1258 
Hence the total time required for the reflection, or the 
duration of the pressure on the wall, is approximately 
L/2a + L/2 (c-u). On the other hand, the duration of the 
pulse in free fluid is L/a. Hence, if c-u <a, as in air, 
the pressure on the wall lasts for a longer time than the 
pressure in the incident pulse. Moreover the reflected 
pulse is in this case longer than the incident one. In 
water, where c-u > a, opposite remarks are true. (The 
difference between the behavior of air and water may be 
conveniently referred to the difference of material 
velocity, u, in the two fluids; in air u is large and in 
water it is small.) The result is that in air the dura- 
tion of the pressure on a wall and the impulse given to 
the wall are both greater than the acoustic approximation 
would indicate, and in water they are doth less than the 
predictions of acoustic theory. These qualitative con- 
siderations are corroborated by the detailed calculations, 
the results of which are given in par, 9 and 10. 
6. One other general remark may be made. Since the 
velocity of the shock S with respect to Mo is greater 
than its velocity with respect to the wall, the wall may be 
regarded as a piston which helps te sustain the shock. 
For this reason and because the total time of reflection 
is short, the assumption that decey during reflection is 
negligible may be a good approximation. The importance of 
this point will be seen in par, 2l. 
7. To compare different fluids it is convenient to 
= 6e— 
