Both domes are founded in their section on the pointed arch. Their internal span is nearly the same, but their proportions differ greatly; for while that at Florence is internally only two of its diameters in height, that at Rome is two and a half; and while the former is externally one and three quarters of its diameter in height, the latter is two diameters—each irrespective of the lantern. Strangely enough, however, the great external defect of the dome of St. Peter’s is its want of height. It is so encumbered by the surrounding building that its height, from near points of view, is greatly lost. Like the mountain—which seems to be its prototype—though towering nobly in the distant view, it becomes as you approach it entangled among the nearer though smaller heights. This is obviated at Florence—at least from the eastern points of view—by the more favourable distribution of the subordinate buildings.
The boast attributed to Michael Angelo—that he would raise the Pantheon upon the top of the Temple of Peace—has more meaning than at first appears. The Temple of Peace (so called), now known to have been the Basilica of Maxentius, consists of a vast nave with aisles. The nave is divided into three square bays of between 80 and 90 feet in width, and these bays are groined. Had the pendentive dome been then known, each bay might have been covered by such a dome as that which spans the central bay of St. Sophia, and in such a case the dome of the Pantheon might, in
Fig. 453.—Half Elevation and Half Section looking East, St. Paul’s, London. (From Fergusson.)
loose language, have been said to be placed upon four piers and four arches of the Temple of Peace. But Michael Angelo aimed at much more than this. It was not the dome only, but the whole structure of the Pantheon, which (in a figure of speech) he thus intended to raise upon a square substructure open on all sides to view from other parts of the interior. Thus he raised upon his pendentives what he compared to the circular wall of the Pantheon, and on that he raised its dome. This was not, however, the whole of his task, for over the eye of the dome (as of the Pantheon) he erected again another structure—a domed rotunda—into which the eye reaches from below, and through whose windows the light penetrates into the dome. And, more than all this, instead of allowing his dome, as in the Pantheon, to be half buried within the walls of the building, he made it rise boldly from their upper surface, and gave it such a proportion as to render it an august and beautiful object from every reasonably distant point from which it is visible.
The task was indeed one of which the greatest genius might fairly boast!
Nearly every subsequent dome of any magnitude seems to have been founded, more or less, upon St. Peter’s; and, so far as I can judge, our own St. Paul’s is the noblest of them all.
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.