The Forbes Log, Forbes Speed Indicator and Forbes Distance Recorder formed a family of devices to measure the passage of one's own ship through the water and to relay its speed and integrated distance traveled to useful spaces in the ship. One might think of them respectively as an axis-mounted revolution sensor, a speedometer, and an odometer.
- Manual of Gunnery in H.M. Fleet, Volume III, 1920, Plate 67.
- Manual of Gunnery in H.M. Fleet, Volume III, 1920, Plate 68.
- Admiralty, Gunnery Branch (1920). Manual of Gunnery (Volume III) for His Majesty's Fleet, 1920. (G. 13018/10) Copy No. 2 is Ja. 013 at Admiralty Library.