In both, the narrow arches are greatly stilted; and at first sight the two may appear to be similarly treated; but on closer examination there will be found to be much difference between them. In the Tower Chapel the transverse ribs are made to increase prodigiously in width towards the outer wall, so as to reduce the want of parallelism of the groined compartments, a very unsightly expedient; and the capitals of the columns are square, which makes the backs of the arches they support nearly double the width they present in front: while at St. Bartholomew’s, the ribs are of uniform width, and the capitals, instead of being square, have their sides radiating from the centre of the apse, so as to share with their arches the spreading of their outer side. The difficulty is really increased in the later work, but is met by more skilful workmanship. Somewhat similar to the case of the aisle round a semicircular apse, is the case of vaulting a circular building with a central pillar. In each, the main surrounding vault, if uncut by others, would assume the form of a portion of an _annulus_ or _ring_. In the aisle such a ring would be wide in the opening it surrounds, but in the circular building its opening would be reduced to the diameter of the central column or its capital. This annulus, or curved vault, would become divided in plan into triangular portions by the transverse ribs which would meet on the central pillar and the cross vaults, proceeding from the surrounding arches, would intersect with only the _outer portion_ of the vault, the _inner portion_ which rests on the pillars being uncut by them, and assume the form of a concave conoid, something like the flower of the convolvulus. [Illustration: Fig. 330.--Chapter-house, Worcester Cathedral.] [Illustration: Fig. 331. Plan of Crypt, Worcester Cathedral.] 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.