310 
CONCENTRATING THE PIKE OP A GROUP OF GUNS, ETC. 
centre B. A series of circles is described whose centres are all on BD 
and all touching ABC at B, and radii successively = 4. • . ^ i. —— 7 
6 ; J sm 2i sin 7V 
&c., on any convenient scale of yards. From B draw the line BE, 
making an angle with ABC equal to the angle between the line of fire 
and the line of pivots, on BE set off BF to represent the range, on the 
same scale as was used to construct the circles. Then if F is on the 
circumference of the circle marked 124 , the following relation holds 
BF = -.-i* sin ABB 
sm 12 4 
that is 12^ minutes is the correction necessary for that range and angle. 
If F falls between the 74 and 124 circles it is sufficiently near to take 
10 minutes as the correction required. If now we graduate CDEA in 
degrees (from left to right), and number the graduations, so that the 
reading at D corresponds to that on the gun arc when the line of fire 
is at right angles to the line of pivots, the reading at E will correspond 
to the training of the gun ; and if, instead of drawing a line, we have 
a scale pivotted at B having a straight edge graduated in yards, wo 
have all that is required for a practical diagram. All that has to bo 
done, then, to make such a diagram, is to ascertain d in yards, and 
the reading of the pointer on the graduated arc when the line of 
fire is at right angles to the line of pivots; this can be done either from 
a plan, or by means of a sextant or compass. The diagram is then 
constructed as described. 
Such a diagram for a group of guns, 10 yards apart, and having arcs 
graduated from 290° to 80° is shown at diagram A. 
The diagram should be drawn out on paper, mounted on a board, and 
varnished; the scale may be of sheet metal with a paper scale, also 
varnished. 
The deflection bar and movable pointer are of the form shown in 
diagram B; a short cylinder fits into the lower socket for the bolt 
Diagram B. 
pointer; a set-screw on its front fits into the slot in the socket, and 
prevents it from turning round ; a bar is fixed at right angles to the 
cylinder and is graduated on its rear face in minutes; the size of these 
graduations depends on the radius of the arc, and can be ascertained 
by measuring the length of a degree of the arc. A pointer of the form 
