This form of vault is well seen in two instances in the Cathedral at Worcester. The best known of these is the Chapter-house ([Fig. 330]), a circular building, between 50 ft. and 60 ft. in diameter, whose circumference is divided into ten parts, from which small ribs run across to the central pillar. The intersecting cells of groining are at present pointed, possibly the result of a subsequent alteration, and simply intersect with the surrounding vault. In this case the central conoid is broken into a polygonal form to give piquancy to its otherwise too unbroken surface. This may be considered the father of our beautiful polygonal chapter-houses, of which I shall have more to say as I proceed.
The other instance I have alluded to at Worcester is in the crypt (Figs. 331, 332). In this, the case in question occurs not in a distinct form, but in combination with an apsidal aisle on the one side, and a vaulted span, with a central range of pillars, on the other; the last pillar forming the central point of the semicircular apse, is exactly parallel in position, and forms very similar groining to that of the Chapter-house.
Fig. 332.—View of Crypt, Worcester Cathedral.
The same problem, when applied to a polygon instead of a circle, is open to two different modes of solution. In the one, the main vault is always supposed to run from each side towards the central pillar; in the other, from each angle towards the pillar. I shall, however, have to go more minutely into this when I come to pointed-arch vaulting, to which the last-named system more especially applies.[44]
Having now briefly touched upon the most prominent forms of round-arched vaulting in its more normal form, as resulting from the barrel vault and its intersections, I will digress for a short time to consider some of the conditions which relate to what I in my last lecture stated to be the other most simple kind of vault—the dome. I do so, however, not with any idea of treating at large on a form which should be made the subject of a separate lecture, but merely to facilitate the explanation of certain indirect influences which it exercised upon ordinary vaulting.
A dome in its most typical form stands upon a circular wall; this, however, is by no means a necessary condition. It may in reality cover a square or polygonal space just as well; for, suppose a square or a polygon inscribed within the base of a hemisphere, it is clear, from the properties of a sphere, that vertical planes erected on the sides of such square or polygon will cut the hemisphere in semicircles of the diameter of those sides ([Fig. 333]). It follows, therefore, that the walls of a square or polygonal building would intersect with a dome in the form of semicircular arches standing on each of its sides; and, consequently, that such a square or polygon will carry a hemispherical dome, or rather the remainder of it left after cutting the base into a square or polygon.
