THE FLOW or GASES. 175 



thatj however much the pressure might be suddenly dimi- 

 nished 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 v,^=a, 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 con- 

 dition as regards pressure, density, and temperature of 

 that in the orifice ; and this is precisely what it is found 

 to be on examining the equations. 



7. The following is the definite expression of the fore- 

 going argument. 



The adiabetic laws for gas are : p being pressure, p 

 density, r absolute temperature, and y the ratio of specific 

 heats at constant pressure and constant density, 



Ir^-^" (■) 



The equation of motion, u being the velocity and x the 

 direction of motion, is 



da _ dp 

 ^ dx dx' 



or 



P 

 Substituting from equations (i 



!f!^_P*+C (2) 



2 J„ o ^ ^ 



CJ'dp ^ 7 Po 1 



Jo p r-' po -^o 



