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GENERAL PRINCIPLES OF HYDRAULICS — 449 
ocity c /2gh, if there were no friction, is less than the actual head 
y the amount A(1-c?), The ratio =O) _1— is called the 
a 
nt of resistance for the orifice. 
387. Drowned or Submerged Orifices.—It was shown in Art. 381 
. 728) that when the water stands above the orifice on both sides, 
2 effective head to be used in calculating the velocity through the 
fice, neglecting friction, is the difference between the levels on the two 
s. This is also the head to be taken in calculating the discharge 
hrough a drowned orifice when friction and the contraction of the jet are 
considered, but it has been found by experiment that in the case of a 
_ drowned sharp-edged orifice the coefficient of discharge is slightly less 
bo as per cent.) than when the discharge is directly into the 
atmo sp ere. 
$88. Time of Flow through an Orifice for a given Change of 
iter Level in a Vessel.—Let a be the area of the orifice, and & its 
coefficient. of discharge. 
___ The simplest case (Fig. 733) is where the level changes from AB to 
CD, the water flowing in under a constant head h. Let V denote the 
volume of water ABCD. The discharge through the orifice per second is 
_ ket ,/2gh, and therefore the time in seconds to discharge the quantity V is 
Vika /2gh. - 
A common case is that in which the level is to be raised from AB to 
‘CD (Fig. 734), the water passing through an orifice at O, the level EF 
4 Fig. 733. Fia. 734. 
of the water on the inflow side being at a constant height above O; or 
_ the level is to be lowered from AB to CD (Fig. 735), the water passing 
through an orifice at O, the level EF of the water on the outflow side 
being at a constant height above O. When the free surface of the water 
in the vessel is at a distance y from the level EF, let its area be Y. In 
_ the time dé let y change to y — dy, then 
ane H Vdy 
ka ./2qy:dt=Ydy. Hence t= \, ie toe 
where ¢ is the time required to change the level from AB to CD. In 
_ practical cases the vessel ABCD is generally either a vertical cylinder or 
4 2A 
: s Agboeesy prism, and Y is a constant = A, then ¢= ica J2g' JH - J/h). 
2F 
