184 BULLETIN OF THE 



expressed the opinion that although vibrations may be produced in 

 the disks by the action of the intermittent beam, s«ch vibrations 

 are not the cause of the sonorous effects observed. According to 

 him the serial disturbances that produce the sound arise spontan- 

 eously in the air itself by sudden expansion due to heat communi- 

 cated from the diaphragm ; every increase of heat giving rise to a 

 fresh pulse of air. Mr. Preece was led to discard the theoretical 

 explanation of Lord Raleigh on account of the failure of experi- 

 ments undertaken to test the theory. 



He was thus forced, by the supposed insufficiency of the explan- 

 ation, to seek in some other direction the cause of the phenomenon 

 observed, and, as a consequence, he adopted the ingenious hypoth- 

 esis alluded to above. But the experiments which had proved 

 unsuccessful in the hands of Mr. Preece were perfectly successful 

 when repeated in America under better conditions of experiment, 

 and the supposed necessity for another hypothesis at once vanished. 

 I have shown in a recent paper read before the National Academy 

 of Science,* that audible sounds result from the expansion and 

 contraction of the material exposed to the beam, and that a real to 

 and fro vibration of the diaphragm occurs capable of producing 

 sonorous effects. It has occurred to me that Mr. Preece's failure 

 to detect with a delicate microphone the sonorous vibrations that 

 were so easily observed in our experiments, might be explained 

 upon the supposition that he had employed the ordinary form of 

 Hughes' microphone shown in Fig. 1, and that the vibrating area 

 was confined to the central portion of the disk. Under such cir- 

 cumstances it might easily happen that both the portions (A B) of 

 the microphone might touch portions of the diaphragm which were 

 practically at rest. It would, of course, be interesting to ascertain 

 whether any such localization of the vibration as that supposed 

 really occured, and I have great pleasure in showing to you to-night 

 the apparatus by means of which this point has been investigated. 

 [See Fig. 2.] 



The instrument is a modification of the form of microphone 

 devised in 1827 by the late Sir Charles Wheatstone, and it consists 

 essentially of a stiff wire, (A,) one end of which is rigidly attached 

 to the centre of a metallic diaphragm (B.) In Wheatstone's origi- 

 nal arrangement, the diaphragm was placed directly against the ear 



* April 21, 1881. 



