What is "drift sight"

The Drift Sight was a bombsight developed by Harry Wimperis in 1916 for the Royal Naval Air Service (RNAS). It used a simple mechanical device to measure the wind speed from the air, and used that measurement to calculate the wind's effects on the trajectory of the bombs. The Drift Sight eliminated the need for a stopwatch to perform this calculation, as on earlier devices, and greatly eased the bomb aimer's workload.

The Drift Sight was quickly introduced into RNAS service and quickly thereafter by the Royal Flying Corps (RFC) as well. In British service, Wimperis' Course Setting Bomb Sight (CSBS) started replacing the Drift Sight in 1917, but it remained in widespread use in the US Army Air Service into the 1920s. In US use the Drift Sight is often referred to as the Wimperis sight, but this name is more commonly applied to the CSBS, especially in Commonwealth air forces.