108 



HYDRAULICS AND ITS APPLICATIONS 



reducing the effective head, and also of slightly reducing the value of the 

 coefficient of discharge. 



The reduction of the coefficient of discharge is probably a secondary 

 effect of the eddy formation which occurs in the dead water surrounding 

 the jet at A and B, Fig. 54, the dead water comprising the eddy projecting 

 into the stream and reducing its effective area. 



In the sketch (Fig. 55), if the suffix d) refer to the free surface in the 

 upper vessel and suffix ( ) to the vena contracta, we have, neglecting 

 viscosity, 



- 

 W 





FIG. 55. 



Taking the datum level at the centre of the orifice, and assuming the 

 area at the surface in the upper vessel to be large, we have 



Pl = 0; v, = 0; z. = 0; z, = H, ; ^ = 7/ a 

 .-. //! - H* = feet 



where h = difference in level of the free surfaces on the two sides of the 

 orifice. 



ART. 40. EXPERIMENTAL DETERMINATION OF THE COEFFICIENTS. 

 (1) Coefficient of Velocity, for an Orifice in a Vertical Plate. Keeping h 

 constant, mark on a flat board parallel to the plane of the jet, points 1, 2, 

 3, 4, etc., by means of a set square, these points marking as nearly as 



