ECHOES OF BATS AND MEN 



off. The combined sound lasted much longer than the 

 time required for it to make the round trip to the bot- 

 tom. In other words, there were severe problems of dis- 

 cnw/wa//(7«— separating relatively faint echoes from the 

 continuing, original emitted sound. The instruments 

 were confronted with the same problems as those that 

 make a blind man less skilled at echolocation than a 

 porpoise or a bat. This engineering problem was solved 

 in part by learning how to make underwater sounds of 

 shorter duration. 



By the 1950s, however, echo sounders had been per- 

 fected to a level of reUability where they have become al- 

 most essential for safe navigation. They even became so 

 sensitive that they began to indicate "false bottoms" be- 

 tween the ship and the true bottom. "Finding" two or 

 three extra ocean bottoms above the real one was a 

 rather disconcerting discovery, but after a time the fish- 

 ermen who used echo sounders began to notice that some ' 

 of the "false bottoms" were really echoes from schools 

 of fish. Still later, mysterious layers of faint echoing, or 

 sound scattering, were noted almost everywhere in deep 

 oceans at several hundred feet below the surface. These 

 have been called the deep scattering layers and they were 

 later found to migrate up and down with dawn and dusk. 

 This fact provided the clue to their identity. 



Oceanographers had already discovered by systematic 

 netting operations that large populations of shrimp and 

 other small marine animals five at depths where sunlight 

 barely penetrates. This depth is greater at noon than at 

 midnight; hence there is a massive vertical migration of 

 these animals upward during the evening and down 

 again at daybreak. The physical records of the deep 

 scattering layers turned out to match the known be- 

 havior of the animals. Once this additional fact was es- 

 tablished, the echo sounder became a valuable tool for 



110 



