ECHOES AS MESSENGERS 



Water Waves and Surface Echoes 



There are many and detailed parallels between water 

 waves and sound waves in air or, for that matter, light 

 waves; but aside from their serving as slow-motion 

 models, we are likely to think of surface waves on water 

 as of little interest and certainly as wholly devoid of the 

 abiUty to carry information. Who would think of trying 

 to signal back and forth across the ocean by means of 

 water waves? They die out too soon and are too easily 

 confused with the natural waves from winds or water 

 currents. A leaf that falls to the surface of a quiet pond 

 may produce a few ripples, but how could one hope to 

 detect this event a hundred feet away? Yet from their 

 very similarity to sound or light we might expect the 

 water waves we study in the physics laboratory to have 

 some message-carrying function. Such cases can be 

 found if we look for them in nature and, in this instance, 

 the search leads to the so-called whirligig beetle which 

 bridges the gap between the ripple tank and the most 

 compUcated radar installations. 



Whirligig beetles are common inhabitants of small 

 ponds and quiet streams. While these aquatic insects 

 often dive and swim below the surface, they are usually 

 noticed most easily when darting about on the surface 

 film of the water. They are light enough in weight so 

 that they are supported by the surface tension of the 

 water— largely because of their fringe of hairs covered 

 with a thin film of waxy material that does not readily 

 become wetted. This ability to support themselves on 

 water could easily lead us into a digression about sur- 

 face tension and why water is a uniquely suitable liquid 

 for the flotation of water beetles. But this is a subject 



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