SURVEYING THE EOUTB. 3 



while ship is going ahead, although the actual time of recovery- 

 is greater owing to surface friction. The friction on 1,000 

 fathoms of wire drawn through the water at the rate of 100 

 fathoms per minute is about equivalent to 251b. pull on the 

 wire. 



Thus, if the ship is going at a speed of nine knots per hour 

 (150 fathoms per minute), the strain due to this motion alone 

 will be 751b. for 2,000 fathoms of wire out. The weight of this 



Fig. 1. — Sherlock's Detacher for Sinkers. 



length of wire in water is 241b., which, together with a 351b. 

 sinker, makes the total strain 1341b. The maximum strain the 

 wire will stand is 2301b. to 2401b., and therefore hauling-in has 

 to be done slowly at first, or the ship remains hove to until the 

 first few hundred fathoms are reeled in. As the length out- 

 board is reduced the speed of hauling-in can be increased with 

 safety. Of course, when the weight is detached at the bottom, 

 the reeling-in can be done at a higher speed from the first. A 

 simple form of detacher for sinkers is shown in Fig. 1, the 



2 



