tine, and in the fourth century generally, there are only four bricks to the foot, as in a modern building; and while the brick itself is coarser and more spongy in appearance, the mortar is an inch thick between the courses or more, whereas in the early work it is scarcely shown.

Opus mixtum was another method of construction, in which bricks and rough stones were set in alternate layers at regular intervals. It occurs in many parts of the walls of Rome, and in the Circus of Maxentius, A.D. 310. It is met with also in the wall built by Theodoric, in the centre of the Stadium on the Palatine. It is known in Rome as the style of the decadence, because it was much used in late Imperial times.

There were several varieties of stone employed by the Romans in the buildings. (1) Tophus, tufa, a volcanic sandstone, called by Vitruvius lapis ruber. The ancient quarries of this stone are in the Cælian, Aventine, and Capitoline hills. The walls of opus quadratum on the Palatine are entirely of this material, and were probably built from quarries on the Palatine Hill itself. (2) Lapis albanus, now called peperino, also a volcanic sandstone, not hard and rough, the surface covered with flinty nodules, like peppercorns: hence its modern

name.

The ancient quarries are near Alba Longa, on Monte Cavo. The wall-backing of the agger of Servius Tullius, part of the Mamertine prison, and the temples of Antoninus and Faustina are of this material. (3) Lapis Gabinus - Sperone. This resembles the peperino, but with nodules of larger size,

and somewhat blacker in colour. The ancient quarries are at Gabii. The triple arch of the Cloaca Maxima, the substructure of the Tabularium, are of this material. (4) Lapis Tiburtinus, called travertino or travertine, a species of limestone. It is white when new, and becomes after exposure a warm yellow. The ancient quarries are at Tibur, Tivoli, near the Anio. The tomb of Cæcilia Metella, the Colosseum, present fine examples of this stone. (5) Selce, a basaltic lava, black, and almost as hard as iron. Its origin is traced to the ancient crater now the lake of Albano. A current of this lava occurs near the tomb of Cæcilia Metella, where quarries of this stone are still in use. It was chiefly used for the pavements of roads and streets.

Pumice-stone was used for internal vaulting on account of its great lightness. The vaults of the Colosseum, the Pantheon, and the baths of Caracalla are made of this material.

In late imperial times jars of pottery were introduced in the masonry in order to lighten the walls. Examples of their use occur in the walls of the Circus of Maxentius.

Lime mortar was not brought into general use at Rome until two or three centuries before the Christian era; the earliest dated example of the use of lime is believed to be the Emporium on the bank of the Tiber, B.C. 175. Necessity must have made the use of mortar familiar to every people. Time, which has hardened it, has caused it to be considered more perfect than the modern. Its extreme hardness may

probably be accounted for by merely referring to the circumstance that the long exposure which it has undergone, in considerable masses, has given it the opportunity of slowly acquiring the carbonic acid from the air, which converts it again into limestone, and upon which its hardness and durability depend. The chief excellence of the mortar of the ancients lay in their knowledge of the art of mixing lime with sand, and in the lime being burnt on the spot and used quite fresh. In Rome the mortar, when well made, consisted of three parts of sand and one of lime. The sand was of two kinds, fossicia, or river sand, and pozzolana (pulvis puteolanus), volcanic ashes, quarries of which extend over the Roman Campagna. So scrupulous were the ancient masons in the mixing and blending of mortar, that the Greeks kept ten men constantly employed for a long space of time in beating the mortar with wooden staves, which rendered it of such prodigious hardness, that Vitruvius tells us that slabs of plaster cut from the ancient walls served to make tables.

WALL OF SERVIUS TULLIUS.

THE city of Rome having gradually extended by successive enlargements over its neighbouring hills during the reigns of the early kings, Servius enclosed the seven hills, over which the city had grown, within one wall.

The seven hills were not occupied all at once, but one after the other, as they were required. The Palatine held the arx of the primitive inhabitants, and was the original nucleus of the town, round which a wall or earthen rampart was raised by Romulus. The hill of Saturn, afterwards the Capitoline, is said to have been united, after the death of Titus Tatius, by Romulus, who drew a second wall or earthen rampart round the two hills. The Aventine, which was chiefly used as a pasture ground, was added by Ancus Martius, who settled the population of the conquered towns of Politorium, Tellena, and Ficana upon it. According to Livy, the Cælian Hill was added to the city by Tullus Hostilius. The population increasing, it seemed necessary to further enlarge the city. Servius Tullius, Livy tells us, added two hills, the Quirinal and the Viminal, afterwards extending it

further to the Esquiline, where, he says, to give dignity to the place, he dwelt himself.

The city having reached such an extent, a vast undertaking was planned by the king, Servius, to protect it. A line of wall was built to encircle the seven hills over which the city had extended. The king, as Mr. Burn writes, having the whole control of the finances of the state, could appropriate large sums of money for works of public utility, and could also, doubtless, command the labour of immense gangs of workmen.

The course of the wall and the sites of the gates may be thus traced:-The wall began on the banks of the Tiber, and thence joined on to the south-west end of the Capitoline. In this portion of the wall there were two gates; one, as its name indicates, must have been near the river, the Porta Flumentana, and the other under the south-west extremity of the Capitoline, the Porta Carmentalis, which derived its name from an altar being erected near it to the nymph Carmentis, the mother of Evander. Between these two there is said to have been a third, which was only opened on the occasion of the triumphant entrance of a victorious general, and hence was called Porta Triumphalis. The wall thence passed round the north-west side of the Capitoline Hill, with which it evidently coincided, until it reached the Porta Ratumena, at the north end of the Capitoline Hill. This gate was so called from the name of a charioteer in the races at Veii, who was unable to stop his runaway horses until they reached Rome, and threw