The dome of St. Paul’s is clearly founded on that of St. Peter’s, though subject to extensive changes (Fig. 453). The object of these changes seems to have been threefold: 1. to render it more conspicuous externally, especially from near points of view; 2. to avoid disproportionate internal height, which was the more desirable from the smaller size of the openings through which the interior is viewed; and 3. a desire to substitute _eight_ arches and pendentives for the _four_ at St. Peter’s. Thhe two former motives acting together led to the greatest peculiarity which this dome possesses, viz.--it being in fact, _two_ domes, one to be seen internally, and the other externally, with the consequent necessity for providing some independent means for the support of the culminating lantern. In this case, the proportions of the interior and exterior are alike, each having two of its own diameters from the base to the top of the dome. The external height is consequently equal in proportion to that of St. Peter’s, while its internal height is half a diameter less. We have seen at Florence and St. Peter’s that the domes consist of a double shell, connected at intervals by ribs--a very excellent method of supporting a lantern when necessity demands so difficult a piece of construction as its direct support by the dome itself. The space, however, which Sir Christopher Wren left between his external and internal domes rendered this constructional effort needless. He accordingly provided for the load on the apex by a mere _cone_ of brick intervening between his domes, giving it a threefold structure--a dome proper within forming the covering of the church below; a cone of brick above this, carrying the lantern; and a dome of timber over that, to give comeliness to the exterior, and to serve as its roof.