40 W. B. Taylor— Recent Researches in Sound. 



direction, (acoustic " non-reversibility,") is obviously inadmis- 

 sible. Nor is the supposition of acoustic " diffraction" around 

 the defined edge of a vapor cloud, more available. 



Professor Tyndall in his recent Preface to the last edition of 

 "Sound" remarks upon this observation of Henry [9] — "a 

 sufficient 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." {Preface, p. xxi.) 

 But he offers no suggestion how this "sufficient reason" is 

 supposed to apply. As it is well-known that an ordinary wind 

 cannot increase the range of sound more than two or three per 

 cent (an amount quite inappreciable), this circumstance alone is 

 wholly inadequate to account for the complete suppression of 

 the shore-signal (a ten-inch steam-whistle) from the distance of 

 three miles to a quarter of a mile, while the feebler sound of the 

 ship-signal (a six-inch steam-whistle) was making itself dis- 

 tinctly heard throughout the three miles. Something more 

 therefore than the direct or convective action of the wind 

 must be invoked to explain the facts. 



Tyndall's observation [7] on the aerial or ocean echoes, cot- 

 responds with Henry [10] excepting as to the direction of the 

 principal echo. This difference is doubtless due to the special 

 arrangement of the surfaces or points of reflection in the re- 

 spective cases observed. Professor Tyndall connects this phe- 

 nomenon with that of acoustic opacity [6] ; and here again his 

 fine experimental skill is brought into requisition to demon- 

 strate the reality of artificial " aerial echoes." By so simple a 

 device as the employment of the flat side of a " bat-wing" gas- 

 jet, the sound beam from a reed instrument was shown to be 

 entirely deflected from one sensitive flame, and reflected back 

 toward another. 



This view of a relation between the acoustic opacity outward 

 or seaward, and the reinforcement or reflection of sound in- 

 ward, is in striking accord with Duane [G], who however in 

 referring to the " reflection ' of sound, does not specifically 

 allude to the ocean " echo." On the refraction theory also, a 

 necessary result is that a deflection of the sound-beam upward 

 in one direction, must be attended with a downward deflection 

 and consequent incn 



Professor Henry 

 crests and slopes of distant waves; (in conjunction probably 

 with a curvature of the sound-beams, constituting a kind of 

 acoustic "mirage.") To this suggestion. Professor Tyndall op- 

 poses the observation that " the echoes have often manifested 

 an astonishing strength, when the sea was of glassy smooth- 

 ness." {Sound, Pre/., p. xxiii.) 



