The whole scheme of this dome was a daring innovation of one man, and in this it differs from former architectural innovations, which were the comparatively slow outcome of corporate endeavour, progressive changes being so gradual that no wide or sudden departures from habitual modes of building were made at any one time, or by any one person.
It was a prodigious undertaking. The span of the dome is nearly a hundred and forty feet, the springing level is a hundred and seventy-five feet above the pavement, and the height of the dome itself, exclusive of the lantern, is about a hundred and twenty feet. Such a project might well appall the most courageous of building committees, and we need not wonder that the Board of Works drew back in dismay when it was first laid before them.1 The successful accomplishment of the work, and the stability which it has thus far maintained, show that the architect was a constructor of great ability,2 and the fact that he managed to raise the vast fabric without the use of the ponderous and costly kind of centring that had been commonly employed in vaulting, makes the achievement still more remarkable. The precise manner in which he did this is not clear, but of the fact there appears no question.3
1 For a full account of the deliberations held, as well as for much else of importance relating to the building of this dome, see Professor C. E. Norton's Church Building in the Middle Ages, New York, Harper & Brothers, 1880.
2 But while Brunelleschi appears to have had great natural constructive aptitude, he had not had a sound training or experience in construction. Such training would have taught him that it would not do, under any circumstances, to spring a vault from the top of a wall, and he ought to have learned this from his study of the ancient Roman monuments.
3 Nelli, Discorsi di Archilettura, Florence, 1753, p. 74, reproduces an old drawing which purports to show the form of the scaffolding that Brunelleschi employed. This drawing bears the following inscription : "Questa Dimostrazione e di Filippo Brunelleschi Architetto fatta per e Ponti della Cupola di S. M'ra. del Kiore di Firenze nell' Anno MCCCCXIX e fu quella che mostrd quando fu lasciato in liberta di dover esser solo nell' operazione di d.a cupola senza il Ghiberti suo com-pagno non avendola voluta dar fuori prima di non essere libero Architetto di.
The dome of Florence is indeed a remarkable piece of construction, and it is no less remarkable as a work of art. In beauty of outline it has not, I think, been approached by any of the later elevated domes of which it is the parent. Yet with all of its mechanical and artistic merit, the scheme is fundamentally false in principle, since it involves a departure from sound methods of dome construction. A bulging thin shell of masonry on a large scale cannot be made secure without abutment, much less can such a shell sustain the weight of a heavy stone structure like the lantern of this monument, without resort to the extraneous means of binding chains. A builder having proper regard for true principles of construction in stone masonry would not undertake such a work. For although it may be possible to give the dome a shape that will be measurably self-sustaining as to thrusts, as Brunelleschi clearly strove to do,1 it is not possible to make it entirely so, and therefore if deprived of abutment it must be bound with chains. But a structure of masonry which depends for stability on binding chains is one of inherent weakness, and thus of false character.2
From these considerations it appears to me that Brunelleschi led the way in a wrong direction, notwithstanding the nobility of his achievement from many points of view. And in following his example modern designers of elevated domes have wandered still farther, as we shall see, from the true path of monumental art.
Da Opera; come sentiranno nella sua Vita scritta da Diversi." Brunelleschi, in his account of his intentions before the Board of Works (note, p. 19), would not explain his scheme for the scaffolding. He said merely that the vault was to be raised, without centring, to the height of 30 braccia, and from that level upwards, in the manner that should be advised by those who might then have the work in charge.
1 In his explanation of his scheme before the Board of Works, as given by Vasari, Brunelleschi begins as follows: "Considerato le difficulta di questa fabbrica, magnifici Signori Operaj, trovo che non si pud per nessun modo volgerla tonda per-fetta, atteso che sarebbe tanto grande il piano di sopra, dove va la lanterna, che mettendovi peso rovinerebbe presto." Op. cit., vol. 2, p. 347.
2 It may be thought that this would condemn the use of metal clamps in masonry, such as were inserted in the walls of the Parthenon, or the wooden ties that were, in some cases, used in parts of Gothic buildings. But there is a wide difference between such use of clamps and ties, and the binding chains of the great domes of the Renaissance. In the Greek and Gothic work the masonry forms are favourable to stability independently of the clamps and ties. These were inserted either for security against unusual dangers, as from earthquakes, or for temporary security against rupture while the work was in progress, before the interaction of the parts of the system was fully established ; but a dome without abutment violates the constant conditions of stability.
Moreover, when we consider that a dome set within its drum is not only stronger, but that it is also much better for interior effect, the dome of the Pantheon still remaining the grandest and most impressive arched ceiling of its kind in the world, the unbuttressed modern domes, with their manifold extraneous and hidden devices for security, appear still less defensible.
But in the architectural thought of the Renaissance little heed was given to structural propriety or structural expression, and the Italian writers, who have largely shaped our modern architectural ideas, have not only failed to recognize the inherent weakness of such a building as the dome of Florence, but have even considered the work praiseworthy on account of those very characteristics which make it weak. Thus Sgrilli lauds Brunelleschi for having had the " hardihood to raise to such a height the greatest cupola which until its time had ever been seen, upon a base without any abutments, a thing that had not before been done by any one." 1 And Milizia says, " It is worthy of special notice that in the construction of this cupola there are no visible abutments."2
As to the permanent stability of this dome various opinions have been held by the experts among the older writers.3 Its form is, as we have seen, as favourable to stability as it would be possible to make that of any vault which could be properly called a dome. It appears to the inexperienced eye as stable as a crest of the Apennines. Every precaution as to material and careful workmanship seems to have been taken to make it secure. The wall of the drum on which it rests is five metres in thickness, and the solid base of the dome itself is built, if the architect's scheme was carried out as he had stated it before the Board of Works, of large blocks of hard stone, thoroughly bonded and clamped with iron. The lower system is sufficiently strong, and appears to rest on a solid foundation. But nevertheless there are ruptures in various parts of the structure which have caused apprehensions of danger,1 and its future duration must be regarded as uncertain. The writers Who have maintained that it is secure have argued on the assumption that the parts of a dome all tend toward the centre.2 These writers overlook the fact that the force of gravity above, especially when the dome is heavily weighed by a lantern, neutralizes the inward tendency of the lower parts and causes a tendency in those parts to movement in the opposite direction. This neutralizing force is lessened by giving the dome a pointed form, as Brunelleschi has done, but, as before remarked (p. 22), it can hardly be overcome entirely so long as any real dome shape is preserved.3
1 Discrizione e Studj delV Insigne Fabbrica di S. Maria del Fiore, Florence, 1733, p. xxi.
2 Afemorie degli Architette, etc., Florence, 1785, vol. i, p. 190. 3 Fontana, Nelli, Cecchini, and others.