As matters eventuated, he raised the internal sphere so that the disproportion with the external might be reduced. The whole dome has three shells. (_a_) The majestic exterior visible to the eye, an outward roof of wood covered with lead and ribbed for the sake of ornament. (_b_) The intermediate brick cone which supports the lantern and its accessories of 700 tons weight. This springs from the level of the stone gallery, and rises in straight lines which converge at the circular opening beneath the lantern. This, although seen neither from the outside or from within, constitutes the most solid and substantial part. Between this and the outside visible shell is an ingenious network of beams supporting the latter, and at the base of this network a strengthening of which the account had better be given in Stephen's own words: "_Altho' the Dome wants not Butment, yet for greater Caution, it is hooped with Iron in this Manner; a Chanel is cut in the Bandage of Portland-Stone, in which is laid a double Chain of Iron strongly linked together at every ten Feet, and the whole Chanel filled up with Lead._"[87] (_c_) The interior dome, also of brick. The height of this third and smallest shell reaches only to the level of the curved lines of the fluted patterns of the exterior shell, a difference of from fifty to sixty feet.
Since the outside cupola does not bear the heavy weight of the lantern it has been denounced as a sham, but this is an exaggeration. It is evident, as we look at it, that it is incapable of bearing any such weight. Much more practical is the objection of Gwilt that the elaborate framework of beams supporting this outside cover is certain to decay in course of time. A third objection is that of deception--the exterior and interior are presumed to be one and the same. This is not correct. Neither roof nor steeple is assumed to have such correspondence, and Wren might surely be allowed a like liberty with his dome. As Mr. Wightwick very properly says, it will be time enough to find fault when the roofs of churches are the same outside as within.
The Romans are credited with first applying the Dome to larger buildings. It travelled eastward to Constantinople, and was in use in Italy during mediæval times. The word "Dome" is derived from the _Duomo_ of Florence, where Brunelleschi covered in the octagon with his famous cupola in the earlier part of the fifteenth century.[88] But Wren's particular study was the Pantheon, which we have no evidence whatever that he saw; and, indeed, he erected his dome without having ever seen, so far as we know, anything like it.