2IO DISCOVERY REPORTS 



throwing the button out of ahgnment. This was no doubt due to the freezing of the fresh water in the 

 chamber under the hammer, as it was against the hull and the temperature of the sea was about 

 — 1-5° C. A new stalloy plate was fitted, glycerine added to the fresh water in the chamber to 

 prevent freezing and no further trouble was experienced till 1932. When the hydrophone was replaced 

 in the fresh-water tank in September 1 93 1 , glycerine was again added, but there appears to have been 

 a small leak in the joint where the water chamber butts on the hull. This joint consists of a dermatite 

 ring, and the hydrophone and water chamber are braced down on it by a strongback clamp. When 

 the ' Discovery II ' was again in water of — i-o° C, the echoes, as before, became very faint, and on 

 examination of the hydrophone it was found that the stalloy plate was again bent inwards and that 

 all the glycerine had diffused into the fresh-water tank through the fault in the joint. This water 

 chamber, which is fitted under both hammer and hydrophone in the shallow-water machine, is 

 of great importance, as by pumping water into it till a pressure of about 20 lb. to the square inch is 

 reached, the chamber acts much in the same way as a lens does with rays of light : it concentrates 

 the sound waves and prevents the scattering of sound noticeable in the earlier models. 



Oceanic echo-soimding tnachmes. The deep-water echo-sounding machine fitted in the 

 'Discovery' in 1925-6 was an Admiralty experimental model. Owing to the difficulty 

 of fitting this machine in a wooden ship, the hammer had to be slung outboard from a 

 davit, and it was found that the rolling of the ship caused excessive water noises in the 

 hydrophone. This, together with mechanical troubles due to the unusual position of 

 the hammer, prevented the machine from working properly, and it was removed in 1926. 

 The deep-water echo-sounding machine at present fitted in the 'Discovery II' is of 

 the listening type and has a much more powerful hammer than the shallow-water in- 

 strument. The hammer is operated by compressed air, and controlled electrically from 

 the receiving gear in the chartroom. Impulses can be sent out at three different rates, 

 one, nine, and ten in 11-25 ^ec, which is the time taken to complete one circuit of the 

 transmitter switch. Here, as in the shallow-water machine, the speed is controlled by 

 a governor in conjunction with a resistance. In the ' Discovery II ', however, this control 

 was not always satisfactory when the machine was running for long periods, and it was 

 necessary to check the speed of the transmitter switch at frequent intervals and correct 

 the soundings accordingly. It was found after several experiments that the temperature 

 inside the case of the receiving gear rose in 20 min. from 11-5" to 15° C, and that the 

 time for a revolution of the transmitter switch fell from 11 -22 to 10-93 sec. If the 

 machine is only running for 3 or 4 min. at a time the error from this cause is, however, prac- 

 tically negligible, and the bulk of the oceanic soundings were taken under these conditions. 



The impulse rates are selected at will by the operator, and to obtain a sounding it is 

 usual to get the approximate time interval between hammer blow and echo on the one- 

 impulse circuit and then tune in more accurately on the nine- or ten-impulse circuit. The 

 reading obtained is in seconds and gives the time taken by the sound waves for their 

 double journey, i.e. from the hammer to the bottom and back to the hydrophone. A 

 skilled operator can easily determine the time correctly to o-oi sec, which roughly 

 corresponds to an error of 7 m., and the depth is quickly calculated after applying the 

 necessary corrections. A book of correction tables^ has been calculated by Mr D. J. 



1 Tables of the velocity of sound in pure water and sea ivater for use in echo-sounding and sound-ranging, 

 H.D. 282, H.M. Stationery Office, London (1927). 



