SURVEYING THE ROUTE. 



35 



shown in Fig. 24. The illustration represents the position 

 assumed by the "kite " when towed by the ship. The pecu- 

 liarity of its action is that it causes the tow line (of steel wire) 

 to take up a concave downward curve, as in Fig. 25, and 

 that the form of this curve remains practically unchanged for 

 any variation in ship's speed between 5 and 1 3 knots. Conse- 



FiG. 26. — Sentry overturned. 



quently, the apparatus stays at any fixed depth in the water 

 (at which it is originally set), notwithstanding that the ship 

 may alter her speed within the above limits. The moment 

 the lower end of the striker touches ground the upper end is 

 forced away and releases the pin. The forward sling then 

 slips oflf the pin, and the "kite" is towed by the hinder 

 sling only. Under these circumstances it no longer has a 



Fig. 27.— Sentry Winch. 



" kite " action, and simply turns over and floats to the 

 surface like any ordinary piece of wood (Fig. 26). The 

 release of the strain on the towing wire resulting from this 

 action causes a bell to ring, announcing to those on board that 

 they have only so many fathoms under them. The apparatus, 

 therefore, keeps a continuous under-water lookout and auto- 



d2 



