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 feet, board measure, yellow pine timber, at 30, - 11,400 10,000 lineal feet round piling, at 20 cents, - 2,000 $35,800 CROTON AQ.UEDUCT. 169 133,610 20,000 pounds wrought iron bolts, straps and spikes, at 12 cents - - $2,400 Pin timber and treenails 250 Carpenter and other work, framing, sinking, and securing frame - 30,000 Driving 2,000 feet lineal of sheet piling, at $2 4,000 Driving 320 round piles at $3 - 960 380,000 cubic yards earth filling and embanking coffer, at 40 cents - 15,200 Estimated cost of pumps, steam engines, and working, (very uncertain) 30,000 Removing frame, piling, earth of dam from the channel — estimated 15,000 1,800 cubic yards brick masonry, at $18 - 32,400 8,000 cubic yards stone masonry and concrete, in abutments and tun- nel, at $10 - 80,000 460 cubic yards coping and side facing of tunnel, at $30 - 13,800 2 entrance houses on abutments - 2,000 6,500 cubic yards stone embankment in river, between abutments and shores, at $2 - 13,000 3,000 cubic yards foundation and protection wall, at $2 50 - 7,500 700 tons iron pipes, delivered, at $75 - - 52,500 Lead, yarn, tallow, and work putting down 3,100 feet, at $4 - 12,400 13,000 cubic yards earth covering, at 20 cents - - 2,600 2 pipe chambers, as per detailed estimate of December, 1837 - 15,642 Waste cocks and man hole plates - 7,500 90,642 $424,492 Add for contingencies, 50 per cent. 212,246 128,200 20,500 $636,738 Aggregate estimate for bridge, - Aggregate estimate for tunnel, - $836,613 00 636,738 00 Difference in favor of tunnel, $199,875 00 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 hy- draulic 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 oc- cur 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, 43 170 MEMOIR OF THE and consequently have greater durability. The work of repairs, or removal of the tun- nel 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, ren- dering 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 ex- pensive 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 re- quired 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 rather the work should be so well guarded as to leave no apprehension of such a result. Well completed, the bridge, in the simplicity and economy of its operation, and its architectural appearance, would, no doubt, be the most satisfactory structure. In the foregoing estimates and remarks, I have endeavored to call your attention to all the essential circumstances and considerations that have a bearing in deciding on the plan most appropriate to be adopted. The question is one of great importance, and surround- ed with embarrassing difficulties, which in some respects are of a nature that do not ad- mit of exact, or even hardly approximate, computation. In relation to the