Chap. IV.
ROMAN.
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portion of the aqueduct was formed by opening a trench 5 feet wide and 10 feet deep,with a uniform fail of 12 inches for every hundred toises. The channel for the water waslined with masonry, the bottom of which was 12 inches thick; on this two walls, 18 inchesin thickness, were raised to the height of 5 feet; the space between, being two feet in width,was covered by a semicircular arch, 12 inches in thickness, over which earth was laid to anaverage thickness of 2 feet.
At the bottom of the watercourse a coat of cement, 6 inches in depth, w as laid, andthe sides were coated in a similar manner, to the thickness of an inch and a half; the widthbetween the walls being reduced to twenty-one inches. The walls were formed of smallangular stones, laid in mortar, composed of coarse rough sand ; the cement used for thelining was made of very fine sand, lime, and powdered brick; that employed for the firstcoat was of a coarser kind, the powdered brick being as large as a pea; but the lime seemsto have been fresh from the kiln, and well-tempered at the time.
'Where the aqueduct was tunnelled in the side of the mountain, and its watercourse wasconsiderably below the surface, putei or wells were sunk, to allow the vapours whicharose to disperse freely ; these shafts were made at the distance of an actus , or 120 feet,many of which are found with their sides steined in very perfect order ; they admittedlight and air, and also the workmen who were required to repair any defects, or removeany deposit that might by accident accumulate within the channel. The putei must notbe confounded with the columnarize, or vent-pipes, which were perpendicular tubes, thetops of which rose considerably above the aqueduct, for the purpose of maintaining anequal pressure and ventilation throughout. Health was the consideration which caused somany contrivances to maintain the water pure ; the ancients knew that pent up, and notallowed its due proportion of air, it lost that sparkling effect for which it is admired ; theypreferred the walled aqueducts on this account, as they delivered the water in a whole-some state.
Where the works are above ground, the walls, 2 feet in thickness, are faced with reti-culated work, the size of the lozenges varying from 3 to 6 inches square, and workedwithout any intermediate course of brick. The arches were roofed over, to throw off therain ; and the entrance to the aqueduct was by iron doors placed to open internally. Thoseportions underground were approached by man-holes, brought up a little above the levelof the soil, and the entrance of the water into the aqueduct was by a valve, which, slidingup and down, could be regulated at pleasure.
When the aqueduct was formed above ground, a footing of masonry 6 inches in thicknesswas set out, and the span of the arches varied according to their height; all were semi-
Fig. 209.
circular, and if of 12 feet span, the piers were made equal to half, and where the archesrequired to be built to the height of 31 feet, the width of the opening was 15 feet 6 inches,and the piers 7 feet 9 inches ; so that the piers were half the opening, and the opening orspan made equal to half the height.
Where the canal for the water was formed on this portion of the aqueduct, an offset wasmade on each side, increasing the thickness to 6 feet.
The piers and masonry throughout are of a corresponding style, formed of small squarestones, laid in a thick bed of mortar, and known as the opus reticulatum, a beautiful kind ofwork, described by Vitruvius in lib. ii. cap. 8. : it is, however, very subject to split, from itsnot having the necessary bond; its whole strength depends upon its quoins being propor-tioned to the space given to the reticulated division — which must always be regarded as asimple filling in. The two or three courses of tiles or stones which lay horizontally throughthe entire wall serve to tie on the two faces of the work, in part, but there is not that sta-bility in a pier so constructed, as is found with either English or Flemish bond ; the cement,
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