1254 
specify the pressure, material velocity, and entropy 
distribution in it. The customary experimental descrip- 
tion of a shock in terms of pressure alone is therefore 
incomplete. In this report it is assumed that the 
entropy is constant on both sides of the shock front, 
and that it is continuous across this front. Three 
particular kinds of incident pulses are considered here. 
At the instant the wall is first struck, they may be 
described as follows: 
(a) Both the pressure and the velocity fall 
off linearly behind the shock; 
(b) they both fall off exponentially; 
(c) the velocity falls off linearly and the 
shape of the pressure pulse is determined 
by the condition that the wave is pro- 
gressive, i.e., by the condition that the 
incident disturbance propagates only toward 
the wall, rather than both toward it and 
away from it. (It will be shown in par. 16 
how the progressive assumption fixes the 
pressure.) 
Some remarks will now be made about these assumptions. 
3. Although a continuous wave of finite amplitude 
can propagate without increase of length, the same is no 
longer true as soon as it has evolved into a shock (See 
for example, reference 2). After it has become a shock, 
2 
