W. B. Taylor— Recent Researches in Sound. 39 



It must be borne in mind that the investigations by Profes- 

 sor Tyndall were concladed before the publication of the 

 United States Lighthouse Eeport. And it is noticeable that 



strikingly confirm each other, 



Tyndall's notice [1] of the inconstant relative range of dif- 

 ferent instruments corresponds with Henry (2), though indi- 

 cating a much more marked variability. 



Tyndall's notice [2] of the sound shadow, corresponds gene- 

 rally with Henry [7], and Duane [E], but assigns a sharper 

 definition to its limit ; probably in consequence of the inter- 

 vention of a larger obstacle (a cliff), and an observation within 



Tyndall [3] confirms Henry [3] and [11]. 



Tyndall [4] corresponds with Henry [7] and Duane [E]. 



Tyndall [5] confirms by a series of careful observations, the 

 opinion of Henry [5] and Duane [I]. 



Tyndall [6] confirms Duane [A and F], and in like manner 

 adopts and extends the suggestion of Humboldt as to the 

 cause of acoustic opacity. Professor Tyndall's admirable skill 

 in experimental physics enabled him to illustrate and fortify 

 his hypothesis by exhibiting in a popular lecture an apparatus 

 for producing in an elongated box or tunnel, aerial laminae of 

 unequal density, through which the sound from a small alarm 

 box failed to excite a sensitive flame. That this mottled 

 condition of the air is therefore a true cause of acoustic ob- 

 longer doubtful. To what extent a similar 



condition of the atmosphere actually prevails, in view of the 

 law of the diffusion of gases, and "how far such usual or un- 

 usual inequalities of density in the air are capable of entirely 

 dispersing the powerful sound of a steam trumpet or siren, at 

 the distance of a quarter of a mile, are not so positively deter- 

 mined. With a continuous wind any such condition of aerial 

 *' flocculence" might be expected to be very speedily dissipated. 

 This theory, however, fails entirely to explain the interesting 

 observations of Henry [4, 8, and 9]. It is scarcely credible 

 that a local screen of aerial flocculence could obliterate on the 

 deck of a schooner, a fog-signal audible at the mast-head. 

 Atmospheric refraction on the other hand, completely satisfies 

 the observed condition ; an opposing wind blowing at the time. 

 Still less successful is the theory, in dealing with the abnormal 

 phenomenon of simultaneous audibility at long range, with the 

 intermediate '' belt" of acoustic opacity, first observed by 

 Duane [D]. And lastly, the assumption of simultaneous trans- 

 mission of sound through a flocculent air-screen in one direction 

 ami its absorption or dissipation by the screen in the opposite 



