Strabo, the great Greek geographer, visited Rome several times in the last decades of the first-century BCE. His examination of the city convinced him that “the Romans have made provision above all for those things that the Greeks have slighted: the construction of roads and aqueducts, and of sewers capable of washing away the city’s waste into the Tiber. And they have laid roads throughout the country, cutting through hills and building up over valleys so that their wagons can carry veritable cargoes. Their sewers, covered with vaulting of cut stone, offer in places a channel haycarts could pass through. So much water is piped in through aqueducts that it flows like streams through the city and its drains.”

The Romans, no question about it, were excellent engineers. Though the skill was all their own, they owed much of their know-how to their neighbors to the north, the Etruscans. The Etruscans were masters in the art of tunneling channels through rock - some of them run for over a mile and a half without a break - to carry off water or to bring it in. They covered the region they occupied with a network of fine roads, at times cutting great gashes fifty feet deep in stony hills to provide a gradient suitable for wheeled traffic. Rome’s engineers for a long time simply carried on Etruscan methods. Not until the third-century BCE did they make their own prime contributions, arched bridges of stone and continuous paving.

The Via Appia, the regina viarum (“queen of roads”) was begun in 312 BCE under Appius Claudius, commissioner of public works for the year. It went south to Capua and later was extended to Brindisi, the port that served as the jump-off point for travel to the east. By the middle of the second-century BCE, two trunk roads to the north had been completed, and the Italian boot was traversed its entire length by first-class highways. They were models of road engineering; Plutarch did not exaggerate by reporting that they “were carried straight across the countryside without deviation, were paved with hewn stones and bolstered underneath with masses of tight-packed sand; hollows were filled in, torrents or ravines that cut across the route were bridged, the sides were kept parallel and all on the same level - all in all, the work presented a vision of smoothness and beauty.”

The paving was of polygonal blocks of hard rock such as basalt or granite; massive pieces often measuring a foot and a half across and eight inches thick, they were fitted together as cunningly as a jigsaw puzzle to form an absolutely smooth surface. They were laid on a bed, usually of packed stone or gravel and clay but also of other materials depending on the terrain. The bed was prepared with extreme care so that the blocks would stand up to any weather or traffic without sinking or erupting. The main roads were two-lane, at least eight feet wide and sometimes ten; where needed they broadened out to three lanes for stretches. Milestones all along the way let the traveler know precisely how much distance he had covered. He crossed streams on arched bridges of stone whose weight and construction guaranteed that they would not wash out.

Rome’s roads and bridges involved no revolutionary technology - the Etruscans knew how to make fine roads and both they and the Greeks had used the arch - but rather bold planning, masterly organization of labor and resources, and the skill to adapt what others had done on small- to large-scale endeavors. For a revolution, we must turn to Rome’s buildings.

When the city of Rome was still a mere village, the Greeks were putting up impressive temples and public buildings, even theaters capable of seating thousands. Yet none of these exhibited any advances in the method of construction: This was always post and lintel - that is, based on uprights supporting horizontal beams, and the material was almost always squared stone. The Greeks’ most celebrated architectural achievement, their temples, were all rectangular structures of stone blocks with pitched roofs of wooden beams and rafters covered with tiles. Their glory lay in their exquisite proportions and decoration, not their engineering. Greek theaters, which by the fourth-century BCE had grown to hold 20,000 people, were set against hillsides in order to let nature furnish the underpinning for the rising tiers of seats.

Rome’s engineers had at their disposal all that the Etruscans and Greeks could teach them. Though they quickly improved on what they had learned from Etruria, it took them quite a while to emerge from under the shadow of Greek influence. When Augustus came to power, he launched a program that gave the city of Rome buildings, forums, and monuments worthy of the metropolis that had become the capital of the Mediterranean world. There were some important new elements in the program, but in the main it was a continuation of what had gone on before, and Greek inspiration is unmistakable everywhere, particularly in the decoration.

Yet Roman engineers for two centuries had known a material and a building technique that were radically different and would eventually bring forth a new style of architecture destined to be among Rome’s prime contributions to the western world. The material was concrete, used either by itself or with stone, and the building technique was one based on the arch and vault rather than the post and lintel. Once architects had begun to put the two new elements together to design arches and vaults of concrete, they were started on the path that was to lead to the Colosseum, the Pantheon, the Baths of Caracalla, and other celebrated structures.

