SOUNDING MACHINES 209 



oceanic and the shallow-water machines differ considerably, but the principle is the 

 same : a blow from a hammer operating on the ship's hull produces a sound wave which 

 is reflected from the bottom and picked up by a very sensitive hydrophone, and from 

 the time interval between the transmission and the reception of the sound the depth of 

 water can be calculated. Observations made at various times on the salinity and tempera- 

 ture of sea water have been used for the calculation of the velocity of sound in that 

 medium. 



Shallozc-water echo-sowiding machine. In this machine the hammer is electrically 

 operated and the speed of the transmitter switch is automatically adjusted by means of 

 a governor and moving resistance to ensure approximately 180 blows per minute. The 

 receiving gear, which is situated on the bridge, houses the transmitter switch and the 

 telephone gear; from it the depth can be read off directly in metres, the corrections 

 for salinity, temperature and pressure being negligible up to the extreme range of this 

 machine — 220 m. In practice this machine was seldom used for depths beyond 160 m. ; 

 for it was found more convenient to change over to the oceanic machine at this point in 

 order to avoid any doubt as to whether soundings were shoaling or increasing when the 

 limit of 220 m. was reached. 



All the ships were fitted with the listening type, as distinct from the recording type, of 

 shallow-water echo-sounding machine, but only in the 'William Scoresby' and the 

 ' Discovery II ' was it extensively used. In the former ship it proved very useful during 

 the trawling survey of the Falkland Islands in 1927, as by taking soundings every few 

 minutes when trawling, the length of warp paid out could be adjusted to any consider- 

 able change in depth. A later pattern of the same type of machine was fitted in the 

 'Discovery 11', which gave considerably better results than the earlier model, though 

 when in constant use for long periods, a considerable amount of attention was required 

 to keep it in adjustment. Apart from some slight delays caused by minor mechanical 

 defects, which were easily made good on board, and the leakage of water into the hydro- 

 phone through faulty assembling, the machine was in continuous use for weeks at a time 

 over a period of fifteen months and gave excellent results. 



The hammer was fitted under the stokehold on the port side of the double bottom between the 

 oil-fuel tanks and the stokehold fresh-water tanks. Ahernative positions were provided for the 

 hydrophone, one at the after end of the double-bottom fresh-water tank on the starboard side of the 

 fore hold and the other about 6 ft. farther aft in the double-bottom between the fresh-water tank 

 and the starboard oil-fuel tank. After the hydrophone had been flooded in the fresh-water tank, 

 it was shifted to the alternative position, but although there was still a tendency towards several 

 false echoes, it worked quite satisfactorily. Neither position was entirely free from false echoes at 

 certain points on the scale, but with practice the operator could easily distinguish the true echo from 

 the false by the difference in note, and even when the true echo coincided on the scale with the false 

 the note was unmistakable. When the ' Discovery II ' was re-commissioned in 193 1, the hydrophone 

 was replaced in the fresh-water tank, where, with the exception of the recurrence of a trouble 

 previously experienced, it has worked quite satisfactorily. 



When the hydrophone was being overhauled in February 193 1, while endeavouring to trace the 

 cause of very faint echoes, it was found that the stalloy plate which forms the base of the hydrophone 

 and on which the micro-button is mounted had been bent inwards towards the centre, thereby 



