344 MEMORIAL OF JOSEPH HENRY. 



much care various phenomena of acoustics, and added much to our 

 practical as well as theoretical knowledge of that important agency 

 sound. In 1851, he read a communication before the American 

 Association, "On the Limit of Perceptibility of a direct and 

 reflected Sound," in which he gave as the result of experimental 

 observations, the subjective fact that a wall or other reflecting sur- 

 face if beyond the distance of about 35 feet from the ear, or from 

 the origin of the sound, gives a distinguishable echo from the sound ; 

 but that if the ear or the sounding agent be placed within this 

 distance, the reflected sound appears to blend completely with the 

 original one. From a number of experiments, he found that under 

 the same circumstances, this limit of perceptibility did not vary 

 more than a single foot; but that under differing conditions the 

 limit of distance ranged from 30 to 40 feet, ( equivalent to a differ- 

 ence of from 60 to 80 feet of sound travel,) depending partly on 

 the sharpness or clearness of the sound, and partly on the pitch or 

 the length of the soniferous wave, which affected the amount of 

 overlapping of the two series. These results imply a duration of 

 acoustic impression on the ear of about one-sixteenth of a second ; 

 serving to show that 16 vibrations to the second must be about the 

 lower limit of a recognizable musical tone. * As applied to Lecture- 

 rooms, he pointed out that the ceiling should not be more than 

 about thirty feet high, within which elevation, a smooth ceiling 

 would tend to re-inforce the sound of a speaker's voice, f 



Many experiments were afterward made on the resonance of dif- 

 ferent materials, by means of tuning forks. While a tuning fork 

 suspended by a fine thread continued to vibrate for upward of four 

 minutes with scarcely any appreciable sound, if placed in contact 

 with the top of a pine table, the same vibration continued but ten 

 seconds, but gave a loud full tone. On a marble topped table the 

 sound was much more feeble, and the vibration continued nearly 

 two minutes. While the tuning fork against a brick wall gave a 



* FELIX SAVART some twenty years previously, concluded from observations 

 with the siren, " that sounds are distinctly perceptible, and even strong, when 

 composed of no more than eight vibrations in a second." (Rev. Encycl. July, 1832. 

 Quoted in Silliman's Am. Jour. Sci. for 1832, vol. xxii. p. 374.) This does not seem 

 to agree with ordinary observations, as it is certain that intervals of one-eighth 

 of a second would give a very appreciable rattle to almost every ear. 



t Proceed. Am. Assoc. Cincinnati, May, 1851, pp. 42, 43. 



