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HISTORY OF ENGINEERING.

Book I.

round the city were thoroughly drained, and by this means all the cities in Cisalpine Gaul ,Ravenna, Aquileia , and others, were rendered healthy. It is recorded that the inhabitants ofSalopia in Apulia , being continually out of health, applied to Marcus Hostilius for permissionto remove their town to a more healthy spot, when a site near the sea was purchased byconsent of the senate and Roman people, and the new work commenced, each citizenhaving a portion of ground allotted him at a moderate price. A communication was after-wards made between the lake and the sea, by which the former was converted into anexcellent harbour for shipping. The Salopians thus acquired a healthy situation four milesfrom their former city, and had all the advantages they desired. The Romans alwaysthought of the best means for providing the inhabitants with the necessaries of life, and forholding communication with other portions of the empire. They would not have followedthat advice which the architect gave to Alexander, to cut Mount Athos into the figure of aman holding a city in one hand, and collecting the waters of the mountain in the other, ifthe country around had been barren or unprofitable. A region more accessible both byland and water would have been preferred by the Roman engineers, where water was of aquality not injurious to health, and sufficiently abundant for the wants and luxuries ofthe citizens. They carefully examined into the health of the people that occupied theregion where a new city was to be founded, they observed the effects on the buildingswinch existed, and the trees, whether they indicated, by their bending in one direction, anyprevalent or continued high winds. They also observed the surfaces of the natural stonesand those used in the dwellings of the natives, whether any decomposition had takenplace, or the atmosphere had acted injuriously upon them.

After the site was determined upon, wells were sunk to try the nature of the soil; if fitto bear the weight of the constructions intended to be put upon it, they commencedtheir foundations for the outer walls, which were always based upon a solid stratum ; theworkmanship as well as the materials employed for this purpose were the best that couldbe obtained on the spot or in the immediate neighbourhood,—square stones, flint or rubble,burnt or unburnt bricks, bonded together with timber, as that of the olive, or some otherequally imperishable wood. The works were conducted by the military or civil engineers,and every means adopted that could render them proof against the attack of an enemy.Vitruvius recommends that the plan of the city should be polygonal, because the angles of asquare or parallelogram are defended with more difficulty. The towers should, he observes,be also round or polygonal, the square towers being more easily injured at the quoins by thebattering-ram. When the wall which surrounded the city was constructed on the side of ahill, buttresses were added, distant from each other as much as the height of the wall, or, ifrequisite, nearer together. They were gathered in gradually, to give additional strength.Every precaution was taken against assault by an enemy; angles jutting out wereavoided, as they were considered to assist the enemy when making an assault, and to be in-jurious to the inhabitants in the defence, and that they were weak against the militaryengines brought to act upon them. Whatever form was adopted for the walls, they weremade of such thickness as would permit two armed men to pass each other behind theparapet, of a sufficient height to prevent the scaling ladder from being applied to them,and built so firmly as to resist the battering-ram.

Varieties of military engines were employed to demolish or break down the walls, andsome to undermine the foundations ; as a security against these, a deep ditch usuallysurrounded the outside of the wall. These ditches were often without water, and madesufficiently wide and steep to defy the approaches of the moveable towers, oi tortoise,brought to act against the fortifications. The city had two walls, if it was of much im-portance, a space being left about 20 feet in width, or more, between them, which spacewas filled with the earth taken out of the ditch, well rammed in : this inner wall wasprovided with flights of steps in every convenient position, that it might be easilymounted from the side towards the city. When there was no ditch, the space betweenthese two walls was left open, where the soldiers were ordinarily assembled. Caesarinforms us, that in Gaul most of the walls had beams of timber laid within them atregular distances, in a parallel direction, braced together and tied diagonally, the spacesbeing filled in with large stones, so that neither fire nor the battering-ram could injurethem.

The towers which project from the outer walls, whether round or square, were usuallycarried up higher than the wall itself. And the inner wall of these towers, on the facetowards the town, was left open, that, if an enemy got possession of them, he was notprotected from the assaults of the inhabitants: they were often crossed by wooden plat-forms or bridges, which could be easily demolished, and thus the walk along the top of thewall, or behind the battlements, destroyed.

The walls and towers externally were finished by a bold projecting cornice, which re-ceived the battlements, and gave greater width at the top of the wall, as well as preventedthe easy application of the scaling-ladder. Where the city was entered by the inhabitantson ordinary occasions, the gate was flanked by two towers, larger than the others, and more