SMALL ORIFICES 



111 



ART. 41. 



Theoretically we may always calculate the flow from an orifice in the 

 side or bottom of a vessel, by equating the flux of momentum across 

 the plane of the orifice per second to the unbalanced statical pressure on 

 the opposite vertical side in the one case, and to weight of the un- 

 balanced column of water in the other, i.e., by expressing the fact that 

 the total unbalanced force in the direction of flow is taken up in producing 

 this flow. With an ordinary opening in the side or bottom of a vessel, 

 however, the calculation of this unbalanced force is impossible, since, due 

 to the fact that there is stream line motion along the face containing the 



FIG. 59. 



FIG. 60. 



opening, giving the water kinetic head, the pressure head at any point in 

 this face is less than that simply due to the statical head at that point by 

 an amount which is indeterminate. 1 The resultant unbalanced pressure 

 is therefore in this case indeterminate. In the case of an opening in the 

 flat, horizontal bottom of a vessel, the pressure over this face is not 

 uniform, since the kinetic head is greatest near the orifice, the distribution 

 of pressure being as represented in Fig. 59, where the ordinates from the 

 base to the curve A B C D measure the pressure on the base. 



"When the orifice G H is closed, the pressure on G His that represented 

 by the shaded area E F H G, but when open the unbalanced pressure 

 becomes that represented by the whole shaded area A B G H C D. 



Similarly in the case of an opening in the vertical side A K of a 

 vessel (Fig. 00). Here the area G E F H represents the pressure on G H 



Since part of the head produces acceleration towards the point. 



