THE IMPROVEMENT OF RIVERS. 



masses A BCD and FGHJ being taken as those which resist the pressure from the 



gates. 



The forces acting are the weight of these masses, IV and W, and the resultant 



thrusts, T and T', from the gates. Let Q be the pressure on the leaf and its miter- 

 wall, the angle of inclination of the gates, 

 H the depth of water on the floor of the lock,* 

 / the half width of the chamber, and e the 

 depth of the gate recess. The surfac FK 

 is subject to a pressure P from the upper 

 pool in the chamber, but that on A E, if E is 

 above the crest of the dam, is balanced by 

 the equal pressure on the outside of the 

 wall. 



The maximum of T occurs at E when the 

 chamber is pumped out for repairs, and the 

 upper pool is full, or more than full; the 

 maximum of T' at K when the upper pool is 

 in the chamber and the lower pool is drawn 

 off. In the case of A BCD, if the crest of the 

 dam is below CD, there will be pressure from 

 the upper pool on the outside of the wall, 

 which we will neglect here, but which makes 

 for safety. If the foundation is porous, there 

 will be an upward pressure on the base from 

 the lower pool, which must be deducted from 

 W. The upward pressure from the upper 

 pool is supposed to be cut off by sheet- 

 piling. The pressure of the lower pool on 

 the outside of the wall will be neglected, as 

 it is small. 



T and T' should be calculated for the 

 head of the water on the floor of the lock, 

 since the pressure below the gates on the 

 miter-wall is transmitted to the main walls, 

 We then have, as found in the cal- 



FIG. 5. 



and they have thus to support the entire thrust. 

 culations for lock gates, 



Q 



T or T = 



2 sin 



* Where the miter-wall is high, it usually possesses sufficient mass to resist the pressure of water 

 against it without any support from the main walls. In such a case H is measured from the bottom of the 

 gate instead of from the floor. 



