Dumaresq

A brass disc and sliding bar turned the deadliest weapons of the industrial age from lottery tickets into precision instruments. By 1900, naval guns could hurl shells 10,000 yards, but the calculations required to aim them—accounting for the speed and course of both ships, wind, shell flight time, and the rotation of the Earth—overwhelmed human cognition under combat stress.

Lieutenant John Saumarez Dumaresq of the Royal Navy recognized that the core problem was geometric. If you know your ship's speed and heading, and the enemy's bearing, course, and speed, trigonometry can tell you how fast the range is changing (range rate) and how much the enemy is moving sideways relative to your guns (deflection). But solving those equations with paper and pencil, while shells exploded around you, was impractical.

Dumaresq's insight was to build the geometry into metal. His 1902-1904 device used a circular dial plate representing your ship, with a movable bar representing the enemy's course. Pointers and scales mechanically computed range rate and deflection as operators turned knobs to input current data. The machine didn't calculate—it demonstrated, making the trigonometric relationships visible and tangible.

The adjacent possible for mechanical fire control had been assembling since the 1880s. Barr & Stroud's coincidence rangefinders could measure distance to 10,000 yards with reasonable accuracy. Precision machining could produce gears and scales with the tolerance required for meaningful computation. The all-big-gun battleship concept demanded that all main armament fire at the same target, requiring centralized fire control. And the arms race between Britain and Germany created intense pressure for any advantage in gunnery.

Elliott Brothers manufactured the device, patenting it in Dumaresq's name in August 1904. The Royal Navy ordered approximately 1,000 units across various marks by 1913, at a total cost of £10,000. When HMS Dreadnought commissioned in 1906—the ship that rendered all previous battleships obsolete—a Dumaresq sat in her transmitting station, feeding data to her Vickers range clocks.

The device evolved rapidly. The Mark II was larger for easier reading. The Mark IV was designed for turret mounting, allowing independent fire control if the main system failed. Frederic Dreyer added gears in 1908 so the enemy bar would automatically adjust as the home ship changed course. By World War I, the Dumaresq had been integrated into the Dreyer Fire Control Table, a room-sized mechanical computer that continuously generated firing solutions.

But the Dumaresq also reveals path dependence in military technology. The Admiralty rejected Arthur Pollen's more sophisticated Argo system in 1913, partly because officers had already invested years learning the Dreyer approach. Some historians argue this decision contributed to poor British gunnery at Jutland in 1916, where several battlecruisers were lost. The simpler tool, already embedded in training and doctrine, won over the technically superior alternative.

After World War I, the Dumaresq's functionality was absorbed into larger automated systems. The device itself became obsolete, but its descendants—mechanical fire control computers, then electronic, then digital—trace a direct lineage. The USS Iowa's analog fire control computers of World War II were conceptual descendants. Today's naval combat systems, calculating missile intercepts at supersonic speeds, still solve the same geometric problem Dumaresq mechanized in 1902.