the disappointment in regard to the extent of the rock in the river, it was found difficult to obtain the desired length of the water way, and it was concluded to raise the abutment to 12 feet at the lower end, and 15 feet at the upper end, and allow the water way to remain an average length of 90 feet for this height. The natural rock formed the southern abutment, and the aqueduct being on this side, the water was conducted to the gateway at its head, by a tunnel, cut 180 feet through the rock ; this allowed the gateway to be located on solid rock, in a situation not exposed to the floods of the river. The water enters the gate chamber by an archway through the second bulk-head. The gate chamber is provided with a double set of gates ; one set of guard gates, of cast iron set in cast iron, frames, and one set of regulating gates, made of gun metal, set in frames of the same material ; the gates are all 18 by 40 inches, and there are nine gates in each set. They are all operated by means of wrought iron screw rods. The gate chamber and bulk-heads are constructed of well dressed masonry, laid up in hydraulic cement. In the north abutment a waste culvert has been constructed, with suitable gates of cast iron, to draw the water down in the reservoir at such times as it may be necessary, to facilitate the making of any repairs that may be required, and to discharge the river at or- dinary times during the construction of the work. From this abutment the old channel of the river was filled by an embankment, with a heavy protection wall on the lower side, which was raised 15 feet above the waste weir of the dam, and designed to be 50 feet wide on the top, but was not completed the full width, when the unprecedented flood of January, 1841, carried it away. The embankment stood well, and gave no indications of failure, until the water rose to near the surface, and passed through between the frozen and unfrozen earth about 20 inches below the top. After the breach was made in the embank- ment, large masses of heavy ice came down from the reservoir, which soon broke down the unfinished protection wall, and carried off nearly the whole embankment. The ma- sonry of the dam and abutment sustained but little injury. Such a flood had not been anticipated, and the water way proved insufficient to pass it off. Had the embankment been completed the full width, and the protection wall carried up to the full height it was intended to carry it, the work might have proved adequate to the emergency. It was determined to fill the gap made by this breach, (about 200 feet long,) by a structure of hy- draulic stone masonry, adapting 180 feet as waste weir. 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 height, demanded the most permanent and durable structure that could be made. 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 ver- tically, 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 dove- tailed together, the joints cut to the line of radius of curve. Above the masonry an embankment of earth is filled in, and extends to 275 feet in width on the bottom, with a slope of one to five on the up-stream face. The north end of