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 convenient 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 foundation 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.