Suppose, for example, an oblong compartment with pointed arches of similar proportions on all its sides and on its diagonals, and the vaults of each cell generated by the motion of the curve of the surrounding arches towards the point of intersection, guided by the diagonals, we obtain at once a vault with pointed arches and raised ridges, the precise correlative of that before described with round arches and raised ridges, and one in which the filling of the vaulted spaces assumes a systematic and accurately geometrical form ([Fig. 354]).
Fig. 354.
The proportions of the arches are not essentials, but where the narrower arches approach the height of the wider ones, a curious effect is produced on the form of the ridge, which, always elliptical, becomes then so obviously so as to be unpleasing.
This form of vault was of very frequent use, though the exact method of filling in the spaces was not rigidly adhered to. Its disadvantages are, that it either limits the height of the walls available for windows, or runs up so high into the roof as to interfere with its construction. It is in many cases, however, a very convenient, as it is a very sightly, form of vaulting.
Even the simple form of vaulting with level ridges is not always convenient for windows, particularly in clerestories, where they have often to fill the whole space. This led to the practice of stilting the wall-rib to such a degree, as to have the effect of twisting the groined surface of the cross vault to an extraordinary extent. This may be seen in the vaulting of the cells adjoining the clerestory at Westminster Abbey, and at St. Saviour’s, Southwark ([Fig. 355]). This twisting of the surface has received the very appropriate name of ploughshare vaulting.
Fig. 355.—St. Saviour’s, Southwark. Vaulting of cells adjoining the clerestory.