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

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, consequently 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 difficulty, 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 capable 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. ;