COEFFICIENT OF EFFLUX. 249 



131. Coefficient of Efflux. When air issues from an 

 orifice, the section of the current undergoes u contraction 

 similar to that observed in the efflux of water (Art. 91). If 

 the orifice of efflux is in a thin plate, the stream of air has 

 a smaller cross-section than the orifice, and the practical 

 discharge is less than the theoretical. 



Denoting the coefficient of contraction by , we have, as 

 in the case of water (Art. 92), = the ratio of the cross- 

 section of the stream of air to that of the orifice. 



Denoting the coefficient of velocity by 0, we have, as in 







Art. 93, d> = , where v, is the actual and v the theoret- 

 v 



ical velocity of discharge. 

 Denoting the coefficient of efflux by ft, we have, as in Art. 



94, \L = j^ = a<f>, where Q s is the actual and Q v the 



Vi 

 theoretical discharge. 



The older experiments upon the efflux of air through ori- 

 fices vary considerably from each other. According to the 

 experiments of Koch,* p = 0.58 when the air issues from an 

 orifice in a thin plate ; p = 0.74 when the air issues from a 

 pipe about six times as long as it is wide; and p = 0.85 

 when the air issues from the conical nozzle of a bellows 

 about five times as long as it is wide and having a lateral 

 convergence of 6. D'Aubuisson, Poncelet, and Pecqueur 

 found values somewhat different^ 



Weisbach has found the following values of the coefficient 

 of efflux n : J 



Conoidal mouth-pieces of the form of the 



contracted vein, with effective p = 



pressures of 0.23 to 1.1 atmosphere, 0.97 to 0.99 



* Tate'e Mech. Phil., p. 829. t Weisbach's Heche., p. 945. 



t Ency. Brit., VoL XII., p. 481. 



