CHALLENGER 



of the hydrographic surveyor's work. The traditional method of 

 sounding is by leadUne, that is lowering a weight to the sea-bed 

 on the end of a line marked in fathoms and feet. This would be 

 easy enough if the ship could be stopped for every sounding, but 

 as this is not practicable, the lead must be heaved far ahead so 

 that it reaches the sea-bed at the moment the leadsman passes 

 over it. An ingenious mechanical apparatus for accomplishing 

 this had been devised many years previously by Admiral 

 Somerville, a well-known surveyor, and this gear was fitted 

 in Challenger as standard equipment. By means of a steam 

 winch operated aft the lead was hauled forward after each 

 sounding, the leadsman being stationed near the winch on the 

 quarterdeck. A trailing counterweight kept the leadline nicely 

 taut. 



In 1 93 1 a shallow water echo sounding set was being introduced 

 in surveying ships. This machine emits a sound toward the sea-bed 

 and measures the time taken for the echo to return to the ship. 

 These early machines employed a formidable hammer in the 

 bottom of the ship which banged out a signal at frequent and 

 regular intervals; the operator on the bridge then ran a drum 

 marked with the various depths backwards and forwards until he 

 received the loudest returning signal in his earphones, when 

 he read off and reported the sounding showing on the dial. 



In Challenger, in addition to shallow water sets, there was 

 fitted a new deep echo sounding machine designed to take 

 soundings in the deeper ocean. The first thing to be done before 

 the season started was to carry out trials on the echo sounding 

 sets and to do this the ship would have to proceed to seaward of 

 the continental shelf to find deep enough water for trying out 

 the deep sounder. 



The continents are surrounded by a comparatively shallow and 

 gently sloping shelf formed over millions of years by deposits 

 carried seawards by water flowing off the land. At a depth of 

 about loo fathoms the edge of this shelf falls rapidly away to 

 depths of 2000 fathoms or more, and the true oceans have been 

 reached. The whole of the English Channel lies within the con- 

 tinental shelf, and to reach the edge of the shelf the ship had to 

 steam south-westward from Plymouth for a distance of 200 miles 

 or more. 