In the beginning, the arch or vault and concrete were used separately and only for utilitarian works. By the early second-century BCE, Rome’s engineers were well aware of the manifest possibilities of the arch and, in addition to arched bridges of stone, had begun to put arched gateways in walls, to erect long lines of stone arches for carrying the conduit of an aqueduct across country, and to set up that uniquely Roman form of commemorative monument, an arch standing on its own. Even earlier, they had employed concrete, in a very coarse form, for foundations, walls, parts of private houses. In the second-century BCE, they made their first important use of the two together when they introduced barrel vaults of concrete into their designs, as in a vast warehouse along the Tiber that was roofed with lofty barrel vaults. By the first-century BCE, they had introduced the barrel vault into the construction of amphitheaters and theaters with revolutionary effect - no longer did a theater have to lean against a hillside as in Greek cities; it could be placed anywhere its builders wanted it, with a semicircle of radially disposed barrel vaults holding up the rows of seats. At the same time, they turned to another type of concrete roof, the dome; it makes its appearance in Roman architecture in the small round rooms common in bath complexes.

Vaults, domes, concrete - all had been in use in Greece and the Near East, but only for small and minor works. The Romans were the first to put them together and use them on a grand scale, thereby realizing to the full the potentialities of concrete. This was in part because they had available a superior form of it, thanks to a special type of volcanic sand that first was found near the town of Puteoli just west of Naples (we call the sand Pozzolana, after Pozzuoli, the modern name for the town) and later right around Rome. Pozzolana produced a mortar that was not only exceptionally strong but would set under water - a characteristic that opened the way to Rome’s achievements in the building of bridges and harbor installations. The mix the Romans worked with was a concrete rubble, a combination of mortar with chunks of stone, rather than concrete as we know it; once set, however, it formed a monolithic mass as strong as any masonry of today.

The Roman way of erecting a wall of concrete was to build up, in a masonry of either small cut stones or bricks, the inner and outer faces of the wall to a height of a few feet; then they filled in the gap between with concrete rubble. This was repeated until the planned height was reached. Before Augustus’s day, the masonry preferred for the facing had been close-set diamond-shaped stones, reticulate as it is called; during and after his reign, brick became increasingly popular. Either left a pattern that was decorative enough for utilitarian works such as the barrel vaults of a warehouse, but not for the walls or ceilings of buildings of higher status. These would be stuccoed over and painted or, if money was no object, clothed in marble veneer. One of the reasons why many Roman ruins look so plain, even unsightly, is because they have lost their stucco coat or have been stripped of their marble sheath, and all that is visible is the facings enclosing the core of concrete.

In certain public buildings, such as theaters and amphitheaters, the façade was of cut stone and the guts of the structure behind it of concrete. Roman architects felt that an expanse of cut stone was not attractive enough by itself; it needed something by way of decoration. And the something they added proved so successful that it became a hallmark of western architecture, lasting until the sweeping changes of the twentieth century. They took from post and lintel architecture the column and pediment and certain other elements, which it had used structurally, and made them purely decorative. They took columns, including such allied versions as half-columns and pilasters, and pediments and disposed them so as to frame openings or to break up an expanse of a wall in aesthetically satisfying ways. The façades of the palaces and churches of Italy, of the classical-style buildings that stud Europe from Britain to Russia, of the colonial buildings of America, are, in effect, so many variations on this Roman theme.

Until the reign of Nero, concrete had been limited to workaday structures - warehouses, shops, apartment houses, aqueducts, the underpinnings of theaters, and so on. Under Nero, it was promoted to higher uses, and the effect was revolutionary. Nero has had some bad press. His bizarre behavior during his last years has been allowed to blot out not only the decade of able and intelligent rule that preceded them but certain other claims to a better fame. He was cultivated, artistic, and possessed of a dimension shared by no other emperor, indeed by no other Greek or Roman intellectual - an avid interest in matters scientific and technological. He sent an expedition to explore the upper reaches of the Nile, devised his own pharmaceutical recipe for a stomach medicine, and had a special lens to improve his shortsighted vision. At the very moment when gathering about him were the storm clouds of the revolt that was to end his reign, he was absorbed in perfecting an improved type of water organ. Vast building projects were a particular interest. He gave Rome the first of its great imperial baths. He undertook the herculean tasks of cutting a canal through the Isthmus of Corinth and of creating an inland waterway between Naples and Rome; though neither was brought to completion, they were feasible projects and not wild chimaeras. When the better part of Rome was wiped out by fire in CE 64, he personally planned the city that was to rise from the ashes, one marked by wide streets instead of the tortuous old alleys and with houses properly built of fireproof materials. At the same time, being Nero, he took over a huge portion of the burnt-out area right in the heart of town to build himself a country-style villa complex set in the midst of synthetic woodland and meadow (the Colosseum stands on what was once an artificial lake adorning it). In the villa mansion, the famous Golden House, concrete was used throughout and, in one particular room, it made possible a new and imaginative effect: The room was given an octagonal shape, which permitted it to be roofed with a dome. It is quite possible that the innovation was the brainstorm of the art-minded, technology-minded emperor himself.

