The bed of the river is about 18 feet below common high water mark. The channel is not this depth for its whole width, but slopes very flat on the sides. The outline of the plan, is to make a tunnel of masonry, of sufficient dimensions to allow four iron pipes, each three feet diameter to be laid within it. The tunnel to be made of two sections, or of two arched vaults, laid close to each other, and each sufficient for two pipes. The tunnel to terminate at each end, in abutments of masonry, having such slopes and curves as will afford the most favorable flow of water, that the situation will permit. A horizontal vault in each section will carry any water that may gather in the tunnel, to the vertical welts in the abutments, where it may be cleared by pumps. The tunnel at each end will open in the top of the abutments, and thence the iron pipe will pass from the tunnel, rising to a suitable level above the river, and enter the embankment by which it will be covered. To protect the pipes from frost and provide for convenient entrance into the tunnel, a house is to be erected over this opening.

It is proposed to make the tunnel, by first excavating the mud which forms the bed of the river, by means of a dredging machine, so as to allow a frame for a coffer dam to be sunk on the sand which underlies the mud. After the mud has been removed, a frame for a coffer dam is to be sunk, embracing the area of tunnel and abutments. The coffer dam being required to be about 400 feet long and 40 feet wide, it is obvious it would be extremely difficult, if not impracticable, to make this frame on land and launch it, as is proposed to be done for the comparatively small coffers required for single piers.