1856] WRITINGS OP JOSEPH HENRY. 407 



on the other hand empirical data alone are in this case 

 entirely at fault, and of this any person may be convinced 

 who will examine the several works written on acoustics by 

 those who are deemed practical men. 



Sound is a motion of matter capable of affecting the ear 

 with a sensation peculiar to that organ. It is not in all 

 cases a motion simply of the air, for there are many sounds 

 in which the air is not concerned ; for example, the impulses 

 which are conveyed along a rod of wood from a tuning- 

 fork to the teeth. When a sound is produced by a single 

 impulse, or an approximation to a single impulse, it is 

 called a noise; when by a series of impulses, a continued 

 sound, &c.; if the impulses are equal in duration among 

 themselves, a musical sound. This has been illustrated by 

 a quill striking against the teeth of a wheel in motion. A 

 single impulse from one tooth is a noise, from a series of 

 teeth in succession a continued sound ; and if all the teeth 

 are at equal distances, and the velocity of the wheel is uni- 

 form, then a musical note is the result. Each of these sounds 

 is produced b}' the human voice, though they apparently 

 run into each other. In speaking however a series of 

 irregular sounds of short duration is usually emitted, — each 

 syllable of a word constitutes a separate sound of appreciable 

 duration, and each compound word and sentence an assem- 

 blage of such sounds. It is no little surprising that in listen- 

 ing to a discourse, the ear can receive so many impressions 

 in the space of a second, and that the mind can take cogniz- 

 ance of and compare them. 



That a certain force of impulse and a certain time for its 

 continuance are necessary to produce an audible impression 

 on the ear, is evident, but it may be doubted whether the 

 impression of a sound on this organ is retained appreciably 

 longer than the continuance of the impulse itself, certainly 

 it is not retained the Yijth. of a second. If this were the case 

 it is diflScult to conceive why articulated discourse, which so 

 pre-eminently distinguishes man from the lower animals, 

 should not fill the ear with a monotonous hum; but whether 

 the ear continues to vibrate, or whether the impression re- 



