CHAPTER XXVII 



SOUND 



249. The Senses. All the information which we possess 

 of the world around us comes to us through the use of the 

 senses of sight, hearing, taste, touch, and smell. Of the five 

 senses, sight and hearing are generally considered the most 

 valuable. In preceding Chapters we studied the important 

 facts relative to light and the power of vision ; it remains for 

 us to study Sound as we studied Light, and to learn what we 



can of sound and the power to hear. 



250. How Sound is Produced. If 

 one investigates the source of any 

 sound, he will always find that it is 

 due to motion of some kind. A sud- 

 den noise is traced to the fall of an 

 object, or to an explosion, or to a col- 

 lision ; in fact, is due to the motion 

 of matter. A piano gives out sound 

 whenever a player strikes the keys 

 and sets in motion the various wires 

 within the piano ; speech and song 

 are caused by the motion of chest, 

 vocal cords, and lips. 



If a large dinner bell is rung, its 

 motion or vibration may be felt on 

 touching it with the finger. If a tuning 

 fork is made to give forth sound by striking it against the knee, 

 or hitting it with a rubber hammer, and is then touched to the 



266 



FlG. 164. Sprays of water 

 show that the fork is in 

 motion. 



