168 



SEVENTEENTH CENTURY. 



PT. III. 



backwards and apart; an instant later, 4, 5, and 6 will be 

 drawing apart, in their turn, while 7, 8, and 9 will be close 

 together, and so on to the end of the train. In this way a 

 movement will pass along the whole line, every three or 

 more carriages being huddled together, while the set behind 

 them are separating, and this oscillation will go on until 

 the train comes to rest. A movement exactly similar to 

 this takes place in a wave of sound. The particles of air set 

 in motion at one end are pushed forward, strike, and rebound 

 just as the carriages do, and thus the movement is passed 

 along the whole mass of air, while the particles themselves 

 are oscillating backwards and forwards within very narrow 

 limits. This movement, if visible, would have the appear- 

 ance of Fig. 33 : a condensation or crowding of the particles 

 a, and a rarefaction or separation of the particles ^, occurring 



FIG. 33. 



Waves of Sound, a, A condensation ; b, a rarefaction. From a to a, or b to b. is 

 called a sound-wave, and varies in length according to the depth or shrillness of 

 the sound. 



one after the other. The space from one condensation to 

 another, or from one rarefaction to another, is called a wave 

 of sound. 



In one way, however, sound travels somewhat differently 

 to the wave of movement along a train. The carriages of 

 a train are confined in one straight line, just as air particles 

 would be if they moved in a tube ; but in the open air the 

 particles can move in all directions, and therefore sound 

 does not only travel in one straight line, but spreads out in 

 all directions in expanding spheres, unless it meets with some 



