117. The True Arch
The true arch differs from the corbelled in needing no counterweight. The blocks that form the under surface or soffit of its span are self-sustaining. The true arch thus yields an æsthetic satisfaction which can be attained in no other way, especially when it soars in magnitude. The fundamental principle of the true arch is the integration of its elements. Such an arch is nothing until completed; but from that moment its constituents fuse their strength. Each block has a shape which is predetermined by the design of the whole, and each is useless, in fact, not even self-supporting, until all the others have been fitted with it. Hence the figure of speech as well as the reality of the keystone: the last block slipped into place, locking itself and all the others. The features of the blocks or “voussoirs” which makes possible this integration, is the taper of their sides. Each is a gently sloping piece of wedge instead of a rectangular block. When bricks replace dressed stone, the mortar takes the place of this shaping, being thinner toward the inner face of the vault and thicker toward the interior of the construction.
A true arch in process of erection would instantly collapse if not held up. It can be built only over a scaffold or “centering.” Once however the keystone has wedged its parts together, it not only stands by itself but will support an enormous weight. The greater the pressure from above, the more tightly are the blocks forced together. Instability in a true arch is not due to the bending stress coming from the superimposed mass, as in the corbelled arch or a horizontal roofing. The blocks are subjected only to crushing pressure, which stone and brick are specially adapted to withstand. The weakness of the arch is that it turns vertical into horizontal thrust. With more weight piled on top, the sidewise thrust, the inclination to spread apart, becomes greater, and must be resisted by buttressing. This is what the Hindus mean when they say that “the arch never sleeps.”
118. Babylonian and Etruscan Beginnings
While the exact circumstances attending the invention of the true arch are not clear, the earliest specimens preserved are from the ancient brick-building peoples of Babylonia, especially at Nippur about 3,000 B.C. Thence the principle of the arch was carried to adjacent Assyria. Both these Mesopotamian peoples employed the arch chiefly on a small scale in roofing doors and in tunnels. It remained humble and utilitarian in their hands; its architectural possibilities were scarcely conceived. They continued to rear their monumental structures mainly with an eye to quantity: high and thick walls, ramps, towers ascending vertically or by steps, prevailed.
The true arch and vault are next found in Italy, among a prosperous city-dwelling people, the Etruscans, some seven or more centuries before Christ. All through the civilization of this nation runs a trait of successful but never really distinctive accomplishment. The Etruscans were receptive to new ideas and applied them with energy, usually only to degenerate them in the end. Whether they discovered the arch for themselves or whether knowledge of it was carried to Italy from Asia is not wholly clear, since history knows little about the Etruscans, and archæology, though yielding numerous remains, leaves the problem of their origin dark. The Etruscans, or Tyrrhenians as the Greeks knew them, were however active traders, and a number of features in their civilization, such as liver divination (§ [97]), as well as ancient tradition, connect them with Asia. It is therefore probable that the principle of arch construction was transmitted to them from its earlier Babylonian source. The Etruscans also failed to carry the use of the arch far into monumental architecture. They employed it in tombs, gates, and drains rather than as a conspicuous feature of public buildings.
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.