

THE SIREN. 



345 



case of a pendulum. If a body makes 500 vibrations 

 in one second, then 500 is called the number of vibra- 

 tions made by the body, and T ^ of a second is called 

 the time of vibration of the body. A knowledge of 

 the absolute number of vibrations made by a sounding 

 body is needful to enable us to make a closer acquain- 

 tance with the essential nature of musical sounds. The 



FIG. 213 (| real size}. 



^termination of the number of vibrations, by the direct 

 )1 nervation of ordinary sounding bodies, presents many 

 lifficulties ; an apparatus used for the purpose, the siren, 

 'acilitates this determination. By the siren a great 

 variety of notes is produced, not by setting a body to 

 Vibrate but by producing a series of puffs of air which 



