150 BULLETIN OF THE 



dependently of any expansion and contraction of the air confined in 

 the cavity behind the diaphragm. 



Lord Rayleigh has shown mathematically that a' two-and-fro 

 vibration of sufficient amplitude to produce an audible sound 

 would result from a periodical communication and abstraction of 

 heat, and he says: " We may conclude, I think, that there is at 

 present no reason for discarding the obvious explanation that the 

 sounds in question are due to the bending of the plates under un- 

 equal heating." (Nature, xxiii, p. 274.) Mr. Preece, however, 

 seeks to prove that the sonorous effects cannot be explained upon 

 this supposition ; but his experimental proof is inadequate to 

 support his conclusion. Mr. Preece expected that if Lord Rayleigh's 

 explanation was correct, the expansion and contraction of a thin 

 strip under the influence of an intermittent beam could be caused 

 to open and close a galvanic circuit, so as to produce a musical 

 tone from a telephone in the circuit. But this was an inadequate 

 way to test the point at issue, for Lord Rayleigh has shown (Proc 

 of Roy. Soc, 1877,) that an audible sound can be produced by a 

 vibration, whose amplitude is less than a ten-millionth of a centimetre, 

 and certainly such a vibration as that would not have sufficed to 

 operate a "make-and-break contact" like that used by Mr. Preece. 

 The negative results obtained by him cannot, therefore, be consid- 

 ered conclusive. 



The following experiments (devised by Mr. Tainter) have given 

 results decidedly more favorable to the theory of Lord Rayleigh 

 than to that of Mr. Preece : 



1. A strip (A) similar to that used in Mr. Preece's experiment 

 was attached firmly to the centre of an iron diaphragm, (B,) as 

 shown in Figure 5, and was then pulled taut at right angles to the 

 plane of the diaphragm. When the intermittent beam was focussed 

 upon the strip (A) a clear musical tone could be heard by applying 

 the ear to the hearing tube (C,) 



This seemed to indicate a rapid expansion and contraction of the 

 substance under trial. 



But a vibration of the diaphragm (B) would also have resulted 

 if the thin strip (A) had acquired a to-and-fro motion, due either 

 to the direct impact of the beam or to the sudden expansion of the 

 air in contact with the strip. 



2. To test whether this had been the case an additional strip (D) 



