PHILOSOPHICAL SOCIETY OF WASHINGTON. 185 



and the free extremity of the wire was rested against some sounding 

 body, like a watch. In the present arrangement the diaphragm 

 is clamped at the circumference like a telephone-diaphragm, and 

 the sounds are conveyed to the ear through a rubber hearing-tube 

 (C.) The wire passes through the perforated handle (D,) and is 

 exposed only at the extremity. When the point (A) was rested 

 against the centre of a diaphragm, upon which was focussed an 

 intermittent beam of sunlight, a clear musical tone was perceived 

 by applying the ear to the hearing-tube (C.) The surface of the 

 diaphragm was then explored with the point of the microphone, 

 and sounds were obtained in all parts of the illuminated area, and 

 in the corresponding area on the other side of the diaphragm. 

 Outside of this area on both sides of the diaphragm the sounds 

 became weaker and weaker until at a certain distance from the 

 centre they could no longer be perceived. 



At the points where one would naturally place the supports of a 

 Hughes' microphone [see Fig. 1,] no sound was observed. We 

 were also unable to detect any audible effects when the point of the 

 microphone was rested against the support to which the diaphragm 

 was attached. The negative results obtained in Europe by Mr. 

 Preece may, therefore, be reconciled with the positive results ob- 

 tained in America by Mr. Tainter and myself. A still more curious 

 demonstration of localization of vibration occurred in the case of a 

 large metallic mass. An intermittent beam of sunlight was focus- 

 sed upon a brass weight (1 kilogram,) and the surface of the 

 weight was then explored with the microphone shown in Fig. 2. 

 A feeble but distinct sound was heard upon touching the surface 

 within the illuminated area, and for a short distance outside, but 

 not in other parts. 



In this experiment, as in the case of the thin diaphragm, abso- 

 lute contact between the point of the microphone, and the surface 

 explored was necessary in order to obtain audible effects. Now, I 

 do not mean to deny that sound waves may be originated in the 

 manner suggested by Mr. Preece, but I think that our experiments 

 have demonstrated that the kind of action described by Lord Ra- 

 leigh actually occurs and that it is sufficient to account for the 

 audible effects observed. 



