SECT. XVI. VELOCITY OF SOUND. 133 



chord. So that the human ear can appreciate a sound which 

 only lasts the 24,000th part of a second. This note was dis- 

 tinctly heard hy M. Savart and by several people who were 

 present, which convinced him that with another apparatus still 

 more acute sounds might he rendered audihle. 



For the deep tones M. Savart employed a bar of iron, two feet 

 eight inches long, about two inches broad, and half an inch in 

 thickness, which revolved about its centre as if its arms were the 

 spokes of a wheel. When such a machine rotates, it impresses a 

 motion on the air similar to its own, and, when a thin board or 

 card is brought close to its extremities, the current of air is 

 momentarily 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 air 

 to restore equilibrium makes a kind of explosion, 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. Indeed 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 proportionally 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 



