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 describing 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 ;