Living Silver 



since the speed of sound through brine was a known constant. 

 This depth was recorded by a sensitive nib on an unfolding roll of 

 paper in the wheelhouse. But if anything lay between the hull of 

 the ship and the bed of the sea it would interrupt the sound and 

 send back a smaller echo of its own. A submarine, for example, 

 would give a very substantial echo, not only because of its size but 

 also because it was filled with air and was therefore able to act like 

 a kind of drum, booming back the minuscule sound from the appa- 

 ratus. The bodies of many fish contained a similar gas-filled space, 

 the so-called air bladder, and this meant that they too gave a fair 

 indication of their presence even when they were present only as 

 individuals. Other fish, like the dogfish, had no air bladder and 

 their echo was thus less easily recorded and they had to be present 

 in large numbers before they were immediately recognisable from 

 their echo-trace. It was because they were able to decipher such 

 small differences in the quality of the trace rather than because the 

 engineers had given them efficient navigational instruments, that 

 fishermen were able to use the echo-sounder as a fish indicator. 

 It would have been a positive nuisance if it had led them into mis- 

 taking the projection of an old wreck for a shoal of cod. Every- 

 thing depended on the interpretation of the text of the trace and 

 that interpretation was a matter of experience, often of contradic- 

 tory experience. Though a few traces were nearly unmistakable, 

 like the thick black band of shoaling pilchards, a kind of horse's 

 mane slung from an imaginary mid- water neck, there were many 

 others so deceptive that skippers preferred to neglect them alto- 

 gether. As years passed they would be sorted out; every tiniest 

 blip would become a safe fish sign : but that sorting would be the 

 work of a whole generation of fishermen, not of an engineer. 



But the engineers continued to confess, by their superior sil- 

 ence in the face of a cursing skipper, that they, and they alone 

 were responsible for the marvels of echo -fishing. Under these cir- 

 cumstances, hostility grew up among fishermen and it was usually 

 directed against the man with white teeth who popped up out of 

 the engine room in a comic mask of coaldust. But, no matter how 



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