feet 8 inches thick at the spring line of the inverted arch, and 2 feet at the top, or spring line of the roof- ing arch. The inverted arch is of brick, 4 inches thick. The roofing arch is also 8 inches thick. The abutments, or side walls, are of rubble stone, with a brick facing 4 inches thick. Spandrels of stone are carried up solid from the exterior angle of side wall, on a line that is tangent to the arch. When the bed of concrete is formed for the inverted arch, a heavy course of plastering is laid over it, on which the arch is laid. When the stone work of the side walls was up, the face that received the brick lining had its irregularities filled with successive courses of plastering, and finally a uniform course of a quarter of an inch in thickness over the whole, in front of which the brick facing was laid up. A course of plastering was also put over the roofing arch. The concrete masonry was formed by mixing one part hydraulic cement, three parts clean sand, and three parts fine broken stone ; in some cases fine pebbles were in part substituted for broken stone. The masonry was all laid up in hydraulic cement, obtained mostly from the hydraulic lime of Ulster Co. The mortar for the stone work was composed of one measure of cement to three of clean sharp sand, and for the brick masonry and plastering, one of cement to two of sand. It may be proper to remark, that every cargo of cement was tested by actual experiment, after it was brought on the ground, before any was allowed to be used. This precaution has had a very salutary influence on the character of the work ; the cement in all cases where it is exposed to view, in its exterior, or broken up for examination, or otherwise, has appeared highly satisfactory. In the commencement of the work, there was much difficulty in getting the workmen to lay their stone and brick in a thorough, full bed of mortar, which is obviously very important in hydraulic masonry. But a rigid system of inspection, requiring all imperfect work to be taken down and relaid, was successful in ob- taining work of great compactness and solidity. The area of a cross-section of masonry in the aqueduct is : Concrete masonry, - - 4.605 ) Stone in side walls, 21.572 V Do. inspandrils, - 2.690^28.867, Brick in arches and side facing, - - 13.658, Total, - 42,525 sqr. feet A limited departure from the above area has been made where peculiar circumstances seemed to justify or require it ; the most important in extent is, where the aqueduct passes over low grounds or valleys, and a dry wall of stone is raised to the proper level to sup- port the conduit masonry, and generally for the depressed bottom on the 4.949 miles at the upper end of the aqueduct, and in a few other cases in the first contracts, where the bot- tom arch and brick facing is 8 inches thick. On the foundation walls, the concrete ma- sonry is laid one foot extra thickness, and three feet extra width ; the base of the side 52 206 MEMOIR OF THE walls is also increased, and the proportion of cement to sand in concrete and mortar for stone work is one to two and a half. In other respects the masonry in conduit, is similar on foundation walls to that in excavation. The proportion of lime of aqueduct masonry on foundation walls over valleys, to that in excavation, is about as one to eight. The masonry of the aqueduct is covered with earth to a sufficient depth to protect it from frost. Culverts, To pass the streams that intersect the line, and the land floods, there has been constructed under the aqueduct 114 culverts, whose aggregate length is 7959 feet. The span varies from one and a half foot to twenty-five feet. Those of one and a half foot span are of a square form, and are constructed by laying down a foundation of concrete masonry, on which a course of well jointed stone, not less than nine inches thick is laid, forming a stone platform on which well dressed stone is laid, one or two courses high, for the abntments or side walls, and finished by a second course of well dressed flagging, not less than nine inches thick ; all the work laid in hydraulic mortar. The culverts over one and a half foot span, with the exception of three of twenty-five feet span, are con- structed as follows : A foundation of concrete masonry is formed, and in a few instances some timber and plank is