SOUNDING MACHINES 211 



Matthews of the Hydrographic Department of the Admirahy, in which the oceans of the 

 world are divided up, according to the sahnity and temperature of the water, into areas 

 in which the same corrections can be apphed. Instructions are also given for working 

 out new tables for any area from the hydrological observations. In this way tables were 

 worked out in the ' Discovery II ' for the South Georgia and Scotia Sea areas from the 

 average of a large number of salinity and temperature observations, but the results were 

 found to vary very little from those already determined by Matthews. A special table 

 (see p. 236) extending to 7250 m. was compiled from observations in the South Sand- 

 wich Deep and was used for the correction of all soundings in that area. 



The proved effective range of this oceanic machine was from approximately 90 to 

 8100 m., and the maximum depth at which soundings have been obtained with the ship 

 under way was just over 5000 m. The limiting factors in obtaining soundings under way 

 are water noises in the hydrophone caused by wind and sea — factors which may vary 

 considerably. No definite statement can be made as to the best method of obtaining a 

 sounding in bad weather; only actual experience of a ship's behaviour in different 

 weather conditions will enable the operator to find the best way of manoeuvring the ship 

 in order to avoid excessive water noises in the hydrophone. In the area where we were 

 working heavy swells and high seas are unusually prevalent, but soundings up to 500 m. 

 could nearly always be obtained. In deeper water it was sometimes necessary to 

 manoeuvre the ship with regard to wind and sea. In our experience the time taken to 

 complete a 5500 m. sounding under average conditions rarely exceeded 4 min., and even 

 in very bad weather it was unusual to stop for more than 7 or 8 min. Only once during 

 our intensive sounding programme was the weather sufficiently bad to prevent us obtain- 

 ing any soundings and then conditions improved within a few hours. 



The present instrument in the ' Discovery II ' has the hammer fitted in the well between the engine- 

 room and stokehold double-bottom tanks, on the port side. The hydrophone was fitted in the star- 

 board oil-fuel well just beside the alternative position for the shallow-water hydrophone. The ham- 

 mer is mounted on a water chamber in a manner similar to the arrangement of the shallow-water 

 gear, but the base of this chamber consists of a special circular steel casting which is let into one of 

 the bottom plates and consequently exposed to the sea. The hydrophone is enclosed in a sluice 

 valve and can be lowered into the sea or withdrawn into the hull at will. Although the recommended 

 position of the hydrophone for sounding was to lower it well clear of the hull we found that the best 

 position in the ' Discovery II ' was with the stalloy plate of the hydrophone flush with the hull, and 

 we had no trouble in obtaining our maximum sounding of 8102 m. with it in this position. 



During the first season, 1929-30, the machine was not altogether satisfactory mechanically, and in 

 the middle of the South Sandwich Islands survey in March 1930, the hammer cylinder sustained 

 a fracture and broke in half. Fortunately it was a clean break, and our Chief Engineer was able to 

 devise a form of collar to hold the two parts together, enabling us to have the machine running again 

 in a day or so. This fracture, however, necessitated a new hammer being sent out from England and 

 fitted in Simonstown in July 1930. After this the machine worked extremely well, though like the 

 shallow-water instrument it required constant attention during periods of continuous use. Much of 

 its efficiency was due to the care and trouble expended on it by R. W. Mackay, the ship's technical 

 assistant, and the engineering staff. One of their problems was the refusal of the piston of the ham- 

 mer to move in very cold weather or its sudden stopping after it had been running for a few minutes. 

 This was found to be due to the compressed air which comes over from the compressor quite hot. 



