ECHOES OF BATS AND MEN 



radar and sonar systems, complicated mechanisms which 

 send out radio waves or sound waves to locate objects 

 that return echoes of these probing signals. These sys- 

 tems will be discussed later, but the present chapter will 

 be devoted to another group of experts who can draw 

 upon a longer history of reaUstic, operational experience 

 —experts who use echoes not only to find their way but 

 also to obtain their daily bread and butter. If their sys- 

 tems should fail, they would starve to death, and this 

 pressure of necessity has led to great refinement and re- 

 habihty of their methods. 



These experts are animals which Uve where sound 

 replaces light as the best means of finding their way- 

 caves where bats fly by the thousands, or dark waters 

 where light is nearly nonexistent or is so difl^sed that 

 clear images over any distance are impossible. The best 

 known of these animal experts are the whales and 

 porpoises, which often swim in dark or turbid waters, 

 catching fish they cannot see, and the bats, which fly in 

 near or total darkness, getting all their food by aerial 

 interception of invisible flying insects. To have survived 

 at all requhred of these animals and their ancestors 

 enormous skill at echolocation, the location of objects 

 by their echoes. By studying the sounds they use and 

 how they modify them for particular problems of echo- 

 location, we may learn much that can help blind peo- 

 ple. Even aside from this reason, we will find these 

 animals' use of echoes to be a fascinating subject in its 

 own right. 



Echo Experts in the Ocean 



Only in the clearest water does fight travel far enough 

 in straight fines so that objects can be seen at more than 

 a few feet. Dayfight cannot penetrate nearly to the bot- 



18 



