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THE DOME OF FLORENCE
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converge on the curb of an opening at the crown. These ribs extend in depth through the whole thickness of the double vault and unite its two shells. Between each pair of these great ribs two lesser ones are inserted within the interval that divides the two shells, and nine arches of masonry, lying in planes normal
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Fig. 7.—System of the dome.
to the curve, are sprung between the great ribs and pass through the lesser ones on each side of the polygon (Figs. 7 and 8), while a chain of heavy timbers (a, Fig. 8, and Fig. 9), in twenty-four sections, clamped together at the ends with plates of iron, binds the whole system between the haunch and the springing. So much of the internal structure can be seen in
