Dumaresq
Below the bar is a round metal plate inscribed with a coordinate plot, and an angle scale around its outer rim.
The fixed bar is mounted on a bearing that allows it to be turned to represent the direction of motion of the ship, measured against the scale.
This allows it to be used "backwards", a process called a "cross cut", to take sequential estimates of the range and bearing of an enemy vessel and discover its speed and heading that would be consistent.
The design of the dumaresq consists of a circular dial with a cross-bar passing over the centre which is oriented to match the heading of one's own ship.
A sliding assembly can be moved backwards along a scale etched on this bar to indicate the ship's speed in knots.
Suspended below the slider is a second bar, which recorded the speed and heading of the enemy ship by rotating and sliding against a similar scale to that on the main cross-bar.
[2] In 1909 it was proposed to add a compass ring to the dial plate, and another mounted on the cross bar for the enemy ship.
[3] The mark IV version was developed in 1910, intended to be used within a gun turret operating independently from the centralised fire control.
[3] In 1908 Frederic Dreyer suggested an improvement, adding gears so that the enemy bar would alter direction automatically when the dial plate was rotated.
[5] The electrical dumaresq's special features were very particular to its use in the Dreyer FCTs in which it was fitted, sitting atop a range clock.
That this was done by simple thumb work suggests that this dumaresq was meant to operate in the absence of advanced systems such as the Admiralty Fire Control Table that was then in service.
An example of the Spartan dumaresqs that survived beyond World War I, these were very simple, with fixed cross-bars and an own-speed of 12 knots that could not be altered.
A further indication that these were to be used by less intensively trained personnel is that the dial plate helpfully features an image of a gun muzzle which is to be pointed toward the enemy ship.
