Trajan was succeeded by the emperor Hadrian who was very fond of architecture: by Hadrian’s initiative the Roman eastern provinces and Egypt were covered with splendid monuments. Hadrian ordered the construction of the double temple of Venus and Roma which was a decastyle temple (i.e. with ten columns). Each one of these temples had one cella with an apse that housed the statues of Venus and Roma. The particularity of these two cellas was that they were covered with coffered barrel vaults*. A barrel vault* is an architectural element formed by the extrusion of a curve along certain distance. Given that curves are circular in shape the barrel vaults then have a semi-cylindrical appearance; the barrel vault is the simplest type of a vault. A coffer (or coffered) surface is a sunken panel in a ceiling or vault with square, rectangle, or octagon shape; when a series of these coffers were used to decorate a ceiling or a vault they are called caissons or lacunaria because they give the ceiling a “lacunar” appearance.
In the famous villa built by Hadrian in the suburbs of Rome it is clear his predilection for the exotic forms: sometimes he used Egyptian and oriental styles in his constructions. Hadrian’s Villa was a complex with near 30 buildings, covering an area of at least 1 square kilometer (near 250 acres). This Villa had a theater, great libraries, baths, hostels, temples for Oriental and Latin cults, and must have been full of statues and art treasures of all kinds.
The famous Pantheon of Rome is in perfect condition because it has been used as a Catholic church. This building was commissioned by Marcus Agrippa during the reign of Augustus (27 BC-14 AD) to be used as a temple to all the gods of ancient Rome, and was rebuilt by Hadrian ca. 126 AD. It is located next to the ancient Baths of Agrippa. The Pantheon’s portico with its robust porphyry columns must be the same as that for Agrippa’s Baths, that is why Hadrian kept the name of the great minister of Augustus on the inscription of the facade’s frieze. The Pantheon is a circular building with a portico of large Corinthian columns (eight in the first row and two groups of four behind) holding a pediment. A rectangular vestibule or pronaos (built in time of Agrippa) connects the porch to the rotunda or circular room (built in time of Hadrian), which is under a coffered concrete dome with a central opening (called oculus) and is the only window of the building. The Pantheon’s dome* is still the largest unreinforced concrete dome in the world. The interior of the circular room and the great dome of 43.20 meters in diameter seem to be a work probably led by the Syrian Apollodorus of Damascus. The dome’s half sphere has a round opening in the upper part (oculus) where light penetrates. This dome was constructed with ribs and arches of brick later filled with concrete. This dome was the “model” from which the architects of the Renaissance later learned dome construction: Brunelleschi, the author of the first modern dome the Duomo in Florence, and Rafael, were both inspired by its design. The Pantheon still conserves its old marble pavement, but the stucco that decorated the coffered dome have fallen off and also, when it was transformed into a church, the lateral niches were modified and turned into altars. The building still retains its old brass door, originally golden, and the porch’s reliefs were polychrome.
But the most extraordinary fact of the Pantheon is that it was the first building that applied the modern concept of architecture as an art that creates interior spaces. Greek architecture was designed to be seen from the outside, where people gathered to attend the liturgical sacrifice practiced at the altar located always outside in front of the temple. The Pantheon, however, created an inner universe in which the people gathered to commune with the gods and isolate themselves from the outer cosmos. It then represents the artistic expression of a new religious sensibility that will later be the same followed by Christianity. No wonder the Roman Pantheon is the only Roman temple that today is a church.
The proportions of the Pantheon are marvelous. This temple for all gods, in which Romans sought to centralize the huge variety of cults existing throughout their empire, appeared as a synthesis of heaven and earth. That is why it consists of a circular floor plan closed by a dome. Both the height of the interior of the dome as well as the diameter inscribed by the circular wall are 43.20 meters. If we imagine the complete sphere delineated by the vault we will have the whole globe resting on the ground. The sphere that statically rests inside the cylinder has a radius of 21.60 meters which corresponds precisely to both the radius of the cylinder and its height.
The Pantheon retains inside the general lines of Greek architecture but combined with the characteristic Roman vaults. The same combination can be seen in great religious buildings of the following centuries: in the temple of Antoninus and Faustina, the Roman Forum, the Temple of the Sun, the Quirinal, and the great temple of Neptune in Rome.
*Dome: (From Latin: domus) is an architectural element that resembles the hollow upper half of a sphere. A dome can rest upon a rotunda or drum, and can be supported by columns or piers that transition to the dome through squinches or pendentives. A lantern may cover an oculus and may itself have another dome. There are also a wide variety of forms and specialized terms to describe them.