1274 
still one more condition needed to fix u, c, and s. The 
additional condition is easily available only at the wall, 
where u vanishes. Elsewhere the approximation is made that 
the line D is a characteristic of P, ieee, 
Piatt) .t) = (0,0) (20.1 
where x=g6 (t) (20.2 
is the equation of D. The required additional condition 
is (20.1); its accuracy will be discussed in the next para- 
graph. 
ele The best arguments for the condition (20.1) are 
that the decay of P along D, computed by the rigorous numeri- 
cal method, is small and that the pressure=time curve, obtained 
on the basis of (20.1), agrees very closely with the corre- 
sponding curve computed according to the numerical method (as 
shown in par. 9.). In fig. 10 the characteristics of P are 
actually steeper than D. In this figure two such characteristics 
x Fig. 10 are shown. That these lines 
D must be steeper than D may 
be shown as follows. By 
p cvaracreristics (25-6) 
S =u, +ec + 6,0, < uy +c, 
t 
since ¢,< ¢, because c is on the high pressure side of D. 
Since us O, andu>d> O, 
s<c ac oF ue 
- 22 - 
