ENGINEERING BEFORE AND AFTER WAR 47 



tion and the concentration of beams of sound are de- 

 scribed in his specifications, and he recommends fre- 

 quencies ranging from 4,786 to 100,000 complete vibra- 

 tions per second, and also suggests that the rate of 

 approach or recession from the object may be determined 

 from the difference in the pitch of the echo from the pitch 

 of the blast sent out. Hiram Maxim also suggested simi- 

 lar apparatus a little later. 



The echo method of detection was not, however, prac- 

 tically developed until French and English scientists, with 

 whom was associated Professor Langevin, of the College 

 de France, realizing its importance for submarine detec- 

 tion, brought the apparatus to a high degree of perfection 

 and utility shortly before the Armistice. Now, with beams 

 of high-frequency sound waves, it is possible to sweep 

 the seas for the detection of any submerged object, such 

 as icebergs, submarines, surface vessels, and rocks; they 

 may also be used to make soundings. It enables a chas- 

 ing ship to pick up and close in on a submarine situated 

 more than a mile away. 



The successful development of sound-ranging apparatus 

 on land led to the suggestion by Professor Bragg that a 

 modified form could be used to locate under-water ex- 

 plosions. It has been found that the shock of an explosion 

 can be detected hundreds of miles from its source by 

 means of a submerged hydrophone, and that the time of 

 the arrival of the sound wave can be recorded with great 

 precision. At the end of the war the sound-ranging sta- 

 tions were being used for the detection of positions at 

 sea, required for strategical purposes. The same stations 

 are now being used extensively for the determination of 

 such positions at sea as light-vessels, buoys which indicate 

 channels, and obstructions such as sunken ships. By this 

 means ships steaming in fog can be given their positions 

 with accuracy for ranges up to 500 miles. 



Among the many other important technical systems and 

 devices brought out during the war which will find useful 



