CHAP. XVIII.] 



SUBMARINE SENTRY 



407 



time, at any required vertical depth down to 40 fathoms with 

 the red kite, and 30 fathoms with the black kite. Should it 

 strike the bottom, through the water shalloAving to less than the 

 set depth, it will at once free itself and rise to the surface, 

 simultaneously sounding an alarm on board, thus giving instant 

 warning. 



The vertical depth at which the sentry sets itself when a 

 given length of wire is paid out is not changed by any varia- 

 tion of speed between 5 and 13 knots, and is read off on 



Fig. 83. 



the graduated dial-plate on the winch. One set of graduations 

 on the dial indicates the amount of wire out ; the other two 

 sets refer to the red and black kites respectively, and show the 

 depth at which the sentry is towing. 



By this machine single soundings down to 40 fathoms can be 

 taken at any time while the sliip is under way, the sentry being 

 let down slowly. The gong wiU indicate when the bottom is 

 touched, and the dial corresponding to the kite used will show 



