the. Flow of Gases. 193 



the tube in an opposite direction to the gas would be a — u. 



If then a=u, no disturbance could flow back along the tube 

 against the motion of the gas ; so that, however much the 

 pressure might be suddenly diminished at any point in the 

 tube, it would not affect the pressure at points on the side 

 from which the fluid is flowing. Thus, suppose the gas to 

 be steam and this to be suddenly condensed at one point of 

 the tube, the fall of pressure would move back against the 

 motion, increasing the motion till u = a 7 but not further ; 

 just as in the Bunsen's burner the flame cannot flow back 

 into the tube so long as the velocity of the explosive mixture 

 is greater than the velocity at which the flame travels in the 

 mixture. 



According to this view, the limit of flow through an orifice 

 should be the velocity of sound in gas in the condition as 



