The
lower and flatter shell, which appears also in the model, had no
connection with the substantial portions of the edifice. It was an
addition, perhaps an afterthought, designed possibly to serve as a
ground for surface-decoration, or to provide an alternative scheme for
the completion of the dome. Had Michelangelo really planned this
innermost sheath, we could not credit him with the soaring sweep
upwards of the mighty dome, its height and lightness, luminosity and
space. The roof that met the eye internally would have been
considerably lower and tamer, superfluous in the construction of the
church, and bearing no right relation to the external curves of the
vaulting. There would, moreover, have been a long dark funnel leading
to the lantern. Heath Wilson would then have been justified in certain
critical conclusions which may here be stated in his own words.
"According to Michelangelo's idea, the cupola was formed of three
vaults over each other. Apparently the inner one was intended to
repeat the curves of the Pantheon, whilst the outer one was destined
to give height and majesty to the building externally. The central
vault, more pyramidal in form, was constructed to bear the weight of
the lantern, and approached in form the dome of the Cathedral at
Florence by Brunelleschi. Judging by the model, he meant the outer
dome to be of wood, thus anticipating the construction of Sir
Christopher Wren." Farther on, he adds that the architects who carried
out the work "omitted entirely the inner lower vault, evidently to
give height internally, and made the external cupola of brick as well
as the internal; and, to prevent it expanding, had recourse to
encircling chains of iron, which bind it at the weakest parts of the
curve." These chains, it may be mentioned parenthetically, were
strengthened by Poleni, after the lapse of some years, when the second
of the two shells showed some signs of cracking.

From Dr. Durm's minute description of the cupola, there seems to be no
doubt about the existence of this third vault in Michelangelo's wooden
model. He says that the two outer shells are carved out of one piece
of wood, while the third or innermost is made of another piece, which
has been inserted. The sunk or hollow compartments, which form the
laquear of this depressed vault, differ considerably in shape and
arrangement from those which were adopted when it was finally
rejected. The question now remains, whether the semi-sph