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. It is therefore proposed to construct a float of timbers, and securely anchor it at the position for the coffer dam, and erect the frame in horizontal sections, until the work may itself be CROTONAGIUEDUCT. 167 sufficiently strong to bear anchoring, when the float timbers may be loosened and removed from 'the same. The frame to be then completed in such sections as will be most conve- nient for the work. After the frame is sunk to its place and well secured, the driving of the sheet-piling, filling in with water-tight earth, and, in general, all other work, will be similar to that proposed for coffer dams for the piers ; with such additional security as its larger dimensions demand. The coffer dam being completed, it is proposed to erect two fifty horse power steam engines, with suitable pumps, to clear the pit from water. The sand and rock within the coffer dam will then be excavated to the proper level of founda- tion of the masonry ; in the progress of which the bottom of the coffer, below the frame, will require to be secured by suitable prop or stay timbers. MASONRY. — It is proposed to lay down a foundation course of concrete, formed to receive an inverted arch of brick, from which will rise curved walls to form abutments for the upper arches of the tunnel, all of which will be of brick masonry supported by con- crete, and the exterior protected on the sides by well set stone, and the top by a covering of well jointed coping. 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 cham- ber, 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. When it is considered how difficult it is, under ordinary advantages, to construct ma- sonry so as to be