PLATE XXXI 



Fig. I . Example of the elimination of the transmission band, by means 

 of the ' cut-out ' switch, to allow the echo to be recorded at depths 

 varying between 990 and laiofm. (1810 and 2213 m.). In this 

 instance a fault in the transmission, known as ' double-knocking ', 

 also is seen recorded. This trouble normally was caused by the lack 

 of sufficient 'back-pressure' through the reducing valve, which 

 prevented the sharp return of the hammer to the magnetic valve. 

 It could also be caused by the partial failure of the magnetic air 

 valve in the head of the hammer, when a second blow would be 

 struck before the supply of H.P. air was properly cut off. Speed of 

 ship, ID knots. Vertical scale, x io-8. (Part of Record XXXVI/37.) 



Fig. 2. Typical record from M/S. XII type shallow water recorder. 

 Record made during survey of NW coast of Coronation Island, 

 South Orkneys, in 1937. Pencil markings were inserted as soon as 

 the record was dry, in order to facilitate the plotting of the soundings 

 should the record fade. Time scale automatically marked every 

 3 minutes. Speed of ship, 7 knots. Vertical scale, x 7-56. (Part of 

 Record XXVIII/37.) See Text-fig. 3(c), p. 46. 



