HEARING. 297 



will hear two sounds after each hlow, the first descending 

 through the wall, and the second through the air. 



As sounds are weakened by diflfusion over a larger sphere 

 of particles, so they are capable of having tlieir intensity in- 

 creased by concentration into a smaller space; an cflect 

 which may be produced 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 construct- 

 ed: and the same effect sometimes takes place in echoes, 

 which 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 very 

 small, 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 proportion, 

 acute. The former is called a low, the latter a high note; 

 designations which in all probability were originally derived 

 from the visible motions of the throat of a person who is 

 singing these diff'erent notes; for, independently of this cir- 

 cumstance, 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 which we now use 

 them. 



The difierent degrees of tension given to the cord or wire 

 of a stringed musical instrument, as well as its diflferent 

 lengths, determine the frequency of its vibrations; a greater 

 tension, or a shorter length, rendering them more frequent, 

 and consequently producing a higher note; and on the con- 

 trary, the note is rendered more grave by either lessening 

 the tension, or lengthening the cord or wire. In a wind 

 instrument, the tone depends altogether upon the length of 

 the tube producing the sound. 



Vol. II. 3S 



