mile, the grade of the aque- duct was compelled to encounter great irregularities of surface. Very little regular table- land could could be found for its location. While the main ridge of high land, that lay on the left of the line in Westchester county, controlled the general location, numerous spurs of ridges, of various forms and extent, bounded by deep ravines, rendered it im- possible to avoid deep cutting and frequent tunnelling, through ridges, and heavy rilling in the valleys ; leaving a very limited proportion of the line in favorable cutting. On the island of New York, with the exception of about one mile from Harlem river, there was no leading ridge to control or guide the location ; which rendered it more difficult to decide on the conflicting claims of different routes. There are on the line sixteen tunnels, varying in length from 160 to 1 263 feet, making an aggregate length of 6841 feet. The height of the ridges above the grade level at the tunnels ranges from 25 to 75 feet. There are in Westchester county twenty-five streams crossing the line of aqueduct, that are from 12 to 70 feet below the grade line, and from 25 to 83 feet below the top covering of the aqueduct. The most prominent of the valleys, are Loun sherry's, Indian brook, Sing Sing kill, Mill river, Jewel's brook, and Saw Mill river ; the foundations of which are in no case less than 40 feet below the grade line, or 53 feet below the top covering of the aqueduct. Besides those above mentioned, there are numerous brooks and valleys of less depth, requiring culverts, and artificial foundations to support the aqueduct. The level of mean tide at Harlem river is 120.66 feet below the grade line of the aqueduct, of which a more particular description will be given hereafter. On New York Island there are several deep and abrupt valleys, along the precipitous rocky hill that bounds the southerly shore of Harlem river. But the most important valleys on the island are Manhattan valley, Clendening valley, and Bowne's valley. A more particular notice of Manhattan and Clendening valleys will be given when descri- bing the plan of work, the manner of construction for the others being similar to that for the valleys of Westchester county. DESCRIPTION OF THE PLAN OF WORK FOR THE AQ.UEDUCT AND ITS APPURTENANCES. Aqueduct. The form and dimensions of the interior of the aqueduct are as follows : The bottom is an inverted arch ; the chord or span line is 6 feet and 9 inches, and the versed sine 9 inches. The masonry of the side walls rises four feet above the springing line of the inverted arch, with a bevel of one inch to a foot rise, or four inches on each side, which brings the width at the top of the side walls, 7 feet and 5 inches ; forming the abutments of the roofing arch, which is a semicircle, having a radius of 3 feet 8^ inches, or a chord line of 7 feet 5 inches. It will therefore be perceived, the greatest interior width is 7 feet 5 inches, and greatest height 8 feet 5] inches. The area of the interior is 53.34 square. In rock tunnels the roofing arch is generally dispensed with, but the bottom and sides are formed with masonry similar to that above described. There is an exception to this form in the first 4.949 miles of the upper end of the C HO TON ACIUEDUCT. 205 aqueduct, where the side walls have an extra height, on account of the bottom being depressed, to draw the water at a lower level from the Croton reservoir. The plan, dimensions, and character of masonry are as follows : In excavation, a bed of concrete masonry is laid down as a foundation. It is laid level across the bottom, three inches thick at the centre of the inverted arch, and curved on its upper surface, to form a bed for the arch, which brings it 12 inches thick at the spring line, and is carried three inches thick under the side walls or abutments. The abutments are 2 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