1.3 Tivoli, Hadrian's Villa, Canopus, after 130. ( R. G. Calkins.)

leaving open both the lateral and the longitudinal
sides. This vault forms a creased, sometimes domi-
cal, canopy that forms the structural unit known as
the bay. The corner supports need buttressing,
preferably at a forty-five-degree angle, unless
abutted by similar bays, in which case some form of
lateral buttressing must brace the supports against
the outward thrust. In Rome, the main halls of huge
baths constructed of bricks and concrete, such as the
Baths of Caracalla ( A.D. 212-16), the Baths of
Diocletian (298-306), or the colossal Basilica of
Maxentius and Constantine ( ca. 307-12; fig.
1.4 ), also used this same system over their main halls
consisting of a longitudinal barrel vault intersected
by three transverse ones that formed three groin
vaulted bays. In the Basilica of Maxentius, three mas-
sive transverse barrel vaults covered the aisle bays on
each side and the walls between them absorbed the
lateral forces generated by the groin vaults.

When an arch is rotated around a center point, it
forms a dome. The dome requires a hefty supporting
system around its complete circumference because
it transmits its forces radially outward. The Romans
not only perfected the use of the hemispherical
dome in the Pantheon in Rome ( ca. 118-28; figs.
1.5 - 1.6 ), but also wrought exotic and structurally
sound variations on it. The huge coffered dome of
the Pantheon -- a monolithic mass like a giant
saucepan lid, 142 feet in diameter and constructed
of concrete reinforced with interlocking brick arch-
es -- sits on a thick cylindrical wall eroded on the
inside by square and semicircular niches screened by
columns. The mass and weight of this dome is not
immediately apparent, however, because the dimin-
ishing coffering of its surface gives the illusion of
converging ribs and the classical orders and entabla-
ture of the drum give it a logical, visual supporting
base. However, in the vault of the Serapheum ( fig.
1.7 ) opening onto the Canopus of Hadrian's Villa at
Tivoli, built at approximately the same time, a half
dome composed of triangular segments alternating
between wedged shaped barrel vaults and flat
upward curving elements, echoes the lively undula-
tion of arches and entablatures around the Canopus.

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