The massive architectural monuments that have become symbols of ancient Rome were only possible
because of two technical advances: the rounded arch and concrete. The rounded arch could span greater
distances than the lintel because some of the stones’ weight was directed outward. Other structures like
the rounded vault, the cross-vault, and the dome are variations based upon the rounded arch. Concrete
was lighter than stone and offered more flexibility because it could be formed into a variety of shapes.
Together, these two innovations allowed buildings to enclose vast interior spaces.
The Pantheon is a temple dedicated to all the Roman gods and was built in the second century CE. Its
interior is a gigantic rotunda consisting of a cylinder topped with a hemispherical dome. The dome’s weight
was reduced by using lightweight aggregate (like tufa and pumice) in the concrete, and by shaping the
concrete into square indentations called coffers. At the top of the dome is a 27 foot-wide oculus—an
opening to the sky that provides natural light to the interior; the beam of light moves around through the
day reminding viewers of the heavenly gods. The oculus was a real feat of engineering (most arches have a
keystone; most domes have a cap). The center of the dome can be left open because the dome is
constructed as if it were a series of concentric rings laid on top of each other.
The Pantheon; click here for interior (from an
century painting by Panini)
Plan and cutaway view of the Pantheon
On the exterior, the Pantheon is faced with a huge temple front with Corinthian columns and a pediment.
The Pantheon stands on a raised podium and is approached by a single stairway leading to the main door.
The dome structure allows for a vast interior, which would accommodate large numbers of people
worshipping at the many altars within. Because of its grandeur, technical innovations, and perfect
proportions (a perfect sphere would fit in the interior), the Pantheon is the most influential building from
the classical period.