119. The Roman Arch and Dome
From the Etruscans their neighbors, the Romans, learned the arch. They too adopted it at first for utilitarian purposes. The great sewer of Rome, for instance, the Cloaca Maxima, is an arched vault of brick. Gradually, however, as the Romans grew in numbers and wealth and acquired a taste for public undertakings, they transferred the construction to stone and introduced it into their buildings. By the time their polity changed from the republican to the imperial form, the arch was the most characteristic feature of their architecture. The Greeks had built porticos of columns; the Romans erected frontages of rows of arches. The exterior of their circus, the Coliseum, is a series of stories of arches. Much of the mass of the structure also rests upon arches, thus making possible the building of the huge edifice with a minimum of material. On the practical side, this is one of the chief values of the arch. The skill which evolved it eliminates a large percentage of brute labor. Earlier peoples would have felt it necessary to fill the space between the interior tiers of seats and the outer wall of the Coliseum.
Once the fever of architecture had infected them, the Romans went beyond the simple arch and vault. They invented the dome. As the simplest arch, such as a doorway or window, a perforation in a wall, is essentially two dimensional, and a vault is the projecting of this plane area into the three dimensions of a half cylinder, so the dome can be conceived as the extension of the arch into another three-dimensional form, the half sphere. Their relations are those of a hoop, a barrel, and a hollow ball. Imagine a vault revolved on a central vertical pivot, and it will describe the surface of a dome. Two intersecting arches can be served by a single keystone. Theoretically, more and more arches can be introduced to intersect at the same point, until they form a continuous spheroid surface. Neither construction nor the evolution of the dome did actually take place by this method of compounding arches, which however serves to illustrate the logical relation of the two structures.
The Roman engineers put domes on their Pantheon, the tomb of Hadrian, and other buildings. In the centuries in which the Mediterranean countries were Romanized, the dome and the arch, the vault and the row of arches set on pillars, became familiar to all the inhabitants of the civilized western world. After Roman power crumbled, the architectural traditions survived. Even when there was decadence of execution and little monumental construction, the principles once gained were never lost.