SECT. XVI. VELOCITY OF SOUND. 127 



as if its arms were the spokes of a wheel. When such 

 a machine rotates it impresses a motion on the air simi- 

 lar to its own, and when a thin board or card is brought 

 close to its extremities, the current of air is moment- 

 arily interrupted at the instant each arm of 'the bar 

 passes before the card ; it is compressed above the card 

 and dilated below ; but the instant the spoke has passed, 

 a rush of ah* to restore equilibrium makes a kind of ex- 

 plosion, and when these succeed each other rapidly, a 

 musical note is produced of a pitch proportional to the 

 velocity of the revolution. When M. Savart turned this 

 bar slowly a succession of single beats was heard ; as 

 the velocity became greater the sound was only a rattle ; 

 but as soon as it was sufficient to give eight beats in a 

 second, a very deep musical note was distinctly audible, 

 corresponding to sixteen single vibrations in a second, 

 which is the lowest that has hitherto been produced. 

 When the velocity of the bar was much increased the 

 intensity of the sound was hardly bearable. The spokes 

 of a revolving wheel produce the sensation of sound, on 

 the very same principle that a burning stick whirled 

 round gives the impression of a luminous circle. The 

 vibrations excited in the organ of hearing by one beat 

 have not ceased before another impulse is given. In- 

 deed it is indispensable that the impressions made upon 

 the auditory nerves should encroach upon each other in 

 order to produce a full and continued note. On the 

 whole, M. Savart has come to the conclusion, that the 

 most acute sounds would be heard with as much ease 

 as those of a lower pitch, if the duration of the sensation 

 produced by each pulse could be diminished proportion- 

 ally to the augmentation of the number of pulses in a 

 given time : and on the contrary, if the duration of the 

 sensation produced by each pulse could be increased in 

 proportion to their number in a given time, that the 

 deepest tones would be as audible as any of the others. 



The velocity of sound is uniform and independent of 

 the nature, extent, and intensity of the primitive dis- 

 turbance. Consequently sounds of every quality and 

 pitch travel with equal speed. The smallest difference 

 in their velocity is incompatible either with harmony or 

 melody, for notes of different pitches and intensities 



