THE ROYAL ART ELLERY INSTITUTION. 
421 
In the case of a uniform spiral, the copying bar is straight edged, and the 
only thing necessary is to set it at the proper angle. 
The angle is easily calculated thus :— 
c 
Take AB equal length of rifling due to one turn, and BC at right angles 
to it and equal length of one turn, or the circumference of the bore, then 
AC will be length of the total spiral, and 0 the angle of the rifling and of 
the tangent bar. 
^_ BC _ 7r x calibre __ tt 
AB number of calibres x calibre number of calibres 
Tor example, take the 7 -inch gun, whose spiral is one turn in 35 calibres, 
then 
tan $ = £- = • 0897; 
35 
6 = 5° 4' nearly. 
Did we require the actual amount of twist x in the bore, it is easily 
obtained from the proportion 
AB : BC :: l : x, 
l being the length of the rifled part of the gun, while we can form a good 
idea of the course of the groove along the bore, by supposing the position 
which the line AC would assume were the above figure wrapped round a 
cylinder equal to AB in length and BC in circumference. 
For shunt rifling, as in the 64-prs., there must be a corresponding shunt 
on the copying bar, and for a uniformly increasing twist the edges of the 
bar must be curved accordingly ‘ and as it is the property of the parabola to 
increase uniformly in curvature, we have adopted it as the curve of the 
rifling, its well known properties enabling us to easily construct a copying 
bar for any required spiral. 
Thus, suppose we want to make a copying bar for the 9-inch gun, which 
has an increasing twist from 0 at the breech to 1 in 45 at the muzzle. 
55 
[VOL. VI.] 
