Chap. VIII.

BRITAIN.

567

Aqueduct of Pont-y-Cysijllte is 4 miles from Chirk, over the river Dee ; an earthen em-bankment was pushed forward until the perpendicular height became 75 feet. The distance

Fig. 648. AQUEDUCT OVER THE DEB AT PONT-Y-CYSYLLTB.

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between the end of this and the north bank is 1007 feet, and the river Dee is 127 feetbelow the water level of the canal carried over it.

Fig. 540.

AQUEDUCT OVER THE DEE AT PONT-Y-CYSYLLTB.

To construct an aqueduct upon the usual principles, with piers and arches, 100 feet inheight, and of a sufficient breadth and strength to afford room for a puddled water-wav*would have been, not only extremely harardous, but expensive. Telford, who had alreadycarried the Shrewsbury canal by a cast-iron trough 16 feet above the level of the ground,formed the idea of doing the same in the present instance, and made a model of a portionset on two piers, with the towing path and side rails, which was approved of and finallyadopted. The foundation on which the piers are erected is a hard sandstone rock ; theirheight above low water in the river is 121 feet, at the bottom they are 20 feet by 12, andat the top 13 feet by 7 feet 6 inches. For a height of 70 feet from the foundations, theyare built solid, and the remaining 50 feet hollow, the walls being only 2 feet in thickness,with one cross inner wall; by this means the centre of gravity is thrown lower in the pier,and the masonry economised.

The width of the water-way is 11 feet 10 inches, of which the towing-path covers 4 feet8 inches, leaving 7 feet 2 inches for the boat; as the towing-path stands upon iron pillars,the water fluctuates and recedes freely as the boat passes.

There are eighteen of these stone piers, besides those of the abutments, and the totalexpense, including the embankment, was 47,0184

£ s. d.

The embankment cost ----- 8,570 15 8

Masonry ------- 21,162 13 5|

Iron work ------- 17,284 17 5£

£47,018 6 7

This aqueduct almost rivals the works of a similar kind left us by the Romans. Theintroduction of iron, however, for the watercourse, is a novelty with which they were un-acquainted ; in this instance it has proved admirably well fitted for the purpose to whichit is applied; had a stone or brick channel been constructed at this great elevation, itcould not have been rendered so secure for the passage of boats, as there would have beena constant leakage from the motion and friction of the water within it. The cast-ironplates which form the sides are strongly riveted and secured to those at the bottom, and

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