XVI 



ECHO-LOCATION 



Fifty years ago the only way for a sailor to find the depth 

 of the water on which his ship was sailing was to let down 

 a line with a weight on the end until it touched the bottom : 

 the length of line paid out told him the depth. Although this 

 method was rather slow and cumbersome it worked well 

 enough in relatively shallow water, but it was extremely 

 difficult to use it successfully in really deep water. Nowadays 

 no one would dream of wasting time with sounding-lines, for 

 machines have been invented that can give a continuous 

 record of the depth of water below a ship without letting 

 down anything over the side. The echo-sounder which began 

 to come into use in the early nineteen-twenties was the 

 answer to one of the sailor's major problems. 



The principle of this machine is very simple. If the hull 

 of the ship is tapped sharply the sound travels away in all 

 directions and some of it reaches the bottom whence it is 

 reflected back as an echo. The echo can be heard if a suffi- 

 ciently sensitive microphone is used in the ship. If the time 

 interval between sending out the sound and receiving the 

 echo is noted then it is easy to calculate the distance the 

 sound has travelled out and back because the rate that sound 

 travels in water is known — it is about 3,150 miles an hour, 

 or 4,700 feet a second, much faster than its rate of approxi- 

 mately 700 miles an hour in air. In practice the machine 

 does the listening and calculating so that the sailor need 

 only read the depth on a scale, or watch the profile of the 

 bottom being continuously drawn on a chart. It was the 

 invention of sensitive microphones and electronic amplifiers 

 that made this device possible. 



Although the principle of the echo-sounder was invented 



by man and developed by the aid of his civilized technology, 



the same principle had been, all unbeknown to him, in use 



183 



