Range-finders — Chambers's Encyclopaedia

Range-finders (see CANNON, GUNNERY) are used for finding the range or distance of an enemy in the field, that in use in the British army being the Watkin Range-finder, introduced in 1882. It is contained in a rectangular box about $6\frac{3}{4} \times 2\frac{3}{4} \times 1\frac{1}{2}$ inches, and is based on the principle of the sextant, but so constructed that, while the near object is seen by reflection in a mirror, the distant object is seen by direct vision, and it can be used either with or without telescopes at its two eyeholes. Its use may be briefly described as follows: Let O be the object to be observed; a base line AB of known length, from 60 to 120 yards, is taken which is at right angles to AO as seen through the instrument at A. Another point on BC is found which is also at right angles to AB. When the instrument at B is set to observe O the angle CBO is obtained, and this being equal to the angle AOB, the distance between OA can be calculated. To save calculation, however, when the instrument is properly set for the length of base and the angle CBO the distance can be read off at once, the whole operation not taking longer than $2\frac{1}{4}$ minutes, and the error should be less than 1 per cent. of the range. All the officers and certain non-commissioned officers in every regiment are trained in the use of the instrument.