65 
with a velocity u and take a for the velocity with which 
sound would travel in the same gas at rest, the velocity at 
which a wave of sound or any disturbance would move 
along the tube in an opposite direction to the gas would be 
a — 'll. 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, 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 ex- 
plosive 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 
regards pressure, density, and temperature of that in the 
orifice, and this is precisely what it is found to be on examin- 
ing the equations. 
7. The following is the definite expression of the foregoing 
argument. 
The adiahetic laws for gas are : ^ being pressure, p density 
T absolute temperature, and 7 the ratio of specific heat — 
the equation of motion u being the velocity and x the direc- 
tion of motion 
( 1 ) 
IS 
du dp 
01 ’ 
( 2 ) 
