ing of the vault by a series of inverted curves sweeping upwards
along the height of the shallow attic. The omission of these details
not only weakened the support given to the arches of the dome, but it
also lent a stilted effect to the cupola by abruptly separating the
perpendicular lines of the drum and attic from the segment of the
vaulting. This is an error which could even now be repaired, if any
enterprising Pope undertook to complete the plan of the model. It may,
indeed, be questioned whether the omission was not due to the
difficulty of getting so many colossal statues adequately finished at
a period when the fabric still remained imperfect in more essential
parts.

Vasari, who lived in close intimacy with Michelangelo, and undoubtedly
was familiar with the model, gives a confused but very minute
description of the building. It is clear from this that the dome was
designed with two shells, both of which were to be made of carefully
selected bricks, the space between them being applied to the purpose
of an interior staircase. The dormer windows in the outer sheath not
only broke the surface of the vault, but also served to light this
passage to the lantern. Vasari's description squares with the model,
now preserved in a chamber of the Vatican basilica, and also with the
present fabric.

It would not have been necessary to dwell at greater length upon the
vaulting here but for difficulties which still surround the criticism
of this salient feature of S. Peter's. Gotti published two plans of
the cupola, which were made for him, he says, from accurate
measurements of the model taken by Cavaliere Cesare Castelli,
Lieut.-Col. of Engineers. The section drawing shows three shells
instead of two, the innermost or lowest being flattened out like the
vault of the Pantheon. Professor Josef Durm, in his essay upon the
Domes of Florence and S. Peter's, gives a minute description of the
model for the latter, and prints a carefully executed copperplate
engraving of its section. It is clear from this work that at some time
or other a third semi-spherical vault, corresponding to that of the
Pantheon, had been contemplated. This would have been structurally of
no value, and would have masked the two upper shells, which at present
crown the edifice. The model shows that the dome itself was from the
first intended to be composed of two solid vaults of masonry, in the
space between which ran the staircase leading to the lantern.