THE IMPENETRABLE SEA 



down, tripled at sixty-six feet, quadrupled at ninety-nine 

 feet, and so on. We must accept the evidence of modern 

 times, that naked divers can descend without apparatus 

 to over a hundred feet, steeling themselves against the 

 tremendous pressure at such depths. 



Men had no knowledge of the real depths of the ocean 

 until quite recently. Not until 1504 were soundings — 

 made in shallow waters — first shown on a map: one 

 drawn by Juan de la Costa. Deeper soundings were 

 shown on Mercator's maps in 1585; after which it be- 

 came the practice, increasingly, to incorporate them in 

 most maps. Captain Cook, in his voyagings round the 

 world, was the first to employ them systematically — 

 using pieces of lead or cannon-balls. Captain Ross followed 

 his example by using them in his Antarctic explorations. 



The last use of hempen cord for soundings on an ex- 

 tensive scale was on the famous Challenger voyage of 

 1872-76. This Admiralty ship, in her explorations of the 

 under waters in many parts of the world, often trailed 

 her sounding line — sometimes as much as eight miles of 

 it dragging behind her. 



Lord Kelvin, the noted British mathematician and 

 physicist devised the modern sounding apparatus in 

 1872, using piano-wire, which superseded the use of rope 

 and enabled mankind to plumb greater depths more 

 efficiently. Less than one-twentieth of an inch in diameter, 

 the wire has tremendous tensile strength, more than ten 

 times that of the finest hemp. 



Sounding lines are supplementary aids to both classes 

 of divers — fishmen and "sphere passengers". They have 

 two purposes. They register depths and they also disclose 

 the type of life below, and the nature of sediments, etc., 

 samples of which they can bring to the surface for inspec- 

 tion and analysis. That is why fathometers (echo sound- 

 ing devices) can never entirely replace the use of the line. 



Countless centuries elapsed between primitive man's 

 use of his fishing line to measure the depth of the shore 



168 



