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 roofing arch.