62 
Then it would follow at once from what has been already- 
said that when ^2 was less than ’527^91, the narrowest portion 
of the stream would be at N, for since the stream converges 
to N the pressure above N can be nowhere less than •527pi 
and since emerging into the smaller surrounding pressure ^2 
the stream would expand laterally, N would be the mini- 
mum breadth of the stream, and hence the pressure at N 
would be *527^1. Tn 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 fiuid 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 boundaries do not change 
their position they may be considered as fixed. 
