374 THE SENSORIAL FUNCTIONS. 



As sounds are weakened by diffusion over a 

 larger sphere of particles, so they are capable of 

 having their intensity increased by concentration 

 into a smaller space ; an effect which may be pro- 

 duced by their being reflected from the solid walls 

 of cavities, shaped so as to bring the undulations to 

 unite into a focus. It is on this principle that the 

 ear-trumpet, for assisting persons dull of hearing, is 

 constructed ; and that echoes occasionally reflect a 

 sound of greater loudness than the original sound 

 which was directed towards them. 



If the impulses given to the nerves of the ear be 

 repeated at equal intervals of time, provided these 

 intervals be not greater than the sixteenth part of a 

 second, the impressions become so blended together 

 as not to be distinguishable from one another : and 

 the sensation of a uniform continued sound, or 

 musical note, is excited in the mind. If the intervals 

 between the vibrations be long, the note is grave ; 

 if short, that is, if the number of vibrations in a 

 given time be great, the note is, in the same pro- 

 portion, acute* The former is called a loiv, the 

 latter a high note; designations which were, per- 

 haps, originally derived from the visible motions of 

 the throat of a person who is singing these differ- 

 ent notes; for, independently of this circumstance, 

 the terms of high and low are quite arbitrary ; and 

 it is well known that they were applied by the 

 ancients in a sense exactly the reverse of that in 

 w^hich we now use them. 



The different degrees of tension given to the 



* In tones produced by the friction of the teeth of a revolving 

 wheel against a hard body, Savart found that the highest audible 

 note consisted of 24,000 impulses in a second. 



