water from the bottom. The reservoir is designed for 36 feet of water, and, when full, will stand 115 feet above mean tide. The walls rise 4 feet above the water line. An iron railing is to be placed around the walls on top of the cornice. The capacity of this reservoir is 20,000,000 imperial gallons. GRADE LINE OF AQUEDUCT. The general declivity of the aqueduct in Westches- ter county is 0.021 foot per hundred, or a fraction over 13j inches per mile.* The top of the conduit pursuing this grade, corresponds with the top of the dam on the Croton ; but in order to adapt the aqueduct to draw at a lower level from the Croton reservoir, the bottom grade was depressed at the head 2.93 feet, and a declivity of 0.0113 foot per 100 feet, or 0.59664 foot per mile for 4.949 miles, where it intersects the regular grade. The top of the aqueduct was depressed only 0.583 foot, and carried level 2276 feet, where it intersected the regular grade line of the top. - This gave an extra height ot side walls, enlarging the capacity of the aqueduct, more than equivalent to the diminu- tion of declivity, and provides for drawing -from the reservoir to a depth of 6 feet, and still leave the capacity of the conduit 35,000,000 of imperial gallons. The grade of the aqueduct from Harlem river to Manhattan vfilley. is the same as the general declivity in Westchester county .; but that from Manhattan valley to the re- ceiving reservoir 2.1727 miles, is 9 inches per mile. It has been stated^ the extra fall given for the pipes at Harlem river, is 2 feet, and at Manhattan valley 3 feet ; between the reservoirs there is a fall of 4 feet, when both are full ; but it is expected something more than this will be required at times, to furnish the full quantity that will be wanted. The pipes are provided .with waste cocks at the depressions, and air cocks at the summit bends. The first contracts for work on the aqueduct, were made in April, 1^37, at which < time about ten miles of the upper end was advertised, but in consequence of the propo- sals being considered too high, only about half of it was contracted for. About the mid- dle of May, the contractors generally entered on the work contracted for. In September following, about 16 miles more was put under contract ; in May, 1838, the balance of the line to Harlem river was put under contract; and the work on the island, except the grading of the 5th Avenue for pipes, in October following. In consequence of legislative interposition, the Harlem river work was re-let in August, 1839. The 'work was so far completed, as to permit the'water to be let in from the Croton dam on the 22d day of June, 1842 ; and it was admitted into the distributing reservoir on the 4th of July following. There is yet considerable work to be done at the Croton dam. the receiving and distributing reservoirs, which will probably all be completed in Sep- tember or early in October. The Harlem river bridge will require at least two years to complete it. At the latter place, a temporary pipe conveys the water across the valley. The following extract is taken from my report of the 12th inst. to the Board of Wa- ter Commissioners :— " Since the water was introduced into the aqueduct, -the velocity ' has several times been ascertained, though not in so perfect a manner as I intend to have it done, as soon as other duties will allow the time necessary. Sufficient data, however, have been ob- * As an illustration of the extreme sensibility of water to the smallest declivity, it may be stated that on a long reach of the Erie canal from Lockport, for a distance of sixty miles, the fall is only of half an inch in a mile, aqueducts had an .average declivity of one foot in six hundred. — [Ei CROfON AdUEDUCT. 215'' tained to show, that the capacity of the aqueduct for delivering water will be at least 15 per cent, greater than the calculated flow. I have not been much disappointed in finding the flow of water in the aqueduct, to exceed the calculation, as all my observations on the currents in canal feeders, have led me to believe the formulas laid down give rather less than the actual result. The flow of water through the pipes across Manhattan valley, and also the temporary pipe across Harlem river, being attended with circumstances somewhat different, has led some very intelligent persons to predict, that our expectations would not be realised in these cases ; it