THE ROYAL ARTILLERY INSTITUTION. 
179 
“ The angle-finders are identical in general construction, but inverted in 
details, one instrument being right and the other left-handed. 
“ Each angle-finder consists of a long telescope, round each end of which 
is a band, turned truly round. The bands rest on two Y’s, fixed on 
the outer side of the barrel of the gun, so that when the telescope 
is laid in them, the axis of the telescope is parallel to that of the gun ; 
above the telescope is an index plate, graduated in a manner which will be 
hereafter described; at the rear of the index plate is a pivot, on which a 
steel limb revolves. Above the steel limb, at the pivot, is fixed a short 
telescope, which revolves with the limb, and remains constantly at right 
angles to it. The limb and short telescope receive their motion from a 
screw fixed on the index plate, and working through a nut on the steel limb. 
The short telescope is protected by means of a tin case, to the inner lip 
of which is pasted a white paper ring, This case does not revolve, but 
is fixed at right angles to the index plate, a hole being cut in its side so 
as not to interfere with the index bar. 
“ The tape is an ordinary measuring tape, working on a reel. At the 
loose end is a hook, which, when the tape is used, is hooked to the inner 
trail-handle of one gun, and the reel is then carried over to the trail of 
the other. 
“ To measure the range, by means of the instrument, two guns a,re used. 
They are drawn up at an interval of about 40 yds., and are generally 
dressed so that the object will be directly in front of some point in the 
inverval. Each gun is then laid on the object, and the interval from 
inner trail-handle to inner trail-handle measured. The short telescope 
of each finder is then turned on the vertical axis of the white ring on 
the lip of the case of the other. This axis is marked by a red spot on 
the highest and lowest points of the circumference of the ring. The 
angles between the long and short telescopes will consequently be the 
base angles of the triangle ABC, formed by the object and the two guns, 
and their sum will be the supplement of the angle BC. (See Diagram I). 
“Now, as we may fairly assume our range over 500 yds., AB and AC 
are very large in proportion to BC; and as the point A is directly 
opposite some point in the base BC, the length of the side BA, CA, will 
be much the same for any particular magnitude of the angle BAC, 
whether the triangle BA C be isosceles or not; and we may therefore 
treat the triangle BAC as an isosceles triangle of known base and known 
angles, of which it is required to find the sides. (The error resulting 
from this assumption will, hereafter, be proved too small to matter in 
practice.) 
“ Our formula will be 
. 7r — a 
sin a 
a 
C ° S 2 
sin (/? + y) 
r — range = AB = AC, 
b— base = BC, ■ 
a = /. BA C. 
