The iron pipes are proposed to be of metal, 1 3-8 inches in thickness, for the part that passes the tunnel, and 1^ inches until they extend to a point 40 feet above the level of the river. Branch pipes and waste cocks will be required in the main pipe, above the high water mark of the river, as a means to force out the sediment that may be deposited in the bottom of the pipe. This means of clearing the pipes would probably be sufficient, if the waste pipes could be placed in, and discharge freely from the bottom of the bend ;

but as the cocks must be about 32 feet above the iowest part of the bend, and about 300 feet in line of pipe from the commencement of the lowest part, there is doubt whether the sediment would be raised by the current that could be given through the depression. On the first opening of the stop cock, with the pipe full, there would be a powerful rush of water but it must be kept in view, that this force would only be of momentary, or of ;

very short, duration as the quantity discharged under this great head, would exhaust ;

the head of the pipe much more rapidly than it could be supplied from the influent chamber, when the action in entering the pipe would be under comparatively a very small head. It is therefore obvious, the discharge at the cock would very shortly be reduced to the quantity that could enter the pipe from the pipe chamber. To provide, therefore, for removing sediment that may not be carried out by the force of current, discharging from the stop cocks, it is proposed to put in the pipes, at convenient distances, man hole plates, by which the pipes may be entered and cleared by manual labor.