This dam sets the water of the river back five miles, and forms a reservoir of about four hundred acres, and has rendered it necessary to construct several new roads and bridges as a substitute for those covered by the flow, the principal of which is the Somerstown turnpike. The grounds lightly flowed on the margin, have been excavated so as to give 4 A feet for the least depth of water. From this reservoir the water flows into the bulk-head, at the upper end of the tunnel, from a level averaging 10 feet below the surface.

AQUEDUCT BRIDGE AT SING SING. The Sing Sing kill,, where it crosses the line of aqueduct, runs in a deep and narrow gulf, the bottom of which is 63 feet below the grade line, or 76 feet below the top covering of the masonry. Over this gulf an aqueduct bridge has been constructed. Near the north end of the valley that spreads out from this gulf^ a road culvert, or arched viaduct has been constructed under the conduit. The work here is the arch over the It is 88 feet span, and 33 principal large directly gulf. feet rise the form is an oval drawn from five centres the abutments are commenced on ; ;

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the solid rock, near the bottom of the gulf. The work is constructed of well dressed stone masonry, laid in hydraulic cement. Open hance walls are carried up over the interior and above the solid spandrels, and united at the top by brick arches. The spaces between the hance walls are carried entirely across the crown of the arch, to afford as much facility as possible for any water that might leak from the conduit to pass off. The depth of arch stone at the spring line is four feet, and at the crown three feet.