Arthur Wellesley White

“ Rangefinders are, however, sometimes employed. The most worthy of notice is the Watkin depression rangefinder used by the British artillery in coast batteries. The instrument resembles in principle the Watkin field rangefinder, the height above the sea-level being a vertical base. The range is found by observing the angle of depression to the object. This is done by a powerful cross-wire telescope, which forms part of the instrument. The fastest steamer can be continuously followed, and even the successive grazes of shot and shell can be observed. ”

“ Fig. 4.—Weldon rangefinder.
The merits of different field rangefinders depend mainly upon the balance of advantages they offer with respect to accuracy, suitability to variety of ground, simplicity, portability, and durability, these conditions being of a more or less conflicting character. The following are recognized principles: (1) the naked eye cannot with certainty appreciate less than one minute difference of angle, therefore telescopic power is necessary in proportion as the base is short compared with the range; (2) telescopes of high power cannot be steadied by hand alone ”

“ Rangefinders for Coast Batteries.—Rangefinding is less adapted to the requirements of coast defence than “position finding,”—a method which furnishes every gun with its proper training and elevation so that it can be fired without sighting the target. ”