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thereby marks the end of the reflection. During this 
interpenetration S is moving away from the wall with 
one speed, and T is moving toward it with another speed, 
namely, a, the speed of sound in undisturbed fluid. 
When S and T meet,the original pulse is very much com= 
pressed. Since both head, 5, and tail, T, travel at 
nearly sonic velocity, a, the pulse has about half its 
initial thickness when they meet. At this instant the 
entire region of disturbance is contained in M, and the 
fluid here is at high and nearly uniform pressure. After 
passing through S, the tail of the incident wave travels 
relative to M with the new velocity, c, that of sound in 
Me Its velocity with respect to the wall is c-u, where 
u is the material velocity in M away from the wall. The 
velocity c-u may be either greater than or less than a. 
In our examples c-u <a in air; c-u > a in water. When 
T reaches the wall, the pressure at the wall has fallsn 
to its value before the impact, the incident pulse has 
reversed its direction, and the reflection is finished. 
5. Let L be the initial length of the pulse. Then 
the time from the start of reflection until head and 
tail meet is nearly L/2a, since the velocity of both is 
nearly sonic for shocks of the strength donsidérea here. 
The time for the tail to reach the wall after it has 
passed through tke head is approximately L/2(c-u), where 
c-u is the average velocity of the tail in M. 
5s Beco 
