144 Mr THEVELYAX'S Experiments on the 



heave will be so modified as to create an inclination to one side. 

 The bar thus thrown off its balance to the right on ascending, 

 will incline as far to the left on descending, and there receiving 

 the expansive impulse, it will be driven back, and thus the prin- 

 ciple of rocking will be created. 



With regard to the sound, which constitutes the second por- 

 tion of the remarkable phenomenon, the following remarks have 

 presented themselves. It seems indispensable to its production 

 that there be an uneven surface, along which a current of air 

 may pass between the bar and block : this roughness may exist 

 either in the block or in the ridge of the bar. 



The manner in which the sonorous undulations of the air, 

 which, reaching the internal ear, cause the sensation of sound, may 

 be occasioned, is not very manifest. 



By attending to the usual modes in which acoustic aerial 

 pulses are produced, we may perhaps be able to assign the one 

 which proves the source of the sound in our experiment. 



The first mode is, when an elastic substance, thrown into vi- 

 bration by an impulse given to it, communicates the tremor to 

 the ambient air, as when a bell, or a simple musical instrument, 

 for instance the stoccado, is struck. 



The second is, when the sonorous tremor is acquired by the 

 air passing through one or more apertures varying in size, and 

 with varying degrees of force and velocity. I consider the ani- 

 mal voice of this description ; for though some physiologists 

 maintain that the voice is produced in the third mode, imme- 

 diately to be mentioned, I am disposed to think, that the sound, 

 and all its modulations, depend upon the form and condition of 

 the aperture of the glottis, and of the passage through which the 

 expired air passes. For though the numerous cartilages, and 

 more especially the arytenoid, may be considered as vibratory bo- 



