62 THE REALITIES OF MODERN SCIENCE 



shoves of the policemen at the center, pictures a some- 

 what similar action. The push or pulse, started at the 

 center, travels outward through the crowd. Figure 6 



represents a series of layers. 

 When the pulse, which origi- 

 nates at the center, reaches a 

 layer, as 6 of the figure, it 

 results in a momentary crowd- 

 ing of this layer against the 

 next outer one. The layer 6 is 



thus crowded or condensed between layer 5 which is 

 moving outward and layer 7 which has not yet started to 

 move, and we speak of this pulse as one of condensation. 

 Now suppose that, just after those at the center have 

 started to push out, conditions change so that they 

 may again move in. They do 

 so, increasing the distance be- 

 tween their layer and the next. 

 This layer in its turn moves 

 inward, and successively the 

 other layers adjust them- 

 selves to the new condition. 



Whereas before we had a con- '_ 



densation we now have a pulse of rarefaction spreading 

 outward from the center, as shown in Fig. 7 by the 

 relative positions of the layers when the pulse has 

 reached 7. In the figure the previous pulse of con- 

 densation is shown as just reaching layer 13. The 

 vibrating card gives rise in the surrounding air to a 

 succession of alternate condensations and rarefactions 

 which travel outward to the ear of the listener. The 

 thin_drum of the ear is pushed in by each pulse of 



