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 . History . Early bombsights . Prior to the introduction of the Drift Sight , bombsights were generally very simple systems of very limited accuracy . The primary pre - war device in RNAS service was the " Lever Sight " which had to be held out of the cockpit in one hand by the pilot while flying the aircraft with the other . The Central Flying School Bombsight ( CFS ) replaced this in 1915 , but was difficult to install in the cockpit . The CFS was in turn replaced by the Equal Distance Sight ( EDS ) designed by in 1916 by Warrant Officer Scarff , better known for the Scarff ring . The EDS allowed the bomb run parameters to be entered once and then left the pilot free to fly the plane . Adjusting for the effects of wind was a difficult process . None of these sights had a way to calculate " drift " , the sideways motion of the bombs due to wind . This meant the aircraft had to attack their targets along the wind line . Even in this direction , the wind would cause the bombs to fall long or short . To correct for this , on an extended run the bomb aimer would measure their speed over the ground using a stopwatch , calculate the wind speed , and then adjust their aim point based on that speed and their current altitude . This solution was difficult , time consuming , and prone to error . Drift Sight . In 1916 , Henry Wimperis started design of a new bombsight , working in collaboration with Scarff . His " Drift Sight " included a simple system for calculating the effects of wind . This measurement was taken prior to the bomb run , using a secondary sighting system are the rear of the main bombsight . The pilot would first determine the direction of the wind , and estimated its speed . The estimated speed was dialled into the sight , which moved a metal rod at the back of the sight so it laid at an angle to the fuselage . The aircraft would then fly at right angles to the bomb line , which would cause the wind to push the aircraft sideways . Comparing the angle of motion of objects on the ground to the angle of the rod , the bomb aimer would adjust the angle of the rod until the two were equal . A scale at the end of the bar directly read the wind speed . This adjustment also automatically moved sights at the front of the bombsight fore and aft , directly setting the bombsight to calculate the drift of the bombs due to the measured wind . Two sights were used , one for bombing directly upwind , and another for bombing directly downwind . The timing of the drop was set by dialling in the measured air speed , which moved the entire sighting system fore or aft , carrying the two " backsights " along with it ( as well as the drift bar mechanism ) . Setting the altitude moved the foresight , below the backlights , setting the correct bombing angle . The original design was only suitable for low - level use and was later known as the Low Height Drift Sight Mk . I . At higher altitudes the indicated airspeed - being measured by pitot tube instruments - was affected by differences in outside air pressure that rendered it increasingly inaccurate . The Mk . IA was introduced for this role , including a simple adjustment between the airspeed and altitude settings that accounted for this effect . A third version was also introduced for use by Navy airship s , which worked at much lower speeds and also had the advantage of being able to directly measure the windspeed by throttling their engines until they lay still over the water . Although the Drift Sight was a significant improvement over earlier designs , it still required the aircraft to fly up or downwind on the final bomb run . For the RNAS this was a serious problem , as a submarine or ship would attempt to maneuver away if it spotted the attack , and thus upset the bomb run . Over land , as anti - aircraft gun s grew more proficient , this became a serious concern as they would pre - sight for firing along the wind line , and using the bombsight under fire was difficult . There were instructions on how to use the Drift Sight for cross - wind bombing , but this was complex and apparently rarely used . In service . The Drift Sight was introduced in 1916 , and the simplicity of the device in terms of manufacture and mounting to the aircraft allowed it to quickly equip RNAS forces . By 1917 it was in widespread use in the RNAS , and was selected for the Handley Page O / 400 bomber in RFC service as well . However , Wimperis was well aware of the shortcomings of the Drift Sight in terms of flying along the wind line , and was already testing his solution , the Course Setting Bomb Sight . The CSBS was only slightly more complex than the Drift Sight to build , adding a compass and another adjustment to the sights to account for cross - drift . Hundreds were in use by 1918 , and had supplanted the Drift Sight by the war ' s end . A major difference between the Drift Sight and CSBS was that the later needed to sight below the aircraft , and was not as suitable for use on the side of the fuselage . This kept the Mk . IA in US Army Air Service use for some time , as they did not have dedicated bomber aircraft until after the war . Oddly , when development of new bombsights started in the 1920s , these were based on the Drift Sight design , not the CSBS . The US Navy , contrarily , developed their Mk . III design from the CSBS , and it was a borrowed Mk . III sights that were used to sink the former German battleship " Ostfriesland " in 1921 . Description . The Drift Sight was mounted to the side of the aircraft using two brackets at the front and back of the sights . The forward bracket had a pivot point , and the rear a screw jack that rotated the entire sighting system up or down around the front bracket ' s pivot . This allowed fine adjustment of the levelling of the sight during flight to account for changes in trim ; after making these adjustments the system was locked in position using wingnuts . This mounting system was common among early British bombsights , notably the Equal Distance Sight that led to the Drift Sight . The main sighting system rode on two rods running fore and aft between the mounting brackets , allowing the bomb aimer to move it to a comfortable position for sighting . In the Equal Distance design , this movement was itself the main aiming mechanism , and could not be adjusted for ease of use as it was in the Drift Sight . Sighting was accomplished in the manner of iron sight s on rifles , and the Drift Sight used the same terminology of " foresight " and " backsight " , although " lower " and " upper " would be more accurate physically . The fore and backsights were thin metal wires stretched across the opening of a C - shaped metal plate . The bombs were dropped when the wires of the foresight , backsight and target overlapped as seen from the bomb aimer ' s position . A separate " direction wire " ran fore - aft on a plate below the foresight , providing left - right aiming . Altitude was set by moving a lever fore - and - aft against a scale , which rotated the foresight fore or aft , setting one half of the sight ' s bombing angle . Behind the altitude setting was an airspeed scale and a wheel to finely select the airspeed . This moved the backsights fore or aft to account for the other half of the sight ' s " range angle " . In this respect the Drift Sight was similar to earlier bombsight designs . Where it differed was in the addition of the " drift bar " setting at the extreme rear of the sighting system . The drift bar was a metal rod that extended rearward from the sight and was pivoted so it could rotated outward , away from the aircraft fuselage . Prior to the bomb run , the pilot or bomb aimer would dial in an estimated wind speed , which rotated the bar outward with increasing values . They would then look past the bar to any convenient objects on the ground , comparing its direction of motion to the line of the bar . Fine adjustments to the wind speed were made until the observed drift was directly along the line of the bar . Changing the setting of the wind speed had the effect of moving the two backlights closer or further apart . This accounted for the effects of the wind on the trajectory of the bombs in addition to the airspeed setting . Two sights were used , one for bombing upwind and another for downwind . As the Drift Sight only worked correctly if flying straight and level , the bombsight also includes two spirit level s .