THE IMPROVKMK\r OF RIVERS. 



The next form of gate, and the one which is practically universally used, consists 

 of two leaves, and is known as the mitt-ring type. 



Gates of this type are subject to tin- following strains: 



(1) The pressure of the water. 



(2) The reactions from the miter, the quoins, and the miter-sill. 



(3) The weight of the leaf. 



(4) The upward pressure from the water under the bottom beam. Shocks from 

 craft, twisting from drift caught U-tween the miters orl>ehind the heel, will also induce 

 strains, but they cannot be calculated. K\j>erience has shown, however, that a gate 

 designed to meet ordinary strains and properly connected, is very rarely disarranged 

 by drift, and will stand in addition a considerable blow from a boat. 



The pressure (4) seldom needs consideration, as it is usually offset by the weight 

 of the framing, and where it is not, a proportion of the friction against the quoin may 

 be taken into account. This upward pressure is a maximum when the gate has its 

 maximum load, and at that time the friction is also a maximum. If it is not desired 

 to make any allowance for its effect weights must be added to the gates, or other means 

 taken to counterbalance the upward pressure. 



The weight (3) in wooden gates is supported by wooden struts or by diagonal ties 

 of iron, as will be mentioned later on. 



The strains from (i) and (2) are usually those which determine the proportions 

 of the framing. The reaction from the miter-sill affects only vertically framed gates, 

 except in so far as it supports the lowest beam of a horizontal framing. 



In determining the head to be provided for in calculating gates for river locks one 

 of the most imj>ortant points to be considered is that in stationary dams there is usually 

 a difference of level between the pools until, and sometimes after, the water has risen to 

 the top of the walls. We are acquainted with one lock where there is a difference of 

 9 feet between the pools when the lock is "drowned out," or flooded by the river. 

 This fact will, of course, require the upper part of the gate to be made strong 

 enough to stand the head produced by the difference between the pools at the 

 varying stages of the water, a difference which, in locks already built, can generally 

 be determined from gauge records. In new locks it can only be approximated 

 from a study of general conditions, or by comparison with other locks similarly 

 placed. On canal locks and those on rivers with movable dams these variations 

 do not occur. 



The lower gates, both with fixed and movable dams, should be proportioned so that 

 they will withstand a full pool above, and a reduced pool below, since the latter condi- 

 tion will occur sooner or later through the leakage in fixed dams caused by wear, and 

 the possibility of repairs to the movable dams. In certain cases the upper gates may 

 have to be used as a coffer-dam, and it should be seen that they will stand the accom- 

 panying head of water without danger. 



