1255 
reflection of the continuous portion of the pulse from 
“the discontinuity begins; as a result the disturbance 
propagates backward, as well as forward, and the peak 
pressure steadily decreases. This phenomenon is called 
"decay" of the shock-wave. Hence the progressive assump- 
tion (c) can not be rigorously satisfied by a shock any 
time after it is formed. It is, however, a good approxi- 
mation; in fact, shocks of the tyre (a) and (b), of the 
strengths considered here (pressure-ratio across shock 
S1.6) are very nearly progressive. Further, one may 
verify that a shock of type (c) has a pressure distri- 
bution which is nearly linear. That is, the difference 
between types (a) and (c) is insignificant for shocks of 
the strengths considered here. It way also be remarked 
that the descriptions (a), (b), and (c) apply to the in- 
cident shock only at the moment the wall is first struck. 
Before and after this time (a) is not linear, (b) 1s not 
exponential, and (c) is not progressive, again because 
of decay. 
4. The general course of a reflection may be 
described qualitatively. When a normally incident wave 
strikes a rigid wall, the overpressure on the wall is at 
first increased to more than twice the peak overpressure 
previously existing in the wave. (In the limiting case 
of a weak shock the overpressure is exactly doubled.) 
After the instant of impact the head of the wave reverses 
direction and begins to travel toward its tail (fig. 1.) 
i} 
ui 
1 
