408 HYDROGRAPHICAL SURVEYING [chap, xviii. 



at once the vertical depth at the place where the sentry struck. 

 The winch is secured to the deck a short distance from the stern ; 

 the towing wire passes from the drum under a roller-fairlead at 

 the foot of the winch, thence through an iron block with sheaves 

 of large diameter, suspended from a short davit on the stern 

 rail, and secured to the sling of the sentry. 



The dial being set to zero, with the sentry at the water's 

 edge, the ship's speed is reduced to 8 or 9 knots, and the 

 wire paid out freely until the kite is fairly in the water, when 

 the brake should be applied steadily and without jerking, 

 veering slowly until the required depth is attained, when the 

 pawl is put on the ratchet wheel and the speed increased to 

 12 knots, if desired, when using the black kite, or 10 knots 

 with the red kite. 



The kite in its position when being towed is indicated in 

 Fig. 83. The point of the catch C, passing through a thimble M 

 in the short leg of the sling, is slipped into the hole at the top 

 of the trigger T, which is hinged at K and kept in its place by 

 the spring S attached to the hook H. On the trigger striking the 

 bottom the catch is released, the short leg of the sling slips off, 

 and the sentry, which then rises to the surface, is left towing 

 by the long leg. 



The wancli is fitted with two handles for heaving in the wire ; 

 one gives great power and slow speed, and the other, acting on 

 the drum spindle direct, winds in quickly. The wire supplied 

 with the machine has a steady breaking strain of about 1,600 

 pounds. Using the black kite at a speed of 7 knots, the 

 strain on the wire is about 150 pounds, and at 10 knots about 

 300 pounds. The red kite increases the strain largely. 



