CHAPTER 2 

 Echoes as Messengers 



Since bats and porpoises leam so much by listening to 

 echoes, it is important to examine the properties of 

 sound waves that make them such useful messengers. 

 Before sounds or other types of wave motion can tell us 

 anything at all, they must interact with something— the 

 surface of the earth, the walls of a house, a human lar- 

 ynx, or the intricate mechanism of an ear that listens. 

 Only by its doing something to some piece of matter, 

 directly or indirectly, can wave motion be detected in 

 the first place. Try to imagine a kind of radiation which 

 penetrates whatever may stand in its way, traveling on 

 and on without being changed, distorted, or deviated in 

 its direction of travel. How could we leam that such 

 rays even existed? High-energy cosmic rays and the sub- 

 atomic particles called neutrinos have only the very 

 slightest effect on matter under ordinary conditions, and, 

 therefore, they were most difficult to discover and still 

 are almost impossible to measure with any precision. 

 Radio waves from natural sources have always existed 

 at low levels of intensity and they penetrated the bodies 

 of our ancestors just as they do our own. But only in 



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