Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/432

App. II. B. Now it is evident that the whole of this reasoning rests on the assumption that the reflector, while impervious to the waves, is freely pervious to the medium; an assumption that may be true in the case of light, but is certainly not true in the case of sound.

Poynting evidently appreciates this difficulty, for he says:—

“It is essential, I think, to Larmor's proof that we should be able to move the reflecting surface forward without disturbing the medium except by reflecting the waves.” But further on he says:—

“But for sound waves I venture to suggest a reflector which shall freeze the air just in front of it, and so remove it, the frozen surface advancing with constant velocity $$u .$$ Or perhaps we may imagine an absorbing surface which shall remove the air quietly by solution or chemical combination.”

Now this is the first time that the author has heard it seriously suggested that portions of any dynamic system, essentially involved in that system, may be stolen away without affecting the sequence of events; it is, at least, evident that any such assumption totally invalidates Larmor's theorem as a generalisation, and in particular in its application to ordinary dynamic wave motion. It is very surprising to find that Poynting subsequently states that he finds Larmor's proof quite convincing.

In the address from which the above quotations have been given, Poynting cites an experiment by Prof. Wood intended to demonstrate the reality of sound pressure. In this experiment the sound waves from a strong induction- spark are focussed by a concave reflector on to a set of vanes as used on a radiometer, causing them to spin round. Now it is fair to assume that the cause of the emission of sound waves by an induction-spark is the heating of the air suddenly and locally by the spark energy, and consequently the wave will primarily be a compression wave. If steps were taken to cool the air immediately after it