SOUNDINGS TAKEN DURING THE DISCOVERY INVESTIGATIONS, 1932-1939 77 



southern ridge was 305 m. (167 fm.) in 61° 36' S, 53° 35' W. In tiie Bransfield Strait many new 

 soundings have made Uttle difference in the general outhnes of the bottom, but in our present bathy- 

 metric chart (PI. XXIV) we have introduced a contour at 1500 m. (820 fm.) in order to show more 

 clearly the deeper parts of the basin south of King George Island and south of Gibbs Island. It is 

 now apparent that the width of the trough south of Bridgeman Island is much less than we had hitherto 

 supposed, but there is no doubt of the continuity of the basin at depths between 1000 and 1500 m. 



On the northern side of the Strait it will now be seen that the 250 m. (137 fm.) contour enters all the 

 bays and straits on the southern side of the South Shetland Islands; in two instances, depths of more 

 than 500 m. (273 fm.) occur just inside the entrance to a strait. Short slopes of considerable magnitude 

 exist off this side of the islands, and details of some of these will be given when we consider the 

 application of our soundings to the various surveys carried out by the 'Discovery II'. 



Unfortunately we still lack evidence from soundings whether Bridgeman Island is connected with 

 King George Island (see Herdman, 1932, p. 230). Such further soundings as have been obtained 

 add but little to our previous knowledge, but in view of the opinion given by Tyrrell (1945), that from 

 its geological structure Bridgeman Island may be considered to be an island of the South Shetland 

 Group, we have now shown the 500 m. (273 fm.) contour as running south of this island. 



SOUNDINGS DURING HYDROGRAPHICAL SURVEYS 



The 'Discovery II' has been engaged mainly in oceanographical work, and little time could be 

 allotted to hydrographic survey. Usually about a month in each commission was available for 

 surveying in the Dependencies of the Falkland Islands, and it was often possible to spend a few days 

 in such work elsewhere when opportunities arose. In the Dependencies it was decided that the 

 method of ' running survey ' would be most suitable having regard to the time available, the imperfection 

 of existing charts, and the great extent of the coastline in the Dependencies. By this method a skilled 

 navigator can obtain excellent results, and much ground can be covered in a short time. 



The immense value of echo sounding during this type of work was clearly demonstrated during the 

 running survey of the South Sandwich Islands by the 'Discovery IT in 1930. A full description of 

 this survey has already been given by Kemp & Nelson (193 1). In subsequent years the ' Discovery II ' 

 carried out a complete running survey of the South Orkney Islands and made very considerable 

 progress with a survey of the South Shetland Islands by the same method — work which could not 

 have been attempted without continuous echo soundings, on account of the numerous off-shore dangers 

 common to nearly all islands in this part of the world. 



At the time of the South Sandwich and the main South Orkneys surveys the 'Discover}^ 11' was 

 fitted only with 'listening' type echo-sounding equipment, with which the maintenance of a proper 

 sounding routine entailed long hours of tiring work. Although some preliminary work had been done 

 in 1929-30 the main South Shetlands Survey was not begun until December 1934, by which time the 

 automatic recorder had been fitted to the deep-water set. Shallow soundings during this season still 

 required the use of the ' listening ' gear, but its use could be restricted to the minimum necessary for 

 entering or leaving an anchorage or when shoal water was met with on passage round the islands. 

 When the survey of the South Shetlands was continued in December 1936 both deep and shallow 

 echo-sounding sets were fully automatic. 



Since the fitting of the deep-water recorder other running surveys carried out by the ' Discovery II ' 

 include work on the three northernmost of the Balleny Islands (in 1936 and 1938), the examination of 

 the neighbourhood of Tristan da Cunha and various passages through the lesser known channels of 

 Magellan Strait. Many little known or imperfectly surveyed Antarctic and sub-Antarctic islands were 



