S4 



THE IMPROVEMENT OF RIVERS. 



joints from bottom to top, these should be arranged so as to avoid a joint near the 

 hollow quoin, and to place behind the gate a monolith at least equal in mass to that 

 required by the calculations. The foundations of the lower end of the walls must be 

 designed with ample strength to resist force X', as, if any weakness exists, the gates 



will tend to separate the concrete blocks at 

 the nearest joint, a case of which we have 

 met with. This will sometimes occur in 

 masonry locks also, several such cases being 

 on record. One of them, which occurred in 

 this country, is worth noting because of the 

 unusual circumstances. The lock, which was 

 built about 1840, carried a maximum head 

 of some 15 feet. The lower end of the river 

 wall, where the parting occurred, was built 

 on squared timbers placed on bed-rock, a 

 peculiar method of construction of which 

 several examples are to be found on the same 

 river. The mass of the masonry appeared 

 ample, but the wall had parted in an irregular 

 seam across the gate recess and moved visibly 

 as the water in the chamber rose or fell. 

 The cause lay in the timber footing, which 

 had been worn away where exposed to the 

 eddies of the current. The wall was satis- 

 factorily repaired by building a concrete buttress on the outside. 



Upper Coffer-wall. The upper coffer-wall is only used when the lock is to be 

 repaired. Its top is provided with a wooden sill which supports the needles used for 

 the coffer-dam, their heads being supported against a beam just above the upper pool. 



Thus, let ABDE (Fig. 7) be a section of a wall supporting needles CB. Let h be 

 the head of water on the needles, and d the height of the wall. The pressure on CB 



/t* 



- X 62^ Ibs., of which two- 



2 



per foot run, supposing the needles to be vertical, is P 



thirds go to B. The pressure on BD per foot run, which may be taken for the sake 

 of simplicity as acting at the center of BD, is 



Q-d 



H D 



-] X 62 J Ibs. 



Combining Q and the proportion of P at B into their resultant S, and finding the weight 

 of the wall W, the position of the final resultant R may be determined as shown 

 for the river chamber wall. 



The space behind AE usually remains filled by water when the head bay is pumped 



