36 



THE IMPROVEMENT OF RIVERS. 



For the wicket to be on the point of swinging, the resultant of P and 1< must 

 pass through B. This resultant Q! P R, or 



(H \ h ///* /t*\ 

 + d]u>h sec a X 62$ Ibs. X wsecaX62i Ibs.f \-dH ) 



Taking moments about C, we have 

 Q.T - P. k - R- 



H'(H 

 - 



- h' 



Substituting the values of P, k, etc., as before found, we have 



u/T/'sec* a (sd + H) X 62} Ibs. _ wh* sec 8 a X 62$ Ibs. 



V _ 2 ~ 2 



/ / ~ h *\ 

 3/ sec a X 62} Ibs. ( + dH J 



In order therefore to maintain the stability of the wicket the axis of rotation 

 must be above the base a distance equal to or greater than /'. 



In the earliest dams constructed the axis of rotation was placed above the base a 



little over one-third of the length AC. It was found, 

 however, that the wickets were entirely too sensitive, 

 and would swing with very little provocation, lowering 

 the pools suddenly and causing a general disturb- 

 ance of levels. In the dams of the upper Seine and 

 other rivers, constructed bertween 1860 and 1870, the 

 axis was accordingly placed at T < * ff of the length above 

 the sill; in the pass of Port-a-1'Anglais at Paris (1870) 

 it was further raised to J f \ of this length; while at 

 the dam of La Mulatiere at Lyons (1879) it was 

 placed at ^ of the length. On the weirs of the 

 Kanawha River in this country it is generally located 

 at -,4^, and on the passes at Jfo of the height above 

 the sill. De Lagren6 recommends that in navigable 

 passes the axis be placed at one-half the height above 

 the sill, and on weirs at a little more than one-third 

 of the same distance. 



Horse. On the earlier dams, where maneuvered 

 by a tripping-bar, the horse BD was slightly in- 

 clined up stream, but when used with hurters of the Pasqueau type it must be inclined 

 down stream, and so placed that it cannot be pulled up-stream far enough to pass the 

 vertical and so throw the weight of the wicket on its up-stream side. A neglect of 

 this precaution will sometimes cause trouble in lowering tne wickets. 



FIG. 19. 



