Sound by Radiant Energy. 517 



In these cases the wooden box and mouth-piece of the 

 transmitter should be removed, so that no air-cavities may be 

 left on either side of the diaphragm. 



It is evident, therefore, that in the case of thin disks a real 

 vibration of the diaphragm is caused by the action of the inter- 

 mittent beam, independently of any expansion and contraction 

 of the air confined in the cavity behind the diaphragm. 



Lord Eayleigh has shown mathematically that a to-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 presemt no reason for discarding the obvious ex- 

 planation that the sounds in question are due to the bending 

 of the plates under unequal 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 con- 

 clusion. Mr. Preece expected that if Lord Bayleigh's ex- 

 planation 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 

 Eayleigh has shown (Proc. of Boy. Soc. 1877) that an 

 audible sound can be produced by a vibration whose ampli- 

 tude is less than a ten-millionth of a centimetre; and cer- 

 tainly 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, there- 

 fore, be considered conclusive. 



The following experiments (devised by Mr. Tainter) have 

 given results decidedly more favourable to the theory of Lord 

 Eayleigh than to that of Mr. Preece: — 



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

 ment was attached firmly to the centre of an iron diaphragm 

 (B), as shown in figure 5, and was then pulled taut at right 

 aDgles to the plane of the diaphragm. When the intermit- 

 tent beam was focused upon the strip (A), a clear musical 

 tone could be heard by applying the ear to the hearing-tube 



litis seemed to indicate a rapid expansion and contraction of 

 the substance under trial. 



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

 sulted 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. 



