the head of the pipe much more rapidly than it could be supplied from the influent chamber, when the action in entering the pipe would 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 masonry 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 undoubtedly 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 tunnel, and protect the top by a roof that will turn off any water that might fall from the of the tunnel, and be roofing arch, and thus allow the whole to settle in the bottom 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