168 



LIGHT AND SOUND 



fork is put into water, it throws the water into sprays; 

 this shows that its sounding is due to its motion (Fig. 162). 



Sounds- are ordinarily carried by the air. If an electric 

 bell is placed inside a vessel of air (Fig. 163), and the air 

 is nearly all removed by an air pump, the sound given 

 out by the bell is feeble; but it becomes stronger again 

 when air is admitted. This shows that a vacuum would 

 not carry sound. But other kinds of matter besides 

 air carry sound. Thus the tap of a pencil or the scratch 

 of a pin against a long steam pipe is heard readily, if we 

 put the ear close to the pipe. Iron conducts sound 

 better than air. Water is also a better sound conductor 



than air : the clashing together of two stones under water produces a 



louder sound than in air. In air at C. 



sound travels about 1,100 feet in a second; 



in water, about 4,600 feet, and in iron, 



about 17,000 feet. 



FIG. 162. 

 A sounding 

 body is in 

 rapid vibra- 

 tion. 



FIG. 163. 



A bell ringing in a vacuum 

 would give forth no sound. 



190. Sound Waves. When a 

 sound is produced (Fig. 164), it is 

 heard in all directions. This sug- 

 gests to us that sound waves start 

 as small spheres which get larger 

 and larger as they move outward from the sounding 

 body. But as the sphere grows larger, more and more air 

 must be set in motion. Naturally the motion of a given 

 volume of the air must become smaller and smaller. Fi- 

 nally the motion of the air is too 

 small to affect the ear. We say the 

 sound " dies out " with distance. 



How does the air move when it 

 ;f: carries sound? It cannot go as in a 

 paS 1 ?d. compressed and ex ~ breeze or wind, by the forward 



