i6o THE IMPROVEMENT OF RIVERS. 



used a single beam is employed for the top support, its ends resting in horizontal slots 

 in the main walls, and the needles rest against this and against a wooden sill in the 

 upper coffer-wall. The slots should be placed about i feet above pool level, so the 

 beam can be put in at any ordinary stage of water. For very small locks a simple 12" 

 X 12" timber will suffice; for locks of 36 feet in width the timber must be trussed or 

 provided with a middle strut and a tie-rod, while for locks of wider opening the timber 

 usually rests on movable iron trestles hinged to the coffer-wall. In this case it may 

 be necessary to provide counterforts or buttresses from that wall, to support the down- 

 stream ends of the trestles. A steel beam may be used instead of a wooden one, if so 

 desired. 



Another, and in many ways a better method, is to omit the trestles, which some- 

 times rust away before they are needed, and to span the opening with a horizontal 

 truss supported on temporary posts, and designed so it can be taken apart into two 

 or more sections for easy handling. Trestles also possess the disadvantage of liability 

 to injury when the lock entrance has to be dredged. 



Guide Cribs or Walls. The upper and lower entrances to a lock should be pro- 

 vided with at least one guide crib each, to assist boats in entering or leaving, and to 

 which barges can be tied while waiting their turn to lock. They should be flush with 

 the chamber face where they meet the lock walls and may either continue in the same 

 line or flare outward from it. Where the lock is used by towboats the former align- 

 ment is preferable for the land side, as the fleet can then be pushed straight into the 

 chamber. The length of the cribs should be not less than the length of a tow which 

 can pass through at one lockage. 



In Europe such structures are built of cut stone or of concrete, and in this coun- 

 try the example is now being followed gradually, the entire wall being made of masonry, 

 or of masonry under water, surmounted above water by a wooden crib filled with rip- 

 rap. Where wood is used a very convenient size for the timbers is 10 inches square, 

 with sets of ties 10 feet apart, and spiked together with a drift bolt at each intersec- 

 tion. Sometimes these timbers are dapped or notched into each other, but expe- 

 rience has shown that equally good results are obtained by simply spiking the ties and 

 stringers upon each other, with butt joints where the latter come end to end. In 

 putting in the stone, large pieces should be picked out and set with flat faces on 

 edge against the openings between the timbers at the front and end of the crib, as this 

 will give a much neater appearance than if the stone are left as they are dumped, and 

 the extra cost is very small. 



The life of these structures is from ten to fifteen years. A frequent cause of 

 failure, especially where the cribs are high, lies in want of care in providing for the 

 weight of the filling. Actually, a very large portion of the riprap is upheld by the 

 timber, since its projections catch in the spaces between the ties or stringers. After 

 a year or so the whole crib becomes choked with sediment, and the result is that an 

 almost solid mass of masonry has to be carried by the ends of the ties, and this is where 



