PANTHEON, ROME

The Pantheon, built as a temple to the Roman gods, stands today as one of the most famous engineering feats ofAncient Rome. Begun in AD 118 by the Emperor Hadrian, this building features a huge portico lined with eightcolumnsthat sup-port a triangular pediment inscribed with the name of Marcus Agrippa, the Roman statesman who commissioned an earlier pan-theon on this site. Rising above the pediment is adome, which al-though unremarkable from the exterior, given its simple drum and shallow dome shell, was the largest built in antiquity. The dome, 143 feet in diameter, is made of a volcanic rock calledtufa, used to cre-ate an early form ofconcrete.This concrete was carved out into con-cave squares to reduce the weight of the material and to direct grav-ity down into the 20-foot wide walls of the building. The center of the dome has a round open window called an oculus, which, together with the entryway, provides the only sunlight into the large, unen-cumbered interior. The walls of this round temple, which act as the drum to support the dome, are covered in coloredmarble. Seven niches in the wall originally held statues of various ancient gods.

The dome, actually anarchrotated on its axis, is a Roman inven-tion. Although the Romans are also traditionally credited with the in-vention of the arch, the arch was in fact developed over time and within a variety of cultures that employed the use of the keystone. The arch is a much more sophisticated structural system than the ear-lierpost-and-lintelstructure because its shape, in addition to the keystone, directs more weight to the posts than would a straight lin-tel. Because the space beneath the lintel or arch is a void, almost all weight must be shifted to the posts or side walls, which become in-creasingly thicker as the arch or lintel becomes wider. Since lintels are inherently weaker than arches, columns might be used to support large interior rooms that have flat roofs, but an arched room need not be encumbered on the interior with wall divisions or internal columns. Instead, thick external walls can be used to buttress the in-terior space. The round arch can therefore be used in a variety of ways to manipulate interior space. An arch that is repeated creates a barrel vault, while two barrel vaults that intersect at a 90-degree an-gle form a cross vault, or groin vault. Finally, the arch that rotates in a circle becomes the dome. Because of their invention of the arch and the dome, Ancient Romans earned a great reputation for architecture that was overwhelming in scale and vast in its open interior spaces.

In the early 600s, Pope Boniface designated the Pantheon a Chris-tian church, which preserved it from destruction. The design of the massive dome later became a great source of inspiration for classi-cizing architects from theRenaissanceonward, and today the Pan-theon is one of the most visited sites in all of Italy.