ON THE SOUECES AND EFFECTS OF SOUND. 295 



Observations on Sound, Nich. Jour. 8vo, i. 329. Armi, Ristretto di Fatti Acustici, 

 4to, Rom. 1821, Append. 1822. 



Propagation of Sound. Walker on the Velocity of Sound, Ph. Tr. 1698, xx. 

 433. Mairan, Hist, et Mem. de Paris, 1737, H. 1. Cassini, ibid. 1738, p. 128, 

 H. 1, 1739. La Condamine, ibid. 1745, p. 448, and Introd. Hist. &c. 1751, p. 98. 

 Euler, Hist, et Mem. de Berlin, 1765, p. 335. Winckler, Tentamina circa Soni 

 Celeritatem, 4to, Leipz. 1763. Blagden, Ph. Tr. 1784, p, 201. Miiller, Gotting. 

 Gelehrte Anzeigen, 1791. Espinosa and Bauza, Annales de Chimie, vii. 93. Ben- 

 zenburg, Gilbert's Annalen, new series, v. 383. Arago, &c. Connoissance des 

 Temps, 1825, p. 361. Goldingham, Ph. Tr. 1823, p. 96. Moll, &c. ibid. 1824, 

 p. 424. Gregory, Trans. Camb. Ph. Soc. 1824, ii. 120. Myrbach and Stampfer, 

 Jahrbuch des Instit. zu Wien, vol. vii. 



Propagation in Gases. Perolle, Melanges de Turin, 1786, iii. Corr. 1 ; 1790, 

 v. Corr. 195. Dulong, Annales de Chimie, vol. xli. 



LECTURE XXXII. 



ON THE SOURCES AND EFFECTS OF SOUND. 



THE examination of the origin of sound might naturally be deemed an- 

 terior to the inquiry respecting its propagation ; but it will appear, that the 

 properties of many of the most usual sources of sound depend immediately 

 on the velocity with which an impulse of any kind is transmitted through an 

 elastic medium ; it was therefore necessary to consider this velocity, before 

 the production of sound in general could be discussed. 



The origin of a simple sound, without any alternation, requires very 

 little investigation : it appears that the only condition necessary for its 

 production is a sufficient degree of velocity in the motion or impulse which 

 occasions it. A very moderate velocity must be sufficient for producing an 

 impression on the ear ; there is reason to believe that, when the sound is 

 continued, it may remain audible with a velocity of no more than one hun- 

 dredth of an inch in a second, and perhaps even with a velocity much 

 smaller than this ; but at its origin, it is probable that the velocity of the 

 motion, constituting a sound, must always be considerably greater. 



A continued sound may be produced by a repetition of separate im- 

 pulses independent of each other, as when a wheel strikes in rapid succes- 

 sion the teeth of a pinion, so as to force out a portion of air from between 

 them ; when a pipe, through which air is passing, is alternately opened 

 and shut, either wholly or partially, by the revolution of a stopcock or 

 valve ; or when a number of parallel surfaces are placed at equal distances 

 in a line nearly perpendicular to them, and a noise of any kind is reflected 

 from each of them in succession ; a circumstance which may often be ob- 

 served when we are walking near an iron railing, an acute sound being 

 heard, which is composed of such reflections from the surfaces of the 

 palisades. 



Musical sounds are, however, most frequently produced by the alternate 

 motions of substances naturally capable of isochronous vibrations, and these 



