258 SCIENCE PROGRESS 



leave the timbre of the vowel sound not materially affected, and 

 that it may therefore fulfil the second desideratum. This 

 conclusion is arrived at by the study of the forms of manometric 

 flames, and by sections of the depth of the indentations in the 

 plate of a gramophone, and is borne out by the observation 

 that the vowel sound emitted by a gramophone is entirely 

 altered if the disc is run at a different speed. 



It is, however, only right to say that there is an alternative 

 theory of vowel quality, according to which a given vowel 

 requires the existence of partials of a prescribed order to be 

 present, and not partials of fixed and prescribed pitch. To 

 what extent this latter theory may be true (and according to 

 Auerbach both characteristics must be fulfilled) it is apparently 

 impossible that any resonator of fixed shape can satisfy the 

 conditions demanded by this second theory. 



In short, both quantity and quality of the sound heard 

 must be taken into account in considering the efficacy of any 

 instrument for hearing. 



For purposes of convenience the existing types of mechanical 

 aids to hearing may be attemptedly classified, so far as there 

 is any principle at all underlying their construction, under 

 the following headings : — 



A. Simple transmission tubes. 



B. Cone-shaped or tapering reflectors. When short these 

 are called by various names, such as trumpets, cornets, auricles, 

 etc. 



C. "Trumpets," i.e. a conical tube like the brass trumpet 

 used on the Continent by the station master in starting a train, 

 but with the narrow end usually doubled three times on itself. 



D. " Resonators," so called — bell-shaped, with a bent 

 tapering tube attached at one point in the side. 



E. Instruments in which the idea of " bone conduction " 

 is aimed at. 



F. Table instruments, in which no definite principle is 

 aimed at, such as resonance, reflection, or conduction through 

 a solid. 



G. Telephonic instruments. 



Class A is familiar to every one in the form of the speaking- 

 tube used from one room to another in a house. Provided 

 that certain conditions are fulfilled, and especially that sharp 

 angular bends are avoided, such simple instruments are of 



