SCIENCE OF COMMON THING8. 125 



Ancient fable of echo. Distance requisite to produce echo. 



7*00 What beautiful fiction existed among the ancients relative to the 

 production of echo f 



They. supposed that Echo was a nymph who dwelt 

 concealed among the rocks, and who repeated the 

 sounds she heard. 



791 At what distance must the body reflecting the sounds be situated in 

 order to produce an echo f 



It is requisite that the reflecting body should be 

 situated at such a distance from the source of sound, 

 that the interval between the perception of the original 

 and reflected sounds may ~b& sufficient to prevent them 

 from being blended together. 



793 When the seunds become thus blended together, what is the effect 

 called ? 



A resonance, and not an echo. 



793 Why do not the watts of a room of ordinary size, produce an echo f 



Because the reflecting surface is so near the source 

 of sound that the echo is blended with the original 

 sound / and the two produce but one impression on 

 the ear. 



7*94 Why do very large buildings (as cathedrals) often reverberate the 

 voice of the speaker ? 



Because the walls are so far off from the speaker, 

 that the echo does not get oack in time to blend with 

 the original sound ; and therefore each is heard sepa- 

 rately. 



7*95 Why do some echoes repeal only one syllable ? 



Because the echoing body is very near. The farther 

 the echoing body is oif, the more sound it will reflect : 

 if, therefore, it be very near, it will repeat but one syl 

 lable. 



790 Why does an echo sometimes repeat two or more syllables f 



Because the echoing body is far off ; and therefore 

 there is time for one reflection to pass away before an- 

 other reaches the ear. 



All the syllables must be uttered before the echo of the first syllable \ 

 reaches the ear : if, therefore, a person repeats 7 syllables in 2 seconds of \ 

 time, and hears them all echoed, the reflecting object is 1142 feet distant ; 

 because sound travels 1142 feet in a second, and the words take one 

 second to go to the reflecting object, and one second to return. 



6* 



