§ II. It is evident that if we are to place a continuous roof above the line of arches, we must fill up the intervals between them on the tops of the columns. We have at present nothing granted us but the bare masonry, as here at _a_, Fig. XXXV., and we must fill up the intervals between the semicircle so as to obtain a level line of support. We may first do this simply as at _b_, with plain mass of wall; so laying the roof on the top, which is the method of the pure Byzantine and Italian Romanesque. But if we find too much stress is thus laid on the arches, we may introduce small second shafts on the top of the great shaft, _a_, Fig. XXXVI., which may assist in carrying the roof, conveying great part of its weight at once to the heads of the main shafts, and relieving from its pressure the centres of the arches. [Illustration: Fig. XXXVI.] § III. The new shaft thus introduced may either remain lifted on the head of the great shaft, or may be carried to the ground in front of it, or through it, _b_, Fig. XXXVI.; in which latter case the main shaft divides into two or more minor shafts, and forms a group with the shaft brought down from above. § IV. When this shaft, brought from roof to ground, is subordinate to the main pier, and either is carried down the face of it, or forms no large part of the group, the principle is Romanesque or Gothic, _b_, Fig. XXXVI. When it becomes a bold central shaft, and the main pier splits into two minor shafts on its sides, the principle is Classical or Palladian, _c_, Fig. XXXVI. Which latter arrangement becomes absurd or unsatisfactory in proportion to the sufficiency of the main shaft to carry the roof without the help of the minor shafts or arch, which in many instances of Palladian work look as if they might be removed without danger to the building. § 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.