CHAPTER 1 

 Voices of Experience 



Doing something in the dark is ahnost always difiBcult; 

 the darker it is the more troublesome an otherwise sim- 

 ple task becomes. Worst of all is to be bUnd. It is also 

 a formidable task to build machines to trace the move- 

 ments of distant objects which we cannot see— airplanes 

 flying above the clouds or submarines hundreds of feet 

 below the surface of the ocean. Finding your way on a 

 dark night is obviously related to the problems of re- 

 adjusting to a life of blindness, and instruments for 

 searching out invisible targets must solve similar prob- 

 lems. All these solutions are based on the sending out of 

 some form of energy and the sensing of a part of this 

 energy as it echoes back from the object at a distance. 

 When we wish to learn about a difficult subject, such 

 as the use of waves for searching out the invisible, we 

 naturally look first for an expert who can explain its 

 complexities for our benefit. There are experts who have 

 extensive practical experience in the use of echoes. Some 

 of them make their hving using echoes to locate small 

 moving objects which they cannot see. One group are 

 the physicists and engineers who design and operate 



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