" Sieno fatti ventiquattro sproni, che otto ne sieno negli angoli e sedici nelle fac-cie: ciascuno sprone negli angoli grosso dappie braccia sette. Dalla parte di dentro, e di fuori, nel mezzo di detti angoli, in ciascuna faccia, sia due sproni; ciascuno grosso dappie braccia quattro ; e lunghe insieme le dette due volte, piramidalmente murate insieme insino alia sommita dell' occhio inchiuso dalla lanterna, per iguale pro-porzione.

" I detti ventiquattro sproni con le dette cupole sieno cinti intorno di sei cerchi di forti macigni, e lunghi, e bene sprangati di ferro stagnato; e di sopra a detti macigni, catene di ferro che cingano d' intorno la detta volta, co' loro sproni. Hassi a murare di sodo, nel principio braccia 5^ per altezza; e poi seguano gli sproni, e dividansi le volte.

" El primo e secondo cerchio, alto braccia 2 ; e '1 terzo e quarto, alto braccia l|; e '1 quinto e sesto cerchio, alto braccia 1: ma '1 primo circhio dappie sia, oltre a cio, afforzato con macigni lunghi per lo traverso, si che 1' una volta e 1' altra della cupola si posi in su detti macigni.

" E nelP altezza d' ogni braccia 12, o circa, delle dette volte, sieno volticciuole a botte tra P uno sprone e P altro, per andito alia detta cupola; e sotto le dette volticciuole, tra P uno sprone e P altro, sieno catene di quercia grosse, che leghino i detti sproni e cingano la volta dentro; e in su detti quercie una catena di ferro.

" Gli sproni murati tutti di pietra di macigno e pietra forte, e le facce della cupola tutte di pietra forte, legate con sprone insino all' altezza di braccia 24: e da indi in su si muri di mattoni o di spugna, secondo che si deliberera per chi allora P avra a fare, piii leggieri che pietra.

"... Murinsi le cupola nel modo di sopra, senz' alcuna armadura, massime irsino a braccia 30 ; ma da indi in su, in quel modo che sara consigliato e deliberato per quei maestri che P avranno a murare : perche nel murare la pratica insegna quello che si ha da seguire." Guasti, La Cupola di Santa Maria del Fiore, Florence, 1857, pp. 28-30.

1 Durm, Die Dotn Kuppel in Florenz, etc., Berlin, 1887, Plate I, gives an admirable illustration of the internal system of this remarkable dome, and shows the masonry of the solid base with its clamps and chains, as described in the document quoted by Guasti (note, p. 18).

And if it exists, it must be embedded in the masonry of the vault, like the chains at the base.

It will thus be seen that while Brunelleschi's scheme is essentially different from that of the Baptistery, its structural system is little more than an ingenious modification of it. The parts of the one answer to those of the other with singular completeness. The attic wall and pyramidal roof of San Giovanni are transformed into the external shell of the cathedral dome, the angle buttresses of the older monument become the great angle ribs of Brunelleschi's vault. The intermediate abutments of the Baptistery are changed into the intermediate ribs of the great dome, and the inclined barrel vaults of the Baptistery scheme are represented in the cathedral dome by the arches sprung between the great angle ribs.

It has been thought by some writers that the rib system of the dome of Florence gives the structure a somewhat Gothic character, and it is sometimes called a Gothic dome.1 But there can be no such thing as a Gothic dome. It is impossible for a dome of any kind to have the character of a Gothic vault. The difference between the two is fundamental. A Gothic vault is a vault of concentrated thrusts, and it requires effective concentrated abutments. A dome is a vault of continuous thrust, and for sound construction it requires continuous abutments, as in the Pantheon. Whatever use the ribs of Brunelleschi's vault may have, they do not, and cannot, perform the function of the ribs in Gothic vaulting. Their use is to strengthen the angles of the dome, and to augment its power of resistance to the weight of the lantern which crowns it. They do not support the vault as the ribs of a Gothic vault do. Being composed of very deep voussoirs, they have more strength to withstand thrusts, as well as to bear crushing weight, than the enclosing shells have, and thus to some extent they may hold these shells in. But it appears plain that the architect did not feel confidence in their power to perform this function without reenforcement by a chain, or chains, which, in his own words, " bind the ribs and hold the vault in " (che le-ghino i detti sproni e cingano la volta dentro). However this may be, the ribs of a dome cannot have any function like that of the ribs in Gothic vaulting. The shell of a Gothic vault is not held in by the ribs, nor is it in any way incorporated with them. Both shell and ribs are held in by the buttresses. This point will be considered further in connection with the dome of St. Peter's.

1 This idea finds expression in the latest work that I have seen on the subject: Die Kuppel des Domes Santa Maria del Piore zu Florence. Von Paul Wenz, Berlin, 1901, p. 52; also in Durm, Die Baukunst der Renaissance in Italien, p. 406.