INTRODUCTION. 137 



aoove the great level at the tunnels, ranges from 25 feet to 75 feet. In West- 

 chester county, the line of the aqueduct is crossed by twenty-five streams, at 

 depths varying from 12 to 70 feet below the grade line. Besides these there 

 are numerous other brooks and valleys of less depth, over which culverts are 

 constructed. The most important valleys on the Manhattan island, over which 

 the aqueduct passes, are the Manhattan valley, Clendinning valley and Bowne's 

 valley. 



The bottom of the aqueduct is an inverted arch ; the chord or span line is 6 

 feet 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, vnth a bevel of one inch to 

 a foot rise, or four inches on each side, which makes the width at the top of the 

 side walls 7 feet 5 inches. These walls form the abutments of the roofing arch, 

 which is a semicircle, having a radius of 3 feet 8J inches, or a chord line of 7 

 feet 5 inches. The greatest interior width of the aqueduct is 7 feet 5 inches, 

 and the greatest height 8 feet 5 J inches. The area of the interior is 53 '34 

 square feet. 



The plan, dimensions and kind 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, 3 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 3 inches thick under the side walls, or abutments. The 

 abutments are 2 feet 8 inches thick at the spring Une of the inverted arch, and 

 2 feet at the top or spring line of the roofing arch. The inverted arch is of brick 

 4 inches thick ; the roofing arch is also of brick 8 inches thick. The abutments 

 or side walls are of rubble stone, with a brick facing of 4 inches thick. Span- 

 drels, of stone, are carried up solid from the exterior angle of side walls 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 Uning had its irregularities filled with successive courses of plastering, and 



Intr. 18 



