It may be thought that the object which Brunelleschi had in view, of producing a vast dome that should be an imposing feature of the cathedral externally, justifies the unsound method of construction to which he resorted (the only method by which the effect that he sought could be attained). But structural integrity is, I think, so fundamental a prerequisite of good architecture that in so far as this gifted Florentine was obliged to ignore sound principles of construction in order to attain an end not compatible with such principles, the result cannot be properly considered as an entirely noble and exemplary work of art, however much beauty and impressiveness it may have.
1 These ruptures were first observed in the year 1693 (Nelli, op. cit., p. 13), and it was then advised by the architect Carlo Fontana to add a new chain of iron. Nelli, however, argued that the fissures had not arisen from thrust, but were due to a slight yielding of the foundations, and he urged that no chain be added, but that a bit of marble be dove-tailed into the vault across the opening, in order that any further movement might be detected by the breaking of this marble. For three years no further sign of disturbance was noticed, but a slight earthquake in 1697 broke a portion of the masonry of the outer face of the dome opposite the fissure across which the marble had been placed. It appears, however, to have been concluded that there was still no danger from thrust, and no new chain was inserted. Cecchini (Opinione intorno lo Stato della gran Cupola del Duomo di Firenze, published together with Nelli's Discorsi, etc., p. 82) speaks of several cracks in both the inner and the outer shells of the vault, and also in the supporting piers, even down to the ground. But he agrees with Nelli in attributing these to movements of the foundations from which he concludes that no further danger is to be apprehended, and he affirms that the structure is entirely safe.
2 Cf. Nelli, op. cit., p. 73.
3 The thrusts of a hemispherical dome are, in some degree, restrained by the binding of its continuous courses of masonry under compression, but this is not enough for security, as experience has shown ; and in a polygonal dome, like Brunelleschi's, there is no such binding force, because there are no continuous circles of masonry.
The example set by Brunelleschi was, in point of construction, a pernicious one, and bore fruit of a still more objectionable character in the works of other gifted men less scrupulous than he, and less endowed with mechanical ingenuity, as we shall see farther on.
Though there is nothing whatever of classic Roman character in this great dome, the lantern which crowns it, built from Brunelleschi's design after his death, has classic details curiously mingled with mediaeval forms. Its eight piers are adorned with fluted Corinthian pilasters surmounted by an entablature, while the jambs of the openings have engaged columns carrying arches beneath the entablature in ancient Roman fashion. From the entablature rises a low spire with finials set about its base, and flying buttresses, adorned with classic details, are set against the piers. None of the classic details have any true classic character, nor has the ornamental carving, with which the composition is enriched, any particular excellence either of design or execution. But these details are invisible from the ground, and in its general form and proportions the lantern makes an admirable crowning feature of this finest of Renaissance domes.