The first idea for obviating it was to lower the springing of the vault for the sake of bringing the thrust to bear upon a portion of the wall more capable of resisting it, and, by raising the nave relatively to its aisles, to obtain space for a range of small windows between the roof of the aisle and the springing of the main vault (Fig. 6). This, however, was a most unsatisfactory arrangement--it compromised the security of the structure, and gained but a very miserable range of lights. [Illustration: 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:--1st, 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_.” [Illustration: Fig. 8.] [Illustration: Fig. 9.]