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LECTURE XXXI, 



ON THE PROPAGATION OF SOUND. 



THE theory of sound, which constitutes the science of acustics, is on 

 many accounts deserving of particular attention, for it not only involves 

 many interesting properties of the motions of elastic substances, but it also 

 affords us considerable assistance in our physiological inquiries respecting 

 the nature and operation of the senses. The subject has usually been con- 

 sidered as exceedingly abstruse and intricate, but the difficulty has in some 

 measure originated from the errors which were committed in the first 

 inquiries respecting it ; and many of the phenomena belonging to it are so 

 remarkable and so amusing, as amply to repay the labour of examining 

 them by the entertainment that they afford. We shall consider first the 

 nature and propagation of sound in general, secondly, the origin of par- 

 ticular sounds, and the effects of single sounds ; thirdly, the consequences 

 of the combinations of sounds variously related, constituting the doctrine 

 of harmonics, and fourthly, the construction of musical instruments, and 

 the history of the science of acustics. 



Sound is a motion capable of affecting the ear with the sensation peculiar 

 to the organ. It is not simply a vibration or undulation of the air, as it is 

 sometimes called ; for there are many sounds in which the air is not con- 

 cerned, as when a tuning fork or any other sounding body is held by the 

 teeth : nor is sound always a vibration or alternation of any kind ; for 

 every noise is a sound, and a noise, as distinguished from a continued 

 sound, consists of a single impulse in one direction only, sometimes without 

 any alternation ; while a continued sound is a succession of such impulses, 

 which, in the organ of hearing at least, cannot but be alternate. If these 

 successive impulses form a connected series, following each other too 

 rapidly to be separately distinguished, they constitute a continued sound, 

 like that of the voice in speaking ; and if they are equal among themselves 

 in duration, they produce a musical or equable sound, as that of a vi- 

 brating cord or string, or of the voice in singing. Thus, a quill striking 

 against a piece of wood causes a noise, but, striking against the teeth of a 

 wheel or of a comb, a continued sound ; and if the teeth of the wheel are 

 at equal distances, and the velocity of the motion is constant, a musical 

 note. 



Sounds of all kinds are most usually conveyed through the medium of 

 the air ; and the necessity of the presence of this or of some other material 

 substance for its transmission is easily shown by means of the air pump ;* 

 for the sound of a bell struck in an exhausted receiver is scarcely per- 

 ceptible. The experiment is most conveniently performed in a moveable 

 receiver or transferrer, which may be shaken at pleasure, the frame which 



* Hauksbee, Ph. Tr. 1705, xxiv. 1902, and xxvi. 367. Biot, Mem. d'Arcueil, 

 ii. 97. See Tr. R. S. E. v. 34. Saussure, Voyage dans les Alpes, vii. 377. 



