374 



INDOOR RECREATION 



When the alarm clock goes off in the morning, it makes 

 no difference what part of the room it is in. Sound travels 

 from it in all directions. If we obtain a tuning fork, an 

 instrument which gives a musical tone, we shall be able 

 to learn something about how sound is produced. If 

 we strike the prongs of the fork on a block of wood, we 

 are all able to hear the sound, although we cannot notice 

 that the tuning fork looks any different than it did before 

 it was struck, but if we hold a pith ball suspended from a 

 string so that it touches the prong of the fork, after the 

 fork has been struck, we notice that the pith ball moves 



violently away from the 

 fork, as if struck by a blow. 

 If we now stick the end of 

 the fork into a vessel of 

 water, we find a fine spray of 

 water thrown from the sur- 

 face. Evidently the fork is 

 in quite rapid motion. As 

 a matter of fact, it is vibrat- 



How to show that a sounding tuning fork 

 is in motion. 



ing several hundred times a 

 second, and in vibrating, it 

 strikes the air just as it strikes the water, and sets up 

 little waves in the air. Just as waves move on the sur- 

 face of water when we drop a stone in, becoming wider 

 and wider as they move away from their source, so a 

 body that makes sound, as the tuning fork, sends these 

 waves off into the air. Only we must remember that 

 instead of the waves moving, as on the surface of the 

 water, in one plane only, these are moving away in all 

 directions from the sounding body, each wave making a 

 hollow sphere in the air. 



