R.R.S. 'DISCOVERY IF 103 



above hammer, a change-over switch from the oceanic machine being provided, and 

 a Mark XII magneto-striction shallow-water sounder with curved scale recorder. The 

 latter has been fitted recently in place of the old Pattern 751 British Admiralty shallow- 

 water machine. 



Soundings of great precision can be taken with the magneto-striction machine. It has 

 a normal range of 0-230 fathoms, but by alteration of the motor speed it can be made 

 to record from o to 230 feet over the same depth of scale. The oceanic machine and 

 the 'Acadia' recorder also are more accurate at great depths than the Lucas machine. 



The oceanic machine is in continual use, and soundings can be obtained from the 

 greatest depths at all times when the weather permits. In rough weather the surge of 

 water on the hull of the ship produces "water noises" which swamp the echo, but 

 except in severe gales, when it is possible that the aeration of the surface water impedes 

 the sound waves, a sounding can always be obtained if the ship is hove-to for a few 

 minutes. The greatest depth so far measured with this machine in the ' Discovery II ' is 

 7882 m.i As long as the ship is at sea routine soundings are taken at hourly or half- 

 hourly intervals, and if rapid changes in depth are observed the intervals are reduced 

 accordingly. The recorders are used mainly in coastal survey work, in shallow water or 

 in any region where a clear delineation of the bottom contour is desired, as when 

 the ship is passing over shoals, deep troughs or continental slopes ; the demarcation of 

 the Scotia Arc would have been quite impossible by ordinary sounding methods in the 

 time at the disposal of the 'Discovery II'. The value of the recorder in uncharted 

 coastal regions need hardly be emphasized. 



Certain practical difficulties in the running of the machines have been encountered 

 from time to time, but in the end all have been successfully overcome. The first and 

 most serious difficulty met with was caused by the ship being mainly in water of which 

 the temperature rarely exceeded 3°C. and was more usually at 0° C. or below. This 

 caused the hot moist air from the compressor to condense on its sudden expansion in 

 the deep-sea hammer, thus forming a sticky emulsion with the oil used to lubricate 

 the piston. After quite short periods of running the piston used to jam and this necessi- 

 tated the removal of the head from the hammer and the cleaning of both piston and 

 cylinder, which at times was most inconvenient and caused serious delay. On the 

 return of the ship to England in 193 1 a steam coil was fitted round the hammer and this 

 minimized the trouble considerably. In 1934, however, it was found necessary to fit a 

 specially designed trap containing calcium chloride to dry the compressed air before it 

 entered the transmitter. This made a considerable improvement, as it was then possible 

 to get twelve to fourteen hours' continuous running in conjunction with the recorder. 



The shallow-water hydrophone, which was enclosed in a small tank of water, was 

 also seriously aftected by the cold water, as the formation of ice in this tank forced the 

 stalloy base of the microphone completely out of alignment. This difficulty was over- 

 come by mixing glycerine with the water. The deep-water hydrophone, which is 

 exposed to the sea, is not aff'ected by the cold water. 



1 Herdman, H. F. P., Report on soundings taken during the Discovery Investigations, etc. 



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