Experiments on Audition* 69 



noise which simultaneously accompanies a very intense sound, 

 proceeds from the same cause, and may be prevented by the 

 same means. This ringing may be produced by applying the 

 stem of a sounding tuning-fork to the hand when covering the 

 ear, or by whistling when a hearing trumpet is placed to the ear. 

 As a proof that the resulting augmentation, which, when great, 

 excites the vibrations of the tympanum, is owing to the recipro- 

 cation of the vibrations by the air contained within the closed 

 cavity, it may be mentioned, that when the entrance of the 

 meatus is closed by a fibrous substance, as wool, &c., no in- 

 crease is obtained. 



If the meatus and the concha of one ear be filled with water, 

 the sounds above-mentioned will be referred to the cavity con- 

 taining the water in the same way as when it contained air, and 

 was closed by the hand ; it will be indifferent whether any par- 

 tition be interposed between the cavity and the external air, as 

 the water is equally well insulated by a surface of air as by a 

 solid body* 



The preceding experiments have shown, that sounds immedi- 

 ately communicated to the closed meatus externus are very 

 greatly augmented ; and it is an obvious inference, that if ex^ 

 ternal sounds can be communicated, so as to act on the cavity 

 in a similar manner, they must receive a corresponding aug- 

 mentation. The great intensity with which sound is trans- 

 mitted by solid rods, at the same time that its diffusion is pre- 

 vented, affords a ready means of effecting this purpose, and of 

 constructing an instrument, which, from its rendering audible 

 the weakest sounds, may with propriety be named a Microphone. 



Procure two flat pieces of plated metal, each sufficiently large 

 to cover the external ear, to the form also of which they may 

 be adapted ; on the outside of each plate directly opposite the 

 meatus, rivet a rod of iron or brass wire about 16 inches in 

 length, and one-eighth of an inch in diameter, and fasten the 

 two rods together at their unfixed extremities, so as to meet in 

 a single point. The rods must be so curved, that when the 

 plates are applied to the ears, each rod may at one end be per- 

 pendicularly inserted into its corresponding plate, and at the 

 other end may meet before the head in the plane of the mesial 



