APPENDIX I 97 



the amplifier due to a defective rectifier. Since spares were not readily available it was necessary to 

 return the amplifier to the makers for repairs, and for some months only the ' listening ' gear could be 

 used. The only other serious trouble met with was the failure of the H.T. dry batteries to maintain 

 a correct voltage. During survey work it was necessary to keep open the door to the echo cabinet and 

 chart room, and the deterioration of the batteries is attributable to unavoidable exposure to much 

 cold and damp weather. Some deterioration, however, also took place in new batteries stored on 

 board. Eventually it was found best to replace the dry batteries with accumulators, and these were 

 charged up every night during periods of surveying. 



The old ' listening' type receiving gear needed little attention, but the transmitter (or hammer) gave 

 considerable trouble, and eventually broke down in December 1934, during a survey of the South 

 Shetland Islands. Temporary repairs were carried out by the ship's engineering staflF, but the hammer 

 was found to be worn out, and a new one was installed on the existing base plate some months later. 

 This unit worked on the whole satisfactorily until 1939, and no replacements were required other than 

 a new balanced head in 1938. The deep-sea hydrophone gave a certain amount of trouble in 1935 but 

 a new unit was fitted at sea, through the sluice-valve provided, and from then until 1939 no further 

 serious trouble was experienced. When the old unit was removed in 1935 it was found that the probable 

 cause of the trouble was damage to the face by careless chipping of the adjacent hull plating in dr>'-dock. 

 Precautions were then taken at other occasions of dry-docking to prevent a recurrence of such damage. 



SHALLOW-WATER ECHO-SOUNDING SET 

 As stated on p. 43 a magnetostriction echo-sounding set, with a Mark XII D recorder, was fitted 

 in the 'Discovery II'. The tanks containing the oscillators forming the transmitter and receiver were 

 fitted to port and starboard of the centre line of the ship in No. 2 double-bottom freshwater tank. 

 This position was 55 ft. from the fore perpendicular, and about 6 ft. forward of the position originally 

 used for the Admiralty Pattern 751 sonic type receiver. The bridge instruments were fitted in the echo 

 cabinet on the port side of the chart-house ; the recorder, amplifier and contactor unit occupying no 

 more space than that originally provided for the sonic receiving gear. H.T. and L.T. supply were 

 arranged from the accumulators already in use for the deep-water set. No trouble was ever experienced 

 with the transmitter or receiver, and when the author of this report had occasion to visit the ship during 

 the course of his war-time duties with the Admiralty it was found that after 5 1 years immersion in 

 a double-bottom water tank the transmitter and receiver were still functioning correctly. The recorder 

 gave little mechanical or electrical trouble except for some difficulty with the change-speed gear box. 

 Finally, to avoid their frequent renewal the train of gears for the high speed was removed and the 

 machine solely used for sounding in fathoms. A serious defect, however, was the rapid fading of the 

 records. In contrast to the 'Acadia' records which, after storage for as much as 14 years, have lost 

 nothing of their clarity, the records from the Mark XII D recorder were liable to fade within a matter 

 of hours, and it thus became essential to ink in the pertinent information as soon as possible. Some 

 few records which were made in 1936, and stored unedited, are now quite illegible. The fading of 

 the Mark XII D records (while the 'Acadia' records remain more or less permanent) is probably 

 attributable to differences in the chemical impregnation and method of wetting in the two instances, 

 the Mark XII D rolls being pre-wetted and supplied in sealed tins. Fading troubles experienced with 

 early pre-wetted paper have since been largely overcome. 



The maximum depths for which this set was designed were not achieved by us in practice and it is 

 possible that the thickness of hull plating of the ' Discovery II ' had a serious effect in diminishing the 

 strength in both the transmission and reception of the soundwaves. 



