I 5 8 THE IMPROVEMENT OF RIVERS. 



18 inches square and a feet high or more above the floor. Their object is to pro- 

 vide an elastic cushion for the bottom of the gates. They should be well bolted down, 

 since they are sometimes subjected to a lifting pressure from the gates, and when 

 once started the upward water-pressure is of course added. This has occurred at the 

 locks of the St. Mary's Falls Canal, at those of the Louisville and Portland Canal, and 

 elsewhere. The sills are also subject to blows from the gates in closing, and some- 

 times, where the dams are leaky and the water low, to the strain of boats being dragged 

 over them. 



They should not be lap-jointed, but should be so constructed that any piece can 

 be taken out and replaced without difficulty, and without disturbing another piece. 



Miter- and Coffer-walls. Where the miter-walls or the coffer-walls are shallow, and 

 consequently of small mass, they should be well secured to the foundations with bolts or 

 otherwise, or the water may leak beneath and force them up, an accident of which 

 several examples have occurred in this country. 



Culverts. See paragraphs on "Valves" in the next chapter. 



Quoins. In locks of cut stone the hollow quoin is cut directly in the pieces, 

 which are set on each other as the building progresses. Especial care must be taken 

 to see that they are set perfectly plumb and level, or it may be necessary to trim the 

 whole quoin after all the stones are in place. The gate bears directly against the surface of 

 these stones, which are made concave to suit the radius of the heel. 



With concrete locks the quoins have usually been made of cast iron, i inch or ij 

 inches thick, in sections 5 or 6 feet long, planed for the gate-bearing surfaces, and bolted 

 to each other and to the walls. This kind requires as much care in setting as those 

 of cut stone, and a more satisfactory method, as shown by recent practice, is to shape 

 the quoin directly in the concrete, using a form in the way employed for any other 

 special surface. This is cheaper than using cast-iron sections, which are expensive, and 

 also gives a better alignment, as it is difficult to keep the iron sections in exact position. 

 If the shop-fitting on them has been at all defective, additional difficulties will be 

 encountered. 



The best shape for ordinary locks consists of an arc of the same radius as the heel 

 of the gate, and in length about one-half of the semicircle. A flat quoin has been 

 used on the Osage River, in Missouri, placed normal to the resultant thrust from the 

 heel, but it is understood that the ordinary circular quoin is to be substituted in any 

 new locks. This should be placed in relation to the heel so that very little space will 

 exist between, or sticks and other floating d6bris, such as is always present, may get 

 behind and strain the gate in closing. 



Wing Walls and Drain. The upper and lower ends of the land wall are usually 

 provided with wing walls, running into the bank from 30 to 60 feet from the 

 chamber face, with the object of preventing the water from cutting round behind the 

 walls. Where the upper wall does not rest on rock, sheet-piling should always be 

 driven along the up-stream side, but with the lower wall this is not necessary. They 



