builders should have conceived the idea of making all the ribs of just the same curvature but of different length according to their several positions. This they did in Sherborne Abbey nave (vaulted 1475-1504).[251] Here the builders very logically used the shortest rib as a measure and connected the points at corresponding distances from the imposts on each rib with liernes. A central space was thus left, which at Sherborne was covered by prolonging a number of the radiants and adding a tracery of liernes and mouldings. The vault as thus constituted is not yet of pure fan type. It was first necessary to replace the ring of straight liernes by those of curved plan and to add one ring above another at the various points of intersection of the tiercerons and transverse ridge ribs, until practically the entire space to the vault crown was filled. Thus, in certain of the fan vaults of Peterborough (second half of the fifteenth century) (Fig. 39), there are three such rings leaving but a small diamond shaped central space which is largely filled by the keystone of the bay.[252] Others down the side aisles where the bays are smaller have but a single ring and a much larger central space. In vaults of the Peterborough type, the radiants are continued through this central panel in a decorative way, but in the cloister at Gloucester (before 1412) (Fig. 40), this portion of the vault is left entirely flat and decorated with tracery patterns in raised mouldings such as are usually found in window heads. The conoids, also, are covered with tracery rather than continuous ribs and the term “Fan-Tracery Vaults” might properly be used to distinguish them from the more common type.[253] [Illustration: FIG. 40.--GLOUCESTER, CATHEDRAL, CLOISTER.] In the matter of construction, fan vaulting differs from any preceding method. Its ribs are all of precisely the same curvature, their length being determined by the position which they occupy, and they are no longer supporting but rather decorative members. The lower portions of some of the vaults still resemble true ribbed vaulting in that the tas-de-charge is used, and also in the fact that the ribs still rise in a single long voussoir from their imposts to the first horizontal ring. But from this point to the crown, the ribs and mouldings are merely carved in relief upon the jointed masonry, which they therefore in no way support. In some fan vaults, as, for example, in Islip’s chapel in Westminster Abbey,[254] and in Gloucester cathedral cloister (Fig. 40), the rib is even carved upon the vault masonry for its entire length.