This work presented all the difficulties it was originally intended to avoid, by carrying the work partially into the hill. It was necessary to form an artificial foundation, and carry up a heavy body of masonry, in the channel of the river, which in some parts had 15 feet in depth below its ordinary level; subject in ordinary seasons to frequent and sudden floods, and affording no means to form another channel for it to pass, until the work could be accomplished.

208 MEMOIR OF THE Had it been admissible to construct the dam with timber, the difficulties would have been far less. But the importance of the object it was designed to secure, arid its great that could be made. height, demanded the most permanent and durable structure The greatest height of the weir of the dam is 40 feet above the low-water level, and 55 feet above the bed of the river. The width of masonry, at low-water line of river, is 61 feet. The form on the lower face commences on a curve, described by a radius of 55 feet, and continues to within about 10 feet of tho top, when a reversed curve, on a radius of 10 feet, carries the face over and meets the back line of the wall. The back line is carried up vertically, with occasional offsets. The main body of the work is laid up of rough stone ;

the curved face of large and closely cut stone, with four heavy courses at the bottom dovetailed together, the joints cut to the line of radius of curve.