THE FLOW OF GASES. 177 



which is the velocity of sound in the gas at the absolute 

 temperature r. 



It thus appears that the velocity of gas at the point of 

 minimum area of a stream along which the pressure falls 

 continuously is equal to the velocity of sound in the gas 

 at that point. 



8. From the equation of flow (5) it appears that for 

 every value of A other than its minimum value, there 

 are two possible values of the pressure which satisfy the 

 equation_, one being greater and the other less than 



It therefore appears that in a channel having two equal 

 minima values of section A and C, as in fig. 6, the flow 



from A to B may take place in either of two ways when 

 the velocity is such that the pressure at A and B is 

 'S'^lPi, i- €• the pressure may either be a maximum or a 

 minimum at C. In this respect gas difi'ers entirely from 

 a liquid, with which the pressure can only be a maximum 

 ate. 



9. For air through an orifice, since y= 1*408, when the 

 pressure in the receiving vessel is less than "527^0,, the 

 numerical value of U„, the velocity in the neck of the 

 orifice, is 



U„ = 99 7 (feet per sec.) \/--'; . . . (13) 



SER. III. VOL. X. N 



