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. [Illustration: Fig. 355.--St. Saviour’s, Southwark. Vaulting of cells adjoining the clerestory.] The liberty which was felt in dealing with the surfaces of vaulting-spaces, when once the salient lines became emphasised by ribs, led to the practice of the “doming up,” as it is called, of those spaces, whether the ridges were raised or level; that is to say, that each course of the filling-in stonework was often laid on a curve, so as to increase the strength of the work, by rendering every course a kind of arch from rib to rib. I must, however, reserve to my next lecture a description of many of the forms which the vaulting of this period assumed, and a number of practical facts relating to it; as well as the pursuit of the subject into its more advanced history; where, instead of limiting its features to such as originated in obvious and functional utility, others were added for purely decorative purposes. The subject is so extensive that I am compelled to divide my lecture upon it abruptly. Let us, then, pause here and consider for a moment the artistic sentiment and character of the stage at which we have arrived. I will suggest, in passing, that this stage, in which no architectural features were introduced for mere purposes of decoration, and which consequently leaves wide vaulting-spaces, is peculiarly suited to the extensive introduction of the works of the sister arts of painting and mosaic, which may be used almost as freely here as in the Byzantine domes. The point to which, however, I desire to direct your attention is rather the _purely architectural_ sentiment.