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Deflection is an ambiguous term in many sources covering the World War I period, especially British ones owing to the twisted etymology of the word.

Be careful when contemporary British sources use the word "deflection", as it might mean either speed-across or gun deflection depending on the context.

When a British source says... means
"Deflection" (ambiguous)
"Gun Deflection" Lateral Aim-off
"Sight Deflection" Lateral Aim-off
"Dumaresq Deflection" Speed-across

Torpedo Control Usage

To make matters even worse, when "deflection" is spoken of in the sense of how to set a Torpedo Deflection Sight or similar device, they are generally speaking of the speed-across attributable to only the movement of the target vessel, discounting that attributable to movement of own ship. On this site, you may find that I employ my own term to make this distinction explicit: Torpedo Deflection.

See Also