410 WRITINGS OP JOSEPH HENRY. [1856 



longation of the sound was observed. By accurately meas- 

 uring this distance and doubling it we find the interval of 

 space within which two sounds may follow each other without 

 appearing separately. But if two rays of sound reach the 

 ear after having passed through distances the difference 

 between which is greater than this, they produce the efifect 

 of separate sounds. This distance we have called the limit 

 of perceptibility in terms of space. If we convert this distance 

 into the velocity of sound, we ascertain the limit of percep- 

 tibility in time. 



In the experiment first made with the wall a source of 

 error was discovered in the fact that a portion of the sound 

 returned was reflected from the cornice under the eaves, and 

 as this was at a greater distance than the part of the wall 

 immediately perpendicular to the observer the moment of 

 cessation of the echo was less distinct. In subsequent ex- 

 periments with a louder noise, the reflection was observed 

 from a perpendicular surface of about 12 feet square, and 

 from this more definite results were obtained. The limit of 

 the distance in this case was about 30 feet, varying slightly 

 perhaps with the intensity of the sound and the acuteness 

 of different ears. This will give about the sixteenth part of 

 a second as the limit of time necessary for the ear to sepa- 

 rately distinguish two similar sounds. From this experi- 

 ment we learn that the reflected sound may tend to strengthen 

 the impression, or to confuse it, according as the diff'erence 

 of time between the two impressions is greater or less than 

 the limit of perceptibility. An application of the same 

 principle gives us the explanation of some phenomena of 

 sound which have been considered mysterious. Thus, in 

 the reflection of an impulse from the edge of a forest of trees 

 each leaf properly situated within a range of 30 feet of the 

 front plane of reflection will conspire to produce a distinct 

 echo, and these would form the principal part of the reflect- 

 ing surfaces of a dense forest, for the remainder would be 

 screened ; and being at a greater distance, any ray which 

 might come from them would serve to produce merely a low 

 continuation of the sound. 



