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ECHO-SOUNDING AND HYDROGRAPHICAL STUDIES 



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Recording echo-sounder. The pointer rotates a 

 little more than once before the sound returns 

 from the bottom. There is a correction of syg 

 metres for temperature, salinity, and pressure. 



The sound reaches the bottom and returns 

 in one second. 



tirely on the installation, and in particular on the stability of the motor 

 and paper. Echo-sounders of this type are now installed in nearly all 

 ships, cargo as well as passenger, though usually their range is restricted 

 to a depth of a few hundred metres. 



It is vital to an oceanographical research ship that she should be able 

 to obtain the maximum possible knowledge of the topography of the ocean 

 floor, if she is to avoid the risk of having her gear destroyed. A cardinal 

 feature of the Galathea's equipment, therefore, was an echo-sounder of 

 the very latest type, a self-registering unit specially constructed for this 

 expedition and powerful enough to record the greatest depths without 

 difficulty. The makers were Kelvin-Hughes Ltd. of London. 



The sound impulse is a high-pitched whistling tone of 10,000 oscilla- 

 tions per second, produced by a column of nickel rings surrounding an 

 electric coil. Behind the rings is an air-charged reflector that concentrates 

 the sound waves in a beam which, through the ship's bottom, is directed 

 vertically into the water, with a dispersal of only 8° from the vertical. 

 This transmitting mechanism is mounted on one side of the ship's bottom, 

 where in place of one of the iron plates is an unpainted stainless steel 

 plate six millimetres thick. The receiver is identical in form and is mount- 

 ed on the opposite side of the ship's bottom. The impulse received is 

 transmitted by way of a transformer to the bridge and is there recorded. 

 The right-hand figure gives a rough idea of the recording mechanism. It 



