HOW SOUND IS TRANSMITTED THROUGH AIR 269 



FIG. 167. Elastic 

 balls. 



stirring, the shriek of a railroad whistle miles distant may be 

 heard with perfect clearness. Moreover, the most delicate 

 notes of a violin can be heard in the remotest corners of a 

 concert hall, when not the slightest motion 

 of the air can be seen or felt. 



In our study of the atmosphere we saw 

 that air can be compressed and rarefied ; in 

 other words, we saw that air is very elastic. 

 It can be shown experimentally that when- 

 ever an elastic body in motion comes in con- 

 tact with a body at rest, the moving body 

 transfers its motion to the second body 

 and then comes to rest itself. Let two billiard balls be sus- 

 pended in the manner indicated in Figure 167. If one of the 

 balls is drawn aside and is then allowed to fall against the other, 

 the second ball is driven outward to practically the height from 

 which the first ball fell and the first ball comes to rest. 



If a number of balls are arranged in line as in Figure 168 or 

 Figure 169, and the end ball is raised and then allowed to fall, 

 or if A is pushed against C, the last ball B will move outward 

 alone, with a force nearly equal to that originally possessed 

 by A and to a distance nearly equal to that through which A 

 moved. But there will be no visible motion of the interven- 

 ing balls. The force of the moving 

 ball A is given to the second ball, 

 and the second ball in turn gives 

 the motion to the third, and so on 

 throughout the entire number, until 

 B is reached. But B has no ball to 

 168. Suspended billiard give its motion to, hence B itself 



balls. ' 



moves outward, and moves with a 



force nearly equal to that originally imparted by A and to a 

 distance nearly equal to that through which A fell. Motion 



