CHAPTER IV
THE PROBLEM OF OPTICAL CONVECTION: INDICATIONS TOWARDS A DYNAMICAL THEORY
35. Consider in the first place the propagation of waves—or indeed the course of any kind of disturbance—in a single self-contained medium, with a view to determining the effect on it of a velocity of uniform translation imparted to the medium. The principle of relative motion supplies the solution. Impart to the whole system a velocity equal and opposite to that of the medium; and, because this uniform velocity introduces no new kinetic reactions, the phenomena of the relative motion will pursue the same course as before, but they will now be relative to the medium at rest instead of in motion. In all such cases, therefore, the disturbances in the medium are simply carried on along with the medium itself, with its full velocity of translation, and in other respects pursue their course unaltered.
For example, the velocity of translation of the air through which sound is propagated is added (in algebraic sense) at each instant to the velocity of the sound itself. In the same way, if, adopting the view discussed by Sir G. Stokes, we considered the surrounding free aether as disturbed by the Earth's motion, its velocity at each point would have to be added on to the intrinsic velocity of radiation through it. This is true whether the motion of the medium is uniform or not: when however it is not uniform a simple wave will no longer travel as a simple wave, and to that extent the meaning of the term velocity of the wave is indefinite.
