ROMAN ARCHITECTURE. IT/ with irregular shaped pieces of stone, mainly used in the first and second centuries B.C. ii. Concrete faced with " opus reticulatum " (No. 46 c), so called from its resemblance to the meshes of a net (reticulum) the joints being laid in diagonal lines, iii. Concrete faced with brick (testae), used from the first century B.C. to the end of the Western Empire. The walling was faced with bricks, triangular on plan and usually about i-^ inches thick (No. 46 d). iv. Concrete with " opus mixtum " consisting of a wall of concrete having in addition to the ordinary brick facing bands of tufa blocks at intervals. The majestic simplicity of their edifices gives them a severe grandeur expressing the Roman ideals of conquest, wealth and power. Thus from the time that concrete displaced the ashlar masonry of the Greeks, and allowed of unskilled labour, the style of the Romans tended to become everywhere uniform and generally abo'e the influence of local conditions ; for through the colonies and legionary camps the new methods penetrated to the extremi- ties of the empire, and cities could be improvised, which became in their turn centres whence radiated the architectural ideas as well as the manners and customs of Rome. Vaulting. — Although, as pointed out, the vault had been previously used by the Assyrians, the early Greeks, and the Etruscans, yet the Romans generalized vaulting as a structural system dating from the first century of the present era. They made it simple and practical by the employment of concrete, by which they covered the largest areas even now in existence. The effect was far reaching and gave freedom in the planning of complex structures, which were easily roofed, the vaults being of any form, and easily constructed on rough centres or temporary supports till the concrete was set. It will thus be understood that vaults of concrete had a very important effect on the forms of Roman buildings, and they were employed universally, so much so, that every Roman ruin is filled with their dehvis. The kinds of vault employed were as follows : — (rt.) The semicircular or waggon-headed vault. (b.) The cross vault. {c.) The dome (hemispherical and semidomes). (rt.) The semicircular or waggon-headed vault resting on two sides of the covered rectangle was used in apartments whose walls were sufficiently thick. {h.) The cross-vault was utilized for covering a square apart- ment, the pressure being taken by the four angles. When used over corridors and long apartments the pressure being exerted on points of division (Nos. 58 and 60), left the remainder of the
