Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/425

Rh There is much confusion of thought at the present time on the question of the carrying of momentum by a wave train and the generation of pressure in a fluid region occupied by wave motion. This has probably arisen from too close attention being paid to the special case of a continuous wave train, as in the Kundt's tube.

It is much more difficult to distinguish between direct momentum transference by the wave and momentum transference by the pressure generated by the wave, in the case of the Kundt's tube, where the whole region is occupied by wave motion than in the case of a limited wave train passing to and fro.

Thus in the case of a limited wave train, if it carry momentum, that momentum can be represented by some definite value of mv, and the remainder of the system with which the wave is associated must, relatively to the common mass centre, have an equal and opposite momentum at every instant of time. But a self-contained system consisting of a simple enclosure containing fluid of uniform mean density (regarding the individual waves of the train as small) cannot suffer change of momentum without infringing the third law of motion; consequently the wave train (if of the same mean density as the quiescent fluid) cannot carry momentum. This is in effect the argument of § 5.

In the case of the continuous train, as in the Kundt's tube, we lose touch with this method of argument, for the action is continuous, and a pressure increase can only be distinguished from the true carrying of momentum by the wave train by a process of mathematical analysis that is full of pitfalls.

The case of light pressure, or the carrying of momentum by electro-magnetic radiation, is not a problem in ordinary dynamics, and is untouched by a purely dynamical argument or method of demonstration such as here employed. The reason for this fundamental distinction is that when motions