The items of which the estimate for the bridge is made, are for the greatest part of a character that give confidence in its being a fair approximation to the actual cost. The hydraulic foundations are the principal exception. In relation to the tunnel, the greatest part is peculiarly uncertain ; and it would not be surprising if unforseen difficulties should occur in its construction, that would materially reduce the difference that appears in the estimates. In regard to the comparative maintenance of the work, the tunnel plan will require a pumping apparatus to be kept in constant operation, to clear the tunnel from water that may percolate through the masonry, The extent and expense of this will depend on the success that may be obtained in rendering the masonry impervious. The pipes will be exposed to deterioration from accidental contact with salt water, and from an atmosphere somewhat affected by its contact and contiguity with the same. The pipes on the bridge will be enclosed in earth, be free from these contingencies,

170 MEMOIR OF THE and consequently have greater durability. The work of repairs, or removal of the tunnel pipes, will be attended with greater difficulty and expense than those on the bridge. The Avaste cocks for the tunnel pipe, being 32 feet above the lower part of the bend, rendering it probable the sediment must be removed, to a great extent, by manual labor, through the man holes, will make the clearing of the pipes much more difficult and expensive than the bridge pipe. The supervision and care, necessary to keep the tunnel pipe in good condition and guard against sudden failure, will be much greater than required for the bridsre pipe. In regard to the masonry, a defect or failure in either plan would be very difficult and expensive to remedy and we should not anticipate any, or ;