Depression range finder

The depression range finder was a fire control device used to calculate firing solutions when gun laying in coastal artillery. It was necessitated by the introduction of rifled artillery from the mid-19th century onwards, which had much greater ranges than the old smoothbore weapons and were consequently more difficult to aim accurately.

Initially, a simple method of fire control was adopted in which an officer in an observation position would observe where shots landed and relay the information back to the gunner. It was hampered by the need for ranges to be estimated by eye, which introduced significant inaccuracy. Between 1870 and 1880, various mechanical range-finding devices were developed to provide a more accurate system. They were, however, limited by requiring two observers to be positioned on a piece of flat ground up to 400 m wide, using instruments to measure the two base angles of a triangle between the observers and the target.

A serving British Army officer, Captain H.S.S. Watkin of the Royal Artillery, devised a solution based on surveying principles, exploiting the fact that the observer's height above the waterline could be used as the base of the measuring triangle. A measurement of the angle of depression to the bow-waterline of the target would thus give the range.

While stationed at Gibraltar in the 1870s, he developed a device termed the Watkin Depression Range-finder, derived from the surveyor's level, which could be used in permanently fixed mountings whose height above sea level could be precisely determined. It was trialled by the War Office between 1876 and June 1881, when it was formally adopted, and subsequently became standard equipment in coastal forts and batteries. It was easy to use, highly accurate and was combined with the electric telegraph to enable observers to rapidly relay firing corrections to the guns.