The Romans invented no new principle in architecture, except the arch, which was not known to the Greeks, and carried out by them to greater perfection than by the Romans; but this, for simplicity, harmony, and beauty, has never been surpassed in any age, or by any nation. The Romans were a practical and utilitarian people, and needed for their various structures greater economy of material than large blocks of stone, especially for such as were carried to great altitudes. The arch supplied this want, and is perhaps the greatest invention ever made in architecture. No instance of its adoption occurs in the construction of Greek edifices, before Greece became a part of the Roman Empire. Its application dates back to the Cloaca Maxima, and may have been of Etrurian invention. It was not known to Egyptians, or Persians, or Indians, or Greeks. Some maintain that Archimedes of Sicily was the inventor, but to whomsoever the glory of the invention is due, it is certain that the Romans were the first to make a practical application of its wonderful qualities. It enabled them to rear vast edifices into the air with the humblest materials, to build bridges, aqueducts, sewers, amphitheatres, and triumphal arches, as well as temples and palaces; its merits have never been lost sight of by succeeding generations, and it is at the foundation of the magnificent Gothic cathedrals of the Middle Ages. Its application extends to domes and cupolas, to arched floors and corridors and roofs, and to various other parts of buildings where economy of material and labor is desired. It was applied extensively to doorways and windows, and is an ornament as well as a utility. The most imposing forms of Roman architecture may be traced to a knowledge of the properties of the arch, and as brick was more extensively used than any other material, the arch was invaluable. The imperial palace on Mount Palatine, the Pantheon, except its portico and internal columns, the temples of Peace, of Venus and Rome, and of Minerva Medica, were of brick. So were the great baths of Titus, Caracalla, and Diocletian, the villa of Adrian, the city walls, the villa of Maecenas at Tivoli, and most of the palaces of the nobility; although, like many of the temples, they were faced with stone. The Colosseum was of travertine faced with marble. It was the custom to stucco the surface of the walls, as favorable to decorations. In consequence of this invention, the Romans erected a greater variety of fine structures than either the Greeks or Egyptians, whose public edifices were chiefly confined to temples. The arch entered into almost every structure, public or private, and superseded the use of long stone beams, which were necessary in the Grecian temples, as also of wooden timbers, in the use of which the Romans were not skilled, and which do not really pertain to the art of architecture. An imposing building must always be constructed of stone or brick. The arch also enabled the