CONCENTRATING THE EIRE OE A GROUP OE GUNS, ETC. 311 
shown has a socket sliding on the bar, and is clamped by a milled head 
screw at the top; the shape must be such that, when the arrow head 
points to zero, the point of the pointer must be under the centre of the 
socket. 
The bar must point to the left for the right hand gun, and to the 
right for the left gun, as the correction has to be given outwards; thus 
they are not interchangeable, but the set screw on the front prevents 
the wrong bar being used. 
The bars experimented with were made of hard wood with paper 
scales varnished; the face of the bar being recessed to prevent wear¬ 
ing away of the paper ; metal would of course be better. 
The pointer can be cut out of thick sheet metal and rivetted or 
soldered to a socket. 
The method of giving the correction is as follows 
The predicted range and training is read out by the dial number; 
for instance, “Range 2400 yards, 50 degrees training, Lay ;” the Group 
Officer (or dial number) puts the scale of the diagram over this training 
on the arc, notes between which two of the circles the range lies, and 
gives out the correction marked there; “10 minutes deflection.” 
The gun-layers set the arrows to that figure on the bars and train the 
guns till the pointer is over the training read out. Thus all the guns 
will be laid at the same training on the graduated arc, viz. at 50 degrees, 
but the lines of fire will in reality pass through 50° 10' for No. 1, 50° 
for No. 2, and 49° 50' for No. 3; and these lines will intersect at a 
range of 2400 yards. 
Two cases which may occur require further consideration. 
First, when a group consists of more than three guns; for instance, 
suppose there are four guns in a line at equal distances apart, the two 
outer guns will be three times as far from the centre of the group as 
the inner guns. 
Now we found that the correction varied directly as d, that is as the 
distance from the centre ; therefore, in the case supposed, the correction 
for the inner guns will be one-third that required for the outer guns. 
If we make the graduations on the deflection bars of the inner guns 
one-third as large as those on the bars of the outer guns, only one 
diagram and one order for deflection will be necessary. For instance, 
if the correction to be given by the diagram is 10 minutes, that deflec¬ 
tion is ordered, all the gun-layers set their pointers to 10; but while 
at 1 and 4 guns 10 minutes are really given, at 2 and 3 guns only 3^- 
minutes are actually put on. 
This principle can be extended to include any case of guns which are 
not equi-distant from the centre of the group; one diagram will suffice 
for the whole group, provided the graduations on the deflection bars 
are made proportional to the distances of the guns from the centre; the 
diagram being made out, preferably, for the gun which is farthest 
from the centre. 
When guns are mounted on different pivots, the length of a degree 
on the arcs will not be the same; care must, of course, be taken to 
graduate the bars according to the length of the radius of the arc; 
also reducing proportionately, if necessary, for the above reasons. 
