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 expense of other work.