424 THE POPULAR SCIENCE MONTHLY. 



tion, altlioiigli from conclusive evidence, furnished by the keeper, it 

 was shown that the signal had been sounding during the whole time." 



But, while the ten-inch shore-signal thus failed to make itself heard 

 at sea, a six-inch whistle, on board the steamer, made itself lieard on 

 shore. Prof. Henry thus turns this fact against me : " It is evident," 

 he writes, " that this result could not be due to any mottled condition 

 or want of acoustic transparency in the atmosphere, since this would 

 absorb the sound equally in both directions." Had the observation 

 been made in a still atmosphere, this argument would, at one time, 

 have had great force. But the atmosphere was not still, and a suf- 

 ticient reason for the observed non-reciprocity is to be found in the 

 recorded fact that the wind was blowing against the shore-signal, 

 and in favor of the ship-signal. 



But the argument of Prof. Henry, on which he places his main re- 

 liance, would be untenable, even had the air been still. By the very 

 aerial reflexion which he practically ignores, reciprocity may be de- 

 stroyed in a calm atmosphere. In proof of this assertion I would refer 

 him to a short paper on " Acoustic Reversibility," printed ai the end 

 of this volume.* The most I'emarkable case of non-reciprocity on 

 record, and which, prior to the demonstration of the existence and 

 power of acoustic clouds, remained an insoluble enigma, is there 

 shown to be capable of satisfactory solution. These clouds explain 

 perfectly the "abnormal phenomena" of Prof. Henry. Aware of 

 their existence, the falling off and subsequent recovery of a signal- 

 sound, as noticed by him and General Duane, is no more a mystery 

 than the interception of the solar light by a common cloud, and its 

 restoration after the cloud has moved or melted away. 



The clew to all the difficulties and anomalies of this question is to 

 be found in the aerial echoes, the significance of which has been over- 

 looked by General Duane, and misinterpreted by Prof. Henry. And 

 here a word might be said with regard to the injurious influence still 

 exercised by authority in science. The afiirmations of the highest 

 authorities, that from clear air no sensible echo ever comes, were so 

 distinct, that my mind for a time refused to entertain the idea. Au- 

 thority caused me for weeks to depart from the truth, and to seek 

 counsel among delusions. On the day our observations at the South 

 Foreland began, I heard the echoes. They perplexed me. I heard 

 them again and again, and listened to the explanations oifered by 

 some ingenious persons at the Foreland. They were an " ocean- 

 echo : " this is the very phraseology now used by Prof. Henry. They 

 were echoes " from the crests and slopes of the waves : " these are the 

 words of the hypothesis which he now espouses. Through a portion 

 of the month of May, through the whole of June, and through nearly 

 the whole of July, 1873, I was occupied with these echoes; one of 



' Also " Proceedings of Royal Society," vol. xxiii., p. 159, and " Proceedings of Royal 

 Institute," vol. vii., p. 344. 



