44 
THE WELDON RANGE-FINDER. 
It consists of a plate-glass prism, bound in brass, baying tbe angle 
BAC carefully ground to 88° 34' 3", tbe back, BBEC, being silvered, 
so as to act as a mirror. Its cost is great, owing to tbe care which 
bas to be given to cutting tbe angle accurately. In practice, tbis 
instrument, small as it is, is found to work perfectly, giving ranges 
witb great accuracy. 
Tbe drawing (Fig. 5) will explain its use. Tbe sketch is neces¬ 
sarily much exaggerated. It is required to find tbe distance of tbe 
Fig. 5. 
palm tree, A. Tbe observer moves along tbe line CF until be sees 
the palm tree and tbe eye of tbe soldier at C coincide. Tbe soldier at 
C, provided witb a similar instrument, sees tbe palm tree and tbe eye 
of the observer at B reflected in a similar way. Then each of tbe 
angles ABC, ACB = 88° 34' 3", and AC = 20 BC. 
Major Weldon also works witb another instrument, rather larger 
and perhaps slightly more complicated, but nevertheless extremely 
simple. It is far cheaper than tbe last described—costing about 
15 shillings—and is easy of adjustment. Tbe instrument (Fig. 6) 
consists of a solid brass case, HGIJK, in tbe form of a segment of a 
cylinder, tbe apex angle being taken greater than 89°, having two 
oblong openings, W, in each side, and underneath each of these 
windows a block, K, in wbicb is fixed a mirror, M. Tbe block on tbe 
leffc-hand side is firmly fixed witb screws, but tbe block on tbe right 
is capable of slight adjustment by screws at L. Tbe mirrors are 
inclined to one another at an angle of 88° 34' 3". A handle, P, is 
screwed to tbe case, for convenience in using the instrument, and 
there is a loop at tbe end of tbis handle, to admit of tbe attachment 
of a lanyard, so that it may be carried round tbe neck and stowed 
away in a pocket without being liable to a fall. 
When using tbe instrument, it must be held witb tbe open side 
towards tbe eye, but nearly half-turned in tbe direction of tbe object 
to be reflected, so that on looking into the instrument only one 
reflector is visible, in wbicb will be seen objects to tbe right or left 
of tbe observer. 
