90

HISTORY OT ENGINEERING.

Book I.

sides have each two gates, and the others one, placed in such a position as to permit thestreets to traverse in a parallel direction. The towers, built on the outside of the wall,are square, and placed at from 150 to 200 feet apart. 'Hie whole city was thereforecontained in a parallelogram, whose sides were 2700 and 2200 feet in length.

The six gates resembled each other, and were defended by towers, in which were stair-cases to ascend the walls, the breadth of which at top was upwards of 20 feet. The threeopenings were closed with wooden gates, and above the archways, the wall, which wascarried over them, was pierced with eleven semicircular-headed apertures, so that theguard traversing the walk behind the battlements could see who approached, or was aboutto enter or go out. (See fig. 106.) An architraved cornice surmounted the whole, bearingat each end upon a Corinthian pilaster. As the wall was double that surrounded the city,the space between was strengthened by arches, and the inner was carried up to a con-siderable height above the outer, both being surmounted with battlements.

Another fine example of a very early gate or entrance to a city is at Perugia , whichhas probably undergone very little change since the time of its construction. (Sec fig. 107.)

The walls at Nismes, erected in the time of Augustus , arc 30 feet in height, and, thoughvarying in thickness, were generally about 9 feet; they are faced on both sides with regularcourses of stone laid in cement; the interior is filled up with rubble, strongly united withmortar or cement as hard as the stone itself. The walls were covered with flags of free-stone, which projected over each side ; these flags were about 10 feet in length, and formeda platform for the movement of the soldiers engaged in the defence of the city : an externaland internal parapet was constructed upon them. The towers were generally round, andthe thickness of their walls 5 feet 6 inches, their interior diameter 24 feet 6 inches: theyprojected from the walls, as shown in the other examples.

At Nismes the gateways are constructed of stone; one has two entrances of the samewidth, each about 12 feet, and 20 feet high; on each side was a smaller for footpassengers, 6 feet in width and 14 feet in height; above the side openings are niches.A level cornice surmounts the whole, and terminates against the two round towers;above this was an attic, now destroyed. The towers are 31 feet in diameter.

Another gate remaining has only one entrance, 13 feet in width, and 21 feet in height.

The walls at Pompeii are usually rubble, faced with reticulatum, 20 feet in width,including the thickness of the two walls which crowned the ramparts: they varied