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LECTURE XXXI. 



ON THE PROPAGATION OF SOUND. 



-L HE theory of sound, which constitutes the science of acustics, is on many 

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

 teresting 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 considered as ex- 

 ceedingly abstruse and intricate, but the difficulty has in some measure ori- 

 ginated from the errors which were committed in the first inquiries respect- 

 ing 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 enter- 

 tainment that they afford. We shall consider first the nature and propaga- 

 tion of sound in general, secondly, the origin of particular 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 concern- 

 ed, 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 distin- 

 guished, they constitute a continued sound, like that of the voice in speak- 



