ECHOES AS MESSENGERS 



mosphere exerts the same pressure as does a 76-centi- 

 meter column of mercury). Furthermore, these re- 

 gions of slightly higher or lower pressure travel through 

 the air, so that the zone of higher pressure which at 

 one moment is passing a particular point will later 

 be found some distance away. These pressure changes 

 are much smaller than most people realize; for ex- 



Fig. 3. If you could photograph the molecules of air 

 around a source of sound, you would find some mole- 

 cules grouped closely and others loosely. There is a 

 high pressure in the close-packed areas and a lower 

 pressure elsewhere. A graph of this variation in pres- 

 sure is a sine curve. 



39 



