ARTILLERY IN COAST DEFENCE. 
469 
verifying the length of fuze, two or more shells are blind or burst on 
graze, and all on the same side of ihe object; these shots may then 
be considered as if they formed part of the verifying series, and the 
elevation altered accordingly. 
The possibility of obtaining good effect with this system of fire 
control depends on correct observation of the result of each round, as, 
indeed, is the case with any system ; the most difficult case occurs 
when the object is stationary and the smoke drifts straight from the 
enemy towards the guns, or vice versa-; laying would then have to be 
done either by auxiliary marks, or by using clinometer for elevation 
and marking the racer for direction (if the guns are not fitted with 
index plates and graduated arcs) ; and the observation could be assisted, 
where practicable, by flanking parties, or the ranging may be con¬ 
ducted by salvos instead of by single shots. 
The second system of fire control is that used when an instrument 
is available which can give the ranges continuously of a moving object 
on the water; such an instrument is the depression range-finder. 
This instrument works with a vertical base and, therefore, requires 
to be at a certain height above the water ; it would usually be placed 
on a flank of the battery, sufficiently far away for the levelling not 
to be affected by the shock of discharge. Where the site is suitable 
it is placed behind and above the guns. When flank positions only 
are available, two stations should be provided, one on each flank, and 
that one occupied which is to windward, and is, therefore, not inter¬ 
fered with by smoke. In any case the instrument will be at some 
distance from the guns, and consequently the raDge as given by the 
instrument will not be the true range from the guns in all cases. The 
distance from the centre of a group of guns to the range-finder is 
called the “ displacement ” of that group ; the difference between 
the range from the group to an object, and the range from the depres¬ 
sion range-finder to the same object, is called the “ group difference.” 
This difference is a variable quantity; at 0 on diagram the range is 
the same from the group and from the range-finder, and the group 
difference would be zero ; as the object moves from 0 to A, the differ¬ 
ence is constantly increasing and is a + quantity; as it moves from 0 
to B it is also constantly increasing, and is a — quantity. Thus the 
differences vary with the different angles of training of the guns. 
The difference also varies with the range. When the object moves 
towards the group from 0 to C } the training has, of course, not altered; 
