=The Clerestory.=--This stage again calls for little or no comment. The windows, hidden from the exterior by the curtain wall, are slightly rounded. Above and on either side are sections of spheres, ornamented with festoons. These are the ends of elliptic cylinders in connection with the vaulting. =The Vaulting.=--The great arches overhead divide the vault as the greater pilasters and their continuations do the walls. Between these arches are the small saucer-shaped domes, 26 feet in diameter. The reason for these and their accessories, the pendentives, may best be understood from Wren's own words. He says that his method of vaulting is the most geometrical, and "_is composed of Hemispheres, and their Sections only; and whereas a Sphere may be cut all Manner of Ways, and that still into Circles ... I have for just Reasons followed this way in the Vaulting of the Church of St. Paul's.... It is the lightest Manner, and requires less Butment than the Cross-vaulting, as well that it is of an agreeable View.... Vaulting by Parts of Hemispheres I have therefore followed in the Vaultings of St. Paul's, and with good reason preferred it above any other way used by Architects._"[94] The saucer-shaped domes are sections of spheres, as are both the pendentives and the sides of the clerestory windows. He set to work something in this way. After satisfying himself that he had hit on a better plan than the plain cylindrical or the cross-vaulting of the Romans, or the other forms of intersecting vaults, he seems to have taken a hemisphere as a plan to work upon, and fixed his imaginary centre about the level of the top of the triforium. In the great square western severy of the nave this was easier, but the other severies are oblong. Here he stretched his sections out, so as to include the clerestory windows and their much-needed light. The usual way of expressing this is to say that the vault is intersected across by an elliptic cylinder. The wreaths, garlands, and festoons, and the various conventional patterns with which the edges and surfaces of the various parts of the vaulting is adorned cannot be estimated from the pavement. We may add here that the pendentives were purposely constructed of "_sound Brick invested with Stucco of Cockle-shell lime_," and not of Portland stone, for further ornament if required.[95] So are the circular sections. The nave is connected with the dome by the space between the great piers or walls of more than 30 feet in length. These piers are also broader at their ends than those which support the arcading, the latter covering a square of about 10 feet. The greater massiveness is owing to their assistance being required in supporting the dome. They have large pilasters at the angles, and their coffered wagon vaulting, adorned with geometrical patterns, is very striking.