280 SOUND 



each other at uncertain and irregular intervals and the result 

 was noise. 



Sound possesses a musical quality only when the waves or 

 pulses follow each other at absolutely regular intervals. 



262. The Effect of the Rapidity of Motion on the Musical 

 Tone Produced. If the disk is rotated so slowly that less 

 than 1 6 puffs of air are produced in one second, only sepa- 

 rate puffs are heard, and a musical tone is lacking ; if, on 

 the other hand, the disk is rotated in such a way that 16 puffs 

 or more are produced in one second, the separate puffs will 

 blend together to produce a continuous musical note of very 

 low pitch. If the speed of the disk is increased so that the 

 puffs become more frequent, the pitch of the resulting note 

 rises ; and at very high speeds the notes produced become 

 so shrill and piercing as to be disagreeable to the ear. If the 

 speed of the disk is lessened, the pitch falls correspondingly ; 

 and if the speed again becomes so low that less than 16 puffs 

 are formed per second, the sustained sound disappears and 

 a series of intermittent noises is produced. 



263. The Pitch of a Note. By means of an apparatus 

 called the siren, it is possible to calculate the number of vibra- 

 tions producing any given musical note, such, for example, as 

 middle C on the piano. If air is forced continuously against 

 the disk as it rotates, a series of puffs will be heard (Fig. 177). 



If the disk turns fast enough, the puffs blend into a musical 

 sound, whose pitch rises higher and higher as the disk moves 

 faster and faster, and produces more and more puffs each 

 second. 



The instrument is so constructed that clockwork at the 

 top registers the number of revolutions made by the disk in 

 one second. The number of holes in the disk multiplied by 

 the number of revolutions a second gives the number of 

 puffs of air produced in one second. If we wish to find the 



