GREEK ORIGIN OF ROMAN ARCHITECTURE: THE ARCH.--The architecture of the Romans was, in the main, an imitation of Greek models. But the Romans were not mere servile imitators. They not only modified the architectural forms they borrowed, but they gave their structures a distinct character by the prominent use of the arch, which the Greek and Oriental builders seldom employed, though they were acquainted with its properties. By means of it the Roman builders vaulted the roofs of the largest buildings, carried stupendous aqueducts across the deepest valleys, and spanned the broadest streams with bridges that have resisted all the assaults of time and flood to the present day. SACRED EDIFICES.--The temples of the Romans were in general so like those of the Greeks that we need not here take time and space to enter into a particular description of them. Mention, however, should be made of their circular vaulted temples, as this was a style of building almost exclusively Italian. The best representative of this style of sacred edifices is the Pantheon at Rome, which has come down to our own times in a state of wonderful preservation. This structure is about 140 feet in diameter. The great concrete dome which vaults the building, is one of the boldest pieces of masonry executed by the master-builders of the world. CIRCUSES, THEATRES, AND AMPHITHEATRES.--The circuses of the Romans were what we should call race-courses. There were several at Rome, the most celebrated being the Circus Maximus, which was first laid out in the time of the Tarquins, and afterwards enlarged as the population of the capital increased, until it was capable of holding two or three hundred thousand spectators. [Illustration: THE ROMAN FORUM IN 1885] The Romans borrowed the plan of their theatres from the Greeks; their amphitheatres, however, were original with them. The Flavian Amphitheatre, known as the Colosseum, has already come under our notice (see p. 316). The edifice was 574 feet in its greatest diameter, and was capable of seating eighty-seven thousand spectators. The ruins of this immense structure stand to-day as "the embodiment of the power and splendor of the Roman Empire."