ECHOES OF BATS AND MEN 



at the correct distance. And there are ahnost always 

 other sounds to compete for our attention. Thus obvious 

 echoes have come to seem rather special sounds to be 

 heard only in the most favorable circumstances. 



One situation where echoes have been put to practical 

 use is aboard boats in foggy coastal waters. Usually it 

 is quiet in a fog and, aside from the boat itself, no 

 echoing surface interrupts until the shore is reached. 

 Often fishermen who find themselves in foggy waters 

 and think that steep shore lines or cliffs may be within a 

 mile or so produce a clear echo by making a short, loud 

 sound. Sometimes this is a blast of a horn or whistle, 

 required by law in any case to signal their presence to 

 other boats, or the probing signal may be simply a shout. 

 Some fishermen say they can even hear echoes from 

 channel-marker buoys (about three feet in diameter) 

 at several hundred feet. The usefulness of this method 

 of navigation is often limited by the lack of adequate 

 echoing targets in the air above the actual underwater 

 hazards. Rocks need not reach the surface to be dan- 

 gerous, and most shore lines are too gentle to provide 

 reliable echoes. 



Modem instruments have largely supplanted air- 

 borne sound by transposing the same basic process into 

 the water itself. Sound waves are broadcast from the 

 boat's hull, and echoes of underwater sound from the 

 bottom or from shoals ahead of the boat are recorded 

 by instruments. Such devices for echolocation under 

 water are called echo sounders or fathometers— ih& more 

 refined models can even detect schools of fish. All these 

 methods have in common the emission of a probing 

 sound, the detection of echoes, and, most important, the 

 discerning of the distance and direction of the object that 

 returns the echo. 



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