THE FLOW OF GASES. 171 



determines the pressure at the orifice when the pressure 

 in the receiving vessel is less than '527^0,, that being the 

 pressure in the discharging vessel. 



5. If, instead of an orifice in a thin plate^ the fluid 

 (escaped through a pipe which gradually contracted to a 

 nozzle, then it would follow at once, from what has been 



already said, that when ;?j was less than •527^,, the nar- 

 rowest portion of the stream would be at N, for since the 

 stream converges to N the pressure above N can be no- 

 where less than '527/?,; and since emerging into the 

 smaller surrounding pressure p^ the stream would expand 

 laterally, N would be the minimum breadth of the stream, 

 and hence the pressure at N would be '527^9,. In a broad 

 view we may in the same way look on an orifice in the 

 wall of a vessel as the neck of a stream. But if we begin 

 to look into the argument, it is not so clear, on account of 

 the curvature of the paths in which some of the particles 

 approach the orifice. 



Since the motion with which the fluid approaches the 

 orifice is steady, the whole stream, which is bounded all 

 round by the wall, may be considered to consist of a 

 number of elementary streams, each conveying the same 

 quantity of fluid. Each of these elementary streams is 

 bounded by the neighbouring streams, but as the boun- 



