Before, however, I proceed further, it may be advantageous—though construction does not, perhaps, come within the range of lectures in this Academy, excepting so far as it exercises an influence upon form—to say a few words on certain practical points which are necessary to the full understanding, even of the artistic portion of the subject we are considering.
In the earlier forms of vaulting, the entire strength lay in the continuous arched surface, which was constructed of brick or of stone, or of rubble bonded at intervals with brick or stone; the rubble or stone being often of the cellular material called tufa, which was much used by the early builders on account of its lightness as well as the tenacity with which it united itself to the cement.
Transverse ribs were next introduced at intervals to strengthen the wider spaces; and, at a later period, the angles were similarly fortified.
Fig. 356.
Fig. 357.
These ribs, in early examples, sprang distinctly as separate arches from the impost, the vaulting passing over them (sections, [Fig. 356]). At a later period—even in round-arched vaulting—we find the practice coming into vogue of uniting the ribs, and even the springers of the vaulting itself, by cutting them at the base out of the same blocks of stone. We see an early specimen of this in St. Bartholomew’s Church ([Fig. 357]).
When the ribs became more numerous, it often occurred that five or even eight of them had to spring from one group of capitals; and at times three at least (and subsequently more) from a single capital. It is clear that in such cases the three or more distinct forms could scarcely retain their separate existence, but that being united in their lower portions in a single block, their forms would, more or less, die one into another. Though we can trace this process in Norman work, it was not completely established till some time later.