SONAR AND RADAR 



defects in tire casings, they distort transmitted sound 

 waves. In some cases very short pulses of sound are used 

 to produce distinct echoes in the material being tested. 

 The sound frequencies are often very high, up to 1 

 megacycle per second (10^ c.p.s.)? and this is possible 

 because relatively short distances of transmission are in- 

 volved. It is a comparatively inexpensive method of test- 

 ing compared to structural failure of an important and 

 costly machine, and the material is not damaged in any 

 way. 



Recently this sort of acoustic probing has been ap- 

 plied to the living bodies of animals and men. It is pos- 

 sible to detect discontinuities in our internal organs in 

 this way, using sound waves generated at the surface of 

 the body by suitable sound sources, such as crystals 

 which are vibrated at high frequencies by electric cur- 

 rents. This method is not without its dangers, for intense 

 sound waves in our bodies can produce damage. But, 

 when properly controlled, the method has some advan- 

 tages over X-rays. At least any damage is local and, inso- 

 far as we know, is not a long-delayed effect on our 

 genes— the complex molecules in our reproductive or- 

 gans, some of which may in time determine what our 

 children will be like. One hmitation of this method stems 

 from the large number of discontinuities that are natu- 

 rally present in a human body— those between muscle 

 and bone, digestive tract and blood vessels, heart and 

 lungs, etc. Thus any abnormaUties must be discriminated 

 from a complex background of natural structures, and 

 this makes it more difficult to locate a tumor in a human 

 brain than an air bubble in a cast-iron pipe. Neverthe- 

 less, this new means for studying our invisible insides 

 may lead in time to safer or more effective methods of 

 locating internal disorders in an early and curable stage. 

 The discrimination problems may be no more difficult 



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