§ V. The form _a_ is a more pure Northern Gothic type than even _b_, which is the connecting link between it and the classical type. It is found chiefly in English and other northern Gothic, and in early Lombardic, and is, I doubt not, derived as above explained, Chap. I. § XXVII. _b_ is a general French Gothic and French Romanesque form, as in great purity at Valence.
The small shafts of the form _a_ and _b_, as being northern, are generally connected with steep vaulted roofs, and receive for that reason the name of vaulting shafts.
§ VI. Of these forms _b_, Fig. XXXV., is the purest and most sublime, expressing the power of the arch most distinctly. All the others have some appearance of dovetailing and morticing of timber rather than stonework; nor have I ever yet seen a single instance, quite satisfactory, of the management of the capital of the main shaft, when it had either to sustain the base of the vaulting shaft, as in _a_, or to suffer it to pass through it, as in _b_, Fig. XXXVI. Nor is the bracket which frequently carries the vaulting shaft in English work a fitting support for a portion of the fabric which is at all events presumed to carry a considerable part of the weight of the roof.
§ VII. The triangular spaces on the flanks of the arch are called Spandrils, and if the masonry of these should be found, in any of its forms, too heavy for the arch, their weight may be diminished, while their strength remains the same, by piercing them with circular holes or lights. This is rarely necessary in ordinary architecture, though sometimes of great use in bridges and iron roofs (a succession of such circles may be seen, for instance, in the spandrils at the Euston Square station); but, from its constructional value, it becomes the best form in which to arrange spandril decorations, as we shall see hereafter.
§ VIII. The height of the load above the arch is determined by the needs of the building and possible length of the shaft; but with this we have at present nothing to do, for we have performed the task which was set us. We have ascertained, as it was required that we should in § VI. of Chap. III. (A), the construction of walls; (B), that of piers; (C), that of piers with lintels or arches prepared for roofing. We have next, therefore, to examine (D) the structure of the roof.