14 THE OCEAN 



100-fathoms mark went over the ship's side 

 was carefully noted. When there was a sudden 

 change in the rate at which the line ran out, 

 it was known that the bottom had been 

 reached, and the depth was then recorded. 

 Deep soundings, even in 4000 fathoms, care- 

 fully taken in this way are believed to be 

 correct to within 25 fathoms. 



A marked advance was made when the late 

 Lord Kelvin introduced piano wire for 

 sounding purposes. In the Lucas sounding- 

 machine (see Plate I.), which at the present 

 time is the one most in use, the moment when 

 the weight strikes the bottom is automatically 

 indicated on the machine, and the running- 

 out of the wire is stopped. Soundings taken 

 with this machine are believed to be correct 

 to within one fathom, even in deep water. 

 A sounding in 2000 fathoms now requires about 

 45 minutes from start to finish, z.e., from the 

 time the lead is let go till it is hauled on 

 deck with a sample of the bottom-deposit, 

 and one in 3000 fathoms about 75 minutes. 

 The " Challenger's " deepest sounding in 

 4475 fathoms occupied about 2 J hours. The 

 single-strand piano wire used for sounding is 

 only about one-twentieth of an inch in 

 diameter; a three-strand wire of the same 

 diameter is also used, while a seven-strand 

 wire with a diameter of one-six,teenth of an 



