This reservoir, fourteen feet long by four and a half feet wide, is seven feet high to the summit ol its arch ; the walls are four and a half feet high to the springing of the arch, and two feet three inches thick. The arch is pierced in the centre by an opening two feet square, which serves as a passage into the reservoir. The bottom is lined with a coat of cement, six inches thick, with a curve at the angles of concourse of the sides and bottom ;

there were two ranges of iron rods about one third of an inch in diameter, to strengthen the walls, and probably also to serve as a kind of stair by which to descend into the reservoir.

This syphon bridge is disposed in the same manner, and has the same proportions, as the other arcades, the width of the arches being 18 feet, and the height of their opening 36 feet but this part is somewhat different from the others, from its width, which is 24 ;

feet, and by its piers apparently terminating at the impost or springing of the arches,

forming an elevation of considerable elegance, and a covered passage under the bridge. The arcades pierced in the piers for this road or gallery, are four feet wide, and 21 feet high, their arches being formed of voussoirs of thin stone, alternating with great bricks ;

the facing is of reticulated masonry, and being built with black and grey pieces, has much the appearance of a chess-board. However, these open ings having weakened too much the higher piers, the builders were under the necessity of strengthening some of them, by counter-forts of the same sort of masonry. Two arcades in the valley of Bannau fell in 1757, from this precaution not having been taken from these piers, after their fall, it was ;