PREFACE 



Physical principles operate, as far as we know, through- 

 out a universe which has both astronomical dimensions 

 and a fine grain, some of it close at hand. New horizons 

 can be large and distant or they may lie in the very small 

 and commonplace. The unique properties of water mole- 

 cules present just as interesting, even awesome, phenom- 

 ena as does the history of stellar galaxies. And in be- 

 tween, accessible for convenient study, is a delightful 

 variety of ingenious mechanisms making up the living 

 bodies of plants and animals. Man has been said to 

 "stand between the atoms and the stars," and between 

 molecules and men are to be found many fascinating 

 applications of physics, broadly conceived. Outstanding 

 among these are the ways in which living organisms 

 utilize wave motion of various kinds. Of particular in- 

 terest is the interplay between sound waves and the 

 animals and men who use them. 



Sound waves can teU us a great deal about the world 

 around us, and they are often used for this purpose by 

 both animals and men. Sound exhibits all the properties 

 of wave motion, and these properties can be observed 

 whenever sound travels back and forth from place to 

 place carrying information about the things it touches. 

 This is obviously true when people talk to one another 

 or when birds call from the treetops. But sound waves 

 are also useful as messengers when only one person or 

 animal is present to broadcast them and listen for their 



