Prof. Tyndall on Acoustic Reversibility \ 151 



Two tin tubes (M N and O 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 O, is the sensitive flame /, a second sensi- 

 tive flame (/') being placed in the continuation of the axis of M IN". 

 On sounding the reed, the direct sound through M N agitates the 

 flame/'. Introducing the square of calico a b at the proper angle, 

 a slight decrease of the action on /' is noticed, and the feeble echo 

 from a b produces a barely perceptible agitation of the flame /. 

 Adding another square, c d, the sound transmitted by a b impinges 

 on c d ; it is partially echoed, returns through a b, passes along P O, 

 and still further agitates the flame /. Adding a third square, ef t 

 the reflected sound is still further augmented, every accession to 

 the echo being accompanied by a corresponding withdrawal of the 

 vibrations from /' and a consequent stilling of that flame. 



"With thinner calico or cambric it would require a greater number 

 of layers to intercept the entire sound ; hence with such cambric 

 we should have echoes returned from a greater distance, and there- 

 fore of greater duration. Eight layers of the calico employed 

 in these experiments, stretched on a wire frame and placed 

 close together as a kind of pad, may be taken to represent a 

 very dense acoustic cloud, Such a pad, placed at the proper 

 angle beyond N, cuts off the sound, which in its absence reaches 

 /', almost as effectually as an impervious solid plate* : the flame 

 f is thereby stilled, while / is far more powerfully agitated than 

 by the reflection from a single layer. With the source of sound 

 close at hand, the echoes from such a pad would be of insen- 

 sible duration. Thus close at hand do I suppose the acoustic 

 clouds surrounding Yillejuif to have been, a similar shortness of 

 echo being the consequence. 



A further step is here taken in the illustration of the analogy 

 between light and sound. Our pad acts chiefly by internal re- 

 flection. The sound from the reed is a composite one, made 

 up of partial sounds differing in pitch. If these sounds be ejected 

 from the pad in their pristine proportions, the pad is acoustically 



* January 13th. — Since this was written I have sent the sound through 

 fifteen layers of calico, and echoed it back through the same layers, in strength 

 sufficient to agitate the flame. Thirty layers were here crossed by the sound. 



