SONAR AND RADAR 



is now well known that objects whose size is much less 

 than the wave length of the sound being used yield only 

 faint echoes, but in 1912 this was not a generally appre- 

 ciated fact. Had scientists been less scornful of bats and 

 had they known more about "Spallanzani's bat prob- 

 lem," more progress would have been made by 1912. 

 Furthermore, Maxim proposed to echolocate icebergs 

 through the air, whereas both the actual danger to the 

 ship and the major part of the iceberg lay beneath the 

 surface. The latter consideration led other inventors to 

 investigate the possibility of using underwater sound. 



Two or three years after the sinking of the Titanic, 

 the increasing use of submarines by the German Navy 

 spurred the development of underwater sound devices. 

 At first it was largely a matter of Ustening to the sounds 

 originating from the submarine, particularly from its en- 

 gines and propellers. Much of the naval use of under- 

 water sound is still passive listening for the sounds of 

 other ships. But to a small degree by 1918, and to a 

 much greater extent by 1940, research had led to active 

 probing of the sea with sounds which would yield usable 

 echoes. Enemy submarines were the main military tar- 

 gets, but along with the development of sonar came the 

 echo sounder, or fathometer, a device to measure the 

 depth of the water. In comparison with an enemy sub- 

 marine (or the fish detected by porpoises), the bottom 

 of the ocean would seem to be an easy target, but for 

 many years the idea proved simpler than its realization. 

 In the deeper parts of the ocean even an echo from the 

 bottom was faint and diflBcult to detect with the early 

 sonar devices, but the most critical problem came when 

 the water was shallower and more dangerous. Here the 

 diflSculty was that the ship's hull had a disconcerting 

 tendency to "ring" or prolong the outgoing sound even 

 after the actual generating mechanism had been turned 



109 



