Most of the leading architects in Rome were Romans, not Greeks. One of them, Marcus Vitruvius Pollio, wrote a world classic On Architecture (ca. 27 B.C.). Having served as military engineer under Caesar in Africa, and as an architect under Octavian, Vitruvius retired in old age to formulate the principles of Rome’s most honored art. “Nature has not given me stature,” he confessed, “my face is homely with years, and illness has stolen my strength; therefore I hope to win favor by my knowledge and my book.” As Cicero and Quintilian made philosophy a prerequisite for the orator, so Vitruvius required it of the architect; it would improve his purposes while science improved his means; it would make him “high-minded, urbane, just, loyal, and without greed; for no true work can be done without good faith and clean hands.” He described the materials of architecture, the orders and their elements, and the diverse types of building in Rome; and added discourses on machinery, water clocks, speedometers, aqueducts, town planning, and public sanitation. As against the rectangular design established by Hippodamus in many Greek cities, Vitruvius recommended the radial arrangement used in Alexandria (and modern Washington); the Romans, however, continued to lay out their towns on the rectangular plan of their camps. He warned Italy that in several localities its drinking water led to goiter, and declared that poisoning could come from working with lead. He explained sound as a vibratory motion of the air, and wrote our oldest extant discussion of architectural acoustics. His book, rediscovered in the Renaissance, deeply influenced Leonardo, Palladio, and Michelangelo. The Romans, says Vitruvius, built with wood, brick, stucco, concrete, stone, and marble. Bricks were the usual substance of walls, arches, and vaults, and served as a frequent facing for concrete. Stucco too was often used as a facing. It was made of sand, lime, marble dust, and water, took a high polish, and was laid on in several coats, often to a thickness of three inches; hence it could keep its form for nineteen centuries, as in some parts of the Colosseum. In making and using concrete the Romans were unrivaled until our time. They took the volcanic ash abounding near Naples, mixed it with lime and water, threw in fragments of brick, pottery, marble, and stone, and produced, from the second century B.C. onward, an opus caementicum as hard as rock, and capable of being poured into almost any shape. They cast it as we do, in troughs formed of boards. By its means they could cover large unsupported spaces with rigid domes free from the lateral thrust of an arched roof; in this way they topped the Pantheon and the great baths. Stone was employed for most temples and the more pretentious homes. One variety from Cappadocia was so translucent that a temple built with it was adequately lighted with all its openings closed. The conquest of Greece brought a taste for marble, which was satisfied first by importing columns, then marble, and finally by working the Carrara quarries near Luna. Before Augustus marble was largely confined to columns and slabs; in his time it was used as a facing for brick and concrete; only in this superficial sense did he leave Rome, here and there, a city of marble; walls of solid marble were rare. The Romans liked to mingle in the same building the red and gray granite of Egypt, the green cipollino of Euboea, the black and yellow marbles of Numidia, with their own white Carrara, and with basalt, alabaster, and porphyry. Never had architectural material been so complex or so colorful.