ON THE MOTIONS OF SOUND. 415 



librium of temperature would be established. The non-homogeneous 

 air surrounding Villejuif is experimentally typified by our screen with 

 the source of sound close behind it, the upper edge of the screen repre- 

 senting the place where equilibrium of temperature was established in 

 the atmosphere above the station. In virtue of its proximity to the 

 screen, the echoes from our sounding-reed would, in the case here sup- 

 posed, so blend with the direct sound as to be practically indistinguish- 

 able from it, as the echoes at Villejuif followed the direct sound so 

 hotly, and vanished so rapidly, that they escaped observation. And 

 as our sensitive flame, at a distance, failed to be aiFected by the sound- 

 ing body placed close behind the cardboard screen, so, I take it, did 

 the observer at Montlh6ry fail to hear the sounds of the Villejuif gun. 

 Something further may be done toward the experimental elucida- 

 tion of this subject. The facility with which sounds pass through 

 textile fabrics has been already illustrated,' a layer of cambric or cal- 

 ico, or even of thick flannel or baize being found competent to inter- 

 cept but a fraction of the sound from a vibrating reed. Such a layer 

 of calico may be taken to represent a layer of air diflerentiated from 

 its neighbors by temperature or moisture ; while a succession of such 

 sheets of calico may be taken to represent successive layers of non- 

 homogeneous air. 



Fig. 3. 



Two tin tubes (J/ iVand P, Fig. 3) with open ends are placed so 

 as to form an acute angle with each other. At the end of one is the 

 vibrating reed r, opposite the end of the other and in the prolongation 

 of P is the sensitive flame /, a second sensitive flame (/') being 

 placed in the continuation of the axis oi M N. On sounding the reed, 

 the direct sound through iJfiV agitates the flame/'. Introducing the 

 square of calico ab a.t the proper angle, a slight decrease of the action 

 on /' is noticed, and the feeble echo from a b produces a barely per- 

 ceptible agitation of the flame/. Adding another square, c d, the 

 sound transmitted by a b impinges on c ^y it is partially echoed, re- 

 turns through a b, passes along F 0, and still further agitates the flame 

 /. Adding a third square, ef, the reflected sound is still further aug- 

 mented, every accession to the echo being accompanied by a corre- 



1 "Philosophical Transactions," 18Y4, Pait I., p. 208. 



