intervals at each end of each compartment, over which segments of barrel vaulting, of a form generated by the elliptical lunettes of the clerestory, are turned. The pendentives thus have a peculiar shape, and are segments of a hemisphere cut by four vertical planes coinciding with the sides of the vault compartment, by a horizontal plane at the base of the dome, and by the interpenetrating barrel vaults. The compartments are separated by transverse ribs, and these, together with the groins formed by the meeting of the pendentives and the interpenetrating lunette vaults, give a somewhat mediæval effect to the vaulting conoid. In other words the lines of the groins and the lunette arches form a combination not unlike that of Gothic vaulting. This may have been one of the points in which Wren fancied that he could "reconcile the Gothic to a better manner."
In the great order Wren has departed from the scheme of St. Peter's in giving only one pilaster to each pier of the nave, though in the larger piers under the great dome he has set them in pairs. Under the archivolts of "the great arcade and under the aisle vaulting the smaller pilasters are coupled, while in St. Peter's they are single. With the details of these orders the architect took great liberties in utter disregard of the canons of Vitruvius and the neo-classic authorities. The crowns of the great arches reach high above the capitals of the pilasters, so that a complete entablature cannot pass over them. It would not, of course, do to allow the archivolts to cut into an entablature, and Wren has therefore omitted the architrave and frieze in the intervals of the order, and has included them only in a ressaut over each pilaster, the cornice alone being carried over the arch. To give the vaulting its admirable elevation without unduly magnifying the great order, as Michael Angelo did in St. Peter's, an attic is interposed, but to spring a vault from an attic wall is an architectural barbarism; though it is perhaps no greater one than to spring it from an entablature, as the architects of the Renaissance had done from Brunelleschi down. In the small order of the aisles the entablature is simplified, and has only an architrave and cornice; while a member, like a diminutive attic, in retreat of the cornice, is interposed at the impost. It looks as if this had been done in order to raise the springing of the arches so that no part of them would be cut
