VELOCITY OF SOUND 271 



the distance between the source of sound and the hearer be- 

 comes too great, the sound disappears entirely and nothing 

 is heard. The explanation of this well-known fact is found 

 in a further study of the elastic balls (Fig. 170). If A hits two 

 balls instead of one, the energy possessed by A is given in 

 part to one ball, and in part to the other, so that neither ob- 

 tains the full amount. These balls, having each received less 

 than the original energy, have less to transmit ; each of these 

 balls in turn meets with others, and hence the. motion be- 

 comes more and more distributed, and distant balls receive 

 less and less impetus. The energy finally given becomes 

 too slight to affect neighboring balls, and the system comes 

 to rest. This is what occurs in the atmosphere ; a moving 

 air particle meets not one but many adjacent air particles, 

 and each of these receives a portion of the original energy and 

 transmits a portion. When the original disturbance becomes 

 scattered over a large number of air particles, the energy given 

 to any one air particle becomes correspondingly small, and 

 finally the energy becomes so small that further particles are 

 not affected; beyond this limit the sound cannot be heard. 



If an air particle -transmitted motion only to those air par- 

 ticles directly in line with it, we should not be able to detect 

 sound unless the ear were in direct line with the source. 

 The fact that an air particle divides its motion among all 

 particles which it touches, that is, among those on the sides 

 as well as those in front, makes it possible to hear sound in all 

 directions. A good speaker is heard not only by those 

 directly in front of him, but by those on the side, and even 

 behind him. 



254. Velocity of Sound. The transmission of motion from 

 particle to particle does not occur instantaneously, but re- 

 quires time. If the distance is short, so that few air particles 

 are involved, the time required for transmission is very brief, 



