[24] 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.
[25] Cf. Nelli, _op. cit._, p. 73.
[26] 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.
[27] The term “Byzantine” is often applied loosely to buildings in which only the ornamental details have a Byzantine character. But the primary and distinguishing structural feature of Byzantine architecture is the dome on pendentives. The Byzantine features of the Pazzi are involved with others derived from different systems, but they are very distinct. The central vault, though of Gothic form, is supported on pendentives, and the true dome on pendentives occurs, as we have seen, in the sanctuary and the porch.
[28] The entablature does, however, occur under vaulting in some provincial Roman buildings, as in the Pantheon of Baalbek, where it forms the wall cornice from which the vaulting springs. But this, though not defensible, is less objectionable than the Renaissance scheme of an entablature passing through the imposts of archivolts.
[29] As in the arch of the apse of St. Paul outside the wall at Rome, and in the Baptistery of Florence.
[30] The character of these details will be discussed in the chapter on the carved ornament of the Renaissance.
[31] Cf. Vasari, _Opere_, vol. 2, p. 368 _et seq._, and Milanesi’s foot-note, p. 370.
[32] _Op. cit._, vol. 1, p. 200.
[33] _Op. cit._, vol. 2, p. 541.