LOCKS. 155 



out; but as it may be necessary at some time to empty it, the pressure on AE should 

 not be relied on. 



Elevation and Angle of Miter-sills. The lower sill is usually placed at the depth 

 below pool required by navigation; thus, if a depth of 6 feet is to be provided, the 

 top of the sill is placed 6 feet below pool. It should never be less, as in a dry season, 

 with little water running, the entire pool will closely approximate the level of the crest 

 of the dam below, and if the latter is leaky the depth on the sill will be accordingly 

 reduced. For this reason the sill should be placed as low as practicable, since an excess 

 of draught is much better than too little. 



This principle is generally adopted for the upper miter-sill where the dam is fixed, 

 and if a 6-foot depth is desired, for example, the sill is frequently placed from 8 to 12 

 feet below the upper pool. Should the river have to be improved at any future date, 

 so as to afford a greater navigable depth, a sill so placed will not have to be disturbed, 

 and a large saving will have resulted with no greater first cost. 



The sills of the upper and lower coffer-walls are placed at the same elevations as 

 the upper and lower miter-sills respectively. 



Where the lock is connected with a movable dam the upper and lower sills are 

 generally pkced at, or nearly on, one level, and from 6 inches to 3 feet below the 

 sill of the pass, depending on the navigable depth required and other circumstances. 

 By this arrangement the lock gates can be left open when the dam is lowered for 'the 

 winter, preventing the chamber from filling with deposit, and at the same time afford- 

 ing more area of discharge. 



The economical angle of the sills, or the angle which permits the minimum of 

 material in the gates, will be usually found to lie between 19 and 21 with a normal 

 to the face of the walls. For this reason an angle of 20 is frequently adopted. 



DETAILS OF CONSTRUCTION. 



Floor. The floor of the chamber, where artificial, is a most important part of the 

 construction. Any weakness which it may develop can only be repaired by pump- 

 ing out the pit and stopping navigation entirely, and in extreme cases it may be neces- 

 sary to build' a coffer-dam around the entire lock in order to cut off the outside water. 

 Such repairing, moreover, is always tedious and expensive, and it is difficult to make 

 it thoroughly satisfactory. 



The floor is subject to the downward pressure of the water when the chamber is 

 filled, and to upward pressure from the lower pool when it is pumped out, and also, 

 judging from experience, to the pressure from the upper pool in many cases as well. 

 As it is impossible entirely to cut off leakage from the upper pool through sheet-piling, 

 stone drains have sometimes been laid under floors to allow the water to escape. It 

 may be questioned whether this remedy would be permanent, as in sediment-bearing 

 rivers the drains would probably soon become filled with mud, and our own experience 



