WORLD VOYAGE BEGINS 



187 



fathoms of water separating her from the ocean bed. To carry out 

 such experiments a ship must be fitted with special apparatus. 



6 TO 20 MILES ' 



SONO-RADIO BUOYS 



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 X BASEMENT ROCK x 

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A seismic refraction experiment, showing the paths by which various sound 

 waves travel from explosion to the hydrophones 



A comparatively flat area of the ocean bed having been found, 

 four sono radio buoys are laid about half a mile apart. Each buoy 

 has a hydrophone slung beneath it in the water and within the 

 buoy itself is a receiver and transmitting wireless set, with bat- 

 teries. An aerial is carried on the buoy's mast. Having laid the 

 buoys, the ship moves off to a distance of eight miles or so and 

 fires a depth charge. This forms a source of sound which travels 

 out in all directions in the sea. 



One path of sound travels horizontally through the water to the 

 hydrophones below the buoys whence it returns to the ship as a 

 radio signal. An accurate timing device in the ship records the 

 time of the firing of the depth charge and the returning signal 

 from the buoys. The speed of sound in sea water is known (^000 

 feet per second) so that a range of the buoys has now been found. 



Other paths of sound lead towards the sea-bed and then hori- 

 zontally through the layer of sediment. As this sound passes 



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