6 
CORRECTIONS FOR QUICK TARGETS. 
In order to use a drum of this kind, it is necessary to know the 
velocity of the ship and the minimum value of her range on her present 
course, i.e., the distance G A 3 of Fig, 1, 
a 10 12 14 16 IB 20 6 10 12 /4 16 12 20 0 10 12 14 16 18 20 
If the speed is not known, it can be very fairly determined as fol¬ 
lows, and, as the telescope of the range-finding instrument can generally 
bear on a target before any gun can, time need not be sacrificed. Thus, 
suppose the ship to first come into the field of the telescope at P (Fig. 
1), let her be carefully followed, and let the time, in which the range 
changes by 100 yards, be taken once or twice. As the angle GP A 
can never, in the supposed case, exceed 15° or so, the velocity estimated 
