Sound: A Course of Eight Lectures

...this fact against me. "It is evident," he writes, "that this result could not be due to any mottled condition or lack of acoustic transparency in the atmosphere, since this would absorb the sound equally in both directions." If the observation had been made in a still atmosphere, this argument would have once carried great weight. However, the atmosphere was not still, and a sufficient reason for the observed non-reciprocity A situation where sound is heard clearly in one direction but poorly or not at all in the opposite direction between two points. is found in the recorded fact that the wind was blowing against the signal on the shore and in favor of the signal on the ship.

But Professor Henry's argument, on which he relies most heavily, would be indefensible even if the air had been still. By the very aerial reflection which he practically ignores, reciprocity can be destroyed even in a calm atmosphere. To prove this claim, I would refer him to a short paper on "Acoustic Reversibility," printed at the end of this volume.¹ The most remarkable case of non-reciprocity on record—which remained an unsolvable puzzle before the existence and power of acoustic clouds Tyndall’s term for invisible patches of air with different temperatures or moisture levels that reflect and scatter sound waves. were demonstrated—is shown there to have a satisfactory solution. These clouds perfectly explain Professor Henry’s "abnormal phenomena." Once we are aware of their existence, the fading and subsequent recovery of a signal sound—as noticed by him and General Duane—is no more mysterious than sunlight being blocked by a common cloud and returning after the cloud has moved or evaporated.

The clue to all the difficulties and anomalies of this question is found in the aerial echoes, the significance of which has been overlooked by General Duane and mis-

¹ Also Proceedings of the Royal Society original: "Proc. Roy. Soc." vol. 23, p. 159, and Proceedings of the Royal Institution original: "Proc. Roy. Inst." vol. 7, p. 344.