B 



ll^/r^^M^*^^' 



Figure 8 . --A tracing of a typical recording of underwater sounds, 

 as received by hydrophone and recorded by wire recorder, then 

 transcribed on the recording voltmeter. The section between "A" 

 and "B" is a recording of ambient noise only; "B" to "C" is a 

 frequency of 20 c.p.s. superimposed upon the ambient noise. 



13 



