VOICES OF EXPERIENCE 



torn of the ocean, though this does not mean, as people 

 used to believe, that the ocean depths are totally dark. 

 Oceanographers have recently discovered that lumines- 

 cent animals are so numerous that a suflBciently sensitive 

 light meter can register the flashes of Ught they give off 

 when the meter is lowered far below the deepest pene- 

 tration of sunlight. On the other hand, many rivers and 

 lakes contain enough sediment so that clear vision is im- 

 possible for more than a few inches even in daylight. 

 Yet hosts of fish and other aquatic animals Uve active 

 lives in these waters where vision is nearly impossible, 

 and it is not surprising that some have turned to sound 

 as a medium of communication and a means of orienta- 

 tion, for sound travels farther in water than does light. 



We usually think of the oceans and deep fresh-water 

 streams and lakes (those without outboard motors) as 

 silent, and few people even realize that fish or whales 

 can hear. The chief reason is that our own hearing mech- 

 anism is designed primarily for use in air and so does 

 not function well in water. Our ears can detect an air- 

 borne sound so faint that it approaches the noise level 

 rising from random motion of molecules. The eardrums 

 and the chain of little bones and elastic tissue that con- 

 vey sound waves to our inner ear mechanisms are beau- 

 tifully adapted to accept sound waves arriving through 

 the air, but poorly suited to receive them from the water. 

 When we do hear sounds under water, much of the 

 acoustic energy flows directly from the water through our 

 bodies, which are largely composed of water, to the 

 sensitive portions of the iimer ear, where minute vibra- 

 tions stimulate the auditory nerve. 



Sound waves do not move easily from air to water, 

 or vice versa. The boundary between a gas and a liquid 

 acts as an almost impenetrable barrier, and more than 

 99 per cent of the sound energy is reflected back into 



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