ECHOES OF BATS AND MEN 



their evolutionary history because they live and move 

 under conditions where sound is a convenient or per- 

 haps the only available means for maintaining their 

 orientation. 



Telling whether you are indoors or out on the street 

 by listening to a tape recording of a shrill and monoto- 

 nous tone may seem a clumsy way to accompUsh the 

 obvious. But transpose the situation to a man lost in a 

 pitch-black cave and unable to use a hght of any kind. 

 Sound waves would be one of the most useful means, if 

 not the only means, at his disposal to learn about those 

 parts of the cave beyond the direct reach of his out- 

 stretched hands and feet. Bats do not feel their way; they 

 fly rapidly through complex and tortuous passages of a 

 cave, dodging stalactites and other bats without acci- 

 dents of any kind, and, as I shall explain later, this is 

 one of the less difficult of the many tasks these little 

 animals accomplish by means of sound waves. 



In pursuing these matters further it will be best to 

 return from time to time to the simple experiments with 

 audible sounds such as those we have just conducted. In 

 this way you may have firsthand experience to confirm 

 and support the concepts and theories about which you 

 read. For many purposes the ripple tank used in physics 

 courses provides more convenient types of waves with 

 which the same phenomena can equally well be visual- 

 ized. This is basically because surface waves on water 

 travel slowly enough for you to watch them directly. 

 Furthermore, their velocity varies with the depth of the 

 water, and they can be caused to bend by installing 

 shallow "sandbars" or "reefs" in the ripple tank. The 

 same ripple tank can also be used to study echoes which 

 are closely analogous to those that cause standing waves 

 of sound and to those used by bats or men to find their 

 way about in situations where hght is not available. 



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