Page:A history of architecture on the comparative method for the student, craftsman, and amateur.djvu/348

 2gO COMPARATIVE ARCHITECTURE. doors and windows, and tracery work in panelling, possibly with a desire to harmonize with the important superstructure of vaulting. The special forms of vault used in Chapter Houses are referred to on page 299. '■^Pendant'" vaulting is a later form often used,in connection with fan vaulting, in which pendants as elongated voussoirs are dropped from a constructive pointed arch, concealed above the vaulting, and form abutments to support the pendant conoids. Henry Vn.'s Chapel and Oxford Cathedral are examples of this method of vaulting. Examples of " pendant " but not of " fan " vaulting are frequent in the Flamboyant period (fifteenth century) in France, as at Caudebec, and other places. Bosses. — The bosses, or ornamental keystones, which form such decorative features in Gothic vaulting, were a constructive necessity, primarily used to cover the awkward junction of the various ribs meeting at all angles, in order that the awkward mitres of the rib mouldings might be hidden behind the ornament of the boss. Open Timber Roofs of the Middle Ages. The open timber roofs of the Middle Ages are a special English feature and may be classed in the following five divisions, being illustrated on No. 113: — (i.) Tie-beam Roofs. (2.) Trussed rafter or single-framed Roofs. (3.) Hammer-beam Roofs of various forms. (4.) Collar-braced Roofs, including arch-braced roofs. (5.) Aisle Roofs of several forms, (i.) The "Tie-beam Roof" is the earliest form of which there is any record, and the simplest in construction, being merely two rafters pitching one against another with the tie- beam inserted, holding their lower portions to counteract the outward thrust on the walls. This was probably the only form known at the Norman period, and it was never entirely discarded by mediaeval builders, being used in every succeeding style (No. 113 A, b). In the early examples, the beam is merely pinned to the wall-plate at either end and unconnected with the rafters. Various methods were afterwards adopted in order to make the truss harmonize well with other features. The tie beam was rarely straight, being cambered or curved ; in the later examples this camber governed the pitch of the roof, the purlins resting immediately on it, as at 'ellingborough Church. Curved braces were often inserted, connecting the tie-beam with wall-pieces (No. 113 b), the whole being framed together and