had been heated, doubtless a rarefaction wave of equal displacement would follow, but no such steps are taken. It is true that the air initially heated by the spark is rapidly cooled by giving up its heat to the surrounding air, but this expands the air to which the heat is passed on, so that, on the principle of the author's bottle calorimeter,[1] no loss of volume takes place. There is possibly some minute quantity of heat lost to the conductors by which the current is supplied to the spark, but except for this the waves emitted will, on the whole, be compression waves involving a displacement of matter, and carrying the momentum appropriate to the mass displaced travelling with the velocity of sound. Ultimately the heated air is carried away by convection, but this does not affect the problem.
It is therefore evident that this experiment proves nothing, except that which we know already, i.e., a displacement of matter carries with it momentum.
It is probable that other more or less successful experiments designed to demonstrate the existence of sound pressure involve some similar fallacy. It must be borne in mind that an unsymmetrical design of sound generator may conceivably emit pressure waves containing momentum in one or more directions, and rarefaction waves in others, or perhaps the air displaced by the pressure waves emitted in one direction may be replaced by a steady flow in other directions. On the other hand, it is possible that by some highly refined method the true pressure of a continuous wave train may be detected and measured, and the theoretical result that it is due to the energy passed into the thermodynamic system may some day receive confirmation.
ADDENDUM C.
In the foregoing Appendix and Addenda A and B the assumption has been made that the change of mean pressure within an enclosure containing a perfect gas is directly proportional to the
- ↑ Addendum C.
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