be under comparatively a very small head. It is therefore obvious, the discharge at the cock would very shortly be reduced to the quantity that could enter the pipe from the pipe chamber. To provide, therefore, for removing sediment that may not be carried out by the force of current, discharging from the stop cocks, it is proposed to put in the pipes, at convenient distances, man hole plates, by which the pipes may be entered and cleared by manual labor. When it is considered how difficult it is, under ordinary advantages, to construct ma- sonry so as to be perfectly impervious to water, we cannot expect this tunnel, which must be constructed under circumstances peculiarly unfavorable, and exposed to a head of 18 feet, resting on the bare walls, will be sufficient to exclude all water ; some will undoubt- edly percolate through, and find its way into the tunnel. This water being salt, must be carefully kept from the pipes, or it would soon corrode and destroy them. It is therefore proposed to place the pipes on seats that will keep them clear from the walls of the tun- nel, and protect the top by a roof that will turn off any water that might fall from the roofing arch, and thus allow the whole to settle in the bottom of the tunnel, and be drained by the vaults that connect with the pump wells, where suitable pumps must be erected to raise and discharge it from the same. The duty of preparing an estimate for this tunnel is obviously a difficult one, and to a great extent must be viewed as mere conjecture. The comparatively small coffer dams required for the piers, are a work in which much embarrassment and difficulty must be 168 MEMOIR OF THE expected in the execution, and uncertainty in regard to the estimated expense ; and but few cases are known of equal difficulty. These, however, only enclose about one-tenth the area required for the coffer dam which is necessary for the tunnel. It is believed no experience exists in placing such a coifer, or one of one-quarter the size or difficulty, con- sequently we cannot have the light of experience to any very satisfactory extent. These remarks are made, not because I consider the work impracticable, but as one of great dif- ficulty, the estimate for which must be considered as subject to much contingency and consequent uncertainty in amount. The estimate has been prepared with much care, all I believe that can be useful, and is submitted as my best judgment on a work, not ca- pable of being reduced to ordinary accuracy in anticipating its cost. Had we been permitted to restrict the channel of the river to 100 feet in width and 8 feet in depth at low water, which would probably have been an ample provision for all navigation that it is necessary to provide for, the plan of passing the aqueduct by means of a tunnel, would have been divested of a large portion of the difficulty and expense, and could have been entered upon with far greater certainty in relation to time, and expense of construction ; but this we are not permitted to do. In difficulty and expense of construction, the Thames Tunnel, at London, has more similarity than any other work of which I have any knowledge ; though it is a work of greater difficulty, and in some respects decidedly so, than the proposed Harlem Tunnel. The history of this work is, however, such as to admonish us of the uncertainty in estimating for work done under a heavy pressure of water. It was commenced in 1825, and then estimated to cost 160,000 pounds sterling. November 2, 1837, 12 years after its commencement, there had been expended 264,000 pounds, and it \vas then estimated to require an additional sum of 350,000 pounds to complete it, which, . if correct, will make the final cost 614,000 pounds, or near four times the original estimate. As before observed, we are not to consider the two cases as parallel, though there is a degree of similarity, which very naturally excites attention when looking for some guide in the ex- pense of other work. Estimated Expense of Harlem Tunnel. 11,000 cubic yards earth excavation on shores, at 14 cents, $ 1,540 2,000 cubic yards rock excavation on shores, at $1 50, 3,000 24,000 cubic yards excavation of mud in river, at 1,- 24,000 3.000 cubic yards excavation of sand in river, at 1, - 3.000 4,000 cubic yards excavation of rock in river, at 5, - 20,000 Coffer Dam. 240,000 feet, board measure, white oak timber, at $35, - - $ 8,400 700,000 feet board measure, white pine timber, at 20, - - , 14,000 380,000