296 WRITINGS OF JOSEPH HENRY. [1851 



wave of sound is known to travel in a second, we shall have 

 the limit of perceptibility in time. 



The foregoing plan is the most simple — but not the most 

 accurate — method of arriving at the quantity sought. The 

 better plan is to employ another person to produce the sound, 

 while the observer is stationary at the distance — at least 150 

 feet from the wall. The person who produces the sound 

 being placed between the observer and the wall, at such a 

 distance from the latter as to give a distinct echo, he is then 

 directed gradually to approach the wall until the echo and 

 the direct sound become one. The distance measured, as 

 before mentioned, will give the limit required. 



From a series of experiments on this plan, he found the 

 limit of perceptibility to vary from about 60 to 80 feet, or in 

 other words, the distance from the wall at which the echo 

 ceased was from 30 to 40 feet. This will give from the 2^0 to 

 the yV P^rt of a second, in time, for the ear to distinguish 

 the difference of two successive sounds. 



The experiments, when made under the same circum- 

 stances, gave the same result, almost within a single foot ; 

 but when a different source of sound was employed and dif- 

 ferent observers, there was observed a difference of results, 

 giving the limits between ^V ^^^^ tV of ^ second. The 

 limit was less with a sound produced by an instrument 

 which gave a sudden crack, without perceivable prolonga- 

 tion, such as is produced by an ordinary watchman's rattle 

 when made to emit a single crack. This difference may be 

 explained by taking into consideration the actual length of 

 the sonorous wave. If a sound occupies ^ of a second, (which 

 is about the time required for the utterance of a short, single 

 syllable,) the length of this sonorous wave will be about 300 

 feet, and hence, when the distance travelled by the two sounds 

 is not more than 80 feet to and from the wall, the two waves 

 must overlap through a considerable portion of their whole 

 length, and will be only separated at the two extremities. 

 The portion of over-lapping may therefore determine the 

 limit of perceptibility, and this again is combined with the 

 fact of the continuance of a sonorous impression on the nerve 

 of the ear. 



