170 HYDKAULICS AND ITS APPLICATIONS 



vernier, results accurate to ^of an inch may be easily obtained, and with 

 practice the possible error may be reduced to about ^Jo ^ an inch. Note 

 should always be made of the effect of capillary action in raising the 

 apparent height of the surface at the hook. 



Where it is impossible to stop flow past the weir for this preliminary 

 work, a vertical staff fixed near the weir face on its down-stream side may be 

 graduated by straight edge and level, to give heights above the sill in the 

 neighbourhood of the surface. These heights may then be transferred by 

 means of a straight-edge and level to the measuring staff, or may be used to 

 give the datum level to which to adjust the zero reading of the hook gauge. 



In selecting a formula for use in any particular case, it should be 

 remembered that that of Francis gives accurate results for weirs with 

 perfect bottom contractions and with heads above 6 inches. The formulae 

 of Smith, Fteley and Stearns, and Bazin are better for very small heads, 

 or where the bottom contraction is imperfect, this element tending to 

 decrease the discharge being included in the larger velocity-of-approach 

 correction. Under such circumstances Bazin's formula is probably most 

 reliable. 



Although in expert hands the method of measurement by weirs will 

 give results which may be relied upon as correct within about 2*0 per 

 cent., this degree of exactitude is not to be expected with any but the most 

 careful measurements and consideration of the special conditions of each 

 case. 



ART. 60. TIME OF EMPTYING A VESSEL THROUGH A LARGE ORIFICE. 



If the orifice be situated in the horizontal base of the vessel, the 

 formulae relating to the time of discharge are the same as those for a 

 small orifice (p. 121), except that now the effect of the velocity of approach 

 is to be taken into account. Thus if a c = area of vena contracta and A 

 that of the vessel we have the velocity of efflux corresponding to a head 

 H, given by 



2 



A / ~ a 

 \f ' 



g H 



(1) 



