between the groining and roof of the aisles, was subsequently occupied by a gallery, so well known as the “triforium.”

Fig. 8.

Fig. 9.

A difficulty here presented itself, which I must state before proceeding further, as much stress had been laid upon it, and it unquestionably exercised a strong influence upon the subsequent arrangements. It is this: the simple groined vault being formed by the intersection of demi-cylinders, demanded that the space covered by it should be divided into perfect squares. Now, the aisles of a church being usually about half the width of the nave, it follows that the groining of both cannot be square. If those of the aisles are so, those of the main vault must be about twice as wide as they are long ([Fig. 8]); while if these are made square, those of the aisles will be twice as long as they are wide ([Fig. 9]). The first alternative was that most usually adopted north of the Alps, though the second was more frequent in Italy. The difficulty was how to groin these oblong bays. It was not, however, a new difficulty; it had occurred in Roman structures, where it was met by the simple expedient of raising the springing of the narrower vault so high, that its crown was level with that of the wider one. This answered the purpose, but it produced a most unpleasant line of intersection, reducing the vault, in fact, for a portion of its height, to a mere strip of the arc-doubleau, and giving a winding intersection for the remainder of the height, as two cylinders of unequal diameter do not intersect in a plane. The mathematical solution of the problem would have been to make the section of the narrower vault, an upright semi-ellipse; but this does not appear to have been at any period adopted, or, if at all, in exceptional cases only. The pointed arch would have been an approximate expedient, and its introduction has been very ingeniously attributed to this difficulty,—a theory to which I shall have again to allude.

Another solution of it would be to make all the arches semi-circles, but to raise up the crown of the vaults of a smaller diameter in a curve to meet the others, thus making it (roughly speaking) a portion of an annulus instead of a cylinder.

This had one great disadvantage: that it cut off a considerable portion of the space for the clerestory windows; or, if the level of the main vault was raised to obviate this, it became impossible to have a tiebeam to the roof. The system actually adopted in most instances would appear to have been a union of that last named with the Roman mode of stilting the narrow vaults, the difference of height being made up partly by raising its springing, and partly by elevating the crown ([Fig. 10]).