As the range of coastal artillery increased, there was an accelerating demand for a reliable method of laying guns onto a moving naval target some miles away. With the transition from Rifle Muzzle Loading guns to Breech Loading guns, effective ranges increased from three thousand meters, to well in excess of ten thousand meters. These increased ranges meant that optical instruments were required to accurately lay the guns on a moving targets. Alongside the improvement in the range of coastal defence guns, there was advances in technical equipment for measuring ranges to objects. The most significant of these was the Watkin Depression Range Finder which was formally adopted in 1881 after four years of trial.
The Depression Range Finder (DRF) was based on surveying principals using the known height above sea level of the instrument as the base for measuring the distance to a target vessel. This DRF was quickly followed by the Position Finder (PF) which used two seperate positions on the shore, some distance apart, to give horizontal base (base end station). This gave a more accurate result than a single DRF.
These horizontal PF Cells were developed to work together with one functioning as a transmitter PF while the other acted as a receiver or Depression Position Finder (DPF). The DPF cell has a plotting board mounted in it on a table with three stone legs. This plotting board, or range dial, displayed the range and bearing of the target. This system was adopted by 1887 after trials at Fort Bovisand and Breakwater Fort. Steps were also taken so that guns could be electronically corrected and fired from the PF Cells.
The Position Finding (PF) system tended to be used for 9.2-inch guns while the DRF was used for gun of 6-inches or less.
The Depression Range Finder or D.R.F. was invented by Captain H.,S.S. Watkins in 1870s and introduced in 1881. It was based on the sexton used by by surveyors. The image shows a D.R.F. instrument from about 1900.
Depression Range Finder
D.R.F. Chapel Bay
D.R.F. Pedestal
The introduction of Depression Range Finder spawned a whole school of related range finding methods:
- The Position Finder used two two telescopes as a horizontal base for range finding
- Some of the Positions Finders were developed as Electric Position Finders
- By the 1890s the position finders were able to transmit directly to guns to adjust the laying of the guns
(1911 Encyclopaedia Britannica, Vol. 22, p. 890)
The image on the right shows a plotting table in the Fire Command at York Redoubt. Two of the three concrete legs are partially visible. Similar tables were used in Position Finding Cells.
The image on the right shows a typical position finding station with three cell, on McNab Island in Halifax, Canada. This particular cell is slightly raised to increase its’ field of vision. Often they were dug into the rock or soil.
Plotting Table
Position Finding Cells, Halifax