Under Hadrian, the potentialities of concrete were realized to the full. The monumental villa he built on the flat ground below Tivoli, some fifteen miles east of Rome, was made almost entirely of concrete and included buildings whose walls curved in and out, one part concave and the next convex; after all, with a material such as concrete, curved shapes are just as easy to make as rectilinear. These structures were totally new, different, avant-garde. The story is told that Trajan’s chief architect once snapped at Hadrian, then a young fellow and offering unwanted advice, “Go draw your pumpkins!” The villa shows us what had made the old professional’s gorge rise: A building there does have a melon-shaped semi-dome. Hadrian erected an avant-garde building right in the middle of Rome - the Pantheon. Both Greeks and Romans had built circular temples, but always on a small scale and roofed in traditional fashion. Since the first-century BCE, Roman architects in designing bath complexes had included small windowless round chambers topped by domes that had at their centers circular openings, oculi, or “eyes,” to let in light. The Pantheon is just such a chamber, but it is freestanding like the round temples. What is new and what represents a triumph of engineering is its breathtaking scale: its dome is the biggest ever built of masonry, 142 feet in diameter, three feet wider than St. Peter’s. The foundation supporting this ponderous structure is a ring of solid concrete fifteen feet deep and thirty-four broad. The wall is twenty feet thick. The dome is ingeniously constructed to diminish in weight as it leaves the wall on which it rests and rises toward the center: it grows progressively thinner, reducing to a mere five feet at the oculus, and the aggregate used in the concrete mix gets progressively lighter, going from stones of ordinary size to smaller stones to porous pumice at the oculus. The Pantheon directly inspired Brunelleschi’s dome for the cathedral at Florence and Michelangelo’s for St. Peters, and indirectly all the domes in western Christendom.

Another achievement of Rome’s builders in concrete was the great public bath. Nero may have been the pioneer here, too, since he put up the first of them; however, little of it has survived, so we cannot be sure. In post and lintel structures, the amount of unencumbered space that can be enclosed is limited by the beams that run at regular intervals from side to side; these cannot go beyond a certain length without being supported by columns. A concrete building, on the other hand, roofed with a barrel vault or a cross vault (intersecting barrel vaults) can enclose a vast area, as in Rome’s imperial bath complexes, which boast a monumental central hall covered by three soaring cross vaults. The central hall of the Baths of Diocletian was big enough to be converted by Michelangelo into a sizable church. The original Penn Station in New York City was a replica of the central hall of the Baths of Caracalla.

The Pantheon and the bath complexes and other such “modern” structures stood out in Rome the way a stark construction of steel and glass stands out in Paris or London today. For Rome’s public architecture was still basically traditional. The commonest public building was the temple, and the architects of Rome’s temples were conservative. So were those who designed the basilicas, the edifices that served as law courts. These were rectangular in shape, usually large and lofty, had pitched roofs, and, as support for the roof beams, had lines of columns that divided the interior into a center and side aisles - in other words, the basic features of a Christian basilica. Constantine, when deciding what the pioneer church of St. John Lateran was to be like, chose the basilica as a model, thereby establishing it as standard for Rome’s Christian places of worship. Since the basilica in shape and construction was a traditional building, the architecture of the Church of Rome looked backward, as it were. Rome’s “modern” style traveled with Constantine to his new capital, and with Honorius and Theodoric to Ravenna, to live on in the Byzantine churches with their curvilinear exteriors, their domes and semi-domes.

For Rome’s utilitarian structures, concrete continued to be the material par excellence. The apartment houses that lined her streets were of concrete with vault-roofed shops on the ground floor. The warehouses that lined the Tiber were of concrete. The aqueducts that the emperors added to enable the water supply to keep up with the ever-growing population marched across the open country on concrete arches. The Colosseum consists of a skeleton of cut stone fleshed out with concrete. Rome’s concrete, able to set under water, made possible the mighty new harbor that was built just north of Ostia.

