FIELD RANG-E-FJNDTNG. 
203 
involving neither calculation, adjustments, nor ambiguity, which can be 
worked successfully and understood thoroughly by any intelligent 
soldier with a few hours instruction. A certain nicety of observation, 
however, is always required to attain accuracy, unless the bases are 
inconveniently long. 
Both the instruments adopted by the Range-finding Committee 
belong to the former class (though, perhaps, the best representatives 
of their class), and to this their failure would seem to be attributable. 
We must therefore look in the other quarter for success. 
All trustworthy systems are based upon triangulation—which means 
measuring one or more sides and one or more angles of a triangle, and 
calculating therefrom by some means other sides or angles which 
cannot be measured; in other words, solving the triangle. These 
systems only differ from each other in the variation allowed in angles 
and bases, and in the mode of solving the triangles. 
The more variation allowed, the more intricate becomes the machine, 
and the more calculation of one sort or another is involved; the less 
variation allowed, the simpler the machine, and the less calculation 
involved. 
There are very few systems—if they can be called systems—which 
are independent of triangulation. Amongst them may be mentioned 
the telemeter, depending upon the rate of travelling of sound, and the 
long-wished-for telescope, which ought to have a different length of 
focus for each 10 or 50 yds. range, but has not. 
To return to practicable systems. The only difference between the 
two service range-finders—Nolan's and Watkin's —in principle, is that 
in Nolan's the base, and both angles at the base, vary within certain 
limits; while in Watkin's the base (within certain limits) and one 
angle at the base vary, the other angle being a right angle and 
constant. In the former, the base and base angles being measured, 
the necessary calculation is performed mechanically; while in the 
latter, the calculation is performed by the same mechanical process by 
which the variable base angle is measured, the variable main base 
having been previously measured in a similar manner, by measuring 
from one end of it the angle subtended by a short constant base of 
18 ft. set off at right angles to the other end of the main base. 
The angle-finder in the former is on the principle of a theodolite ; 
in the latter on the principle of a sextant. 
Innumerable systems have been proposed and experimented upon, 
from time to time, differing more in mechanism and name than in prin¬ 
ciple ; and would-be inventors would do well to consider and lay to 
heart this fact before rushing into the thorny path of invention, with 
its costly experiments and inevitable disappointments. 
The following is a summary of nearly all possible systems of trian¬ 
gulation, in which it is assumed (1) that one of the long sides of the 
triangle is an unknown quantity and the required range; (2) that it is 
impossible to measure directly the apex angle of the triangle when the 
object forms this apex; and (3) that the base and base angles are 
known, or can be measured. 
