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.
This expedient (certainly rather complicated), has been very differently dealt with by critics; some have extolled it as an original effort of genius, while others have decried it as artificial and false. Neither party have, as I think, full justice on their side. In the first place, it is not original, the same principle having been, centuries before, acted on at St. Mark’s, Venice. It is true that, in that case, not the external dome alone, but the lantern with its supports, are all of timber. This does not, however, alter the principle in the least; for we have the threefold structure—the dome proper, the supports of the lantern, and the external dome—just as at St. Paul’s, as a glance at the sections of the two will at once prove.[70] In more recent structures, iron supports for the lantern have been substituted for the brick cone, bringing it still more nearly to the type given by St. Mark’s. In the second place, I hardly think, with this Mediæval precedent before us, we need be so squeamish about the expedient being artificial. We constantly find double coverings to our ancient churches—the vault to be seen within and the roof without—and if we desire to place a flèche riding upon the roof, we support it by constructions concealed between the two. This is precisely what Wren has done. The only difference against him is, that his roof is domical, and suggests to the thoughtless observer that it is the same which he sees within—a mere peccadillo, after all—and amply atoned by the fact that you gain by it the power of giving due height both within and without, and avoid the difficulty and danger of supporting a massive structure of stone, as large as some church steeples, upon the apex of a dome. Anyhow, public opinion has decided in favour of the expedient, for a majority of subsequent domes are constructed on the same principle; while I almost defy an architect now designing a dome and experiencing these two difficulties—(1.) the artistic difficulty of making the same dome look well from within and without; and (2.) the constructive difficulty of balancing a steeple on the top of his dome—to resist the temptation presented by this simple expedient; and the more so when conscious of having for it a Mediæval precedent.
Fig. 454.—Ground Plan, St. Paul’s, London.
Fig. 455.—Ground Plan, St. Peter’s, Rome. The shaded portions indicate the Crypt.
The dome of St. Paul’s is, externally, perhaps more successful than any other. Internally, it is good from the supporting arches upwards, excepting that it is damaged by the unreasonable system on which its painted decorations have been designed.