We know about the Roman art of building not only from the ruins that have survived but also from a textbook that has survived - M. Vitruvius Pollio’s On Architecture.

Vitruvius trained himself specifically for the profession of architect. He seems to have started his career in the army; at one time, he was in charge of maintenance and development of engines of war. He served both Caesar and Augustus, particularly the latter, and in his old age wrote the work that made him famous, dedicating it to Augustus. It has ten divisions, or books as they are called: Book One takes up the qualifications of an architect, the nature of architecture, town sites, town planning; Two takes up materials; Three and Four, temples; Five, public buildings; Six, private houses; Seven, interior decoration; Eight, water, its sources and properties, its uses for leveling, how to pipe it in, and so on; Nine, sundials and other means for measuring time; Ten, engines of war and construction machinery. What use Augustus may have made of it, we don’t know. What we do know is that it became a veritable bible for architects of the Renaissance and through them affected generations of followers.

The work is far from a bald handbook of building techniques. To Vitruvius, a good architect was not a narrow professional but an intellectual of wide-ranging abilities, “a man of letters, a skillful draughtsman, instructed in geometry, versed in history, and a diligent student of philosophy; he should understand music, have some knowledge of medicine, know the opinions of jurists, and be acquainted with astronomy.” He must be a man of letters to write up his knowledge; a draughtsman, so that he can draw up his own sketches of his work. He must know geometry since “it teaches us the use of the rule and the compass, and this facilitates the planning of buildings on their sites, and the truing of these by the use of the square, level, and plumb bob”; be versed in history because “he should be able to explain to enquirers the rationale behind many of the ornaments often included in an architect’s design”; know philosophy since it “makes an architect highminded . . . and free of greed, which is all-important for no work can be accomplished without good faith and honesty”; understand music in order to be able to “tune” the tension of the cords that propel catapults; have some knowledge of medicine in order to answer questions “of air, the healthiness and unhealthiness of sites, the water supply, for without such considerations no habitation can be regarded as healthy”; know the opinions of jurists in order to draw up specifications that will conform with regulations and property rights; and he must be acquainted with astronomy in order to set up sundials in the proper way.

Vitruvius’s conception of architecture is no less wide, at times almost approaching what we define as urban studies. Take his pronouncements on the choice of a site for a city. The very first consideration must be salubrity: One must avoid marshy land, places that are troubled by cloud or frost, places that are mercilessly exposed to the sun. Once an appropriate site has been picked, one must keep very carefully in mind the direction of the prevailing winds in orienting the streets. Winds must not blow directly upon a street, for the effect is uniformly bad: “if they are cold, they injure, if hot infect, if humid harm.” In discussing public buildings, he warns that a theater should not be put near marshy areas since the spectators, who consist of whole families, men and women and children, spend long hours seated without moving and will thus undergo extended exposure to the unhealthy marsh winds. Nor should it be oriented toward the south, for then the air, which in a theater has little chance to circulate, will get excessively hot and be damaging to the body. Bath complexes must be in the warmest possible site, sheltered on the north and northeast. The hot rooms should be lighted from the west, or from the south if the site rules out the west, since the usual time for people to come to the baths is between midday and evening rather than during the morning hours. In dealing with private houses, he reminds the architect to take cognizance of climate: In the north, houses should be all closed in and face the direction that offers the most warmth; in the south, they should be open to the air and have a northern or northeastern exposure.

Vitruvius wrote almost 100 years before the architectural revolution that began in Nero’s reign. The architecture he describes is largely traditional, the style in which his patron was rebuilding the capital. In this area, he provides us with a precious store of information, from the proportions to be used in planning public buildings to the techniques of painting and stuccoing, from the dimensions of marble columns to those of lead pipes, from the design of derricks to the design of water-clocks - a detailed exposition, in sum, of one of the areas in which Rome’s genius shone most brightly.

Underneath many of Europe’s present highways are the roads that were part of Rome’s great network. The Roman way of decorating a building set a pattern that lasted until this century. The Pantheon, the Baths of Caracalla and Diocletian, Vitruvius’s descriptions and pronouncements, all inspired architects from Michelangelo to the designers of railroad terminals. Our debt to Rome’s engineers is tangible and ubiquitous.