When we speak of construction, we mean the material used in building and the way it is put together. The different historical periods of building are now classed into distinct dates, which have been arrived at by observing the material used, and the way it is used, in buildings of which there is no doubt as to the date of erection, and comparing it with others. The early Greek Period in Italy is marked by massive walls of masonry—walls built from the stone of the vicinity, the blocks being rough as hewn out of the quarry,—polygonal. The later Greek Period and the Etruscan are identical, being formed of square blocks of stone, headers, and stretchers. In the time of the kings of Rome the stones were squared, and were of tufa, lapis ruber, tophus. In the earliest walls they are close jointed; in the second period the edges are bevelled.

During the Republic the stones were also squared, but the material was of peperino. Lapis Albanus and other forms of working up the material were introduced. Pieces of stone, fixed together with cement, gave a new kind of wall called opus incertum. This was improved upon by facing the outside of the small pieces of stone and making them of one uniform size—small polygonal. Then the stones were cut into wedge shapes: the point being inwards, and being laid in regular rows it has the appearance of network, and is called opus reticulatum. This work, introduced in the last years of the Republic, went out of fashion after the time of Tiberius, but was revived by Hadrian, who always set his reticulated work in bands of brick like a picture frame, thus distinguishing his from the earlier work, the inside of the walls in those cases being concrete. The earliest brick building which we have is the Pantheon. Thus it was under Augustus that brick was first used by the Romans. It was his boast that he found Rome of brick, and left it marble; which is only true in a certain sense, for he did not build of solid marble, but cased veneering marble on to the brickwork.

One period of Roman brickwork can easily be distinguished from the others by measuring the number of bricks in a foot, and noticing their uniformity of size. This, of course, does not refer to ornamental brickwork. The brickwork of Nero is the best in the world—thin narrow bricks, tiles, with very little mortar between them. Before his time it was not quite so good; but after, it gradually declined till the cement is as thick as the bricks.

The stone used during the Empire was travertine, lapis Tiburtinus, but brick was the material generally used then. They are of two colours, red and yellow, according to the clay from which they were made. The walls were not of solid brick all through; but the interior was made of pieces—rubble-work—the outside course being entire brick, whilst at every four or five feet all through the construction were laid the great tie-bricks to keep the rubble-work from shifting. The brickwork was called opus lateritium. The great tie-bricks are usually stamped with the names of the consul or emperor and the maker, and these date the walls by measuring the number of bricks there are in a foot. In the fourth century another system—opera decadence—came into vogue, and walls were built with layers of brick and pieces of tufa-stone a little larger than our English bricks. This work continued down to the thirteenth century, when opera Saracenesca—tufa-stones without the bricks between—came into use. In the stone walls no cement was used; one stone was simply placed upon another, its weight keeping it in its place, and clamps were inserted to keep it from shifting. In the walls of Roma Quadrata we know of no clamps having been found; but in the wall of the two kings wooden clamps were found. In the walls of Servius Tullius iron clamps were found; and in the Colosseum clamps can still be seen in several places where pieces of the facing of the stone have been split off.

Tufa is found all over the Campagna, and is of volcanic origin. When the Alban Hills were active volcanoes, the ashes and scoriÆ thrown up fell into the sea, now the Campagna. The pressure of water on it formed it into stone: where there has been a great pressure, it is very hard; where little pressure, it is softer; and where there was no pressure, it still remains a sort of sand—this mixed with live lime is the celebrated Roman cement. The softer tufa was used by the Greek colonists, and the hard stone by the kings of Rome. Some tufa from the neighbourhood of Gabii is dark gray, the other is brown and reddish. Peperino is also volcanic. It was ejected in the shape of hot mud from the volcano, and on cooling formed a good stone: this comes from the Alban hills, and was used in the time of the Republic.

Travertine comes from Tivoli, and is a petrifaction formed by the action of lime and sulphur on vegetable decay. This was not used as a building material to any great extent before the time of CÆsar. It is white, and becomes yellow on exposure. Silex is another volcanic stone very little used for building, but entirely for paving the roads both ancient and modern. This came out of the volcano as a red-hot stream of lava, and on cooling down became a capital paving material. The bed of the road was first properly prepared, and then it was paved with polygonal blocks of blue basalt called silex. The stones fitted close to one another. Many of the roads are in a good condition to this day; the best specimen is opposite the Temple of Saturn in the Forum, B.C. 175. This stone is used for opus reticulatum in some of the tombs on the Appian Way and at the Temple of Hercules; also for concrete.

TABLE OF CONSTRUCTION.

RAMBLES IN ROME