A common form of construction was that of facing a rubble wall with square pieces of stone, arranged in a wedge-like manner on their angles. This mode of arrangement was united with the commoner one of horizontal courses, so as to form a kind of pattern, which produced a pleasing effect, still imitated in some of our own buildings. Thick walls among the Romans were often formed by facing the outer and inner surfaces with stones squared and fitted, or with brick, while the interior was filled with rough fragments, strongly imbedded in a mass of their admirable mortar. To bind together the two encasing surfaces, large stones were introduced, extending through the whole thickness of the wall. But the most perfect kind of wall was that which we call ashlar work, and is still to be seen in the temples of Athens, Corinth, and other Greek cities. The stone or marble was quarried, and then accurately worked with the chisel, so that the eye could scarcely trace the union of the large adjacent blocks. These blocks were connected with those above them by dovetailing; and the stones lying side by side were firmly united by iron cramps fixed with lead. So constant and abundant was this employment of metallic fastenings, that the vast remains of ancient buildings have proved perfect mines for peculators. In a marble temple at Cyzicus, the lines of union of the slabs were covered with gold. In contrasting the Greek and Roman masonry, we see that the great works of the former were mostly of marble and highly finished, while many Roman remains of great magnificence are composed of rougher stone-work united by mortar, or of a union of stone and brick in alternate courses. The chief superiority of the Romans was in their complete knowledge and application of the principle of the arch, with which the Greeks were not acquainted. There was, it is true, a kind of pointed arch in use among them for corridors; but this was probably formed by cutting a passage through the solid walls when built, not by building the stones up archwise. Such passages are found in the vast Cyclopean walls before mentioned. Before subjoining any particular account of edifices, we may mention that the lever, the capstan, the crane, pulley, and other simple machines for raising or adjusting stones, were known to the Greeks and Romans. Though they could not pretend to a knowledge of machinery and mechanics even remotely approaching our own, still they had sufficient to answer the ordinary requirements of building. In carpentry, too, the Romans must have possessed considerable skill, or they could never have connected, by a structure of timber, arches so wide as those of Trajan’s bridge over the Danube.