SMALL ORIFICES 107 



as indicated in Fig. 22 b, curtails the full flow to the orifice, and tends to 

 prevent the full contraction of section at the vena contracta. In any such 

 case the value of C c and of C will be greater than indicated above, the 

 true values being entirely dependent on the circumstances governing the 

 flow to the orifice. 



From experiments on a rectangular orifice, *177 feet long and *089 feet 

 wide, in which the contraction could be suppressed on part or the whole 

 of the perimeter by means of flat plates perpendicular to the plane of the 

 orifice and *22 feet long, Biiloin- 

 deduced the formula 



C = '608 + -0925 x, 

 where x is the fraction of the peri- 

 meter over which the contraction is 

 suppressed. For circular orifices 

 C = -608 -f -079 x. 



With this device, however, contrac- . 

 tion of section takes place at the inner 



I 



edges of the baffle plates, and when ^4sS> 5 ~=^=. 



the orifice is entirely surrounded this 

 simply forms a Borda's mouthpiece 

 running full (p. 113) with C about '75. 



Experiments by Lesbros on a 

 vertical orifice, '656 feet square, 



showed mean values of the coefficient, for heads between 1*5 and 10 feet, in 

 the ratios TOO, T032, T043, 1*116, according as the contraction was un- 

 affected, suppressed at the lower edge, at one side, and at the lower edge 

 and sides, and thus agree fairly well with Bidone's results. 



Where an orifice is made in the thick side of a vessel, or is fitted with 

 an external pipe so that the. jet, after springing clear of its inner edges, 

 again touches the sides of the orifice before escaping, or where fitted with 

 re-entrant mouthpiece, the physical conditions governing the flow are 

 entirely changed, and the coefficients assume new values. Thus the 

 coefficient of contraction may vary from '5 to 1/0 under different condi- 

 tions of working, with the same size of orifice and the same head. There 

 is, however, nothing mysterious about this change it is governed 

 entirely by the conditions of flow, and, as will be seen, may be predeter- 

 mined with some exactitude when those conditions are known. 



Submerged Orifices. If an orifice discharges below the surface of the 

 water in a second vessel instead of into the air, this has the effect of in- 

 creasing the pressure against which discharge takes place, and thus of 



